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August 2010
March March 1999 1999
New New Zealand. Zealand.
at at the the University University of of Otago, Otago, Dunedin, Dunedin,
Master Master of of Science Science
A A thesis thesis submitted submitted for for the the degree degree of of
Craig Craig Irwin Irwin
Clam Clam (Austrovenus. (Austrovenus. stutchburyi) stutchburyi) in in Waitati Waitati Inlet. Inlet.
Population Population Biology Biology of of the the New New Zealand Zealand Littleneck Littleneck
The The Effects Effects of of Harvesting Harvesting on on the the Reproductive Reproductive and and
ii ii
followed followed by by biomass. biomass.
levels levels within within treatment treatment populations, populations, leading leading to to a a recovery recovery firstly firstly in in density density then then
resources. resources. This This allows allows for for an an increase increase in in growth growth rates rates and and higher higher settlement settlement
about about by by harvesting harvesting and and the the consequent consequent liberation liberation of of both both space space and and food food
ability ability is is likely likely to to be be due due to to the the decrease decrease in in intraspecific intraspecific competition competition brought brought
to to recover recover from from severe severe stock stock depletion depletion in in a a relatively relatively short short period period of of time. time. This This
These These results results indicate indicate that that Austrovenus Austrovenus stutchburyi stutchburyi is is quite quite resilient resilient and and able able
were were found found to to spawn spawn longer longer and and more more intensely intensely than than those those on on control control sites. sites.
gametogenesis gametogenesis or or spawning spawning was was found found on on treatment treatment sites sites although although clams clams
pattern pattern was was retained retained over over the the study study period. period. No No major major changes changes in in the the onset onset of of
clams clams having having been been recruited recruited to to the the population. population. On On control control sites sites a a unimodal unimodal
treatment treatment sites sites from from a a unimodal unimodal to to a a bimodal bimodal pattern pattern with with a a cohort cohort of of young young
was was evident evident on on control control sites. sites. The The distribution distribution of of length length frequencies frequencies altered altered on on
estimates estimates on on treatment treatment sites. sites. An An increase increase of of 1% 1% in in density density and and 6% 6% in in biomass biomass
Density Density levels levels were were found found to to recover recover to to 87% 87% and and biomass biomass 81% 81% of of initial initial
indices, indices, maturity maturity indices indices and and oocyte oocyte area area analyses analyses being being utilised. utilised.
reproductive reproductive analyses analyses were were made made during during the the study study period period with with condition condition
also also undertaken undertaken prior prior to to and and following following the the harvesting harvesting treatment. treatment. Monthly Monthly
biomass biomass and and density density estimates estimates were were made. made. Length Length frequency frequency analyses analyses were were
removed. removed. A A year year after after the the completion completion of of this this harvesting harvesting treatment treatment further further
estimates estimates were were made, made, 60% 60% of of the the biomass biomass from from one one site site on on each each area area was was
half, half, forming forming two two study study sites sites on on each each area. area. After After initial initial biomass biomass and and density density
Three Three study study areas areas within within Waitati Waitati Inlet Inlet were were selected selected and and each each area area divided divided in in
distributions distributions and and the the reproductive reproductive biology biology of of Austrovenus Austrovenus stutchburyi. stutchburyi.
biomass biomass and and density density levels, levels, the the consequent consequent shifts shifts in in length length frequency frequency
to to the the population. population. This This study study focused focused on on the the direct direct impact impact of of harvesting harvesting on on
structure structure of of the the remaining remaining populations populations and and the the recruitment recruitment of of new new individuals individuals
Waitati Waitati Inlet Inlet as as to to how how commercial commercial harvesting harvesting affects affects biomass, biomass, the the size size
Austrovenus Austrovenus stutchburyi stutchburyi and and local local recreational recreational and and traditional traditional fishers fishers of of
There There has has been been considerable considerable debate debate between between local local commercial commercial harvesters harvesters of of
Abstract. Abstract.
·~. ·~. ·~. ·~. ·~.
Acknowledgements.
From go to whoa this work has taken almost two and a half years to complete. In this time I have had a great deal of assistance from a number of sources. Of these the most influential has come from my two supervisers Mike Barker and John Jillett whose expert advice and guidance has been supurb. I would also like to thank the management team of Roger Belton and Simon Gilmour at Southern Clams Ltd (SCL) for their interest and investment in research in regards to their industry. Worthy of special thanks, also from SCL is both Dave Redshaw and Dave Pirri whose assistance in collecting field data was invaluable. David Fletcher was a godsend with has statistical advice as was Bev Todd, Evan Hunt and Ken Turner in helping to set up and undertake my lab and histology work. Last but not least a special thanks to my family and friends whose patience and support while completing this work has been fantastic.
iii
iv iv
Discussion Discussion ...... 39 39
Commercial Commercial Size Size Classes Classes ...... 36 36
Length Length Frequency Frequency Analysis Analysis ...... 32 32
Mean Mean Length Length ...... 30 30
Results Results ...... 30 30
Methods Methods and and Materials Materials ...... 28 28
Aims Aims ...... 27 27
Introduction Introduction ...... 25 25
Chapter Chapter Three: Three: Effects Effects of of Harvest Harvest on on Size Size Frequency Frequency Distribution Distribution
Discussion Discussion ...... 21 21
Density Density ...... 18 18
Biomass Biomass ...... 15 15
Treatment Treatment Effects Effects ...... 15 15
Homogeneity Homogeneity ...... 12 12
Biomass Biomass Estimates Estimates ...... 12 12
Results Results ...... 12 12
Methods Methods and and Materials Materials ...... 10 10
Aims Aims ...... 9 9
Introduction Introduction ...... 8 8
Chapter Chapter Two: Two: Harvest Harvest Effects Effects on on Biomass Biomass and and Density Density
Study Study Area Area Design Design ...... 6 6
Study Study Location Location ...... 6 6
Aims Aims ...... 4 4
Fishery Fishery History History ...... 2 2
General General Biology Biology ...... 1 1
Chapter Chapter One: One: Introduction Introduction
List List of of Figures Figures ...... : : ...... viii viii
List List of of Tables Tables ...... vi vi
Table Table of of Contents Contents ...... iv iv
Acknowledgements Acknowledgements ...... iii iii
Abstract Abstract ...... ii ii
Table Table of of Contents. Contents.
·~. ·~.
. . ,, ,, Table of Contents
Chapter Four: The Effect of Harvesting on Reproduction
Introduction ...... 42 Aims ...... 45 Methods and Materials ...... 46 Results ...... 49 Sex Ratios ...... 49 Histological Staging ...... ·...... 49 Maturity Indices ...... 60 Condition Indices ...... 63 Oocyte Analysis ...... 66 Yearly Patterns ...... 66 Comparisons Between Sites ...... 7 4 Discussion ...... 77
Chapter Five: General Discussion ...... 82 References ...... 88
v List of Tables.
Table 1. Reported commercial catch of A. stutchburyi from Waitati Inlet...... 3
Table 2. Total biomass and date of removal from each area ...... 11
Table 3. Total estimated biomass and 95% confidence limits from sample surveys carried out in October. The target weights for harvest are also shown with the actual weight removed shown in brackets ...... 12
Table 4. ANOVA on mean sample biomass (kg 0.099m-2), testing for variation between study sites and areas from the initial sample survey (Oct, 1996) ...... 13
Table 5. ANOVA on mean sample density (0.099m-2), testing for variation between study sites and areas from the initial sample survey (Oct, 1996) ...... 13
Table 6. ANOVA on mean biomass (0.099m-2) comparing study sites and areas before and after a harvesting treatment...... 16
Table 7. ANOVA on mean density (0.099m-2) comparing study sites and areas before and after a harvesting treatment...... 19
Table 8. Analysis of variance on mean clam length comparing treatment and control values for changes over time ...... 30
Table 9. Standardised residuals from Chi-squared analyses testing for variation between control and treatment sites in October, 1996 and February, 1998 ...... 34
Table 10. Standardised residuals from Chi-squared analyses testing for variation between control and treatment sites in October, 1996 and February, 1998 ...... 37
Table 11. Significant(+) and non significant (NS) results from ANOVA on clam maturity indices comparing treatment and control sites for monthly changes...... 62
Table 12. ANOVA on clam Maturity indices comparing monthly values for treatment and .control sites ...... 62
Table 13. Analysis of variance and L. S.D. tests on clam condition indices data comparing treatment and control sites for monthly changes ...... 64
Table 14. P-values from ANOVA on clam condition indices data comparing monthly values between control and treatment sites (Site) and study areas (Area). Where NS signifies an insignificant result, +significant to <0.05, ++significant to <0.001 and +++ significant to <0.0001 ...... 64
vi
vii vii
or or less less than than negative negative two two indicate indicate significant significant variation variation ...... 76 76
observed observed and and expected expected frequencies. frequencies. Standardised Standardised residuals residuals greater greater than than two two
also also shown shown where where t>7.81 t>7.81 indicates indicates significant significant variation variation in in distribution distribution between between
and and treatment treatment sites sites at at each each stage stage of of sampling. sampling. Chi-squared Chi-squared values values are are (X2) (X2)
Table Table 18. 18. Standardised Standardised residuals residuals from from Chi-squared Chi-squared tests tests comparing comparing values values from from control control
significant significant to to <0.05, <0.05, ++significant ++significant to to <0.001 <0.001 and+++ and+++ significant significant to to <0.0001 <0.0001 ...... 74 74
each each month month of of sampling. sampling. Where Where NS NS signifies signifies an an insignificant insignificant result, result, + +
Table Table 17. 17. ANOVA ANOVA on on clam clam oocyte oocyte areas areas comparing comparing treatment treatment and and control control site site values values at at
variation variation ...... 70 70
residuals residuals greater greater than than two two or or less less than than negative negative two two indicate indicate significant significant
frequencies frequencies between between months months on on control control and and treatment treatment sites. sites. Standardised Standardised
Table Table 16. 16. Standardised Standardised residuals residuals from from Chi- squared squared tests tests comparing comparing size size class class
comparing comparing treatment treatment and control control and sites sites for for monthly monthly changes changes ...... 67 67
Table Table 15. 15. Analysis Analysis of of variance variance L. L. and and S. S. D. D. tests tests on on clam clam mean mean Oocyte Oocyte area area (f..lm2) (f..lm2)
List List of of Tables Tables
. . . . ' ' ·~ ·~
viii viii
based based size size classes classes ...... 38 38
areas areas 8, 8, D D and and E E in in October, October, 1996 1996 and and February, February, 1998 1998 using using commercially commercially
Figure Figure 13. 13. Observed Observed length length frequencies frequencies for for clams clams from from control control and and treatment treatment sites sites on on
classes classes ...... 37 37
sites sites in in October, October, 1996 1996 and and February, February, 1998 1998 using using commercially commercially based based size size
Figure Figure 12. 12. Observed Observed and and expected expected length length frequencies frequencies for for clams clams from from control control and and treatment treatment
·three ·three study study areas areas B, B, D D and and E E in in February, February, 1998 1998 ...... 35 35
Figure Figure 11. 11. Observed Observed frequencies frequencies for for clams clams from from control control and and treatment treatment sites sites on on each each of of the the
increments increments ...... 33 33
treatment treatment sites sites in in October, October, 1996 1996 and and February, February, 1998 1998 using using 2.5mm 2.5mm size size
Figure Figure 10. 10. Observed Observed and and expected expected frequencies frequencies for for clams clams from from pooled pooled control control and and
E E in in October, October, 1996 1996 and and February, February, 1998 1998 ...... 31 31
Figure Figure 9. 9. Mean Mean clam clam length length (±95%CI) (±95%CI) for for the the control control and and treatment treatment sites sites on on areas areas B, B, D D and and
1996 1996 and and February, February, 1998 1998 ...... 31 31
Figure Figure 8. 8. Mean Mean clam clam length length (±95%CI} (±95%CI} for for pooled pooled control control and and treatment treatment sites sites in in October, October,
treatment treatment sites sites on on each each area, area, before before and and after after harvesting harvesting has has taken taken place place ...... 20 20
Figure Figure 7. 7. Mean Mean sample sample density density (0.099m-2) (0.099m-2) with with 95% 95% confidence confidence intervals intervals for for control control and and
control control and and treatment treatment sites sites before before and and after after harvesting harvesting took took place place ...... 19 19
Figure Figure 6. 6. The The pooled pooled mean mean sample sample density density (0.099m-2) (0.099m-2) with with 95% 95% confidence confidence intervals intervals for for
place place ...... 17 17
and and treatment treatment sites sites on on each each area, area, before before and and after after harvesting harvesting has has taken taken
Figure Figure 5. 5. Mean Mean sample sample biomass biomass (kg.0.099m-2) (kg.0.099m-2) with with 95% 95% confidence confidence intervals intervals for for control control
for for control control and and treatment treatment sites sites before before and and after after harvesting harvesting took took place place ...... 16 16
Figure Figure 4. 4. The The pooled pooled mean mean sample sample biomass biomass (kg.0.099m-2) (kg.0.099m-2) with with 95% 95% confidence confidence intervals intervals
1996 1996 ...... 14 14
intervals intervals for for control control and and treatment treatment sites sites within within each each study study area area in in October October
Figure Figure 3. 3. The The mean mean sample sample biomass biomass (kg) (kg) and and density density (0.099m-2) (0.099m-2) with with 95% 95% confidence confidence
confidence confidence intervals intervals for for control control and and treatment treatment sites sites in in October October 1996 1996 ...... 14 14
Figure Figure 2. 2. The The overall overall mean mean sample sample biomass biomass (kg) (kg) and and density density (0.099m-2) (0.099m-2) with with 95% 95%
Figure Figure 1. 1. Study Study area area design design ...... 7 7
List List of of Figures. Figures. ·~. ·~.
ix ix
600~1 600~1 second second 0001Jm2 0001Jm2 etc etc ...... 73 73
October October 1996 1996 to to January January 1998. 1998. Where Where the the first first size size class class is is 0-600!Jm2, 0-600!Jm2, the the
Figure Figure 25. 25. Observed Observed relative relative oocyte oocyte size size frequencies frequencies for for samples samples from from treatment treatment sites sites for for
600~1 600~1 second second 0001Jm2 0001Jm2 etc etc ...... 72 72
October October 1996 1996 to to January January 1998. 1998. Where Where the the first first size size class class is is 0-6001Jm2, 0-6001Jm2, the the
Figure Figure 24. 24. Observed Observed relative relative oocyte oocyte size size frequencies frequencies for for samples samples from from control control sites sites for for
1998 1998 ...... 68 68
control control and and treatment treatment sites sites an an each each study study area area from from October October 1996 1996 to to January January
Figure Figure 23. 23. Seasonal Seasonal variation variation in in mean mean oocyte oocyte area area 1Jm2 1Jm2 (±95% (±95% Cl) Cl) for for the the combined combined
treatment treatment sites sites from from October October 1996 1996 to to January January 1998 1998 ...... 68 68
Figure Figure 22. 22. Seasonal Seasonal variation variation in in mean mean oocyte oocyte area area 1Jm2 1Jm2 (±95% (±95% Cl) Cl) far far control control and and
1996 1996 to to January January 1998 1998 ...... 65 65
control control and and treatment treatment sites sites an an each each of of the the three three study study areas areas from from October October
Figure Figure 21. 21. Seasonal Seasonal variation variation of of mean mean condition condition indices indices (±95% (±95% Cl) Cl) far far the the combined combined
control control sites sites from from October October 1996 1996 to to January January 1998 1998 ...... 65 65
Figure Figure 20. 20. Seasonal Seasonal variation variation of of mean mean condition condition indices indices (±95% (±95% Cl) Cl) for for treatment treatment and and
treatment treatment sites sites ...... 61 61
calculated calculated from from developmental developmental staging staging of of histological histological slides slides for for control control and and
Figure Figure 19. 19. Seasonal Seasonal variation variation of of mean mean monthly monthly male male and and female female maturity maturity indices indices
histological histological sections sections on on control control and and treatment treatment sites sites ...... 59 59
Figure Figure 18. 18. Seasonal Seasonal distribution distribution of of female female developmental developmental stages stages determined determined from from
histological histological sections sections on on control and and control treatment treatment sites sites ...... 57 57
Figure Figure 17. 17. Seasonal Seasonal distribution distribution of of male male developmental developmental stages stages determined determined from from
spawning spawning (x425} (x425} ...... 55 55
development development (x425); (x425); (c) (c) late late development development (x425); (x425); (d) (d) ripe ripe (x425) (x425) and and (e) (e)
Figure Figure 16. 16. Female Female developmental developmental stages stages (magnification): (magnification): (a) (a) spent spent (x425); (x425); (b) (b) early early
(x1 (x1 060); 060); and and (f) (f) spawning spawning (x425) (x425) ...... 52 52
development development (x1060); (x1060); (c) (c) late late development development (x1060); (x1060); (d) (d) ripe ripe (x425); (x425); (e) (e) ripe ripe
Figure Figure 15. 15. Male Male developmental developmental stages stages (magnification): (magnification): (a) (a) spent spent (x425); (x425); (b) (b) early early
clams clams over over sixteen sixteen months months ...... 49 49
Figure Figure 14. 14. Observed Observed and and expected expected sex sex ratios ratios (Male: (Male: Female) Female) from from histologically histologically examined examined
List List of of Figures Figures
-~. -~.
,, ,, . . List of Figures
Figure 26. Observed relative size frequencies for samples from control (white bars) and treatment sites (hatched bars) from October 1996 to January 1998. The expected frequencies(-- o--)are from Chi-squared analyses comparing the size class frequency distributions between control and treatment sites at each month ...... 75
X
1 1
stutchburyi stutchburyi does does vary vary with with shore shore dynamics. dynamics.
important, important, however, however, to to acknowledge acknowledge that that the the distribution distribution A. A. pattern pattern of of
shore shore distribution distribution as as such, such, but but the the site site specific specific effects effects of of harvesting. harvesting. It It is is
Pakawau/Puponga Pakawau/Puponga beach. beach. The The present present study study however however is is not not concerned concerned with with
Otago Otago Harbour Harbour by by Dobbinson Dobbinson et et al al (1989) (1989) and and by by Bull Bull 1985 1985 on on two two areas areas of of
These These trends trends have have been been found found throughout throughout the the country country by by Larcombe Larcombe (1971), (1971), in in
the the mean mean size size decreases decreases with with distance distance from from the the entrance entrance of of a a harbour harbour or or inlet. inlet.
mean mean A. A. size· size· of of stutchburyi stutchburyi increases increases from from high high to to low low shore shore levels. levels. Second, Second,
zone zone (Larcombe, (Larcombe, 1971 1971 ). ). Two Two trends trends in in shore shore distribution distribution are are evident. evident. First First the the
water water neap neap (Stephenson, (Stephenson, 1981) 1981) and and may may extend extend 6-8m 6-8m below below the the sublittoral sublittoral
A. A. stutchburyi stutchburyi distribution distribution is is restricted restricted to to the the shore shore below below the the lowest lowest high high
(Morton (Morton and and Miller, Miller, 1968). 1968).
most most abundant abundant bivalves bivalves on on New New Zealand Zealand intertidal intertidal mud mud and and sand sand flats flats
around around New New Zealand Zealand where where suitable suitable conditions conditions are are present, present, and and is is one one of of the the
deeper deeper than than 1 1 Ocm Ocm below below the the substrate substrate A. A. surface. surface. stutchburyi stutchburyi is is found found all all
sheltered sheltered shallow shallow water, water, soft soft shore shore habitats. habitats. They They are are rarely rarely found found buried buried
A. A. stutchburyi stutchburyi is is an an infaunal infaunal filter filter feeder feeder and and is is a a common common inhabitant inhabitant of of
stutchburyi. stutchburyi.
Chione, Chione, Anomalocardia Anomalocardia and and Venerupis) Venerupis) will will be be compared compared to to the the A. A. findings findings for for
the the biologyof biologyof other other Venerid Venerid clams clams (i.e. (i.e. Mercenaria, Mercenaria, Protothaca, Protothaca, Tapetinae, Tapetinae,
environmental environmental and and ecological ecological niche. niche. Therefore Therefore for for the the purposes purposes of of this this study, study,
Northern Northern Hemisphere Hemisphere Chione Chione and and has has thus thus adapted adapted to to a a similar similar
genus genus it it Chione Chione is is suggested suggested A. A. that that stutchburyi stutchburyi evolved evolved convergently convergently to to the the
though though Jones Jones ( ( 1979) 1979) A. A. suggests suggests that that stutchburyi stutchburyi may may not not belong belong to to the the
Mercenaria Mercenaria and and Protothaca Protothaca and and some some members members of of the the Tapetinae. Tapetinae. Even Even
characteristics characteristics however however Jones Jones (1979) (1979) A. A. found found stutchburyi stutchburyi to to resemble resemble
significantly significantly from from species species of of the the genus genus Chione Chione (Jones, (Jones, 1979). 1979). In In some some
Clam. Clam. It It has has been been found found that that the the anatomical anatomical A. A. structure structure of of stutchburyi stutchburyi differs differs
stutchburyi stutchburyi is is currently currently fished fished and and exported exported as as the the New New Zealand Zealand Little Little Neck Neck
the the Veneridae Veneridae (Bivalvia) (Bivalvia) family" family" commonly commonly known known as as Venus Venus A. A. clams. clams.
the the New New Zealand Zealand cockle cockle and and formerly formerly known known as as Chione Chione stutchburyi stutchburyi belongs belongs to to
Austrovenus Austrovenus stutchburyi stutchburyi (Gray, (Gray, 1828, 1828, Veneridae: Veneridae: Bivalvia) Bivalvia) often often referred referred to to as as
GENERAL GENERAL BIOLOGY. BIOLOGY.
CHAPTER CHAPTER ONE: ONE: INTRODUCTION. INTRODUCTION. ·~.
Chapter One: Introduction General Biology
Densities of over 4000 clams. m2 have been reported on Cheltenham beach near Auckland (Larcom be, 1971) and populations of A. stutchburyi are often found at such high densities that neighbouring individuals are touching and even mutually deformed. Growth is greatest over the spring-summer period and in smaller size classes (larcombe, 1971; Blackwell, 1984 and Martin, 1984).
The sexes have been found to be separate and at a ratio of approximately 1:1 (Larcom be, 1971 ). Sexual maturity appears to be a function of size rather than age with maturity being reached at 18-20mm in length (Larcombe, 1971). Spawning occurs once a year but varies over time and amongst localities (Larcombe, 1971; Booth, 1972 & 1983; and Stephenson, 1981). Settlement is variable from year to year and between different areas (Larcom be, 1971) with most populations being found to be dominated by one or two size classes. Recruitment has also been found to be temporally and spatially highly variable. Extensive movement of smaller size classes has been observed but is rare in larger (;;:: 25mm) specimens, which appear to move only in response to disturbance. (larcombe, 1971; and Stephenson, 1981).
FISHERY HISTORY.
A. stutchburyi has traditionally been an important food source to Maori They are also taken by recreational fishermen. Maori and amateur harvesters show a predilection for larger shellfish usually with a length in excess of 30mm. Accurate estimates for biomass removed by recreational harvesters is not available.
In 1980 a memorandum of understanding on shellfish was signed by the Food and Drug administration of the United States and the New Zealand Ministry of Agriculture and Fisheries Meat Division allowing exports of shellfish to US markets (Holdsworth and Cryer, 1991 ). In 1982 165 tonnes (live weight in shell) of A. stutchburyi were exported to the US, all of which were taken from Snake Bank in the Whangarei Harbour, Northland (Holdsworth and Cryer, 1991 ). Being one of the first areas to be commercially fished, Snake Bank has been extensively studied. Biomass surveys on Snake Bank since the inception of a commercial fishery there in 1982 found that while landings increased dramatically between 1982 and 1991, recruited biomass (shellfish over 30mm in length) fell to approximately one-third of its virgin level (Holdsworth and Cryer, 1991 and Cryer, 1996). The proportion of very large
2 "'" "~.
Chapter One: Introduction General Biology
(:=::35mm) A. stutchburyi fell to less than 10 percent of virgin stocks. Growth rates of the remaining shellfish and recruitment levels appear to have increased since 1982 (Holdsworth and Cryer, 1991 and Cryer, 1996). Commercial harvesting of A. stutchburyi continues to be carried out on Snake Bank (Whangarei Harbour), as well as Tapu Bay (Tasman Bay), Pahawau Beach (Golden Bay), and in Otago on Papanui and Waitati Inlets (Annala and Sullivan, 1997).
Commercial harvesting in Waitati Inlet is undertaken by a single company, Southern Clams Limited. Harvesting of low quantities began in 1990/91, increased in 1992 to 92t per year, and to 252t in 1994, and has remained at this level since (Table 1). Commercial picking is undertaken all year round with no particular seasonality. The fishery is currently managed under a maximum constant yield (MCY) approach. Management of Southern Clams Limited have recently applied to have this changed to a constant annual yield (CAY) approach. Where MCY allows harvesters a set catch level which remains constant over time and CAY sets an annual catch limit calculated from yearly surveys.
Table 1. Reported commercial catch of A. stutchburyifrom Waitati Inlet.
Year Landings (t) 1990/91 16 1991/92 14 1992/93 92 1993/94 109 1994/95 252 1995/96 252 1996/97 252 1997/98 252
The first biomass survey of Waitati Inlet was undertaken over the summer of 1983-1984 (Wildish, 1984). This first survey obtained a total biomass estimate of 12080 tonnes. Over the 1991-1992 summer a second survey undertaken by Stewart et al (1992) was carried out and a biomass of 11207 ± 707 tonnes was estimated. This estimate was to within 10 percent of that for the initial survey. Stewart et al (1992) suggested however that as a greater sampling effort was possible in their survey, about twice the number of quadrats were able to be sampled and that the 1983/1984 survey appeared to be a slight overestimate. The total commercial catch for the intervening eight years was around 259 tonnes (Belton, pers. comm; from Stewart et al, 1992). Recent biomass surveys carried out by the National Institute of Water and Atmospheric Research (Carbines, pers. comm.) has estimated a total
3 Chapter One: Introduction General Biology biomass of 10070 tonnes which is not significantly different to that found by Stewart et al (1992). Significant variation in the biomass of juvenile clams was found however, with significantly fewer being found in the 1998 survey. This was accounted for by poor recruitment during the period between sampling (Carbines, per. comm.).
Harvesting at Waitati Inlet is done by hand. First the top layer of sand and debris is swept to reveal a clear. layer of shellfish. Then a steel basket is worked into the sand and pulled through the substrate. The basket is then lifted, shaken and washed to remove smaller size classes and the contents tipped into onion bags for transportation to the factory. This operation takes place in association with the low tide to enable harvesters to work in water saturated sand. Depending on varying market preferences orders different size classes are targeted from day to day. This is achieved by site selection and a change in grill size of the baskets being used. Generally shellfish >30mm in length are targetted. This method of harvesting leaves small strips (approximately 60cm wide by 2-3m long) with a very low level of biomass remaining.
Aims.
Since the inception of the fishery at Waitati Inlet there has been continual lobbying against the harvesting of A. stutchburyi from this area. Recreational and traditional users are concerned that commercial harvesting is depleting the biomass and stripping the larger size classes from the area. The management of Southern Clams Limited believe that the harvesting of A. stutchburyi encourages faster growth, higher settlement and recruitment levels in the remaining shellfish populations. It is also contended that commercial harvesting at present levels has little effect on the overall biomass of the inlet.
The questions being addressed in this thesis all relate to the effect of commercial harvesting on A. stutchburyi populations, and can be divided into three areas. First, how commercial harvesting directly affects the biomass and number of shellfish in a harvested area, second, how commercial harvesting affects the remaining A. stutchburyi population size structure and third, how commercial harvesting affects the reproductive cycle of the remaining shellfish.
4 Chapter One: Introduction General Biology
Through assessing the impacts of commercial harvesting on population biomass, number, structure and reproduction, relevant information will be produced for use in the future management of the fishery.
5 STUDY LOCATION.
Waitati Inlet is a broad expanse of intertidal flat lying behind a spit at the mouth of an embayment which receives several small rivers, most notably the Waitati Stream and Carey's Creek.
Waitati Inlet is situated beyond a coastal promontory north of the Otago Harbour. The estuary is enclosed by the southward oriented Warrington sand spit and consists of a broad tidal flat-channel complex. Waitati Inlet covers about 6.5km2. The tidal compartment is approximately 4*106m3, and at low tide around 97% of the estuary is exposed.
STUDY AREA DESIGN.
Three commercially virgin study areas were located within Waitati Inlet. Each of these areas was located on sections of commercially unharvested beds which were considered to be of high enough density to allow commercial harvesting to be undertaken. Study areas were located on Bank B, Bank E and Bank D. Each study area was measured and marked out into a square 30m by 30m in size. Each area was then divided into two sites (one and two) along a line perpendicular to the main channel (Figure 1). Each site now 15m by 30m was then divided into three strata (a, b and c) of 15m by 1Om in size, moving parallel to and away from the main channel (Figure 1). Stratification allowed for an even sampling distribution over each study site.
On each study area study site one was designated as the control site and study site two as the treatment site. The consistency in site labelling was in order to reduce the possibility of confusion to harvesters and prevent harvesting being accidentally carried out on control sites.
6 ·~ "' .
Chapter One: Introduction Study Design
Channel
Strata ....•.---- 30m ------•~
a
30m b • 15m • c
• Site one• • Site two•
Figure 1. Study area design.
7
8 8
reduction reduction in in density density reduces reduces intraspecific intraspecific competition. competition. Food Food has has been been
1989 1989 and and Peterson Peterson and and Black, Black, 1993). 1993). It It is is generally generally considered considered that that a a
are are not not strongly strongly affected affected by by shellfish shellfish density density (Oiafsson, (Oiafsson, 1986; 1986; Dobbinson Dobbinson et et al, al,
and and Peterson Peterson and and Beal, Beal, 1989). 1989). Relatively Relatively few few have have found found that that growth growth rates rates
Blackwell, Blackwell, 1984; 1984; Martin, Martin, 1984; 1984; Bertness Bertness et et al, al, 1985; 1985; Peterson Peterson and and Black, Black, 1987 1987
reductions reductions in in shellfish shellfish growth growth rate rate with with increasing increasing densities densities (Peterson, (Peterson, 1982; 1982;
A A large large number number of of previous previous studies studies utilising utilising density density manipulations manipulations have have found found
Mercenarif) Mercenarif) mercenaria mercenaria at at 1-2 1-2 adults.m-2. adults.m-2.
size size of of about about 3.8cm 3.8cm and and K K rhytiphora rhytiphora 4.7cm. 4.7cm. Maurer Maurer (1983) (1983) found found densities densities of of
rhytiphora rhytiphora of of around around 30 30 clams.m-2. clams.m-2. With With K. K. approaching approaching scalarina scalarina a a maximum maximum
found found densities densities of of Katelysia Katelysia of of scalarina scalarina around around 130 130 clams.m-2 clams.m-2 and and Katelysia Katelysia
almost almost 6cm 6cm in in length. length. Again Again in in Western Western Australia Australia Peterson Peterson and and Black Black (1993) (1993)
different different clam clam species, species, the the largest largest of of which, which, Callista Callista ranged ranged impar impar up up to to
densities densities up up to to 41 41 clams.m-2. clams.m-2. This This comprised comprised a a combination combination of of several several
Western Western Australian Australian site site on on Shark Shark Bay Bay Peterson Peterson and and Black Black ( ( 1987) 1987) found found mean mean
present present at at a a mean mean density density of of 0.5 0.5 clams.m-2 clams.m-2 (mean (mean length, length, 4.3cm). 4.3cm). At At a a
densities densities up up to to 9 9 clams.m-2 clams.m-2 (mean (mean length, length, 2.9cm), 2.9cm), while while Chione Chione was was undatella undatella
California, California, Peterson Peterson (1982) (1982) found found Protothaca Protothaca staminea staminea living living in in sand sand at at
considerably considerably lower lower A. A. than than found found for for stutchburyi. stutchburyi. At At Mugu Mugu Lagoon Lagoon in in southern southern
In In populations populations of of clam clam species species overseas, overseas, densities densities have have been been found found to to be be
place place (i.e. (i.e. mid- low low shore shore level). level).
been been calculated calculated only only on on areas areas appropriate appropriate for for commercial commercial harvesting harvesting to to take take
variation variation of of shore shore heights. heights. The The mean mean densities densities for for the the present present study study have have
that that the the densities densities from from these these studies studies have have been been taken taken from from over over a a wide wide
and and 100 100 clams.m-2 clams.m-2 in in It It Western Western Golden Golden Bay Bay (Bull, (Bull, 1985). 1985). should should be be noted noted
Pilkington, Pilkington, 1995), 1995), 580 580 clams.m clams.m -2 -2 at at Pauatahanui Pauatahanui Inlet Inlet (Richardson (Richardson et et al, al, 1979) 1979)
350 350 clams.m-2 clams.m-2 on on the the Avon-Heathcote Avon-Heathcote Estuary Estuary in in Christchurch Christchurch (Marsden (Marsden and and
Auckland Auckland (Larcombe, (Larcombe, 1971) 1971) have have been been found found along along with with mean mean densities densities of of
in in Otago Otago Harbour Harbour (Dobbinson, (Dobbinson, 1986) 1986) and and on on Cheltenham Cheltenham beach beach near near
species species on on NZ NZ intertidal intertidal sand sand and and ·mudflats. ·mudflats. Mean Mean densities densities of of 600 600 clams.m-2 clams.m-2
Austrovenus Austrovenus stutchburyi stutchburyi has has been been found found to to be be one one of of the the dominant dominant bivalve bivalve
INTRODUCTION. INTRODUCTION.
DENSITY. DENSITY.
CHAPTER CHAPTER TWO: TWO: HARVEST HARVEST EFFECTS EFFECTS ON ON BIOMASS BIOMASS AND AND Chapter Two: Biomass and Density Introduction suggested as being more likely than space to be the limiting resource for which sedentary suspension feeders compete. Studies on hydrodynamics have shown suspension feeders are capable of depleting local food resources in their immediate surroundings (Carlson et al, 1972; Peterson and Black, 1987; Peterson and Beal, 1989; and Peterson and Black, 1993).
Peterson ( 1982) found that the recruitment rate of Protothaca in sand was never related to adult density. Maurer (1983) was unable to demonstrate any affect of varying densities of M. mercenaria on larval recruitment. Locally Martin (1984) produced tenuous evidence that the removal of exploitable size classes of A. stutchburyi led to higher recruitment levels. Martin (1984) did find strong evidence that recruitment levels were lower on areas of total removal. Re-colonisation of areas that have been entirely stripped have been shown to recover extremely slowly. Stephenson (1981) found that after 18 months areas stripped of A. stutchburyi had only recovered to 15% of their original biomass. Martin (1984) suggested this may be caused by larval substrate selectivity or possibly gregarious settlement patterns.
Peterson (1982) found that the rate of migration of P. staminea in sand was density dependent, and suggested that the normal response to high local densities is lateral migration. In New Zealand juvenile A. stutchburyi have been found by Larcombe (1971) to be capable of migration up to 1.5m between tides but he suggested that adult A. stutchburyi rarely moved at all. Larcombe (1971) suggests that movement in dense beds would be nearly impossible. To move, the animal would have to travel over the top of other shellfish, placing it in danger of not being able to dig itself back into the substrate. Stephenson ( 1981) found the mean size of immigrants to be smaller than the mean size in the population into which they move and that migration is especially frequent around the edges of disturbances.
Aims.
It is the aim of this study to observe how commercial harvesting affects a population of A. stutchburyi. How the population reacts to a size specific reduction in biomass was monitored by observing the biomass and density of shellfish on commercially unharvested areas before and twelve months after a harvesting treatment took place. This was undertaken to observe how harvested populations would react in comparison to control populations over a period of one year.
9 ";" "~.
METHODS AND MATERIALS.
Population Sampling.
Sampling was undertaken at the onset of the study (October, 1996) and again twelve months after the conclusion of the harvesting treatment (February, 1998). A total of twenty-one quadrat samples were taken from the treatment and control site on each of the three study areas (as described in chapter one). Within each site the twenty-one quadrat samples were evenly divided between three strata (a, band c), giving seven quadrat samples from each stratum. Quadrat location was randomly allocated from computer generated coordinates within each stratum. All material to a depth of 1OOmm was removed from a 0.099m2 (300*330mm) quadrat and put through a 2.0mm mesh sieve. The clams from each sample were weighed, using a Salter (1 Okg, 20g unit) set of field scales, counted and recorded on site.
Biomass Estimates.
From the means found from the initial and final sample surveys it was possible to estimate the total biomass and density of clams at each study site. This was achieved by converting the numbers in each sample mean to numbers m-2 and multiplying by the area of each site (450m2). Ninety five percent confidence intervals were produced using the same method as in Stewart et al 1992. Using the equation
Si2 = L (Xij- Xi)2J (ni -1) where Si2 = sample variance for stratum i; Xij = the difference between each total biomass value of each quadrat within a stratum (xij ) and the mean quadrat biomass value (xi); ni = number of quadrats taken in stratum i.
These sample variances were then used to produce 95% Cl for the biomass estimate in each area using the following equation
where N i = the number of possible quadrats that could be placed in stratum i.
10 0'.
Chapter Two: Biomass and Density Methods
Harvesting.
Once homogeneity and biomass estimates were established the harvesting treatment went ahead. Harvesting was carried out by Southern Clams Limited harvesters under normal procedures over a four month period (as described in Chapter One). An arbitrary amount of 55 to 60% of the estimated original biomass level was decided to be removed from the treatment site on each of the three study areas. Table 2 shows the harvesting regime with the date and number of sacks taken over the four month harvesting period.
Table 2. Total biomass and date of removal from each area
Study area D Study area E Study area B Date Sacks taken Date Sacks taken Date Sacks taken 20/10/96 33 18/10/96 20 10/10/96 64 25/11/96 24 24/10/96 37 23/1/97 28 26/11/96 12 28/10/96 34 30/1/97 59 29/12/96 18 28/11/96 23 26/2/97 37 9/1/97 66 12/12/96 30 20/1/97 14 29/12/96 33 21/1/97 20 10/1/97 28 27/1/97 24 4/3/97 60 28/1/97 12 20/2/97 50
Total sacks 273 265 188 Total weight 5596.5 5432.5 3854 (kg)* *Total weight (kg) has been calculated using an estimate of 20.5kg average weight per sack.
Statistics.
Analysis of Homogeneity.
From the initial sample survey, mean biomass and density values on control and treatment sites were analysed for homogeneity. A linear model ANOVA utilising the Datadesk statistical software package was used.
Treatment Effects Analysis.
Analysis of biomass and density utilised linear ANOVA from the Datadesk statistical software package to compare mean values between control and treatment sites before and one year after harvesting.
11
12 12
P P to to found found not not was was = = 0.9605) 0.9605) 5, 5, (Table (Table variation variation significant significant contrast contrast In In 0.160). 0.160).
± ± (2.249 (2.249 D D and and 0.126) 0.126) ± ± (2.207 (2.207 E E 0.134), 0.134), ± ± 1.489 1.489 8 ( ( 8 areas areas study study three three the the
between between biomass biomass sample sample mean mean in in found found was was P:::; P:::; 0.0001) 0.0001) 4, 4, (Table (Table Variation Variation
shown. shown.
are are intervals intervals confidence confidence 95% 95% with with levels levels density density and and biomass biomass mean mean the the
where where 2 2 Figure Figure in in observed observed be be can can This This 5). 5). (Table (Table either either evident evident was was 4.62) 4.62)
± ± (77 (77 sites sites control control and and 5.66) 5.66) ± ± (71 (71 treatment treatment on on (0.099m-2) (0.099m-2) density density sample sample
= = (±95%CI) (±95%CI) mean mean the the between between 0.1898) 0.1898) (P (P ± ± variation variation No No 0.142). 0.142). (2.046 (2.046 sites sites
control control and and 0.144) 0.144) ± ± (1.91 (1.91 treatment treatment from from clams clams of of 0.099m-2) 0.099m-2) (kg (kg biomass biomass Cl) Cl)
(±95% (±95% mean mean sample sample between between found found was was 0.1709) 0.1709) (P= (P= variation variation significant significant no no
that that shows shows 1996 1996 October October from from areas areas study study and and types types site site different different between between
density density and and biomass biomass mean mean comparing comparing 5) 5) and and 4 4 (Tables (Tables ANOVA ANOVA way way Two Two
Homogeneity. Homogeneity.
(3854) (3854) kg kg 3700-4000 3700-4000 8 8 area area Study Study
(5432.5) (5432.5) kg kg 5800 5800 5300- E E area area Study Study
(5596.5) (5596.5) kg kg 5900 5900 5400- D D area area Study Study
brackets. brackets. in in shown shown removed removed weight weight actual actual With With biomass). biomass). initial initial
total total of of 55-60% 55-60% (i.e. (i.e. area area study study each each of of site site treatment treatment the the from from removed removed be be to to weights weights Target Target
± ± ± ± 13816 13816 330908 330908 527.1 527.1 6753 6753 Treatment Treatment B B
± ± 14665 14665 360000 360000 ± ± 689.7 689.7 6780 6780 Control Control B B
± ± 13345 13345 341820 341820 ± ± 492.6 492.6 9595 9595 Treatment Treatment E E
± ± 11938 11938 366363 366363 ± ± 449.8 449.8 10465 10465 Control Control E E
± ± 6856.5 6856.5 323640 323640 ± ± 488.9 488.9 9788 9788 Treatment Treatment D D
12394 12394 ± ± 354555 354555 883.2 883.2 ± ± 10655 10655 Control Control D D
Cl Cl ±95% ±95% estimate estimate Cl Cl ±95% ±95% (kg) (kg) estimate estimate
Density Density 1996 1996 Biomass Biomass 1996 1996 Treatment Treatment area area Study Study
brackets. brackets.
in in shown shown removed removed weight weight actual actual the the with with shown shown also also are are harvest harvest for for weights weights target target The The October. October.
in in out out carried carried surveys surveys sample sample from from limits limits confidence confidence 95% 95% and and biomass biomass estimated estimated Total Total 3. 3. Table Table
clams. clams. 000 000 350 350 around around fall fall each each sites sites six six
the the for for estimates estimates abundance abundance Total Total sites. sites. treatment treatment and and control control the the on on OOOkg OOOkg
8 8 7 7 around around are are estimates estimates biomass biomass area area on on while while OOOkg OOOkg 10 10 approximately approximately
are are E E and and D D areas areas on on sites sites treatment treatment and and control control for for estimates estimates biomass biomass total total
the the seen seen be be can can As As 1996. 1996. October October in in intervals intervals confidence confidence 95% 95% relevant relevant with with
site site study study each each for for levels levels density density and and biomass biomass estimated estimated the the shows shows 3 3 Table Table
Estimates. Estimates. Biomass Biomass RESULTS. RESULTS.
13 13
Total Total 61126.8 61126.8 125 125
120 120 Error Error 60178.7 60178.7 501.48~ 501.48~
2 2 SA SA 311.873 311.873 0.311 0.311 155.937 155.937 0.7333 0.7333
A A 2 2 40.444 40.444 0.040 0.040 20.222 20.222 0.9605 0.9605
s s 1 1 595.841 595.841 595.841 595.841 3.821 3.821 0.1898 0.1898
Const Const 713859 713859 35301 35301 713859 713859 S0.0001 S0.0001
Source Source d.f. d.f. m.s. m.s. s.s. s.s. F F PValue PValue
Analysis Analysis of of Variance Variance
0, 0, Study Study area area (A): (A): B, B, E E
Site Site Control, Control, {$): {$): Treatment Treatment
Treatments Treatments
and and areas areas from from the the initial initial sample sample survey survey (Oct, (Oct, 1996). 1996).
Table Table 5. 5. ANOVA ANOVA on on mean mean sample sample density density {0.099m-
testing testing for for variation variation between between ), ), study study sites sites 2
125 125 Total Total 41.841 41.841
120 120 0.214 0.214 Error Error 25.728 25.728
2 2 SA SA 0.237 0.237 0.552 0.552 0.118 0.118 0.5770 0.5770
2 2 7.678 7.678 A A 15.355 15.355 35.810 35.810 S0.0001 S0.0001
s s 1 1 0.521 0.521 4.397 4.397 0.521 0.521 0.1709 0.1709
1 1 Const Const 494.842 494.842 64.453 64.453 494.842 494.842 0.0152 0.0152
Source Source d. d. f. f. F F m.s. m.s. s.s. s.s. PValue PValue
Analysis Analysis of of Variance Variance
Study Study area area (A): (A): B, B, D, D, E E
Site Site (S): (S): Control, Control, Treatment Treatment
Treatments: Treatments:
sites sites and and areas areas from from the the initial initial sample sample survey survey (Oct, (Oct, 1996). 1996).
Table Table 4. 4. ANOVA ANOVA on on mean mean sample sample biomass biomass (kg (kg 0.099m- testing testing for for variation variation between between study study ), ), 2
over over the the three three study study areas. areas.
E. E. D D and and It It is is also also clear clear that that mean mean sample sample density density levels levels are are fairly fairly uniform uniform
seen seen that that the the mean mean sample sample biomass biomass levels levels on on area area B B are are lower lower than than those those on on
sample sample biomass biomass and and density density values values for for each each of of the the study study areas. areas. It It can can be be
8.386), 8.386), ± ± E E (77.91 (77.91 5.38) 5.38) and and D D (74.619 (74.619 ± ± 4.898). 4.898). Figure Figure 3 3 displays displays the the mean mean
be be present present in in mean mean sample sample density density between between the the three three study study areas areas B B (76 (76 ± ±
Chapter Chapter Results Results Two: Two: Biomass Biomass and and Density Density Chapter Two: Biomass and Density Results
Biomass Density 2.5 - 80 T T - T .L T 2 - .L ,...... L l. 60 - ""Ci) ~ 1.5 - 1=! ""Q ~ 40 - 1 - ~ z 0.5 - 20 -
0 I I 0 I I Control Treatment Control Treatment Site Site
Figure 2. The overall mean sample biomass (kg) and density (0.099m-2) with 95% confidence intervals for control and treatment sites in October 1996.
Biomass 0 Control sites
lilll) Treatment sites
8
Figure 3. The mean sample biomass (kg) and density (0.099m-2) with 95% confidence intervals for control and treatment sites within each study area in October 1996.
14 Chapter Two: Biomass and Density Results
Treatment Effects.
Biomass.
The initial mean (±1 SD) biomass kg.m-2 for pooled control and treatment sites was 20.67 (±5.82) kg.m-2 and 19.36 (±5.87) kg.m-2 respectively. These values changed to 21.91 (±7.98) kg.m-2 for control sites and 15.74 (±8.76) kg.m~2 for treatment sites twelve months after the completion of the harvesting treatment. On control sites an increase in mean sample biomass of 1.24 kg.m-2 or 6% of the initial mean site biomass was recorded. On the treatment sites a decrease of 3.62kg.m-2 was recorded which amounts to a 18.7% decrease in mean site biomass (Figure 4). Therefore biomass on treatment sites has recovered to 81.3% of the original estimated biomass one year after the removal of 55-60% of the initial estimated biomass, while little change in biomass has been observed on control sites over time.
Considerable variation in initial (1996) mean biomass (kg 0.099m-2) between study areas was found (Figure 5), with a substantially smaller mean biomass level found on area B than areas D and E. The treatment site on each study area has shown a similar trend with an obvious decrease after the harvesting treatment. However some variation in the pattern of change on control sites between the three study areas is evident. A decrease in mean biomass on the control site of study area B was found and an increase in mean biomass on the control site of areas D and E was evident.
A three way ANOVA (Table 6) comparing biomass at different site, study areas and times found significant variation (P= 0.0338) for the ST interaction. The relationship in mean sample biomass (kg 0.099m-2) for the pooled control and treatment sites over time are shown in Figure 4, which shows that the mean biomass on control sites has remained unchanged over time while a decrease in biomass has been found on treatment sites. This has led to significant variation in mean biomass between pooled control and treatment sites in February 1998. No significant variation was found in the SAT interaction (P = 0.577) thus indicating that the ST interaction does not depend on study area.
A significant AT interaction (P= 0.0438) suggests that the observed pattern of change over time on each of the three areas was not the same. Figure 5 shows the pattern of change over time on control and treatment sites on each of the three study areas. A similar pattern of change to that found for the pooled means was found on study areas 0 and E. On area B the mean
15
16 16
control control and and treatment treatment sites sites before before and and after after harvesting harvesting took took place. place.
Figure Figure 4 4 The The pooled pooled mean mean sample sample biomass biomass (kg.0.099m-2) (kg.0.099m-2) with with 95% 95% confidence confidence intervals intervals for for
Date Date
1996 1996 1998 1998
1.25 1.25
1.5 1.5
~ ~
......
...... <>······· <>······· Treatment Treatment sites sites 1.75 1.75
t t
------·i ------·i
--o-- Control Control sites sites
2 2
~ ~
-a -a
2.25 2.25
2.5 2.5
138.496 138.496 251 251 Total Total
240 240 74.366 74.366 Error Error 0.310 0.310
2 2 SAT SAT 0.253 0.253 0.409 0.409 0.127 0.127 0.6648 0.6648
2 2 0.658 0.658 SA SA 1.316 1.316 2.124 2.124 0.1219 0.1219
2 2 AT AT 1.964 1.964 3.170 3.170 0.982 0.982 0.0438 0.0438
A A 2 2 23.904 23.904 47.807 47.807 77.143 77.143 S0.0001 S0.0001
1 1 3.560 3.560 ST ST 3.560 3.560 28.085 28.085 0.0338 0.0338
s s 1 1 8.452 8.452 8.452 8.452 12.845 12.845 0.0698 0.0698
1 1 0.778 0.778 T T 0.778 0.778 0.792 0.792 0.4674 0.4674
1 1 934.973 934.973 39.114 39.114 934.973 934.973 Const Const 0.0246 0.0246
d. d. Source Source F F f. f. m.s. m.s. s.s. s.s. PValue PValue
Analysis Analysis of of Variance Variance
Site Site (S): (S): Control, Control, Treatment Treatment
lime lime (T): (T): Oct Oct 1996, 1996, Feb Feb 1998 1998
Study Study (A): (A): area area 8, 8, D, D, E E
Treatments Treatments
a a harvesting harvesting treatment. treatment.
Table Table ANOVA ANOVA 6. 6. on on mean mean biomass biomass (0.099m-2) (0.099m-2) comparing comparing study study sites sites and and areas areas before before and and after after
treatment treatment site. site.
and and treatment treatment sites sites in in 1998 1998 as as biomass biomass levels levels decreased decreased on on the the control control and and
sample sample biomass biomass (0.099m-2) (0.099m-2) was was not not significantly significantly different different between between control control
Results Results Chapter Chapter Two: Two: Biomass Biomass and and Density Density
·, ·, > > ·~.
Chapter Two: Biomass and Density Results
Area B Area D Area E 3
2.5
2 !------I~ 1.5 --a- Control sites 1 ········-<>······· Treatment sites
0.5~---T------~---L--~------~--_.----r------.---- 1996 1998 1996 1998 1996 1998
Date Date Date
Figure 5. Mean sample biomass (kg.0.099m-2) with 95% confidence intervals for control and treatment sites on each area, before and after harvesting has taken place.
17
18 18
treatment treatment sites sites on on area area D D is is evident. evident.
significantly significantly different different while while some some variation variation between between the the control control site site and and
levels levels between between control control and and treatment treatment sites sites on on areas areas E E and and 8 8 appear appear to to not not be be
over over time time in in density density levels levels varies varies between between the the three three study study areas. areas. Density Density
A A significant significant TA TA interaction interaction (Ps (Ps 0.0001) 0.0001) indicates indicates that that the the pattern pattern of of change change
conclusion conclusion or or the the onset onset of of the the study study (Figure (Figure 6). 6).
density density between between control control and and treatment treatment sites sites was was not not present present at at either either the the
= = (P (P 0.0701). 0.0701). Thus Thus indicating indicating that that significant significant variation variation in in the the pooled pooled mean mean
showed showed no no significant significant variation variation in in mean mean sample sample density density for for the the ST ST interaction interaction
three three A A way way ANOVA ANOVA (Table (Table 7) 7) comparing comparing study study sites, sites, study study areas areas and and time time
on on the the treatment treatment site site over over time. time.
On On study study area area D D mean mean density density increased increased on on the the control control site site and and decreased decreased
mean mean density density increases increases on on both both sites sites from from October October 1996 1996 to to February February 1998. 1998.
decreases decreases over over time time on on both both control control and and treatment treatment sites sites E E while while on on area area
three three study study areas areas appears appears to to be be (Figure (Figure different 7). 7). On On area area 8 8 mean mean density density
The The pattern pattern of of change change between between control control and and treatment treatment sites sites on on each each of of the the
while while no no change change in in density density has has been been observed observed on on control control sites. sites.
levels levels one one year year the the after removal removal of of 55-60% 55-60% of of the the initial initial estimated estimated biomass biomass
1998. 1998. The The treatment treatment sites sites appear appear to to have have recovered recovered to to 86.9% 86.9% of of initial initial density density
sites sites showed showed a a loss loss of of around around 13. 13. 1% 1% between between October October 1996 1996 and and February February
from from October October 1996 1996 to to February February 1998. 1998. The The mean mean sample sample density density for for treatment treatment
(±316.73). (±316.73). Mean Mean sample sample density density on on control control sites sites has has increased increased by by only only 0.2% 0.2%
had had a a mean mean density density of of 798.79m-2 798.79m-2 (±205.95) (±205.95) and and treatment treatment sites sites 641.41m-2 641.41m-2
the the start start of of the the study study (Figure (Figure 6). 6). At At the the conclusion conclusion of of the the study, study, control control sites sites
clams. clams. m m -2 -2 (±231.43) (±231.43) were were found found for for control control and and treatment treatment sites sites respectively respectively at at
Mean Mean (±1SD) (±1SD) sample sample densities densities of of 800.6 800.6 clams.m-2 clams.m-2 (±189.12) (±189.12) and and 738.34 738.34
Density. Density.
Chapter Chapter Two: Two: Biomass Biomass and and Density Density Results Results
19 19
and and treatment treatment sites sites before before and and after after harvesting harvesting took took place. place.
Figure Figure 6. 6. The The pooled pooled mean mean sample sample density density (0.099m-2) (0.099m-2) with with 95% 95% confidence confidence intervals intervals for for control control
Date Date
1996 1996 1998 1998
50~------~~------r------
60 60
...... <>...... <>...... Treatment Treatment sites sites
~ ~
·- ~ ~ ...... , ,
70 70
-a-- Control Control sites sites
80 80
90 90
251 251 Total Total 155069 155069
240 240 Error Error 128622 128622 535.924 535.924
2 2 TSA TSA 154.369 154.369 308.738 308.738 0.288 0.288 0.7500 0.7500
2 2 SA SA 1583.150 1583.150 1.477 1.477 791.575 791.575 0.2304 0.2304
2 2 TA TA 8322.450 8322.450 4161.230 4161.230 7.765 7.765 0.0005 0.0005
2 2 A A 7034.390 7034.390 3517.190 3517.190 6.563 6.563 0.0017 0.0017
1 1 TS TS 1972.320 1972.320 1972.320 1972.320 12.777 12.777 0.0701 0.0701
s s 6230.190 6230.190 7.871 7.871 6231.190 6231.190 0.1070 0.1070
1 1 T T 996.036 996.036 996.036 996.036 0.239 0.239 0.6731 0.6731
1 1 Const Const 1353294 1353294 1353294 1353294 384.770 384.770 0.0026 0.0026
d.f. d.f. Source Source F F m.s. m.s. s.s. s.s. PValue PValue
Analysis Analysis of of Variance Variance
Study Study site site (S}: (S}: Control, Control, Treatment Treatment
Oct Oct lime lime 1998 1998 (T}: (T}: 1996, 1996, Feb Feb
Study Study area area (A}: (A}: B, B, D, D, E E
Treatments Treatments
harvesting harvesting treatment. treatment.
Table Table 7. 7. ANOVA ANOVA on on mean mean density density (0.099m-2) (0.099m-2) comparing comparing study study sites sites and and areas areas before before and and after after a a
Chapter Chapter Two: Two: Biomass Biomass and and Density Density Results Results ',t. ',t.
20 20
treatment treatment sites sites on on each each area, area, before before and and after after harvesting harvesting has has taken taken place. place.
Figure Figure 7. 7. Mean Mean sample sample density density
(0.099m- with with 95% 95% confidence confidence ) ) intervals intervals for for control control and and 2
Date Date Date Date Date Date
1998 1996 1996 1998 1996 1996 1998 1996 1996 1998 1998 1998
30~--~------r--~~--~------~--~--~------r----
...... 40 40 <>...... <>...... Treatment Treatment sites sites
50 50 --o-
Control Control sites sites .,, .,, '· '·
'· '· ., .,
'· '·
60 60 ., .,
...... I I '· '· ., .,
'· '·
...... ,. ,. I I
'· '· ., ., 70 70
80 80
90 90
100 100
Area Area 8 8 Area Area D D Area Area E E
Chapter Chapter Two: Two: Biomass Biomass Results Results and and Density Density
·~. ·~.
., ., . . DISCUSSION.
The aim of this study was to observe how populations of A. stutchburyi react to a size specific reduction in biomass. Firstly homogeneity was determined between control and treatment sites to ensure that the study areas being used would provide a suitable area in order for later post treatment comparisons to be made. Then the harvesting treatment was undertaken and comparisons made of biomass and density twelve months after the conclusion of harvesting compared to the pre harvesting measurements.
With a maximum density of 1555 clams.m-2 and a mean of close to 800 clams.m-2 the densities found in populations at Waitati Inlet are not too dissimilar to those of Larcombe (1971), Richardson (1979) and Dobbinson et at (1989). The mean densities found on the study sites here are more than twice that found by Marsden and Pilkington (1995) in the Avon-Heathcote Estuary and almost eight times higher than those found by Bull (1985) in Western Golden Bay. Mean A. stutchburyi lengths from populations around New Zealand have been found to be fairly similar (Larcombe, 1971).
Numerical densities on areas at Waitati are nearly 100 and 1500 times greater than those found at Mugu lagoon for P. staminea, and C. undatella respectively (Peterson, 1982). In Shark Bay (Western Australia) mean clam densities are only 5% of that found in this study (Peterson and Black, 1987). K. scalarina and K. rhytiphora in Western Australia were found at densities of only 16.25% and 4% respectively of those found for A. stutchburyi in Waitati Inlet (Peterson and Black, 1993). A. stutchburyi has a mean length of approximately 3cm and a maximum length of up to 6cm. The size of the clams discussed above can not be considered dissimilar enough to discount density comparisons with A. stutchburyi .
Looking at these figures with these comparisons in mind it becomes obvious that A. stutchburyi populations exist at much higher densities than those of clam species overseas. This may be due to continual harvesting over a long period of time by man, natural predators being more abundant, tower quality of environmental conditions, variation in food quality and/or quantity or a combination of these factors limiting densities of overseas clam species. The mechanisms that have been proposed to control population densities of overseas clams (i.e. interspecific competition for food, migration and
21
22 22
both both food food and and space space resources. resources.
competition. competition. The The removal removal of of 57% 57% of of the the biomass biomass should should therefore therefore liberate liberate
intraspecific intraspecific competition competition will will be be due due to to both both space space and and food food resource resource
( ( 1989) 1989) also also suggest suggest that that at at high high densities densities it it is is reasonable reasonable to to expect expect that that
point point where where adjacent adjacent neighbours neighbours are are mutually mutually deformed. deformed. Dobbinson Dobbinson et et al al
growth growth can can be be physically physically retarded retarded by by pressure pressure from from neighbouring neighbouring shells shells to to the the
in in populations populations with with such such high high densities densities space space for for juveniles juveniles may may be be limited limited and and
often often found found hard hard packed packed against against each each other. other. Larcombe Larcombe (1971) (1971) suggests suggests that that
study study population population at at Waitati Waitati Inlet. Inlet. Densities Densities are are so so A. A. high high that that stutchburyi stutchburyi are are
as as important important a a resource resource as as food. food. This This does does not not appear appear to to be be the the case case in in the the
and and Peterson Peterson and and Black, Black, 1993). 1993). These These studies studies also also suggest suggest that that space space is is not not
(Carlson (Carlson et et al, al, 1984., 1984., Peterson Peterson and and Black, Black, 1987., 1987., Peterson Peterson and and Beal, Beal, 1989, 1989,
supplies supplies growth growth rates rates of of remaining remaining shellfish shellfish would would be be expected expected to to increase increase
thus thus with with less less competition competition and and less less shellfish shellfish to to deplete deplete the the immediate immediate food food
Earlier Earlier studies studies indicate indicate that that intraspecific intraspecific competition competition appears appears to to be be for for food, food,
but but will will be be expanded expanded on on in in the the following following chapter). chapter).
harvested harvested patches. patches. (Settlement (Settlement and and migration migration will will be be discussed discussed here here briefly, briefly,
period: period: third, third, that that clams clams from from high high density density areas areas migrated migrated into into lower lower density density
that that a a significant significant settlement settlement event event events events or has has taken taken place place over over the the study study
specific specific reduction reduction in in biomass biomass freeing freeing up up food food and and space space resources: resources: second, second,
attributed attributed to to a a combination combination of of the the following following three three mechanisms. mechanisms. First First the the size size
recovery recovery of of mean mean biomass biomass and and density density levels levels on on the the treatment treatment sites sites can can be be
recovering recovering from from substantial substantial stock stock losses losses in in a a short short period period of of time. time. The The
These These results results indicate indicate that that A. A. populations populations of of stutchburyi stutchburyi are are capable capable of of
recovered recovered to to 86.9% 86.9% of of the the initial initial estimates. estimates.
control control sites sites and and a a drop drop in in density density of of 13. 13. 1% 1% on on treatment treatment sites. sites. Mean Mean density density
over over time time between between control control and and treatment treatment sites sites with with no no change change evident evident on on
however however did did not not show show a a significant significant level level of of variation variation in in the the pattern pattern of of change change
control control and and treatment treatment sites sites were were affected affected by by study study area. area. Mean Mean densities densities
initial initial estimates. estimates. There There is is no no eviqence eviqence that that the the differences differences found found between between
observed observed on on treatment treatment sites, sites, the the treatment treatment sites sites did did recover recover to to 81.3% 81.3% of of their their
displaying displaying a a small small increase increase of of 6% 6% and and a a decrease decrease in in biomass biomass of of 18.7% 18.7%
in in mean mean biomass biomass between between control control and and treatment treatment sites sites with with control control sites sites
Although Although there there was was a a significant significant difference difference in in the the pattern pattern of of change change over over time time
the the large large difference difference in in density. density.
settlement) settlement) may may not not necessarily necessarily act act in in the the same same manner manner in in Waitati Waitati Inlet Inlet due due to to
Chapter Chapter Two: Two: Biomass Biomass and and Density Density Discussion Discussion *'t. *'t.
23 23
density density areas areas surrounding surrounding the the disturbance disturbance affected affected harvest harvest areas areas into into patches patches
around around disturbances. disturbances. It It is is A. A. possible possible that that large large stutchburyi stutchburyi migrated migrated from from high high
Stephenson Stephenson 1981 1981 ). ). ( ( also also 1981) 1981) found found that that migration migration was was especially especially frequent frequent
generally generally only only in in smaller smaller size size classes classes (Larcombe, (Larcombe, and and 1971 1971 Stephenson, Stephenson,
A. A. Locally Locally 1982). 1982). stutchburyi stutchburyi have have been been found found to to be be capable capable of of migration migration but but
normal normal response response to to high high density density levels levels being being lateral lateral migration migration (Peterson, (Peterson,
Migration Migration has has been been found found in in P. P. staminea staminea to to be be density density dependent dependent with with the the
patches. patches.
newly newly settled settled larvae larvae should should have have a a higher higher chance chance of of survival survival on on these these
not not be be affected affected enough enough to to discourage discourage settlement. settlement. VVith VVith space space being being available, available,
smaller smaller individuals, individuals, substrate substrate suitability suitability of of harvested harvested patches patches or or density density may may
removal removal of of larger larger size size classes classes from from the the population population while while leaving leaving behind behind
may may be be affected affected by by substrate substrate selectivity selectivity or or gregarious gregarious settlement settlement patterns. patterns. By By
and and 1981 1981 Martin, Martin, Martin Martin 1984). 1984). has has (1984) (1984) suggested suggested that that larval larval recruitment recruitment
recruitment recruitment to to areas areas that that have have been been entirely entirely stripped stripped is is low low (Stephenson, (Stephenson,
recruitment. recruitment. Strong Strong evidence evidence has has also also been been compiled compiled that that suggests suggests
from from the the treatment treatment sites, sites, harvesting harvesting may may actually actually facilitate facilitate higher higher levels levels of of
Whangarei Whangarei Harbour. Harbour. Thus Thus by by removing removing the the larger larger portions portions of of the the population population
of of larger larger size size classes classes led led to to higher higher recruitment recruitment A. A. levels levels in in stutchburyi stutchburyi on on
Martin Martin found found (1984) (1984) some some evidence evidence weak to to support support the the idea idea that that the the removal removal
comparison comparison to to initial initial values. values.
have have become become more more dispersed dispersed or or variable variable at at the the conclusion conclusion of of the the study study in in
February February This This (1998). (1998). increase increase in in SO SO suggests suggests that that the the samples samples measured measured
in in February February 8.76 8.76 and and (1998): (1998): density density from from in in October October 231 231 to to (1996) (1996) in in 316 316
treatment treatment sites sites standard standard deviation deviation in in biomass biomass from from in in October October 5.87 5.87 to to (1996) (1996)
intermingled intermingled with with high high density density areas. areas. This This can can be be seen seen by by the the increase increase in in the the
untouched. untouched. Thus Thus a a clumped clumped distribution distribution is is left, left, with with lower lower density density strips strips
have have been been harvested harvested and and areas areas surrounding surrounding harvested harvested strips strips are are left left
seldom seldom entirely entirely stripped. stripped. Smaller Smaller size size classes classes are are left left behind behind in in strips strips that that
the the harvesting harvesting has has been been undertaken undertaken in in this this study study means means that that large large areas areas are are
(Larcombe, (Larcombe, unless unless 1971) 1971) older older individuals individuals are are removed. removed. The The manner manner in in which which
densities densities there there is is very very little little room room for for new new recruits recruits to to settle settle and and survive survive
compared compared to to 800 800 clams.m-2 clams.m-2 in in the the populations populations found found at at Waitati Waitati Inlet. Inlet. At At these these
were were found found at at densities densities of of only only 9 9 clams.m-2 clams.m-2 and and clams.m-2 clams.m-2 1-2 1-2 respectively respectively
however however that that P. P. (Peterson, (Peterson, staminea staminea and and 1982) 1982) M. M. mercenaria mercenaria (Maurer, (Maurer, 1983) 1983)
(Peterson, (Peterson, and and 1982) 1982) M. M. mercenan'a mercenan'a (Maurer, (Maurer, It It must must 1983). 1983). be be considered considered
Recruitment Recruitment has has been been found found unlikely unlikely to to be be affected affected by by density density in in P. P. staminea staminea
Chapter Chapter Two: Two: Biomass Biomass and and Density Density Discussion Discussion
·,' ·,' ',1. ',1. ,, '
Chapter Two: Biomass and Density Discussion of lower density. I would also suggest that newly settled A. stutchburyi which found themselves in high density areas within the treatment sites or on surrounding areas, may have migrated to areas within the treatment sites of low density. Migration may also have an influence in maintaining substrate suitability and density levels (at a level) where larval settlement is still high.
A large degree of variation in the results was found between study areas. Initially biomass levels between the three study areas varied, as less was found an area B than areas D and E. This pattern was not found however in regards to density, where similar values were found for each study area. It is of interest that although each study area is located in a different area of the inlet and subject to differing environmental conditions, that similar densities were found on each area. The consequent environmental conditions and the availability and quality of food will no doubt be a leading contributor to the variation in the biomass levels. Variation in the pattern of change over time in mean biomass and density between study areas was also found. Again this is likely to be due to local environmental conditions and their affect on the mechanisms discussed above.
In this study biomass and density levels have recovered to over 80% of their initial estimates after the removal of 55- 60% of the total initial biomass. A combination of decreased competition for food and space resources coupled with possible increases in recruitment levels and lateral migration of smaller A. stutchburyi is undoubtedly what has lead to this recovery. How each of these mechanisms contribute to the recovery of harvested areas can not be answered here, but requires more specific work to look at the function of each process.
24 CHAPTER THREE: EFFECTS OF HARVEST ON SIZE FREQUENCY DISTRIBUTION.
INTRODUCTION.
The size distribution within a population and how it reacts under harvesting conditions is of interest to the management of any fishery. In this study the effects of harvesting on length frequency patterns, the proportion of commercial size clams and settlement was studied. The relative consequences to non-commercial harvesters and future management considerations are discussed.
A. stutchburyi distribution is restricted to the shore below the lowest high water neap (Stephenson, 1981) and may extend 6-8m below the sublittoral zone (Larcom be, 1971 ). Two trends in shore distribution are evident. Firstly, the mean size of A. stutchburyi increases from high to low shore levels, secondly the mean size decreases with distance from the entrance of the harbour or inlet. These trends have been found around the country by Larcombe (1971), in Otago Harbour by Dobbinson et al (1989) and by Bull (1985) on two areas of Pakawau/Puponga beach. The present study however is not concerned with shore distribution as such but the effect of harvesting on site-specific distributions. It is important, however, to acknowledge that the distribution patterns of A. stutchburyi does vary with shore level and distance from the mouth of the inlet or harbour.
Larcom be ( 1971) found that most population distributions around the country consisted of adult A. stutchburyi of a mean size approaching their maximum lengths. Length frequency distributions of A. stutchburyi from surveys on Snake Bank in Whangarei Harbour have shown a unimodal length frequency distribution with a mode larger than 30mm in shell length (Cryer, 1996). This pattern of distribution has changed in later surveys following regular harvesting, where a bimodal length frequency distribution with the first mode at about 10-15mm (assumed to be a recruiting year class) and the second (corresponding to the accumulated biomass of other clams) at Jess than 30mm was found (Cryer, 1996).
The proportion of recruited biomass in the fishable portion of stocks (length >30mm) on Snake Bank has declined from about 90% in the first two surveys when the stock was near virgin (1982 and 1983) to only about 35% in 1996. A
25
26 26
appropriate appropriate than than a a constant constant catch catch approach. approach.
biomass biomass suggesting suggesting a a Constant Constant Annual Annual Yield Yield (CAY) (CAY) approach may may approach be be more more
addition, addition, sporadic sporadic settlement settlement of of juveniles juveniles will will probably probably lead lead to to a a fluctuating fluctuating
through through the the inhibition inhibition of of spatfall, spatfall, despite despite high high levels levels of of egg egg production. production. In In
low, low, however, however, then then recruitment recruitment overfishing overfishing of of this this population population could could still still occur occur
for for the the stock stock against against reproductive reproductive overfishing. overfishing. If If adult adult biomass biomass is is driven driven too too
fisheries fisheries until until 30mm 30mm shell shell length length that that there there is is probably probably considerable considerable protection protection
about about 18mm 18mm shell shell length, length, but but do do not not recruit recruit to to commercial commercial or or non-commercial non-commercial
Cryer Cryer ( ( has has 19~6) 19~6) suggested suggested that that as as clams clams recruit recruit to to the the spawning spawning biomass biomass at at
commercial commercial uses uses than than for for commercial commercial fishers. fishers.
individuals individuals in in the the population population would would have have greater greater implications implications for for non
size, size, Cryer Cryer (1996) (1996) suggests suggests that that reducing reducing the the proportion proportion of of relatively relatively large large
seems seems to to be be some some flexibility flexibility in in the the export export markets markets for for cockles cockles with with respect respect to to
are are much much less less attractive attractive to to non-commercial non-commercial users users than than larger larger ones. ones. There There
by by physical physical constraints constraints (Holdsworth (Holdsworth and and Cryer, Cryer, 1991). 1991). However, However, small small cockles cockles
expected, expected, but but not not necessarily necessarily an an increase increase in in size size which which would would still still be be limited limited
increase increase in in productivity productivity (higher (higher turnover turnover of of mature mature cockles) cockles) would would be be
lf lf mortality mortality of of larger larger shellfiSh shellfiSh (>30mm) (>30mm) is is increased increased through through harvesting, harvesting, an an
(Stephenson, (Stephenson, 1981). 1981).
Settlement Settlement has has been been found found to to be be both both temporally temporally and and spatially spatially highly highly variable variable
more more likely likely to to be be successful, successful, although although the the limits limits of of this this range range are are not not known. known.
some some optimal optimal range range of of adult adult biomass biomass within within which which settlement settlement of of juveniles juveniles is is
consistent consistent since since 1992 1992 (Cryer, (Cryer, 1996). 1996). Cryer Cryer (1996) (1996) suggests suggests that that there there may may be be
1985) 1985) settlement settlement was was not not heavy heavy whereas whereas settlement settlement has has been been relatively relatively
Snake Snake Bank Bank during during the the years years when when adult adult biomass biomass was was close close to to virgin virgin (1982-
selectivity selectivity or or possibly possibly gregarious gregarious settlement settlement patterns patterns (Martin, (Martin, 1984). 1984). On On
1981 1981 ). ). It It has has been been suggested suggested that that this this may may be be caused caused by by larval larval substrate substrate
entirely entirely stripped stripped have have been been shown shown to to be be slow slow (Martin, (Martin, 1984 1984 and and Stephenson, Stephenson,
were were lower lower on on areas areas of of total total removal. removal. Re-colonisation Re-colonisation of of areas areas that that have have been been
1983). 1983). Martin Martin (1984), (1984), however, however, found found strong strong evidence evidence that that settlement settlement levels levels
affected affected by by density density for for some some species species of of clams clams (Peterson, (Peterson, 1982 1982 and and Maurer, Maurer,
As As has has been been discussed discussed in in chapter chapter two, two, settlement settlement has has not not been been found found to to be be
expected expected following following the the development development of of a a commercial commercial fishery fishery (Cryer, (Cryer, 1996). 1996).
the the proportion proportion and and absolute absolute biomass biomass of of larger, larger, older older individuals individuals can can be be
levels levels of of less less than than 10% 10% of of its its virgin virgin levels levels has has also also been been found. found. A A decrease decrease in in
Snake Snake Bank Bank since since the the start start of of the the commercial commercial fishery fishery in in 1982, 1982, down down to to current current
consistent consistent decline decline in in the the biomass biomass of of large large cockles cockles (;::3Smm (;::3Smm shell shell length) length) on on
Chapter Chapter Three: Three: Distribution Distribution Introduction Introduction METHODS AND MATERIALS.
Samples collected from initial and final survey samples as described in Chapter One were stored in labelled bags and frozen. These were measured for length (maximum anterior- posterior margins) using vernier callipers to the nearest 0.1 mm. Mean lengths were compared between control and treatment sites and study areas using linear model analysis from the Datadesk statistical software package. Chi-squared tests were used to compare observed sample length frequencies from control and treatment sites using methods modified from Grant and Tyler's (1983) oocyte analysis techniques. Where the expected frequencies eij were calculated using the following equation.
Where Ri equals the total number of clams in the ith size class summed over both site types (i.e. control and treatment), Cj equals the total number of clams measured in the jth site type and L:Ci equals the total number of clams measured. The data was analysed twice using two different size class increments. The first utilised a 2.5mm size increment to give an overall account of any change in size distribution patterns while the second used a three size class division based on commercial applications, where juvenile clams were classed as 0 to <19mm, mature non-commercial (mature) clams 19 to <30mm and mature commercial (commercial) clams~ 30mm in length.
The following statistic was calculated
G = :L (oij- eij)21eij) where Oij are the observed frequencies. If the samples show the same distribution in length frequencies G will be distributed Chi2 with (r-1 )(c-1) degrees of freedom.
Residuals were calculated using the following equation
rii = (Oij- eii) I "eij and the standardised residuals by dividing by the expected standard deviation Wij (Haberman, 1973) where
Wij ="[1 - (Ri I n)][1 - (Cj In)]
28
29 29
sampling. sampling. of of time time eaGh eaGh at at sites sites between between heterogeneity heterogeneity
for for test test to to 1998 1998 February, February, and and 1996 1996 October, October, from from sites sites control control and and
treatment treatment of of frequencies frequencies size size pooled pooled the the on on performed performed were were analyses analyses These These
expected. expected. than than more more values values positive positive a a and and observed observed was was expected expected than than
less less indicating indicating value value negative negative a a With With class. class. size size that that in in values values expected expected and and
observed observed the the between between variation variation significant significant show show two two than than less less or or than than greater greater
residuals residuals Standardised Standardised measured. measured. clams clams of of number number total total the the is is n n where where
Methods Methods
Distribution Distribution Three: Three: Chapter Chapter
30 30
7921 7921 5247.07 5247.07 Total Total
7910 7910 4301.07 4301.07 0.543751 0.543751 Error Error
2 2 :>0.0001 :>0.0001 24.4898 24.4898 45.039 45.039 48.9795 48.9795
:>0.0001 :>0.0001 62.114 62.114 33.7744 33.7744 2 2 67.5488 67.5488 AS AS
0.2659 0.2659 57.2323 57.2323 2.3370 2.3370 57.2323 57.2323 1 1 TS TS
s s 0.3437 0.3437 1 1 51.1132 51.1132 1.5134 1.5134 51.1132 51.1132
:>0.0001 :>0.0001 18.6006 18.6006 2 2 34.208 34.208 37.2012 37.2012 TA TA
:>0.0001 :>0.0001 537.04 537.04 292.019 292.019 2 2 584.038 584.038 A A
0.6246 0.6246 6.10209 6.10209 0.32806 0.32806 6.10209 6.10209 T T
0.0026 0.0026 381.94 381.94 111533 111533 111533 111533 Const Const
PValue PValue F F m.s. m.s. d.f. d.f. s.s. s.s. Source Source
Variance Variance of of Analysis Analysis
Treatment Treatment Control, Control, (S): (S): site site Study Study
(98) (98) Feb Feb (96), (96), Oct Oct Time Time (T): (T):
E E D, D, B, B, (A): (A): area area Study Study
Treatments: Treatments:
time. time. over over changes changes
for for values values control control and and treatment treatment comparing comparing length length clam clam mean mean on on variance variance of of Analysis Analysis 8. 8. Table Table
increase. increase. small small a a show show both both
sites sites treatment treatment and and control control the the on on length length clam clam B B area area On On increases. increases. length length
sites sites control control on on while while decreases decreases sites sites treatment treatment on on length length clam clam E E and and D D areas areas
on on varies, varies, also also areas areas three three the the between between time time over over change change of of pattern pattern The The
B. B. area area and and E E area area by by followed followed D D area area on on highest highest be be to to appears appears length length Clam Clam
8). 8). Table Table 0.0001, 0.0001, (Ps (Ps present present is is areas areas between between variation variation that that shows shows 9 9 Figure Figure
8). 8). Table Table 0.2659, 0.2659, (P= (P=
significant significant not not were were changes changes these these increases, increases, sites sites control control on on and and decreases decreases
sites sites treatment treatment on on length length clam clam as as time time over over change change of of pattern pattern the the in in variation variation
some some is is there there Although Although 1998. 1998. and and 1996 1996 in in sites sites treatment treatment and and control control
from from samples samples pooled pooled of of length length clam clam (±95%CI) (±95%CI) mean mean the the shows shows 8 8 Figure Figure
Length. Length. Mean Mean
RESULTS. RESULTS. ·,. ·,. ·~. ·,'
Chapter Three: Distribution Results
45
40 -o-- Treatment sites
········<>······· Control sites
35
30~------r------,------1996 1998 Date
Figure 8. Mean clam length (±95%CI) for pooled control and treatment sites in October, 1996 and February, 1998.
45
g...... j e -o-- 0 treatment E 40 -···-·~·-·-·· ~ ____ ...... -·-·~· ...... D control ' 2. ' ' ' ----0--- 8 treatment
8 control 35 ------<>--- E treatment -·-·+·-· E control
30~------T------,r------1996 1998 Date
Figure 9. Mean clam length (±95%CI) for the control and treatment sites on areas 8, 0 and E in October, 1996 and February, 1998.
31
32 32
length. length. shell shell 35-37.5mm 35-37.5mm around around second second the the and and length length shell shell 15-17.5mm 15-17.5mm
around around mode mode first first the the with with found found is is distribution distribution bimodal bimodal a a site site treatment treatment the the
on on while while length length shell shell 42.5-45mm 42.5-45mm around around mode mode a a with with sites sites control control on on evident evident
is is pattern pattern unimodal distribution distribution unimodal a a E E area area On On respectively. respectively. 42.5-45mm 42.5-45mm and and
35-37.5mm 35-37.5mm around around modes modes with with pattern pattern unimodal unimodal a a show show D D and and 8 8 areas areas on on
sites sites treatment treatment and and control control on on distributions distributions frequency frequency length length 1998 1998 February, February,
E E In In 37.652[2Sd.f.J). 37.652[2Sd.f.J). = = val. val. crit. crit. 258.09. 258.09. (X2= (X2= area area on on variation variation considerable considerable
and and 37.652[25d.f.J) 37.652[25d.f.J) = = val. val. crit. crit. 56.302. 56.302. (X2= (X2= D D area area on on evident evident is is sites sites between between
variation variation some some while while 8 8 37.652[25d.f.J). 37.652[25d.f.J). = = val. val. crit. crit. 32.84, 32.84, (X2= (X2= area area on on sites sites
treatment treatment and and control control between between variation variation little little is is there there observed observed be be can can As As
E. E. and and D D 8, 8, areas areas study study three three the the of of each each on on 1998 1998 February February in in sites sites treatment treatment
and and control control the the for for frequencies frequencies length length observed observed the the shows shows 11 11 Figure Figure
sites. sites.
treatment treatment on on found found where where expected expected than than less less while while found found were were classes classes size size
<47.5mm <47.5mm to to 40 40 the the in in clams clams expected expected than than more more significantly significantly sites control control On On
classes. classes. size size these these in in found found were were sites sites control control on on clams clams expected expected than than less less
Significantly Significantly class. class. size size <35mm <35mm to to 32.5 32.5 the the in in as as well well as as <25mm <25mm to to 7.5mm 7.5mm
from from classes classes size size the the in in sites sites treatment treatment on on found found were were expected expected than than clams clams
more more significantly significantly that that indicate indicate 9 9 Table Table in in residuals residuals Standardised Standardised found. found. was was
37.652[25d.f.J) 37.652[25d.f.J) val.= val.= crit. crit. 264.27, 264.27, (X2= (X2= variation variation significant significant 1998 1998 In In 1996. 1996. in in
sites sites treatment treatment and and control control between between existed existed 37.652[25d.f.J) 37.652[25d.f.J) = = val. val. crit. crit. 18.389, 18.389,
(X2= (X2= variation variation significant significant no no that that .showed .showed classes classes size size 2.5mm 2.5mm on on analysis analysis
square square Chi Chi length. length. in in mm mm 40-42.5 40-42.5 and and 15-17.5mm 15-17.5mm around around modes modes with with found found
is is distribution distribution bimodal bimodal a a sites sites treatment treatment on on while while length length shell shell 42.5-45mm 42.5-45mm
around around mode mode a a with with sites sites control control on on evident evident is is distribution distribution unimodal unimodal a a 1998 1998
February, February, In In 0). 0). 1 1 (Figure (Figure 40-42.5mm 40-42.5mm around around mode mode a a with with sites sites treatment treatment
and and control control both both for for found found is is pattern pattern distribution distribution unimodal unimodal a a 1996 1996 October, October, In In
Analysis. Analysis. Frequency Frequency Length Length
Results Results Distribution Distribution three: three: Chapter Chapter Chapter three: Distribution Results
20 October, 1996 15 a .• Control sites ~ 0) • e . D Treatment sites ff. 10 . .. --- -<>---- Expected
5
0
20 February, 1998 15 a ·• ~<(I) e 10 . &. .. .. 5 ..
Size class
Figure 10. Observed and expected frequencies for clams from pooled control and treatment sites in October, 1996 and February, 1998 using 2.5mm size increments.
33
34 34
-1.0013 -1.0013 1.0013 1.0013 62.5 62.5
-0.9802 -0.9802 0.9802 0.9802 60 60
0.999 0.999 -0.999 -0.999 57.5 57.5
-0.9802 -0.9802 0.9802 0.9802 1.413 1.413 -1.413 -1.413 55 55
1.1881 1.1881 0.5756 0.5756 -0.5756 -0.5756 -1.1881 -1.1881 52.5 52.5
-0.2677 -0.2677 1.2695 1.2695 0.2677 0.2677 -1.2695 -1.2695 50 50
-1.2696 -1.2696 1.2696 1.2696 -1.6527 -1.6527 1.6527 1.6527 47.5 47.5
-5.0575 -5.0575 0.3415 0.3415 -0.3415 -0.3415 5.0575 5.0575 45 45
-4.1051 -4.1051 4.1051 4.1051 -0.2679 -0.2679 0.2679 0.2679 42.5 42.5
-2.9979 -2.9979 2.9979 2.9979 -0.2646 -0.2646 0.2646 0.2646 40 40
-1.1215 -1.1215 0.4869 0.4869 1.1215 1.1215 -0.4869 -0.4869 37.5 37.5
-1.2269 -1.2269 1.2269 1.2269 1.2193 1.2193 -1.2193 -1.2193 35 35
2.4492 2.4492 1.163 1.163 -1.163 -1.163 -2.4492 -2.4492 32.5 32.5
1.34 1.34 -1.34 -1.34 0.4508 0.4508 -0.4508 -0.4508 30 30
0.3193 0.3193 -0.3193 -0.3193 0.0623 0.0623 -0.0623 -0.0623 27.5 27.5
0.6135 0.6135 0.7341 0.7341 -0.6135 -0.6135 -0.7341 -0.7341 25 25
2.1716 2.1716 0.9177 0.9177 -0.9177 -0.9177 -2.1716 -2.1716 22.5 22.5
2.669 2.669 -2.669 -2.669 2.2415 2.2415 -2.2415 -2.2415 20 20
4.0347 4.0347 0.0052 0.0052 -4.0347 -4.0347 -0.0052 -0.0052 17.5 17.5
10.061 10.061 0.4433 0.4433 -0.4433 -0.4433 -10.061 -10.061 15 15
7.2269 7.2269 0.0037 0.0037 -7.2269 -7.2269 -0.0037 -0.0037 12.5 12.5
4.3606 4.3606 0.4501 0.4501 -4.3606 -4.3606 -0.4501 -0.4501 10 10
2.4619 2.4619 -2.4619 -2.4619 7.5 7.5
1.226 1.226 -1.226 -1.226 5 5
0.0284 0.0284 -0.0284 -0.0284 2.5 2.5
0 0
Treatment Treatment Control Control Treatment Treatment Control Control (mm) (mm) class class Size Size
1998 1998 1996 1996
residuals residuals Standardised Standardised
variation. variation. significant significant indicate indicate two two negative negative than than less less or or two two
than than greater greater residuals residuals Standardised Standardised 1998. 1998. February, February, and and 1996 1996 October, October, in in sites sites treatment treatment and and
control control between between variation variation for for testing testing analyses analyses Chi-squared Chi-squared from from residuals residuals Standardised Standardised 9. 9. Table Table
Results Results Distribution Distribution three: three: Chapter Chapter
35 35
1998. 1998. February, February, in in E E and and D D B, B, areas areas study study
three three the the of of each each on on sites sites treatment treatment and and control control from from clams clams for for frequencies frequencies Observed Observed 11. 11. Figure Figure
classes classes Size Size
5 5
10 10
,£ ,£
15 15 e e
ol) ol) ~ ~
20 20
a. a.
25 25
AreaE AreaE
30 30
0 0
5 5
10 10
,£ ,£
15 15
e e
ol) ol) t! t!
20 20
a. a.
AreaD AreaD
25 25
30 30
0 0
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0 0 10 10 sites sites Treatment Treatment
,£ ,£
sites sites Control Control • • 15 15 e e
ol) ol) ~ ~
20 20
a. a.
25 25
B B Area Area
30 30
Results Results Distribution Distribution three: three: Chapter Chapter Chapter Three: Distribution Results
Commercial Size Classes.
Chi square analysis carried out using size classes utilised for commercial exploitation found that the size distribution of clams between control and treatment sites did not differ in October 1996 (X2= 0. 7052, crit. val. = 5. 991 [2d.~) while in February 1998 significant variation was present in each of the three size classes (Chi2= 203.14, crit. val.= 5.991[2d.f]). This can be seen in Figure 12. Standardised residuals (Table 10) indicate that in February, 1998 a significantly greater number of clams were found on treatment sites in the juvenile and mature size classes and significantly less in the commercial class. The opposite was found on control sites.
From October, 1996 to February, 1998 the proportion of clams on control sites in the commercial size class increased from 86% to 92% of the population. On treatment sites the proportion of commercial clams decreased from 85% to 80%. The proportion of mature clams on control sites decreased from 12.4% to 6.1 %. On treatment sites mature clams also decreased from 13.4% to 8.1% of the population. The proportion of juvenile clams on control sites remained fairly static from October, 1996 to February, 1998 moving from 1.4% to 1.8% while on treatment sites the proportion of juvenile clams increased from 1.3% to 12.2%.
Figure 13 shows the percentage frequency of clams found in each size class for the control and treatment site of each of the three areas B, D and E. As is evident from this figure and chi square analyses significant variation between sites in February, 1998 is not found on area B (X2= 2.287, crit. val. = 5.991[2d.f.]) while on areas D (X2= 25.33, crit. val. = 5.991 [2d.f.]) and E (X2= 204, crit. val. = 5.991 [2d.f.J) significant variation was evident.
36
37 37
13.79 13.79 -11.987 -11.987 2.576 2.576 Treatment Treatment
11.987 11.987 -13.79 -13.79 -2.576 -2.576 1998 1998 Control Control
-0.7521 -0.7521 -0.1408 -0.1408 0.833 0.833 Treatment Treatment
0.7521 0.7521 0.1408 0.1408 -0.833 -0.833 1996 1996 Control Control
30) 30) (19- Commercial Commercial (~30) (~30) (0-19) (0-19) Mature Mature Site Site Juvenile Juvenile Year Year
(mm) (mm) classes classes Size Size
residuals residuals Standardised Standardised
variation. variation. significant significant indicate indicate two two negative negative than than less less or or two two than than
greater greater 1998. 1998. residuals residuals Standardised Standardised 1996 1996 February, February, and and October, October, in in sites sites treatment treatment and and control control
between between variation variation for for testing testing analyses analyses Chi-squared Chi-squared from from 10. 10. residuals residuals Standardised Standardised Table Table
classes. classes. size size based based 1998 1998 commercially commercially using using 1996 1996 February, February, and and October, October, in in
sites sites treatment treatment and and control control from from clams clams for for frequencies frequencies length length 12. 12. expected expected and and Observed Observed Figure Figure
class class Size Size
Commercial Commercial Mature Mature Juvenile Juvenile
I I I I
0 0
·.·.·.·.·.·.·.·:T ·.·.·.·.·.·.·.·:T I I .V .V
:.·.·;_~ :.·.·;_~
.••.. .••.. • • ..•..•••.. ..•..•••.. : : ·...,,: ·...,,: • • ..,.._.....,..c--·' ..,.._.....,..c--·' • • •• •• J J ...... • • A. A.
25-
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~~~~~r ~~~~~r e e
a a 75- 1998 1998 February, February, 100- I I T T 0 0 ~~I ~~I .----····tl""!"1:f'::O'>":::::::~fJ]-····_· .----····tl""!"1:f'::O'>":::::::~fJ]-····_· ...,;·· ...,;·· Expected Expected ----0---- 25- tf. tf. e e D D sites sites Treatment Treatment . . Jl~~~~~ Jl~~~~~ s s sites sites Control Control (:,;:;i (:,;:;i m1 m1 75- 1996 1996 October, October, . . .·.·.·.·.·.·.· .·.·.·.·.·.·.· 100- Results Results Distribution Distribution Three: Three: Chapter Chapter 38 38 and and E E in in October, October, 1996 1996 and and February, February, 1998 1998 using using commercially commercially based based size size classes. classes. Fig\.lre Fig\.lre 13. 13. Observed Observed length length frequencies frequencies for for clams clams from from control control and and treatment treatment sites sites on on areas areas B, B, D D Size Size class class Juvenile Juvenile Commercial Commercial 0 0 f f 25 25 f f e e ~ ~ 50 50 ~ ~ 75 75 AreaE AreaE 100 100 0 0 25 25 &. &. e e 50 50 ol) ol) ~ ~ ~ ~ 75 75 Area Area D D 100 100 0 0 IIIII IIIII 1998 1998 Treatment 25 25 II II &. &. Control Control 1998 1998 e e 50 50 ol) ol) ~ ~ ITiill ITiill Treatment Treatment 1996 1996 ~ ~ 0 0 Control1996 Control1996 75 75 Area Area B B 100 100 Chapter Chapter Three: Three: Distribution Distribution Res\.llts Res\.llts 39 39 area area D D has has shown shown some some settlement settlement of of new new recruits, recruits, while while on on area area E E B B settlement settlement has has occurred. occurred. On On area area very very little little settlement settlement has has taken taken place, place, found found in in this this study. study. Over Over the the three three areas areas of of study, study, three three different different patterns patterns of of variable. variable. Evidence Evidence to to support support the the spatial spatial variability variability of of settlement settlement has has been been Stephenson Stephenson ( ( 1981) 1981) found found that that settlement settlement is is spatially spatially and and temporally temporally very very differently differently over over time. time. juvenile juvenile biomass biomass on on control control and and treatment treatment sites sites have have been been shown shown to to react react practise practise needs needs further further attention. attention. The The proportion proportion of of mature, mature, commercial commercial and and over over a a longer longer period period of of time time and and a a more more consistent consistent or or continual continual harvesting harvesting reduce reduce the the proportion proportion of of commercial commercial clams clams by by 5%. 5%. How How populations populations react react sampled sampled in in the the present present study study a a single single harvesting harvesting event event has has managed managed to to measured measured over over a a much much longer longer period period of of time time (1982- 1996). 1996). At At the the sites sites decline decline on on Snake Snake Bank Bank was was considerably considerably greater greater (90% (90% to to 35%) 35%) and and a a drop drop in in commercial commercial size size clams clams was was observed observed (Cryer, (Cryer, 1996). 1996). However However the the 1998. 1998. This This result result is is also also consistent consistent with with earlier earlier findings findings on on Snake Snake Bank Bank where where commercial commercial clams clams has has fallen fallen from from 85% 85% in in October October 1996 1996 to to 80% 80% in in February February control control sites. sites. On On treatment treatment sites sites a a drop drop of of 5% 5% is is evident evident as as the the proportion proportion of of The The proportion proportion of of commercial commercial clams clams has has increased increased from from 86% 86% to to 92% 92% on on significantly significantly different. different. developed developed on on control control and and treatment treatment sites sites over over time time has has been been shown shown to to be be accumulated accumulated biomass biomass of of the the other other clams. clams. The The pattern pattern of of change change that that be be newly newly settled settled juveniles juveniles and and the the second second mode mode representing representing the the Bank Bank following following several several years years harvesting harvesting where where the the first first mode mode was was assumed assumed to to around around 40-42.5mm 40-42.5mm was was found. found. Similar Similar distributions distributions were were found found on on Snake Snake frequency frequency distribution distribution with with a a first first mode mode around around 15-17.5mm 15-17.5mm and and a a second second mode mode of of 42.5-45mm. 42.5-45mm. On On pooled pooled treatment treatment sites sites however however a a bimodal bimodal length length in in February February 1998 1998 had had a a similar similar unimodal unimodal distribution distribution but but with with a a slightly slightly higher higher length length (Cryer, (Cryer, 1996). 1996). The The length length frequency frequency distribution distribution for for pooled pooled control control sites sites where where unimodal unimodal length length frequencies frequencies with with modes modes larger larger than than 30mm 30mm in in shell shell in in early early pre-harvest pre-harvest samples samples collected collected on on Snake Snake Bank Bank in in Whangarei Whangarei Harbour Harbour upper upper size size attained attained by by adults adults in in a a population. population. results results Similar were were also also found found findings findings of of length length frequency frequency distributions distributions having having one one major major peak peak near near the the of of approximately approximately 40-42.5mm. 40-42.5mm. This This is is consistent consistent with with Larcom Larcom be's be's (1971) (1971) October, October, 1996 1996 showed showed a a unimodal unimodal length length frequency frequency distribution distribution with with a a mode mode Length Length frequency frequency distributions distributions from from pooled pooled control control and and treatment treatment sites sites in in DISCUSSION. DISCUSSION. '' '' ·~ ·~ ' ' 40 40 surveys surveys on on a a yearly yearly basis basis may may deter deter this this method method being being used. used. to to be be a a sensible sensible one, one, but but the the cost cost and and effort effort needed needed in in order order to to carry carry out out settlement settlement found found here. here. A A CAY CAY approach approach to to the the fisheries fisheries management management appears appears juveniles juveniles will will lead lead to to fluctuating fluctuating biomass biomass which which is is supported supported by by the the patterns patterns of of has has also also been been suggested suggested (Cryer, (Cryer, 1996) 1996) that that the the sporadic sporadic settlement settlement of of areas, areas, but but instead instead strip strip small small patches patches surrounded surrounded by by areas areas of of high high density. density. It It threat. threat. Current Current practices practices used used now now do do not not involve involve strip strip harvesting harvesting of of large large broodstock broodstock are are reduced, reduced, then then the the recruitment recruitment of of juveniles juveniles may may be be under under reducing reducing densities densities to to a a level level where where settlement settlement is is not not favourable favourable or or adult adult areas areas are are stripped, stripped, significantly significantly altering altering the the makeup makeup of of the the substrate substrate or or in in length, length, but but not not commercial commercial fisheries fisheries until until 30mm 30mm in in shell shell length. length. If If large large against against egg egg overfishing overfishing as as clams clams recruit recruit to to the the spawning spawning population population at at 18mm 18mm As As Cryer Cryer (1996) (1996) has has suggested, suggested, there there is is some some protection protection for for clam clam populations populations have have to to face face large large changes changes in in the the sizes sizes available available harvest. harvest. for several several easily easily accessible accessible beds beds untouched, untouched, non·commercial non·commercial fishers fishers should should not not beds beds not not easily easily accessible accessible to to the the public. public. If If this this was was maintained, maintained, thus thus leaving leaving harvesters harvesters in in time. time. Commercial Commercial harvesting harvesting is is currently currently carried carried out out on on isolated isolated effect effect on on commercial commercial harvesting harvesting but but could could impinge impinge on on non-commercial non-commercial has has already already suggested, suggested, these these results results of of harvesting harvesting are are unlikely unlikely to to have have any any along along with with a a reduction reduction in in the the proportion proportion of of commercial commercial clams. clams. As As Cryer Cryer (1996) (1996) decreases decreases in in mean mean length length on on two two of of the the three three treatment treatment sites sites were were observed, observed, also also expected. expected. After After just just one one harvesting harvesting period period and and one one year year of of recovery, recovery, is is expected, expected, a a decrease decrease in in mean mean size size and and the the proportion proportion of of larger larger shellfish shellfish is is With With commercial commercial harvesting harvesting and and the the subsequent subsequent increase increase in in productivity productivity that that larval larval settlement. settlement. study study has has not not altered altered the the substrate substrate or or density density of of shellfish shellfish enough enough to to inhibit inhibit suggested suggested by by Martin Martin (1984), (1984), then then the the harvesting harvesting treatment treatment undertaken undertaken in in this this larval larval substrate substrate selectivity selectivity or or gregarious gregarious settlement settlement patterns, patterns, as as has has been been have have been been made made available available through through harvesting. harvesting. If If settlement settlement is is affected affected by by more more successful successful on on treatment treatment sites sites where where both both space space and and food food resources resources environmental environmental conditions. conditions. So So it it is is fair fair to to suggest suggest that that settlement settlement has has been been presumably presumably both both sites sites on on each each area area are are exposed exposed to to the the same same or or very very similar similar difficult difficult to to explain explain through through variation variation of of environmental environmental conditions, conditions, as as found found between between control control and and treatment treatment sites sites within within each each individual individual area area is is settlement settlement are are undoubtedly undoubtedly linked linked to to the the physical physical environment. environment. Variation Variation settlement settlement of of juveniles juveniles have have been been relatively relatively heavy. heavy. Factors Factors affecting affecting Chapter Chapter Three: Three: Distribution Distribution Discussion Discussion ·~. ·~. Chapter Three: Distribution Discussion Harvesting significantly altered the length frequency distribution and proportion of commercially available clams in comparison to control sites. These changes were due to the increased rate of settlement on treatment sites, which was highly variable between study areas. These results suggest that the liberation of food and space resources through harvesting allows for greater settlement success, but also leads to a decrease in the mean length and proportion of larger clams in the remaining population. [ tl 41 CHAPTER FOUR: THE EFFECT OF HARVESTING ON REPRODUCTION. INTRODUCTION. Knowledge of the reproductive cycle is important to any management plan of a commercially harvested species. In this study the reproductive cycle of A. stutchburyi has been qualitatively and quantitatively monitored and analysed over a sixteen month period. The general reproductive biology of A. stutchburyi is fairly well understood (Larcom be, 1971 and Stephenson, 1981) but detailed histological analysis of A. stutchburyi has not previously been undertaken. Many similar studies on Veneridae overseas have been carried out and have found a changing cycle of gametogenesis dependent on latitude and environmental variability. Descriptions of oogenesis and spermatogenesis in molluscs has been documented by De Jong-Brink et al (1983) and Maxwell (1983) respectively. In Venerid clams Keck et al (1975) give a good description of the cycle of gametogenesis in a temperate population of hard clams (Mercenaria mercenaria). Seasonal cycles of energy storage and utilisation in marine bivalves can generally be attributed to reproductive activities (Barber and Blake, 1981 ). Energy is stored prior to gametogenesis when food is abundant and subsequently utilised in the production of gametes when metabolic demand is high. Condition indices provide a measure of energy storage and can be utilised to quantify the nutritive status of individuals and thus can be a useful method to monitor gametogenesis. Feder et al (1979) suggest that condition index calculations are necessary to determine .periods of peak spawning. VVith histological techniques alone, male littleneck clams in Prince William Sound appeared in spawning condition for most of the year and female clams for four months of the year. The use of a condition index highlighted that the major spawning effort of P. staminea was restricted to the month of June. Therefore condition is an important tool in pinpointing major events and provides more detail to describe gametogenesis. 42 43 43 only only spawned spawned at at temperatures temperatures above above 15°C. 15°C. found found that that the the Manila Manila clam clam Tapes Tapes philippinarum philippinarum under under laboratory laboratory conditions conditions species species specific specific temperature temperature ranges ranges (Hasselman (Hasselman et et al, al, 1989). 1989). Mann Mann (1979) (1979) been been found found to to be be initiated initiated and and spawning spawning to to take take place place between between narrow narrow reproductive reproductive cycle cycle (Borrero, (Borrero, 1987). 1987). In In Mercenaria Mercenaria sp., sp., gametogenesis gametogenesis has has of of abundance abundance and and the the composition composition of of food food have have been been found found to to affect affect the the Geukensis Geukensis demissa demissa differences differences in in the the water water temperature, temperature, and and the the seasonality seasonality food food content content in in the the water water was was high high (Ruiz (Ruiz et et al., al., 1992). 1992). In In the the mussel mussel begin begin when when temperature temperature is is at at its its lowest lowest in in winter winter and and spawning spawning began began when when food food availability. availability. In In the the oyster oyster Ostrea Ostrea edulis edulis gametogenesis gametogenesis has has been been found found to to and and spawning. spawning. Most Most work work has has referred referred to to the the importance importance of of temperature temperature and and Environmental Environmental cues cues are are thought thought to to be be important important in in initiating initiating gametogenesis gametogenesis reproductive reproductive activities activities may may be be due due to to site site specific specific differences differences in in food food supply. supply. has has been been suggested suggested that that microgeographic microgeographic variation variation in in the the timing timing of of both both spatially spatially and and temporally temporally between between sites sites (Marsden (Marsden and and Pilkington, Pilkington, 1995). 1995). It It levels levels A. A. of of stutchburyi stutchburyi from from the the Avon- Heathcote Heathcote estuary estuary were were found found to to vary vary were were lower lower from from high high tidal tidal populations populations than than intertidal intertidal populations. populations. Condition Condition estimates estimates of of gametogenic gametogenic condition condition in in the the ribbed ribbed mussel mussel Geukensis Geukensis demissa demissa al, al, 1980; 1980; and and Hasselman Hasselman et et al, al, 1989). 1989). Borrero Borrero (1987) (1987) found found qualitative qualitative sample sample sites sites has has been been found found (Ansell (Ansell et et al, al, 1964; 1964; Keck Keck et et al, al, 1975; 1975; Eversole Eversole et et In In most most studies studies considerable considerable variability variability in in reproductive reproductive activity activity between between resources resources to to growth growth or or reproduction. reproduction. limitations limitations (i.e. (i.e. high high densities) densities) do do not not show show any any preferences preferences in in reallocation reallocation of of an an increase increase in in gametogenic gametogenic activity activity it it appears appears that that clams clams under under resource resource resources resources to to reproductive reproductive development development over over winter winter months months in in preparation preparation for for reproductive reproductive activity activity of of early early spring. spring. Although Although clams clams may may reallocate reallocate energy energy resource resource allocation allocation by by adults adults in in nutrient nutrient storage storage in in preparation preparation for for the the ( ( 1986) 1986) argue argue that that this this variation variation in in growth growth may may be be due due to to a a winter winter shift shift in in December December and and January January when when growth growth declined declined by by 85%. 85%. Peterson Peterson and and Fegley Fegley in in M. M. mercenaria mercenaria mirrored mirrored that that in in juveniles juveniles over over ten ten months months of of the the year year until until despite despite resource resource limitations. limitations. Peterson Peterson and and Fegley Fegley (1986) (1986) found found adult adult growth growth reproduction reproduction simultaneously simultaneously so so as as to to maintain maintain high high rates rates of of survivorship survivorship development development and and physiology physiology to to reduce reduce energy energy invested invested in in growth growth and and increased increased density. density. Thus Thus it it appears appears that that clam clam species species are are sufficiently sufficiently flexible flexible in in reallocate reallocate energy energy between between growth growth and and reproduction reproduction under under the the stress stress of of staminea. staminea. These These rates rates were were indistinguishable. indistinguishable. P. P. staminea staminea did did not not appear appear to to declined declined with with density density to to the the rate rate at at which which growth growth declined declined with with density density P. P. for for Peterson Peterson ( ( 1982) 1982) was was able able to to compare compare the the rate rate at at which which reproductive reproductive effort effort Chapter Chapter Four: Four: Introduction Introduction Reproduction Reproduction I' Chapter Four: Reproduction Introduction M. mercenaria on the east coast of the United States has been found to have a sex ratio of 1:1 (Eversole and Michener, 1980; Heffernan et al, 1989; and Hasselman et al, 1989). Ponurovsky and Yakovlev (1992) found that Manila clam populations around the north-western parts of the sea of Japan consisted of equal numbers of males and females, although the majority of the oldest clams were females. A sex ratio close to 1: 1 was found for most populations except in Vostock Bay where females were more numerous than L males with sex ratios ranging from 1.2:1 to 1.5: 1. It was suggested that this may be due to clams being alternate hermaphrodites in these areas (Ponurovsky and Yakovlev, 1992). Many studies which follow the reproductive cycles of northern hemisphere clams have been carried out using histological techniques. Undertaking histology and preparing slides of gonads from a representative sample from the population on a frequent basis, allows for an accurate analysis of gonad development. This allows for qualitative observations on the clams reproductive activity or stage at time of sampling as well as qualitative measurements of a constant variable (e.g. oocyte area) to be carried out. Spawning in the Manila clam has been found to occur once or twice each year depending on latitude. In northern populations spawning occurs once each year (Ponurovsky and Yakovlev, 1992). Populations in central and southern Japan spawned twice each year, in late spring to early summer and again throughout the autumn (Bardach et al, 1972). Populations located close to the equator were found to spawn year around with peak spawning periods during December and January in Hawaii (Yap, 1977) and May to July in Israel (Shpigel and Fridman, 1990). Similarly, in the hard clam (M. mercenaria), seasonal temperature differences that exist between latitudes lead to differences in the reproductive cycles of different populations. Mercenaria mercenaria in Georgia (Heffernan et at, 1989), So~th Carolina (Eversole et al, 1980), and North Carolina (Porter, 1964) have been found to spawn twice yearly. However in populations at more northerly extremes, such as Delaware Bay (Keck et al, 1975) and Long Island Sound (Loosanoff, 1937) spawning occurred only once per year. These studies emphasise that reproductive cycles are extremely variable and are influenced by the latitude and consequent environmental conditions. Sexual maturity in A. stutchburyi has been found to be a function of size rather than age with individuals reaching sexual maturity at a length of 18- 44 45 45 unharvested unharvested clams. clams. by by commercial commercial harvesting harvesting would would affect affect gametogenesis gametogenesis cycles cycles of of the the extended extended period period of of time time and and to to observe observe if if the the size size specific specific reduction reduction caused caused document document the the reproductive reproductive cycle cycle of of a a population population of of A. A. stutchburyi stutchburyi over over an an the the A. A. reproductive reproductive cycle cycle of of stutchburyi. stutchburyi. This This was was undertaken undertaken in in order order to to compiled compiled from from histological histological analyses analyses along along with with condition condition indices indices to to investigate investigate In In the the study study pre~ent pre~ent monthly monthly qualitative qualitative and and quantitative quantitative information information has has been been Aims. Aims. and and Stephenson, Stephenson, 1981). 1981). Zealand Zealand have have been been fairly fairly well well described described (Larcombe, (Larcombe, 1971; 1971; Booth, Booth, 1972, 1972, 1983; 1983; histological histological analyses. analyses. The The reproductive reproductive A. A. patterns patterns of of stutchburyi stutchburyi in in New New with with techniques techniques suitable of of analysing analysing reproduction reproduction using using condition condition indices indices and and Overseas Overseas studies studies indicate indicate the the importance importance of of environmental environmental variables variables along along harvested harvested species species is is important to to important future future management management considerations. considerations. A A thorough thorough understanding understanding of of the the reproductive reproductive biology biology of of a a commercially commercially spawning spawning ending ending in in April-May April-May in in a a Heathcote Heathcote Avon Avon Estuary Estuary population. population. January-February. January-February. Stephenson Stephenson (1981) (1981) reported reported an an extended extended period period of of population population with with a a small small peak peak in in October October followed followed by by a a large large peak peak occurring occurring in in a a later later study study Booth Booth (1983) (1983) found found a a similar similar pattern pattern in in a a Bay Bay of of Islands Islands one one in in March March for for populations populations in in Wellington Wellington Harbour Harbour and and the the Bay Bay of of Islands. Islands. In In (1972) (1972) described described two two peaks peaks in in a a yearly yearly spawning spawning cycle: cycle: one one in in October October and and despite despite the the wide wide latitudinal latitudinal range range and and ensuing ensuing temperature temperature variations. variations. Booth Booth Larcombe Larcombe (1971) (1971) found found that that this this pattern pattern was was similar similar all all around around New New Zealand Zealand until until the the end end of of May May by by which which time time all all individuals individuals were were entirely entirely spent. spent. once. once. The The gonad gonad content content was was instead instead slowly slowly reduced reduced throughout throughout the the summer summer Beginning Beginning in in January, January, individuals individuals were were found found not not to to shed shed all all their their gametes gametes at at made made up up of of a a single single spawning spawning event event over over a a protracted protracted summer summer period. period. Larcom Larcom be be (1971) (1971) found found a a consist~nt consist~nt spawning spawning pattern pattern around around New New Zealand Zealand with with dioecious a a 1:1 1:1 male: male: female female ratio ratio (Larcom (Larcom be, be, 1971 1971 ). ). (Stephenson (Stephenson A. A. and and Chanley, Chanley, 1979). 1979). stutchburyi stutchburyi has has been been found found to to be be veliger veliger larval larval development development with with settlement settlement after after approximately approximately three three weeks weeks Booth, Booth, 1983). 1983). Fertilisation Fertilisation is is external external and and is is followed followed by by a a planktotrophic planktotrophic vary vary among among localities localities and and over over time time (Larcombe, (Larcombe, 1971 1971 ; ; Stephenson, Stephenson, 1981 1981 ; ; and and 20mm 20mm (Larcom (Larcom be, be, 1971 1971 ). ). Spawning Spawning periods periods around around New New Zealand Zealand appear appear to to Chapter Chapter Four. Four. Reproduction Reproduction Introduction Introduction METHODS AND MATERIALS. Sampling. Once per month thirty A. stutchburyi, each greater than 20mm in length were taken from each site for reproductive analyses. To allow for any possible shore height differences in biomass or density levels as discussed earlier, ten shellfish from within each of the three strata from each site were taken. These were then stored in a tank with free flowing filtered sea water at the Portobello Marine laboratory for 48 hours in order to eliminate any sediment build up in the tissue. Ten individuals (3-4 from each strata) were then dissected out of their shell and fixed in a 10% formalin solution, for histological analysis. The remaining twenty specimens were frozen awaiting condition analyses. Condition Indices. The clams stored for analysis were carefully thawed. The flesh was dissected out of the shells and any excess moisture absorbed using a paper towel, the flesh weighed using a set of Sartorius electronic scales, and placed in foil trays for drying. The shells were washed to remove any sediment or attached epifauna, rinsed in distilled water and then dried with the flesh for 24h at 100°C in a drying oven. The dry weights of the flesh were recorded and shell volumes measured by filling each valve with plasticine and recording the displacement volume of the plasticine in a cylindrical measuring tube. Then Cl calculated as Condition = DlW * 100 sv 1 where DlW equals dry tissue weight (gms) and SV is shell volume (mls). Histology. Gonadal tissue samples approximately 1cm3 in size were dissected from the area directly above the foot from six individuals of each sample. Tissue samples were then dehydrated in an alcohol series, cleared in xylene and embedded in paraplast (industrial name). Transverse sections (8J.Jm thick) were cut, mounted on glass slides and stained using haematoxylin and eosin Staging. Several random fields of gonadal tissue from histological preparations was examined from every specimen. Each individual was then ascribed to one of 46 Chapter Four: Reproduction Methods five developmental stages: Male or Female Spent; Male or Female Early Development; Male or Female Late Development; Male or Female Ripe; and Male or Female Spawning. Maturity Indices. A monthly maturity index (MI) was computed for samples from each study site type (i.e. control and treatment). A scoring system with male or female Spent = 0; Male or Female Early Development = 2; Male or Female Late Development= 3; Male or Female Ripe = 4; and Male or Female Spawning = 0 was adopted. The monthly Ml for both sexes was determined by multiplying the number of specimens ascribed to each category by the category score, summing all such values and dividing this figure by the total number of males or females analysed (Heffernan et al, 1989, after Kennedy, 1977). Oocyte Analysis. Using NIH image 1.61 software on a Power Macintosh 8600/200, individual mean oocyte areas of 50 nucleated oocytes from each female were measured and recorded. Fields were systematically selected around each preparation to ensure detection of variation in development within individuals (Heffernan et at, 1989). Fields were analysed until 50 nucleated oocytes had been measured. Individual means were used to calculate mean sample oocyte areas. Statistics. Sex Ratios. Sex ratios were compared to a 1:1 ratio with Chi-square tests. Analysis of Variance. Staging, maturity index, oocyte area and condition index results from control sites on each of the three areas were combined as were the results from treatment sites. Analyses of maturity indices, mean oocyte areas and condition indices utilised linear ANOVA from the Datadesk software package to compare mean values between control and treatment sites at each stage of sampling. Comparison within sites in the pattern of change over time using [ linear ANOVA were also made. Both types of analyses also compared differences between study areas at each sampling and variation in the pattern of change over time between study areas. 47 48 48 variation variation in in oocyte oocyte size size frequencies frequencies over over time. time. control control sites sites separately separately over over the the sixteen sixteen month month study study period period to to test test for for each each time time of of sampling sampling and and on on summed summed size size frequencies frequencies treatment treatment for and and control control sites sites within within each each month month to to test test for for any any heterogeneity heterogeneity between between sites sites at at This This was was ·performed ·performed on on the the summed summed size size frequencies frequencies for for treatment treatment and and where where n n is is the the total total number number of of oocytes oocytes measured. measured. = = Wu Wu ~[1 ~[1 - (Ri (Ri In)] In)] I I n)][1 n)][1 - (Cj (Cj ~ij ~ij (Haberman, (Haberman, 1973) 1973) where where and and the the residuals residuals standardised standardised by by dividing dividing by by the the expected expected standard standard deviation deviation ~eij ~eij rij rij = ( ( = eij) eij) I I Oij Oij - Residuals Residuals were were calculated calculated using using the the following following equation equation of of development development G G will will be be distributed distributed Chi2 Chi2 with with (r-1 (r-1 )(c-1) )(c-1) degrees degrees of of freedom. freedom. where where Oij Oij are the the are observed observed frequencies. frequencies. If If the the samples samples are are at at the the same same stage stage = = L L G G (oij- eij)21eij) eij)21eij) The The following following statistic statistic was was calculated. calculated. (late (late development development phase) phase) >1650~m2 >1650~m2 and and (ripe (ripe phase). phase). (initial (initial phase), phase), 550- 1100~m2 1100~m2 (early (early development development phase), phase), 1100- 1650~m2 1650~m2 classes classes were were utilised. utilised. These These included included four four phases phases of of development; development; 0- 550~m2 550~m2 animal animal and and LjCj LjCj equals equals the the total total number number of of oocytes oocytes measured. measured. Four Four size size over over all all individuals, individuals, Cj Cj equals equals the the total total number number of of oocytes oocytes measured measured in in the the jth jth V\lhere V\lhere Ri Ri equals equals the the total total number number of of oocytes oocytes in in the the size size class class summed summed jth jth the the expected expected frequencies frequencies eij eij were were calculated calculated using using the the following following equation. equation. frequencies frequencies from from control control and and treatment treatment sites sites (Grant (Grant and and Tyler, Tyler, 1983). 1983). Where Where Chi-squared Chi-squared tests tests were were used used to to compare compare observed observed summed summed sample sample oocyte oocyte Oocyte Oocyte Size Size Frequency Frequency Analysis. Analysis. of of variation variation found found in in AN AN OVA OVA was was of of interest. interest. Least Least squared squared difference difference (LSD) (LSD) analyses analyses were were carried carried out out where where the the location location Chapter Chapter Four: Four: Reproduction Reproduction Methods Methods 49 49 at, at, and and Ponurovsky Ponurovsky (1989), (1989), and and Yakovlev, Yakovlev, (1992). (1992). Keck Keck et et al al Eversole Eversole et et (1975), (1975), at at Heffernan Heffernan (1980), (1980), et et al al Hasselman Hasselman (1989), (1989), et et A A five five stage stage reproductive reproductive staging staging criteria criteria was was modified modified from from descriptions descriptions of of Histological Histological Staging. Staging. over over sixteen sixteen months. months. Figure Figure 14. 14. Observed Observed and and expected expected sex sex ratio$ ratio$ (Male: (Male: Female) Female) from from histologically histologically examined examined clams clams Month Month ONOJ ONOJ FMAMJ FMAMJ JASON JASON OJ OJ b ...... <> <> ...... -<> -<> -~-· -~-· ¢ ¢ ...... 0.5 0.5 ''.,, ''.,, ,.. ,.. ·: ·: '' <>--···9·· <>--···9·· o-=- ...... o o .. .. '\ '\ ...... o o ·. ·. . . ·" ·" 0 0 ••~ ••~ \~ \~ o/'l' o/'l' I I I I /" /" ...... /' ./' "•-. "•-. .. .. :• :• / / ·... ·... P. P. c c ·. ·. c: c: c c 'P 'P c c c c "'/ "'/ c c .0- \', \', .. .. c c •. •. c c c c 1), 1), .• .• 1:1! 1:1! 1 1 40 ·~··; ·~··; I I ; ; " " 1 1 ...... I I 1 1 ...Q. ...Q. • • 1 1 ; ; ' ' I I \ \ : : \ \ .o.. .o.. () () ...... \. \. :::s :::s 1.5 1.5 . . :' :' . . ~ ~ LL LL 0 0 ' ' . . . . . . . . ~ ~ ' ' . . . . . . 2 2 . . . . . . ~ . . . . ' ' .. .. 0 0 . . . . ' ' • • • • • • • • 0· 0· • • • • • • Treatment Treatment . . sites sites .. .. . . .. .. . . 2.5 2.5 ' ' Q Q .. .. .. .. ...... <>...... <>...... Control Control sites sites 9 9 3 3 c c Expected Expected 3.5 3.5 observations observations of of hermaphrodites hermaphrodites were were made. made. different different to to that that of of the the expected expected ratios ratios (X2 (X2 = = t= t= 7.47, 7.47, No No 3.84[1d.f.J). 3.84[1d.f.J). observed observed sex sex ratios ratios (0. (0. on on treatment treatment 75:1) 75:1) sites sites are are found found to to be be significantly significantly in in October October and and December December are are removed removed 1996 1996 from from the the calculations calculations the the expected expected (X2 (X2 = = t= t= 1.373, 1.373, If If the the very very 3.84[1.d.f]). 3.84[1.d.f]). high high male: male: female female ratios ratios found found the the observed observed male: male: female female ratio ratio 1) 1) was was not not significantly significantly (0.87: (0.87: different different to to that that expected expected ratio ratio of of (X2 (X2 = = t= t= 1:1 1:1 12.84; 12.84; However However 3.84[1d.f.J). 3.84[1d.f.J). on on treatment treatment sites sites male: male: female female ratio ratio 1) 1) was was found found (0.64: (0.64: to to be be significantly significantly different different to to the the to to month month on on both both control control and and treatment treatment sites. sites. On On control control sites sites the the observed observed Figure Figure shows shows that that the the 14 14 male: male: female female ratios ratios found found were were variable variable from from month month Sex Sex Ratios. Ratios. RESULTS. RESULTS. 50 50 r r , , 15f). 15f). (Figure (Figure gonad gonad the the within within stages stages differing differing at at often often Follicles Follicles disjointed. disjointed. or or up up broken broken instead instead appeared appeared but but continuous continuous longer longer no no were were activity activity spermatogenic spermatogenic of of Layers Layers stage). stage). r > r spawning spawning initial initial the the to to through through proceeds proceeds gametes gametes of of (development (development present present stiU stiU are are gametes gametes immature immature although although gametes, gametes, mature mature fewer fewer contained contained Follicles Follicles Spawning. Spawning. Male Male r r e). e). and and 15d 15d (Figures (Figures strands strands in in arranged arranged spermatozoa spermatozoa by by occupied occupied was was gonad gonad the the of of most most however, however, follicle, follicle, the the of of wall wall the the along along present present still still was was layer layer spermatogenic spermatogenic narrow narrow A A lumen. lumen. the the dominated dominated sperm sperm Mature Mature walls. walls. thin thin with with expanded expanded fully fully were were follicles follicles Ripe Ripe Ripe. Ripe. Male Male 15c). 15c). (Figure (Figure spermatozoa spermatozoa containing containing follicles follicles the the of of 50% 50% about about with with follicles, follicles, in in seen seen are are spermatids spermatids and and Spermatocytes Spermatocytes gametes. gametes. proliferating proliferating accommodate accommodate to to expanded expanded rapidly rapidly had had Follicles Follicles Development. Development. Late Late Male Male 15b). 15b). (Figure (Figure spermatocytes spermatocytes of of mass mass arranged arranged loosely loosely a a with with filled filled were were follicles follicles of of lumen lumen The The proliferating. proliferating. were were gametes gametes immature immature early early and and thick thick was was wall wall follicle follicle Tlie Tlie small. small. were were stage stage this this in in Follicles Follicles Development. Development. Early Early Male Male 15a). 15a). (Figure (Figure gametes gametes residual residual phagocytizing phagocytizing amoebocytes amoebocytes follicles; follicles; the the in in remained remained sperm sperm some some empty; empty; almost almost were were and and contract contract to to begun begun had had Follicles Follicles Spent. Spent. Male Male Results Results Reproduction Reproduction Four: Four: Chapter Chapter Chapter Four: Reproduction Results Figure 15. Male developmental stages (magnification): (a) spent (x425); (b) early development (x1060); (c) late development (x1060); (d) ripe (x425); (e) ripe (x1060); and (f) spawning (x425). 51 Chapter Four: Reproduction Results a b c d e 53 53 16e). 16e). (Figure (Figure walls walls follicle follicle the the to to attached attached were were oocytes oocytes and and lumens lumens empty empty completely completely to to partially partially follicles follicles Spawned Spawned stage). stage). spawning spawning initial initial the the through through proceeds proceeds gametes gametes of of (development (development present present still still were were gametes gametes immature immature although although gametes, gametes, mature mature fewer fewer contained contained Follicles Follicles Spawning. Spawning. Female Female 16d). 16d). (Figure (Figure thin thin walls walls follicle follicle and and expanded expanded fully fully Follicles Follicles follicle. follicle. the the within within space space empty empty no no or or little little with with lumen lumen follicle follicle the the in in free free were were oocytes oocytes Most Most follicle. follicle. the the of of most most occupied occupied oocytes oocytes packed packed Densely Densely Ripe. Ripe. Female Female 16c). 16c). (Figure (Figure present present were were 25001Jm2) 25001Jm2) (250- sizes sizes oocyte oocyte of of range range wide wide A A wall, wall, follicle follicle the the to to attached attached remain remain majority majority the the although although follicles, follicles, the the of of lumen lumen the the in in free free were were many many oocytes, oocytes, numerous numerous more more and and larger larger the the accommodate accommodate to to expanded expanded Follicles Follicles Development. Development. Late Late Female Female 16b). 16b). (Figure (Figure season season spawning spawning last last the the from from oocytes oocytes residual residual large large occasional occasional for for except except empty empty area area lumen lumen Central Central peduncle. peduncle. a a by by walls walls follicle follicle the the to to adhering adhering seen seen are are oocytes oocytes some some proliferating, proliferating, (<5501Jm2) (<5501Jm2) oocytes oocytes immature immature .early .early thickness, thickness, wall wall follicle follicle in in increase increase An An Development. Development. Early Early Female Female 16a). 16a). (Figure (Figure gametes gametes residual residual phagocytizing phagocytizing amoebocytes amoebocytes follicles; follicles; the the in in remained remained ova ova some some empty; empty; almost almost were were and and contract contract to to begun begun have have Follicles Follicles Spent. Spent. Female Female Results Results Reproduction Reproduction Four: Four: Chapter Chapter Chapter Four: Reproduction Results Figure 16. Female developmental stages (magnification): (a) spent (x425); (b) early development (x425); (c) late development (x425); (d) ripe (x425) and (e) spawning (x425). 54 55 55 e e c c a a Results Results Chapter Chapter Four: Four: Reproduction Reproduction Chapter Four. Reproduction Results Spermatogenesis. Figure 17 indicates that individuals first appeared to be at spawning stage on control sites from January 1997 (40%). A large proportion of clams continued to spawn until May (57.1%). Spawning individuals were not found again until November 1997 (40%) and continued through December 1997 (70%) and January 1998 (50%). Spawning on treatment sites began a month earlier in December 1996 (11.1 %) and continued at generally higher levels than that found on control sites from January 1997 (71.5%) to May 1997 (71.4%). Spawning on treatment sites continued into August 1997 (11.1%) and began again a month earlier than on control sites in October 1997 ( 12.5%). Spent males were not common and were observed on the control sites only from May to July (42.9% and 50% respectively). On the treatment sites spent clams were found from March to August (20% and 11. 1% respectively). Clams at an early developmental stage were found on control sites in October 1996 (71.5%) and from July (50%) to September 1997 (54.5%). Clams in the same stage on treatment sites were found from October (61.5%) to November 1996 (16.7%) and from August (77.8%) to September 1997 (30%). The late development stage in males was present on control sites from October 1996 (14.3%) to February 1997 (50%) and again from August (14.3%) to December 1997. On treatment sites individuals in this stage were found from October 1996 (23%) through to January 1997 (14.3%), a month less than on control sites. Individuals were found in the late development stage a month later on treatment sites from September (60%) to December 1997 (83.3%). Ripe males were found on control sites from October 1996 ( 14.3%) to January 1997 (10%), April 1997 (25%), October 1997 (33.3%), November 1997 (20%) and January 1998 (50%). On treatment sites ripe clams were found at similar levels from October 1996 to January 1997 but were not observed again until September 1997 (10%) through November 1997 (8.3%) and again in January 1998 (50%). 56 ·~. Chapter Four: Reproduction Results Control sites 100 75 El Early development e>. m Late development ~ 50 II Ripe ~ ;¥ ~ Spawning 25 ~ Spent 0 Treatment sites 100 0 N 0 J F M A M J J A S 0 N D J Month Figure 17. Seasonal distribution of male developmental stages determined from histological sections on control and treatment sites. 57 Chapter Four: Reproduction Results Oogenesis. Figure 18 shows that spawning in females occurred on control sites from January 1997 (8.3%) to July 1997 (11.1%) with relatively heavy spawning being observed between February and May 1997. Spawning began again in November 1997 (7.7%) and continued through to the end of the study in January 1998 (16.7%). Spawning on treatment sites took place from January (36.4%) to July 1997 (60%) with similar proportions to that on control sites being found with the exception of July 1997 were the number of spawning individuals found was considerably higher on treatment sites. Spawning recommenced in September (12.5%), two months earlier than on control sites and carried through to January 1998 (60%). Spent females were found from April (33.3%) to July (33.3%) on control sites and from February (9.1%) to August (22.2%) on treatment sites. On control sites individuals in early developmental phases were recorded from October 1996 (18.2%) to November 1996 (14.3%) and from July (55.6%) to October 1997 (8.3%). On treatment sites early developmental phases were found at similar levels to that of control sites in October 1996 (20%) and November 1996 (14.3%). Individuals in early development on treatment sites were also found between July (20%) and September 1997 (25%) but unlike control sites no individuals in this phase were found in October 1997 while some were present in November 1997 (14.3%). Late developmental phases were recorded on control sites from October 1996 (72.7%) to February 1997 (16.7%) and from August (63.6%) to December 1997 (69%). A very similar pattern was found on treatment sites except that individuals in late development phase were found a month later at the end of the study in January 1998 (30%). Ripe females were recorded on control sites from October 1996 (9.1 %) to February 1997 (33.3%) and from October 1997 (8.3%) to the end of the study in January 1998 (83.3%). The same pattern was found for females in treatment sites at the start of the study but ripe females were found a month earlier in September (12.5%) than on control sites. 58 Chapter Four: Reproduction Results Control sites 100 75 rn Early development . Late development e>. lillJ ~ 50 II Ripe ~ if!. [§3 Spawning 25 tm Spent 0 Treatment sites 0 N D J F M A M J J A S 0 N D J Month Figure 18. Seasonal distribution of female developmental stages determined from histological sections on control and treatment sites. 59 >'. -~. Chapter Four. Reproduction Results Maturity Indices. Males. The Ml of male clams (Figure 19) on the control sites was 2.22 in October 1996. The Ml steadily increased through until December 1996 {3.83), after which there was a steep drop through January (2.58) and February (2.11) to March (1.0). From this time the Ml remained fairly low until July (1.0), after which it rose steadily until October 1997 (3.06). Then a dropwas recorded in November 1997 {2.5) and the Ml remained fairty constant until January 1998 {2.58). On the treatment sites the Ml of male clams followed a very similar pattern except for a peak in November 1997 (3.41). Females. Females from the control sites had a Ml of 2.93 in October 1996 (Figure 19). This remained at a high level through to December 1996 (3.33) followed by a constant drop until March (1.0). From March through to July (1.18) Ml remained fairly low. An increase in Ml was recorded in August (2.61) and was followed by a slight decrease in September (2.28). Ml increas~ over October (2.98) and November (3.23) and then fell away slightly in December (2.71) and remains around this level in January 1998 (2.88). On the treatment sites female clams like males followed a very similar pattern of change to control sites. Except in September where MJ on treatment sites continues to increase and in November where Ml decreases on treatment sites. A four way ANOVA (Table 11) comparing Ml at different times, sites, areas and between sexes found no significant variation in the pattern of change over time between sites (P= 0.9898), areas (P= 0.1 062) or sexes (P= 0.6806). Three factor ANOVA (Table 12) comparing Ml between different sites, areas and sexes at each time of sampling showed no significant variation between control and treatment sites or study areas at any stage of the study. Variation between the sexes was found in October 1996 (P= 0.0417) and April 1997 (P= 0.490). Significant variation was also found in the SX interaction in October 1996 (P= 0.0457) and for the AX interaction in November 1997 (P= 0.0285). 60 61 61 sites. sites. treatment treatment and and control control for for slides slides histological histological of of staging staging developmental developmental from from calculated calculated indices indices maturity maturity female female and and male male monthly monthly mean mean of of variation variation Seasonal Seasonal 19. 19. Figure Figure Month Month JASONOJ JASONOJ ONOJFMAMJ ONOJFMAMJ 0 0 1 1 4 4 Females Females 5 5 <>········ <>········ ...... sites sites Control Control 0 0 -o- sites sites Treatment Treatment 1 1 2 2 3 3 4 4 Males Males 5 5 Results Results Reproduction Reproduction Four: Four: Chapter Chapter 62 62 NS NS NS NS NS NS NS NS NS NS Jan-98 Jan-98 NS NS NS NS NS NS NS NS NS NS Deo-97 Deo-97 + + NS NS NS NS NS NS NS NS Nov-97 Nov-97 NS NS NS NS NS NS NS NS NS NS Oct-97 Oct-97 r r NS NS NS NS NS NS NS NS NS NS Sep-97 Sep-97 NS NS NS NS NS NS NS NS NS NS Aug-97 Aug-97 NS NS NS NS NS NS NS NS NS NS Jul-97 Jul-97 NS NS NS NS NS NS NS NS NS NS May-97 May-97 NS NS NS NS + + NS NS NS NS Apr-97 Apr-97 NS NS NS NS NS NS NS NS NS NS Mar-97 Mar-97 NS NS NS NS NS NS NS NS NS NS Feb-97 Feb-97 NS NS NS NS NS NS NS NS NS NS Jan-97 Jan-97 NS NS NS NS NS NS NS NS NS NS Deo-96 Deo-96 NS NS NS NS NS NS NS NS NS NS Nov-96 Nov-96 NS NS + + + + NS NS NS NS Oct-96 Oct-96 Sex Sex Area* Area* Sex Sex Site* Site* Sex Sex Area Area Site Site Month Month values values P- ANOVA ANOVA sites. sites. control control and and treatment treatment for for values values monthly monthly comparing comparing indices indices Maturity Maturity clam clam on on ANOVA ANOVA 12. 12. Table Table 803.667 803.667 522 522 Total Total 0.615694 0.615694 221.034 221.034 359 359 Error Error 0.1151 0.1151 1.3893 1.3893 0.855361 0.855361 18.8179 18.8179 22 22 MASX MASX 0.8844 0.8844 0.12283 0.12283 0.075628 0.075628 2 2 0.151256 0.151256 ASX ASX 0.4908 0.4908 0.99361 0.99361 0.849891 0.849891 11.8985 11.8985 14 14 MSX MSX 0.9860 0.9860 0.00039 0.00039 0.000030 0.000030 0.000030 0.000030 1 1 sx sx 0.6736 0.6736 0.85163 0.85163 0.524346 0.524346 13.1086 13.1086 25 25 MAX MAX 0.8822 0.8822 0.12543 0.12543 0.077226 0.077226 0.154453 0.154453 2 2 AX AX 0.6806 0.6806 0.77956 0.77956 0.408759 0.408759 5.72263 5.72263 14 14 MX MX 0.8615 0.8615 0.03911 0.03911 0.003021 0.003021 0.003021 0.003021 X X :50.0001 :50.0001 4.0566 4.0566 2.49764 2.49764 59.9433 59.9433 24 24 MAS MAS 0.6081 0.6081 0.49808 0.49808 0.306663 0.306663 0.613327 0.613327 2 2 AS AS 0.9898 0.9898 0.29277 0.29277 0.731239 0.731239 10.2373 10.2373 14 14 MS MS 0.1711 0.1711 4.3909 4.3909 1.34654 1.34654 1.34654 1.34654 s s 0.1062 0.1062 1.3842 1.3842 0.852266 0.852266 21.3067 21.3067 25 25 MA MA 0.0152 0.0152 4.2382 4.2382 2.60946 2.60946 5.21892 5.21892 2 2 A A :50.0001 :50.0001 15.297 15.297 13.0372 13.0372 182.521 182.521 14 14 M M 0.0010 0.0010 1020.3 1020.3 2662.33 2662.33 2662.33 2662.33 Const Const F F PValue PValue m.s. m.s. s.s. s.s. d.f. d.f. Source Source of of Variance Variance Analysis Analysis Female Female Male, Male, (X): (X): Sex Sex Treatment Treatment Control, Control, (S): (S): site site Study Study (98) (98) -Jan -Jan (96) (96) Oct Oct (M): (M): Month Month E E D, D, B, B, (A): (A): area area Study Study Treatments: Treatments: changes. changes. monthly monthly for for sites sites control control and and treatment treatment comparing comparing indices indices maturity maturity clam clam on on ANOVA ANOVA from from results results (NS) (NS) significant significant non non and and Significant(+) Significant(+) 11. 11. Table Table Results Results Reproduction Reproduction Four: Four: Chapter Chapter 63 63 1997. 1997. August August and and February February except except month month every every in in found found was was areas areas study study between between variation variation significant significant that that indicates indicates month month each each for for ( ( areas areas different different at at condition condition comparing comparing 14) 14) (Table (Table ANOVA ANOVA factor factor Two Two B. B. area area study study on on than than higher higher consistently consistently is is E E and and D D areas areas in in condition condition that that shows shows also also 21 21 Figure Figure time. time. over over condition condition in in change change of of pattern pattern the the in in ar:eas ar:eas between between differences differences significant significant are are there there shows shows 13) 13) (Table (Table interaction interaction MA MA s0.0001) s0.0001) (P (P significant significant The The evident. evident. is is time time over over change change of of pattern pattern the the in in variation variation some some areas areas three three the the of of each each on on trend trend similar similar a a follows follows condition condition although although that that seen seen be be can can it it compared compared areas areas between between variation variation the the and and 21) 21) (Figure (Figure combined combined are are area area study study each each of of sites sites treatment treatment and and control control from from indices indices condition condition VVhen VVhen 14}. 14}. (Table (Table 1997 1997 October October and and April April between between found found was was sites sites treatment treatment and and control control between between variation variation Significant Significant sites. sites. control control on on that that than than higher higher consistently consistently was was sites sites treatment treatment on on Condition Condition sites. sites. control control on on 1998 1998 January January to to December December and and 1997 1997 May May and and April April between between and and sites, sites, treatment treatment on on found found was was 1997 1997 April April and and March March between between change change Significant Significant 1997. 1997. August August to to July July and and 1997 1997 March March to to January January 1996, 1996, November November and and October October between between sites sites treatment treatment and and control control on on found found were were condition condition in in changes changes significant significant that that indicates indicates analysis analysis L.S.D. L.S.D. time. time. over over sites sites treatment treatment and and control control between between change change of of pattern pattern the the in in variation variation of of level level significant significant a a not not is is there there shows shows areas areas and and sites sites times, times, different different at at condition condition comparing comparing 13) 13) (Table (Table ANOVA ANOVA factor factor three three A A condition. condition. in in increase increase an an showed showed sites sites treatment treatment and and control control both both on on clams clams observations observations of of month month last last the the Over Over December. December. until until rates rates varying varying at at decrease decrease slowly slowly September September after after and and off off levels levels then then sites sites both both on on condition condition The The August. August. to to sites sites both both on on increased increased condition condition then then and and sites sites treatment treatment and and control control on on July July to to May May from from occurred occurred drop drop slight slight A A increased. increased. sites sites treatment treatment and and control control on on condition condition May May to to April April Between Between April. April. into into moving moving stable stable relatively relatively remain remain sites sites control control on on those those while while increase increase then then sites sites treatment treatment from from clams clams in in Condition Condition March. March. to to January January from from sites sites both both for for recorded recorded were were condition condition in in decreases decreases marked marked which which After After 20). 20). (Figure (Figure 1997 1997 January January until until increase increase gradual gradual a a then then and and drop drop a a by by followed followed was was 1996 1996 November November and and October October between between sites sites treatment treatment and and control control on on condition condition in in increase increase sharp sharp A A Indices. Indices. Condition Condition Results Results Reproduction Reproduction Four: Four: Chapter Chapter 64 64 +++ +++ ++ ++ NS NS +++ +++ +++ +++ +++ +++ + + Area Area +++ +++ NS NS +++ +++ ++ ++ +++ +++ +++ +++ +++ +++ ++ ++ NS NS NS NS NS NS +++ +++ NS NS NS NS NS NS NS NS NS NS Site Site NS NS ++ ++ NS NS NS NS NS NS NS NS N N 0 0 S S F F D D J J D D A A J J M A M M A M J J N N 0 0 values values P- ANOVA ANOVA <0.0001. <0.0001. to to significant significant +++ +++ and and <0.001 <0.001 to to significant significant ++ ++ <0.05, <0.05, to to significant significant + + result, result, insignificant insignificant an an signifies signifies NS NS Where Where (Area). (Area). areas areas study study and and (Site) (Site) sites sites treatment treatment and and control control between between values values monthly monthly comparing comparing data data indices indices condition condition clam clam on on ANOVA ANOVA from from P-values P-values 14. 14. Table Table 0.0005). 0.0005). (P= (P= 97- 98 98 Jan, Jan, Dec, Dec, 0.0001) 0.0001) (P= (P= 97 97 0.003): 0.003): and and (P= (P= 97 97 Aug, Aug, 0.009): 0.009): Jul- (P= (P= 97 97 Feb- May, May, Apr- 0.0001): 0.0001): (PS (PS 97 97 Mar, Mar, Feb, Feb, 0.039): 0.039): 96 96 Oct- Jan (P= (P= Nov, Nov, between between found found were were condition condition in in sites sites control control on on change change Significant Significant (P=0.0002). (P=0.0002). 97 97 0,013) 0,013) (P= (P= 97 97 Aug, Aug, Jul- and and 0.031): 0.031): (P= (P= 97 97 Apr, Apr, Mar- :s0.0001): :s0.0001): (P= (P= 97 97 Mar, Mar, Feb~ Feb~ Feb, Feb, 0.005): 0.005): (P= (P= 96 96 Oct- Jan Nov, Nov, between between found found were were condition condition in in sites sites treatment treatment on on change change Significant Significant I. I. D. D. S. S. L L C. C. clam clam of of ANOVA ANOVA after after tests tests of of Results Results 3841.10 3841.10 1743 1743 Total Total 1.52855 1.52855 2531.27 2531.27 1656 1656 Error Error 1.6161 1.6161 0.0239 0.0239 2.47022 2.47022 66.6961 66.6961 27 27 MSA MSA 2 2 0.1489 0.1489 2.91430 2.91430 1.9066 1.9066 5.82861 5.82861 SA SA 4.09643 4.09643 2.6799 2.6799 S0.0001 S0.0001 110.603 110.603 27 27 MA MA 114.94 114.94 2 2 S0.0001 S0.0001 175.691 175.691 351.382 351.382 A A 1.0890 1.0890 2.69013 2.69013 37.6618 37.6618 0.4086 0.4086 14 14 MS MS s s 9.41147 9.41147 3.2294 3.2294 0.2142 0.2142 9.41147 9.41147 1 1 49.5055 49.5055 12.085 12.085 693.077 693.077 S0.0001 S0.0001 14 14 M M 0.0017 0.0017 103331 103331 588.14 588.14 103331 103331 Const Const F F PValue PValue m.s. m.s. s.s. s.s. Source Source f. f. d. d. Variance Variance of of Analysis Analysis Treatment Treatment Control, Control, (S): (S): site site Study Study (98) (98) -Jan -Jan (96) (96) Oct Oct (M): (M): Month Month E E 0, 0, B, B, (A): (A): area area Study Study Treatments: Treatments: changes. changes. monthly monthly for for sites sites control control and and treatment treatment comparing comparing data data L. L. indices indices condition condition clam clam on on tests tests D. D. S. S. and and variance variance of of Analysis Analysis 13. 13. Table Table Results Results Reproduction Reproduction Four. Four. Chapter Chapter . . ., ., Chapter Four: Reproduction Results 10 -o- Treatment sites ...... <>·····.. Control sites 9 8 7 0 N D J F M A M J J A S 0 N D J Month Figure 20. Seasonal variation of mean condition indices (±95% Cl) for treatment and control sites from October 1996 to January 1998. --o-- AreaB ...... <> ... ····· Area E · · ··0· ··· Area D 0 N D J F M A M J J A S 0 N D J Month Figure 21. Seasonal variation of mean condition indices ( ±95% Cl) for the combined control and treatment sites on each ofthe three study areas from October 1996 to January 1998. 65 66 66 different. different. significantly significantly is is areas areas between between condition condition in in change change of of pattern pattern the the that that shows shows 15) 15) (Table (Table interaction interaction MA MA 0.0033) 0.0033) (P= (P= significant significant A A areas. areas. three three all all at at rapidly rapidly of of dropped dropped area area oocyte oocyte mean mean the the April April and and February February between between although although 23), 23), (Figure (Figure markedly markedly varied varied areas areas between between time time over over change change of of pattern pattern the the compared compared and and combined combined were were areas areas three three the the of of each each on on sites sites treatment treatment and and control control from from area area oocyte oocyte mean mean for for values values When When 1997. 1997. December December to to November November and and 1997 1997 October October to to September September 1997, 1997, February February to to January January 1996, 1996, November November and and October October between between sites sites control control on on found found also also were were changes changes significant significant while while sites sites treatment treatment and and control control on on 1998 1998 January January to to December December and and 1997 1997 April April and and March March between between found found were were area area oocyte oocyte mean mean in in changes changes Significant Significant time. time. over over area area oocyte oocyte mean mean in in 0.0049) 0.0049) (P= (P= variation variation significant significant was was there there however however sites sites control control and and treatment treatment between between difference difference no no indicates indicates areas areas and and sites sites times, times, different different at at area area oocyte oocyte mean mean comparing comparing 15) 15) (Table (Table ANOVA ANOVA factor factor three three A A sites. sites. treatment treatment and and control control both both for for 1998 1998 January January to to increase increase sharp sharp a a then then and and December December until until decrease decrease a a by by followed followed similar similar fairly fairly were were sites sites both both from from values values November November and and October October sites. sites. control control from from samples samples in in decreased decreased and and sites sites treatment treatment on on increased increased area area oocyte oocyte mean mean September September to to August August From From sites. sites. both both on on May May in in increase increase small small a a with with winter, winter, over over low low relatively relatively remained remained area area Oocyte Oocyte sites. sites. both both at at decreased decreased area area oocyte oocyte mean mean April April to to February February from from but but February, February, and and January January between between sites sites treatment treatment and and control control on on recorded recorded was was area area oocyte oocyte in in increase increase An An 1997. 1997. January January into into moving moving decrease decrease a a then then and and December December approaching approaching increase increase small small a a showed showed sites sites Both Both 22). 22). (Figure (Figure 1996 1996 November November to to October October from from increased increased sites sites treatment treatment and and control control on on area area oocyte oocyte Mean Mean Patterns. Patterns. Yearly Yearly Analysis. Analysis. Oocyte Oocyte Results Results Reproduction Reproduction Four: Four: Chapter Chapter 67 67 sites. sites. 0.0108) 0.0108) control control on on 98 98 (P= (P= Jan, Jan, Dec- 0.0228) 0.0228) and and 97 97 (P= (P= Dec, Dec, 0.0104); 0.0104); Nov- 97 97 (P= (P= Oct, Oct, 0.042); 0.042); Sep- 97 97 (P= (P= Apr, Apr, Mar- 0.202); 0.202); 97 97 (P= (P= Feb, Feb, 0.002); 0.002); Jan- (P= (P= 96 96 Nov, Nov, Oct- between between found found area area oocyte oocyte in in change change Significant Significant sites. sites. 0.024) 0.024) treatment treatment for for 98 98 (P= (P= Jan, Jan, 0.028) 0.028) Dec- and and (P= (P= 97 97 Apr, Apr, Mar- between between found found area area oocyte oocyte in in change change Significant Significant clam clam of of ANOVA ANOVA after after tests tests D. D. S. S. L. L. of of Results Results 57113725 57113725 269 269 Total Total 131512 131512 24592806 24592806 187 187 Error Error 0.7347 0.7347 0.79954 0.79954 105149 105149 2523582 2523582 24 24 MSA MSA 0.8446 0.8446 0.16900 0.16900 2 2 22226.1 22226.1 44452.1 44452.1 SA SA 0.0033 0.0033 2.0721 2.0721 272510 272510 6812748 6812748 25 25 MA MA 0.2204 0.2204 1.5247 1.5247 200516 200516 401033 401033 2 2 A A 0.1795 0.1795 1.5154 1.5154 159346 159346 2230842 2230842 14 14 MS MS s s 0.5616 0.5616 0.47573 0.47573 10573.5 10573.5 1 1 10573.5 10573.5 0.0049 0.0049 3.2544 3.2544 886867 886867 12416144 12416144 14 14 M M 0.0004 0.0004 2683.9 2683.9 538157548 538157548 538157548 538157548 1 1 Canst Canst PValue PValue F F m.s. m.s. s.s. s.s. d.f. d.f. Source Source Variance Variance of of Analysis Analysis Treatment Treatment Control, Control, (S): (S): site site Study Study (98) (98) Jan Jan • • (96) (96) Oct Oct (M): (M): Month Month E E D, D, B, B, (A): (A): area area Study Study Treatments: Treatments: changes. changes. monthly monthly for for sites sites control control and and treatment treatment comparing comparing (JJm2) (JJm2) area area Oocyte Oocyte mean mean clam clam L. L. on on tests tests D. D. S. S. and and variance variance of of Analysis Analysis 15. 15. Table Table Results Results Reproduction Reproduction Four: Four: Chapter Chapter Chapter Four: Reproduction Results --o- Treatment sites ········<>...... Control sites ONDJ FMAMJ JASON OJ Month Figure 22. seasonal variation in mean oocyte area J.lm2 (±95% Cl) for control and treatment sites from October 1996 to January 1998. --o- AreaB ...... <>...... Area E ····0···· Area D 0 N D J F M A M J J A S 0 N D J Month Figure 23. Seasonal variation in mean oocyte area J.lm2 (±95% Cl) for the combined control and treatment sites on each study area from October 199$ to January 1998. 68 69 69 observed, observed, were were expected expected than than class class size size >16501Jm2 >16501Jm2 the the in in oocytes oocytes fewer fewer December December In In progresses. progresses. time time as as oocytes oocytes larger larger of of number number the the in in increase increase an an of of trend trend distinct distinct a a shows shows pattern pattern This This 1997. 1997. November November in in >16501Jm2 >16501Jm2 the the in in levels levels oocyte oocyte expected expected than than higher higher significantly significantly by by followed followed class class size size 1100-1650J.Jm2 1100-1650J.Jm2 the the in in found found are are oocytes oocytes of of levels levels higher higher significantly significantly 1997 1997 October October In In class. class. size size 550-1100J.Jm2 550-1100J.Jm2 the the in in found found are are oocytes oocytes of of levels levels higher higher significantly significantly September September to to July July From From period. period. this this over over decreases decreases gradually gradually variation variation this this of of degree degree The The September. September. to to March March from from found found are are class class size size smallest smallest the the in in oocytes oocytes expected expected than than more more Significantly Significantly noticeable. noticeable. is is development development oocyte oocyte of of pattern pattern a a 1997 1997 November November till till through through March March From From present. present. are are class class size size smallest smallest the the in in oocytes oocytes more more significantly significantly as as evident evident is is oocytes oocytes of of cohort cohort new new a a of of development development of of beginning beginning the the and and way way under under well well is is spawning spawning as as found found are are classes classes size size largest largest three three the the in in oocytes oocytes fewer fewer significantly significantly March March In In sizes. sizes. of of range range full full a a exhibit exhibit oocytes oocytes 1997 1997 October October to to March March From From 1997. 1997. March March in in levels levels expected expected than than less less significantly significantly to to February February in in levels levels expected expected than than higher higher significantly significantly in in present present being being from from change change class class size size ripe ripe the the in in oocytes oocytes as as spawned spawned been been have have oocytes oocytes mature mature of of majority majority the the 1997 1997 April April and and February February of of months months the the Between Between 26). 26). and and 24 24 (Figures (Figures class class ripe ripe the the in in more more significantly significantly and and classes classes size size smaller smaller three three the the in in found found oocytes oocytes fewer fewer with with again again increases increases oocytes oocytes ripe ripe of of number number the the February February In In place. place. taken taken have have may may spawning spawning some some that that indicating indicating class class size size ripe ripe the the in in oocytes oocytes fewer fewer are are there there but but expected expected the the a a from from different different significantly significantly not not frequencies frequencies size size with with occurred occurred drop drop January January In In (>1650J,Jm2). (>1650J,Jm2). oocytes oocytes ripe ripe for for value value expected expected than than greater greater significantly significantly a a by by shown shown is is which which spawning spawning for for preparation preparation in in 1996 1996 December December and and October October between between grow grow and and develop develop oocytes oocytes (<550J.Jm2) (<550J.Jm2) pre-vitellogenic pre-vitellogenic immature immature as as evident evident is is sites sites control control from from clams clams in in oocytes oocytes of of development development in in pattern pattern A A Sites. Sites. Control Control 16). 16). (Table (Table residuals residuals standardised standardised using using examined examined is is frequency frequency size size oocyte oocyte in in frequencies frequencies expected expected and and observed observed between between found found variation variation The The found. found. was was sites sites 58.124) 58.124) d.f.]= d.f.]= [42 [42 val. val. crit. crit. 719.26, 719.26, (X2= (X2= treatment treatment and and 58.124) 58.124) val.[42d.f.)= val.[42d.f.)= crit. crit. 862.26, 862.26, (X2= (X2= control control on on times times sampling sampling between between oocytes oocytes of of distribution distribution the the in in variation variation Significant Significant Analysis. Analysis. Frequency Frequency Size Size Results Results Reproduction Reproduction Four: Four: Chapter Chapter 'j. 'j. 70 70 analysis. analysis. histogram histogram scale scale Fine Fine 2.71 2.71 0.93 0.93 0.42 0.42 -4.30 -4.30 16 16 -1.68 -1.68 -7.11 -7.11 -9.74 -9.74 J J -3.98 -3.98 2.68 2.68 -3.19 -3.19 2.20 2.20 -0.10 -0.10 3.2 3.2 1.54 1.54 -0.58 -0.58 D D 2.95 2.95 0.52 0.52 -2.70 -2.70 -1.09 -1.09 3.48 3.48 1.89 1.89 -2.07 -2.07 N N -3.49 -3.49 1.59 1.59 3.42 3.42 0.73 0.73 -5.57 -5.57 -1.74 -1.74 3.77 3.77 0.74 0.74 0 0 -1.92 -1.92 s s -0.26 -0.26 3.19 3.19 2.07 2.07 -4.63 -4.63 1.54 1.54 ~.42 ~.42 6.19 6.19 2.21 2.21 -3.66 -3.66 4.36 4.36 2.45 2.45 -2.20 -2.20 -6.45 -6.45 0.46 0.46 A A 2.91 2.91 2.93 2.93 -7.15 -7.15 1.79 1.79 3.64 3.64 2.83 2.83 -2.15 -2.15 -4.87 -4.87 4.49 4.49 2.95 2.95 J J -5.23 -5.23 -3.89 -3.89 7.42 7.42 1.94 1.94 -1.28 -1.28 -3.43 -3.43 0.15 0.15 4.16 4.16 M M -9.61 -9.61 -3.71 -3.71 0.75 0.75 13.4 13.4 -6.66 -6.66 -2.34 -2.34 A A 0.62 0.62 8.82 8.82 -2.30 -2.30 -4.96 -4.96 -1.92 -1.92 8.94 8.94 -2.63 -2.63 -2.00 -2.00 -2.93 -2.93 M M 7.24 7.24 4.78 4.78 -0.02 -0.02 -3.28 -3.28 -2.08 -2.08 5.46 5.46 -2.70 -2.70 -2.26 -2.26 ·1.69 ·1.69 F F 1.49 1.49 -0.38 -0.38 -1.38 -1.38 0.04 0.04 1.83 1.83 -1.47 -1.47 -0.87 -0.87 0.96 0.96 J J 6.61 6.61 ·1.99 ·1.99 -2.36 -2.36 -3.36 -3.36 5.66 5.66 0 0 -1.51 -1.51 -3.35 -3.35 -1.87 -1.87 -2.17 -2.17 8.53 8.53 -6.71 -6.71 -1.97 -1.97 6.3 6.3 -0.37 -0.37 -1.74 -1.74 N N -5.08 -5.08 -0.27 -0.27 0.29 0.29 ·0.87 ·0.87 0.99 0.99 1.19 1.19 -6.92 -6.92 5.00 5.00 0.84 0.84 0 0 JJm2 JJm2 1650J.1m2 1650J.1m2 JJm2 JJm2 J.lm2 J.lm2 JJm2 JJm2 1650J.1m2 1650J.1m2 J.lm2 J.lm2 1Jm2 1Jm2 >1650 >1650 1100- 550-1100 550-1100 0-550 0-550 >1650 >1650 1100- 550-1100 550-1100 0-550 0-550 Month Month sites sites Treatment Treatment sites sites Control Control residuals residuals Standardised Standardised variation. variation. significant significant indicate indicate two two negative negative than than less less or or two two than than greater greater residuals residuals Standardised Standardised sites. sites. treatment treatment and and control control on on months months between between frequencies frequencies class class size size comparing comparing tests tests squared squared from from residuals residuals Standardised Standardised Chi~ Chi~ 16. 16. Table Table 26). 26). and and 25 25 (Figures (Figures sites sites control control on on than than faster faster little little a a proceed proceed to to appeared appeared development development sites sites treatment treatment from from clams clams in in Except Except sites. sites. control control from from clams clams in in found found that that to to sites sites treatment treatment from from clams clams from from observable observable was was gametogenesis gametogenesis in in development development of of cycle cycle similar similar A A Sites. Sites. Treatment Treatment seen. seen. is is oocytes oocytes large large of of number number the the in in increase increase large large very very a a 1997 1997 January January In In 1997. 1997. January January in in that that as as such such event event spawning spawning small small a a perhaps perhaps indicating indicating Results Results Reproduction Reproduction Four: Four: Chapter Chapter 71 71 sites. sites. control control on on than than 1997) 1997) (November (November earlier earlier month month a a place place takes takes oocytes oocytes small small of of proportion proportion the the in in increase increase an an with with classes classes size size large large in in drop drop small small the the and and onwards onwards June June about about from from evident evident becomes becomes trend trend this this sites, sites, treatment treatment on on quickly quickly more more proceed proceed to to appears appears oocytes oocytes larger larger of of development development or or recovery recovery The The sites. sites. treatment treatment on on earlier earlier month month a a occurs occurs sites sites control control on on March March in in found found classes classes size size large large of of proportion proportion the the in in drop drop The The exceptions. exceptions. notable notable few few a with with 25) 25) (Figure (Figure sites sites treatment treatment on on found found is is sites sites control control on on found found that that to to pattern pattern similar similar A A classes. classes. size size larger larger the the in in increase increase an an and and oocytes oocytes small small of of proportion proportion the the in in drop drop a a with with 1998 1998 January January in in followed followed is is This This classes. classes. size size larger larger the the in in decrease decrease a a with with recorded recorded again again was was oocytes oocytes smaller smaller of of proportion proportion the the in in increase increase an an when when December December until until continues continues pattern pattern This This classes. classes. size size larger larger the the across across themselves themselves distribute distribute gradually gradually and and oocytes oocytes large large into into develop develop these these as as decreases decreases gradually gradually oocytes oocytes small small of of proportion proportion the the April April From From markedly. markedly. increased increased oocytes oocytes small small very very of of proportion proportion the the conversely conversely and and dropped dropped rapidly rapidly oocytes oocytes larger larger of of proportion proportion the the March March to to February February From From February. February. in in present present again again were were oocytes oocytes large large but but evident evident is is oocytes oocytes larger larger of of proportion proportion the the in in decrease decrease small small a a January January In In increases. increases. classes classes size size larger larger in in oocytes oocytes of of proportion proportion the the as as be be developing developing to to appear appear oocytes oocytes 1996 1996 December December to to 1996 1996 October October from from 24) 24) (Figure (Figure sites sites control control On On analyses. analyses. for for chi~squared chi~squared used used those those than than classes classes size size smaller smaller in in frequencies frequencies size size oocyte oocyte relative relative the the show show 26 26 and and 25 25 Figures Figures Results Results Reproduction Reproduction Four: Four: Chapter Chapter 72 72 etc. etc. 600-1000pm2 600-1000pm2 second second the the 0-600pm2, 0-600pm2, is is class class size size first first the the Where Where 1998. 1998. January January to to 1996 1996 October October for for sites sites control control from from samples samples for for frequencies frequencies size size oocyte oocyte relative relative Observed Observed 24. 24. Figure Figure class class Size Size class class Size Size class class Size Size 0-'-r-'-r~~~~~~~ 0-'-r-'-r~~~~~~~ ...... 0 0 -'-r~~~~~~~ -'-r~~~~~~~ 0-'-r-'-t-'-r..,_,.....L.r....L.r-'-r~-'-r-1 0-'-r-'-t-'-r..,_,.....L.r....L.r-'-r~-'-r-1 10 10 10 10 10 10 20 20 20 20 20 20 30 30 30 30 30 30 40 40 40 40 40 40 1998 1998 1997 1997 50 50 January, January, August, August, 1997 1997 February, February, 50 50 50 50 o~~~~~~~~=r o~~~~~~~~=r 0~-'-r~~~~~~~ 0~-'-r~~~~~~~ 0~~~~~~~~~ 0~~~~~~~~~ 10 10 10 10 10 10 20 20 20 20 20 20 30 30 30 30 30 30 40 40 40 40 40 40 December December January January 50 50 50. 50. 50 50 0-'-r~~~~~~~~ 0-'-r~~~~~~~~ ...... 0 0 -'-r-'-r~~~~~~ -'-r-'-r~~~~~~ 0-'-r~~~~~~~~ 0-'-r~~~~~~~~ 10 10 10 10 10 10 20 20 20 20 20 20 30 30 30 30 30 30 40 40 40 40 40 40 November November December December 50 50 50 50 50 50 0~~~~~~~~~ 0~~~~~~~~~ O...a..,~~~~~~~~ O...a..,~~~~~~~~ o_,_,...a..,~~~~~~=r o_,_,...a..,~~~~~~=r 10 10 10 10 10 10 20 20 20 20 20 20 30 30 30 30 30 30 40 40 40 40 40 40 October October November November 50 50 50 50 50 50 0~~~~~~~~~ 0~~~~~~~~~ o_,_,~~~~~~~~ o_,_,~~~~~~~~ 0-'-r-'-t--'-r-4-r....L.r....L.r""'-T..a::;:_,... 0-'-r-'-t--'-r-4-r....L.r....L.r""'-T..a::;:_,... 10 10 10 10 10 10 20 20 20 20 20 20 30 30 30 30 30 30 40 40 40 40 40 40 1997 1997 September, September, 1997 1997 March, March, 1996 1996 October, October, 50 50 50 50 50 50 Results Results Reproduction Reproduction Four: Four: Chapter Chapter 73 73 etc. etc. 600-10001Jm2 600-10001Jm2 second second the the 0-600JJm2, 0-600JJm2, is is class class sb:e sb:e first first the the Where Where 1998. 1998. January January to to 1996 1996 October October for for sites sites treatment treatment from from samples samples for for frequencies frequencies size size oocyte oocyte relative relative Observed Observed 25. 25. Figure Figure class class class class Size Size Size Size class class Size Size 0~~~~~~~~~ 0~~~~~~~~~ 0~~~~~~~~~ 0~~~~~~~~~ 0~~~~~~~~~ 0~~~~~~~~~ 10 10 10 10 10 10 20 20 20 20 20 20 30 30 30 30 30 30 40 40 40 40 40 40 1998 1998 1997 1997 January, January, August, August, 1997 1997 February, February, 50 50 50 50 50 50 0~~~~~~~~~~ 0~~~~~~~~~~ o~~r-'-1~~~~~~~ o~~r-'-1~~~~~~~ O~r-'-1~~~~~,.-J;;;;::a-..- 10 10 10 10 10 10 20 20 20 20 20 20 30 30 30 30 30 30 40 40 40 40 40 40 December December January January 50 50 50 50 50 50 0~~~~~~~~~ 0~~~~~~~~~ 0~~~~~?9~~ 0~~~~~?9~~ 0~~~~~~~~~ 0~~~~~~~~~ 10 10 10 10 10 10 20 20 20 20 20 20 30 30 30 30 30 30 40 40 40 40 40 40 November November 50 50 December December 50 50 50 50 0~~~~~~~~~ 0~~~~~~~~~ 0~~~~~~~~~~ 0~~~~~~~~~~ o~~r-'-l~r-'-l~~r-'-1~~ o~~r-'-l~r-'-l~~r-'-1~~ 10 10 10 10 10 10 20 20 20 20 20 20 30 30 30 30 30 30 40 40 40 40 40 40 October October November November 50 50 50 50 50 50 0~~~~~~~~~~ 0~~~~~~~~~~ 0~~~~~~4r~=? 0~~~~~~4r~=? 0~~~~~~~~~~ 0~~~~~~~~~~ 10 10 10 10 10 10 20 20 20 20 20 20 30 30 30 30 30 30 40 40 40 40 40 40 1997 1997 September, September, 1997 1997 March, March, 1996 1996 October, October, 50 50 50 50 50 50 Results Results Reproduction Reproduction Four: Four: Chapter Chapter Chapter Four. Reproduction Results Comparisons Between Sites. ANOVA (Table 17) on mean individual oocyte areas showed no significant variation in mean individual oocyte area between control and treatment sites for any month of the study (Figure 22). Figure 23 shows the combined control and treatment site values for each area, variation between areas was not present, except in October 1996 (P:;:: 0.0007). Table 17. ANOVA on clam oocyte areas comparing treatment and control site values at each month of sampling. Where NS signifies an insignificant result, + significant to <0.05, ++ significant to <0.001 and +++ significant to <0.0001. ANOVA P- values 0 N 0 J F M A M J A s 0 N 0 J Site NS NS NS NS NS NS NS N$ NS NS NS NS NS NS NS Area ++ NS NS NS NS NS NS NS NS NS NS NS NS NS NS Size Frequency Analysis. Size frequency analysis carried out on the summed sample size frequencies for each month found that clams from control and treatment sites were not at the same stage of development in October 1996, April-October 1997 and January 1998 {Table 18). This can be seen in Figure 26 where the relative observed and expected frequencies for control and treatment sites at each sampling date are shown. In the early size class (0-5501Jm2) significantly more oocytes were .found on control sites from August to October 1997 while significantly fewer were found on treatment sites. The opposite was found in January 1998. Significant variation in January 1998 was found between control and treatment sites in the early development (550-11 001Jm2) size class, where fewer than expected were found on control sites and more than expected on treatment sites. The late deveiC?pment (1100-16501Jm2) size class showed variation between sites in July and August of 1997 where fewer than expected was found on control sites and more than expected on treatment sites. Variation in the ripe (>16501Jm2) oocyte size class was also found. From April to July and January 1998 less than expected oocytes were found on treatment sites while more than expected were found on control sites. In September 1997 this trend was reversed. 74 Chapter Four: Reproduction Results 60 October, 1996 60 March, 1997 60 September, 1997 40 40 40 20 20 20 0 60 November 60 40 40 20 20 0 60 December 60 November 40 40 20 20 60 January 60 December ~ -e 40 40 ~ 20 20 0 60 February, 1997 60 August, 1997 60 January, 1998 40 40 40 20 20 20 A 8 c 0 A B c 0 A B c D Size class Size class Size class Figure 26. Observed relative size frequencies for samples from control (white bars) and treatment sites (hatched bal'$) from October 1996 to January 1998. The expected frequencies( - 0 - -) are from Chi-squared analyses comparing the size class frequency distributions between control and treatment sites at each month. Where A= Q..550~m2; B= 550-1100~m2; C= 1100-1650J,Jm2; and 02: 1650fJm2 oocytes. 75 '. Chapter Four. Reproduction Results Table 18. Standardised residuals from Chi~squared tests comparing observed values from control and treatment sites at each stage of sampling. Chi-squared (X2) values are also shown where t > 7.81 indicates significant variation in distribution between observed and expected frequencies. Standardised residuals greater than two or Jess than negative two indicate significant variation. Standardised residuals Month Site O..S50 ~m2 550- 1100- >1650}Jm2 x2 value 1100}Jm2 1650~m2 Oct- 96 Con 0.96 1.16 -0.44 -1.72 10.992 Tmt -1.36 ~1.64 0.62 2.43 Nov-96 Con 0.25 -0.52 0.11 0.15 0.477 Tmt -0.21 0.43 -0.09 -0.13 Dec-96 Con 0.57 0.36 0.45 -0.99 2.618 Tmt -0.81 -0.51 -0.63 1.40 Jan- 97 Con 0.80 -0.14 0.47 -0.98 2.706 Tmt -0.88 0.15 -0.52 1.07 Feb- 97 Con 0.65 0.26 -1.88 0.58 7.592 Tmt -0.72 -0.28 2.06 -0.64 Mar- 97 Con 0.36 -1.34 0.58 0.36 3.039 Tmt -0.30 1.10 -0.48 -0.30 Apr- 97 Con -1.36 -0.47 0.20 2.15 8.897 Tmt 1.28 0.45 -0.19 -2.03 May-97 Con -0.99 -1.26 -0.37 2.54 11.308 Tmt 0.92 1.17 0.35 -2.35 Jul- 97 Con 0.05 -0.28 -2.23 2.33 19.422 Tmt -0.06 0.35 2.73 -2.85 Aug-97 Con 2.51 -0.41 -2.32 0.03 22.135 Tmt -3.15 0.52 2.91 -0.04 Sep- 97 Con 4.01 1.50 -1.27 -4.17 52.970 Tmt -4.01 -1.50 1.27 4.17 Oct- 97 Con 2.55 -0.46 -0.48 -1.19 13.220 Tmt -2.67 0.48 0.50 1.25 Nov- 97 Con 0.39 -0.58 -0.03 0.19 1.130 Tmt -0.53 0.80 0.04 -0.25 Dec- 97 Con 0.17 -0.86 -0.53 1.14 3.983 Tmt -0.20 1.04 0.63 -1.37 Jan- 98 Con -2.73 -4.42 -1.82 6.38 91.098 Tmt 2.99 4.84 1.99 -6.99 76 Chapter Four: Reproduction Results DISCUSSION. The aims of this chapter were to document a yearly cycle of reproduction and gametogenesis in A. stutchburyi. In addition it was necessary to compare gametogenesis between control and treatment sites to observe how harvesting and the consequent removal of competition for food and space may impact on the reproductive cycle and condition of the remaining population on harvested sites. This was achieved through histological preparations and the reproductive staging of each individual, the calculation of maturity and condition indices, mean oocyte area and size frequency distribution analyses. The male:female ratio for control sites was 0.64:1. Indicating a lower than expected proportion of males in the population. Initial analysis of the male:female ratio on treatment sites indicated that there was not a significant deviation from the expected 1: 1 ratio. With the removal of two uncharacteristically high male:female ratio values from October and December 1996 significant variation was found. A difference in sex ratio has not been found before in studies in A. stutchburyi (Larcombe, 1971) and is generally not observed in clams overseas (Eversole and Michener, 1980; Heffernan et al, 1989; and Hasselman et al, 1989). Ponurovsky and Yakovlev (1992) however found several populations of Manila clams in Vostock Bay where females were more numerous than males. They attributed sex ratios of these populations to the clams being alternate hermaphrodites in these areas. As no instances of hermaphroditism were found in the present study this is not a plausible reason for the sex ratios found here. Ponurovsky and Yakovlev also found that the majority of older clams on all sites were females. The relationship of sex with size was not investigated in this study but it may be a plausible explanation for the sex ratios found. Beds in which there is a high density of large animals have been found to contain low densities of young individuals. Larcombe (1971) found that the size distribution of most populations around the country consisted of adult A. stutchburyi of a mean size approaching maximum length (30-40mm) and that young individuals (less than two years of age) formed less than five percent of the total population. Whether the sex ratio in populations of A. stutchburyi is affected by the age structure of the populations being studied is a topic requiring further attention. 77 ,, ' Chapter Four: Reproduction Discussion As has been found in other clam species (Porter, 1964; Bardach et al, 1972; Keck et al, 1975; Yap, 1977; Eversole et al, 1980; and Shpigel and Fridman, 1990), A. stutchburyi does show a definite annual cycle of reproduction. It has been suggested that species in more temperate or cooler regions show a strong unimodal cycle with one major period of spawning followed by redevelopment towards the next major spawning event (Loosanoff, 1937 and Keck et al, 1975). This cycle of gametogenesis in a temperate population of M. mercenaria has been well documented by Keck et al (1975). The results of this study point to a similar pattern of reproduction and also concur with studies by Larcombe (1971) and Stephenson (1981) which show that A. stutchburyi has a distinct seasonal spawning season made up of a single spawning period beginning in January and ending in April- May. Males and females appear to spawn at a similar time for about the same duration. Staging and maturity indices indicate that spawning individuals were present in samples from mid summer through until mid winter in males and females. Spent individuals were found two to three months after the initiation of spawning, usually from mid autumn to mid winter for both sexes. Individuals in various stages of development from early development to ripe were found from the onset of the study until March 1997 and again from May 1997 to the conclusion of the study. It was not uncommon to find individuals within samples in three to four different phases of development when spawning was not taking place. This suggests a marked lack of synchrony within study populations. Condition indices indicate that spawning began between January and February and continued until April at which stage clam condition began to recover. Another decrease in condition was observed in December 1997. Mean oocyte area analysis indicates that major spawning began between February and March a month later than condition indices suggest, although a smaller spawning event did appear to take place between December and January 1·997 which may have contributed to the drop in condition from January to February 1997. Both condition and oocyte area indicate that the majority of spawning had been completed by April and from then until July individuals continued to discharge remaining oocytes. Oocyte size frequency analysis from the duration of the study period for control and treatment sites also indicates a minor spawning event between December and January 1997 followed by a major spawning event which took place between February and April 1997. A new cohort of oocytes began 78 ·~. Chapter Four: Reproduction Discussion development in March of 1997 and continued to develop into ripe oocytes by November of the same year. A minor spawning event appears to take place between November and December 1997 similar to that between December 1996 and January of 1997 with ripe oocyte numbers recovering in January of 1998. Histological staging data for males and females indicate that spawning individuals were present on treatment sites earlier and later than that found on control sites. Over the months that spawning was prevalent on both sites a larger proportion of spawning individuals on treatment sites was apparent. Spent individuals were evident considerably earlier and spawning was found to recommence earlier in late 1997 on treatment sites. Maturity indices for males and females from control and treatment sites showed a similar pattern. These variations in spawning between control and treatment sites can be attributed to the reduction in competition leading to an increase in space and more importantly in the case of reproductive effort, food availability. Condition remained similar between control and treatment sites throughout the study except in April and October 1997. April followed a major spawning event, clams from treatment sites showed a significant increase in condition from May as they appear to have began to recover while condition in clams from control sites remained relatively low. In October 1997 condition in clams sampled from control sites dropped while those on treatment sites remained fairly stable, leading to a significant difference in condition between control and treatment sites. The higher condition values found on treatment sites after the completion of spawning in April 1997 and in October 1997 when clams were beginning to develop large oocytes on control sites are again probably due to a decrease in competition for space and food resources on treatment sites. No significant variation in mean oocyte area was found between clams from control and treatment sites at any stage of the study. Variation in oocyte size frequency distributions were found however. In October 1996 significantly more oocytes in the ripe size class were found on treatment sites. Significant variation in size class frequency between control and treatment sites was not observed again until April 1997, coinciding with the conclusion of major spawning and the beginning of development of a new cohort of oocytes. In April through July there were significantly fewer than expected oocytes in the ripe size class in clams from treatment sites and significantly more in clams from control sites. This suggests clams on treatment sites have spawned 79 "'" Chapter Four. Reproduction Discussion more intensely than clams on control sites. Chi-square tests of oocyte size frequency distributions carried out separately for control and treatment sites over the study period indicate the new cohort of oocytes develop more quickly on treatment sites. These results indicate that clams on treatment sites have not only spawned more intensely but the development of a new cohort of oocytes has proceeded more quickly than on control sites. Peterson (1982) found that P. st~minea did not reallocate energy between growth and reproduction under the stress of increased density. Energy was allocated between these activities evenly and as density increased reproductive effort and growth declined at similar rates. Presumably if density is decreased then individuals in lower density populations would have more food and thus energy to allocate to both growth and reproductive effort than in high density populations. This is what appears to have happened in the present study as individuals on treatment sites were found to be spawning earlier and later than on control sites. The extent of spawning is also greater on treatment sites as is the recovery and redevelopment of a new cohort of oocytes. It seems reasonable to link these results to an increase in energy due to increased food availability from a reduction in competition through the size specific harvesting of shellfish. Although gametogenesis proceeds more quickly in samples taken from treatment sites, initiation of spawning appears to take place at a similar time for clams from control and treatment sites. Although small numbers of male and female clams have been found on treatment sites to be spawning earlier and later than clams from control sites the majority of spawning occurs at the same time on both sites. This suggests that spawning is triggered by environmental cues. As has been suggested by Mann (1979), Borrero (1987), Hasselman (1989), and Ruiz et al (1992) these are likely to be temperature or food availability. Spawning appears to have started a month earlier at the end of 1997 than at the beginning of the year. This change in timing may be due to a change in water temperature or in present food availability or increased food availability throughout the year. Variation was found between study areas in condition for most months, and in the pattern of change over the course of the study on each of the three areas in condition and mean oocyte area. This indicates that clams sampled from each study area have responded differently over the study period. A number of studies have found variability in condition and reproductive activity between sample sites (Ansell et al, 1964; Keck et al 1975; Eversole et at, 1980; and 80 Chapter Four: Reproduction Discussion Hasselman et al, 1989). Differences in food supply and local environmental conditions have been suggested to be the determining factors in the variation between sample sites. This appears to also be true in the present study as each study area is located in different environmental conditions. The three study areas however, do give a good representation of the range of areas that are being harvested. All study areas however followed a similar overall trend through the course of the study, hence the variation in pattern of change observed here is over a relatively small time scale. A. stutchburyi from sample populations in Waitati Inlet consisted of a higher proportion of females than males. This may be a consequence of the age structure of the population. A unimodal gametogenic cycle was similar to that found in temperate populations of Northern hemisphere clams (Loosanoff, 1937 and Keck et al, 1975) and to previous studies for A. stutchburyi by Larcombe (1971) and Stephenson (1984). The cycle of gametogenesis was found to differ between treatment and control sites in the intensity and duration of spawning and the rate of development of new oocytes. However major efforts between control and treatment sites still appeared to occur at similar times of the year suggesting that the onset of spawning is synchronised by environmental factors. Although harvesting appears to impact on the relative amounts of energy available for reproduction leading to an increase in reproductive effort, spawning cycles do not appear to be affected. 81 82 82 migrate migrate from from high high density density areas areas surrounding surrounding harvested harvested strips strips into into density density low may may be be important important in in the the recovery recovery of of harvested harvested strips. strips. Established Established clams clams may may migration migration to to be be especially especially frequent frequent around around the the edges edges of of disturbances disturbances and and this this as as has has been been suggested suggested by by Larcombe Larcombe (1971). (1971). Stephenson Stephenson (1981) (1981) found found impossible impossible due due to to the the lack lack of of suitable suitable space space for for clams clams to to relocate relocate themselves themselves occurred. occurred. At At densities densities found found at at Waitati Waitati Inlet Inlet migration migration would would appear appear to to be be near near scale scale migration migration of of clams clams from from surrounding surrounding areas areas into into harvested harvested areas areas has has As As density density levels levels on on control control sites sites have have not not varied, varied, it it is is unlikely unlikely that that any any large large unimodal unimodal population population structure structure found found pre pre and and post-harvest. post-harvest. clams clams developing. developing. On On control control sites sites the the status status quo quo was was retained retained with with a a unimodal unimodal distribution distribution to to one one with with a a bimodal bimodal structure structure with with a a cohort cohort of of young young unchanged. unchanged. Size Size frequency frequency distributions distributions on on treatment treatment sites sites changed changed from from a a recovered recovered to to 86.9% 86.9% of of initial initial estimates estimates while while density density on on control control sites sites remained remained found found between between treatment treatment and and control control sites. sites. Density Density on on treatment treatment sites sites Significant Significant variation variation in in the the pattern pattern of of change change over over time time in in density density was was not not space space after after harvesting harvesting had had taken taken place. place. to to the the consequent consequent reduction reduction in in intraspecific intraspecific competition competition for for both both food food and and shown shown a a strong strong resilience resilience to to a a major major harvesting harvesting event. event. This This can can be be attributed attributed this this study study indicate indicate that that the the populations populations being being studied studied in in Waitati Waitati Inlet Inlet have have a a population population with with densities densities as as high high as as that that found found in in Waitati Waitati Inlet. Inlet. Results Results of of that that intraspecific intraspecific competition competition will will be be due due to to both both space space and and food food resources resources in in Black, Black, 1987 1987 and and Peterson Peterson and and Beal, Beal, 1989). 1989). It It is is reasonable reasonable to to expect expect however however 1982; 1982; Blackwell, Blackwell, 1984; 1984; Martin, Martin, 1984; 1984; Bertness Bertness et et al, al, 1985; 1985; Peterson Peterson and and competition competition and and that that space space is is not not as as important important a a resource resource as as food food (Peterson, (Peterson, It It is is generally generally considered considered that that a a reduction reduction in in density density will will lessen lessen intraspecific intraspecific management management considerations. considerations. The The results results with with possible possible causative causative factors factors will will be be discussed discussed along along with with future future aspects, aspects, information information useful useful to to future future management management plans plans has has been been developed. developed. energy energy was was affected. affected. By By assessing assessing the the impact impact of of harvesting harvesting on on these these three three remaining remaining population population and and third third hdw hdw the the reproductive reproductive cycle cycle and and allocation allocation of of shellfish, shellfish, second second how how harvesting harvesting affected affected growth growth and and the the size size structure structure of of the the how how commercial commercial harvesting harvesting directly directly affects affects the the biomass biomass and and density density of of commercial commercial harvesting harvesting of of New New Zealand Zealand littleneck littleneck clams clams stutchburyi). stutchburyi). (A. (A. First First This This study study was was designed designed to to address address three three questions questions relating relating to to the the affect affect of of CHAPTER CHAPTER FIVE: FIVE: GENERAL GENERAL DISCUSSION. DISCUSSION. 83 83 showed showed an an increase increase of of 6%. 6%. estimated estimated biomass biomass removed. removed. Total Total estimated estimated biomass biomass on on control control sites sites recover recover to to 81.3% 81.3% of of initial initial estimates estimates after after having having close close to to 60% 60% of of the the initial initial change change over over time time in in total total estimated estimated biomass biomass was was found. found. Biomass Biomass however however did did Significant Significant variation variation between between treatment treatment and and control control sites sites in in the the pattern pattern of of studied studied more more closely. closely. establish establish themselves themselves or or a a combination combination of of both both is is a a question question that that needs needs to to be be harvesting harvesting action, action, or or by by newly newly settled settled juveniles juveniles searching searching for for space space to to migration migration is is undertaken undertaken by by adults adults in in response response to to a a disturbance disturbance created created by by the the found found as as more more space space is is available available to to allow allow their their establishment. establishment. Whether Whether attract attract settlement. settlement. High High rates rates of of survival survival of of newly newly settled settled clams clams would would then then be be maintain maintain density density levels levels and and substrate substrate suitability suitability at at levels levels high high enough enough to to density density around around the the disturbed disturbed strips strips left left after after harvesting, harvesting, and and therefore therefore after after harvesting, harvesting, established established clams clams may may migrate migrate from from areas areas of of high high to to low low subsequent subsequent increase increase in in the the survival survival of of newly newly settled settled juveniles. juveniles. Alternatively Alternatively not not survive, survive, while while an an increase increase in in space space on on treatment treatment sites sites is is available available with with a a individuals individuals can can not not find find any any space space to to establish establish themselves themselves and and therefore therefore do do surfaces surfaces are are not not available available on on control control sites sites and and therefore therefore newly newly settled settled and and then then migrating migrating to to nearby nearby suitable suitable surfaces surfaces to to become become established. established. These These include include juvenile juvenile clams clams settling settling on on high high density density areas areas which which attract attract settlement settlement combine combine is is another another question. question. Possible Possible scenarios scenarios of of population population recovery recovery probably probably act act in in combination combination to to restore restore population population densities densities but but how how they they Migration Migration and and the the increased increased rate rate of of survival survival of of newly newly settled settled juveniles juveniles population. population. allow allow a a significantly significantly higher higher proportion proportion to to become become established established in in the the would would presumably presumably increase increase the the survival survival rate rate of of newly newly settled settled juveniles juveniles and and enough enough to to discourage discourage settlement. settlement. The The liberation liberation of of space space on on treatment treatment sites sites harvesting harvesting undertaken undertaken has has not not affected affected substrate substrate suitability suitability or or density density levels levels increase increase in in settlement settlement on on treatment treatment sites sites indicates indicates that that the the methods methods of of removal removal A. A. of of larger larger stutchburyi stutchburyi leads leads to to higher higher settlement settlement levels. levels. The The (Martin, (Martin, 1984 1984 and and Dobbinson Dobbinson et et at, at, 1989) 1989) has has been been found found suggesting suggesting that that the the frequency frequency distribution distribution analyses analyses indicate indicate that that this this did did occur. occur. Some Some evidence evidence to to occur occur the the recovery recovery of of density density can can be be attributed attributed to to new new settlement. settlement. Length Length As As the the large large scale scale migration migration of of clams clams from from surrounding surrounding areas areas does does not not appear appear and and clam clam density density at at levels levels which which are are still still attractive attractive to to settlement. settlement. harvested harvested areas. areas. This This could could be be important important in in maintaining maintaining substrate substrate suitability suitability Chapter Chapter Five: Five: OiscuS$ion OiscuS$ion 84 84 to to post post harvest harvest affects affects were were an an increase increase in in the the duration duration and and intensity intensity of of throughout throughout the the study. study. Differences Differences that that were were observed observed and and may may be be attributed attributed timing timing of of reproductive reproductive cycles cycles of of clams clams from from control control and and treatment treatment sites sites through through until until mid mid to to late late autumn. autumn. There There appeared appeared to to be be little little difference difference in in the the major major period period of of spawning spawning beginning beginning in in mid mid to to late late summer summer and and continuing continuing A. A. stutchburyi stutchburyi displayed displayed an an annual annual unimodal unimodal cycle cycle of of reproduction reproduction with with one one to to be be investigated investigated further. further. ratios ratios in in this this study, study, or or some some other other anomaly anomaly has has affected affected these these findings findings needs needs reproductive reproductive analyses. analyses. Whether Whether these these factors factors affected affected the the variation variation in in sex sex noted noted that that clams clams of of reproductive reproductive size size in in length) length) (~18mm (~18mm were were taken taken for for than than 30mm 30mm in in length, length, which which were were presumably presumably fairly fairly old. old. It It should should also also be be studied studied were were originally originally virgin virgin and and consisted consisted largely largely (>85%) (>85%) of of clams clams greater greater higher higher proportion proportion of of females females found found in in the the present present study study as as the the populations populations 1971). 1971). If If female female clams clams do do outlive outlive their their male male counterparts counterparts this this may may explain explain the the been been found found to to contain contain low low (<5%) (<5%) levels levels of of young young individuals individuals (Larcombe, (Larcombe, (Ponurovsky (Ponurovsky and and Yakovlev, Yakovlev, 1992). 1992). Beds Beds of of high high A. A. density density stutchburyi stutchburyi have have clams clams in in Vostock Vostock Bay Bay have have been been found found to to mainly mainly consist consist of of female female clams clams and and Michener, Michener, 1980; 1980; and and Heffernan Heffernan et et at at 1989). 1989). Older Older populations populations of of manila manila Zealand Zealand before before and and is is rare rare in in other other species species (Hasselman (Hasselman et et at, at, 1989; 1989; Eversole Eversole Deviation Deviation from from a a 1:1 1:1 sex sex ratio ratio has has not not A. A. been been found found in in stutchburyi stutchburyi in in New New further further analyses. analyses. valuable valuable and and a a shell shell dying dying or or notching notching method method would would have have been been useful useful in in investigated investigated in in this this study. study. The The consideration consideration of of growth growth rates rates would would have have been been post-harvest post-harvest recovery recovery of of biomass. biomass. Unfortunately Unfortunately growth growth rates rates were were not not combined combined with with newly newly settled settled clams clams are are postulated postulated as as the the causative causative factors factors in in reduced, reduced, again again leading leading to to an an increase increase in in growth growth rate. rate. These These two two factors factors Consequently Consequently the the physically physically retarding retarding forces forces of of neighbouring neighbouring clams clams was was resources resources liberated liberated for for the the remaining remaining population population but but also also space space resources. resources. S. S. 1968) 1968) and and patula patula (Tegelberg (Tegelberg and and Magoon, Magoon, 1969). 1969). Not Not only only were were more more food food (Peterson, (Peterson, A. A. 1982), 1982), brasiliana brasiliana (Monti (Monti et et al, al, 1991 1991 ), ), M. M. mercenaria mercenaria (Ansell, (Ansell, as as a a number number of of other other clam clam species, species, including including P. P. staminea staminea and and C. C. undatella undatella stutchburyi stutchburyi (Larcombe, (Larcombe, 1971; 1971; Martin, Martin, 1984 1984 and and Dobbinson Dobbinson et et al, al, 1989) 1989) as as well well reduction reduction in in intraspecific intraspecific competition competition leads leads to to an an increase increase A. A. in in growth growth rate rate in in new new recruitment recruitment and and subsequent subsequent biomass. biomass. Previous Previous studies studies have have found found that that a a resources resources has has undoubtedly undoubtedly impacted impacted on on growth growth of of the the remaining remaining population, population, growth growth rate rate and and new new recruitment. recruitment. The The liberation liberation of of both both food food and and space space The The biomass biomass recovery recovery found found in in this this study study can can be be attributed attributed to to an an increase increase in in Chapter Chapter Five: Five: Discussion Discussion 85 85 order. order. There There was was also also no no means means of of calculating calculating how how much, much, if if any any biomass biomass was was hampered hampered by by practical practical considerations considerations as as harvesting harvesting was was done done to to commercial commercial rather rather than than the the four four months months it it took took in in this this study. study. But But this this last last problem problem was was condense condense the the harvesting harvesting treatment treatment into into a a shorter shorter period period i.e. i.e. one-two one-two months months harvesting harvesting affect affect is is difficult difficult to to calculate. calculate. It It would would also also have have been been preferable preferable to to harvesting harvesting treatment treatment and and consequently consequently an an accurate accurate estimate estimate for for the the mistake mistake was was also also made made in in not not taking taking a a survey survey sample sample at at the the completion completion of of the the and and density density levels levels on on the the treatment treatment areas, areas, but but this this was was unavoidable. unavoidable. A A duration duration of of the the study study was was too too short short to to fully fully measure measure the the recovery recovery of of biomass biomass Several Several problems problems became became evident evident during during the the course course of of this this study. study. The The variation variation in in results results is is inevitable. inevitable. predation, predation, sediment sediment properties properties may may all all vary vary from from area area to to area, area, so so this this Factors Factors such such as as time time of of aerial aerial exposure, exposure, water water temperature, temperature, food food availability, availability, exposed exposed to to its its own own microgeographical microgeographical set set of of environmental environmental conditions. conditions. found. found. As As each each study study area area is is in in a a different different location location of of the the Inlet, Inlet, each each is is patterns patterns of of change change in in condition condition and and oocyte oocyte area area between between study study areas areas was was Variation Variation in in biomass biomass and and density density levels, levels, settlement settlement rates, rates, and and monthly monthly Hasselman Hasselman et et al, al, 1989 1989 and and Ruiz Ruiz et et at, at, 1992). 1992). and and spawning spawning in in a a number number of of bivalves bivalves (Mann, (Mann, 1979; 1979; Borrero, Borrero, 1987; 1987; which which has has previously previously been been found found to to be be important important in in initiating initiating gametogenesis gametogenesis more more sensitive sensitive to to changes changes in in other other environmental environmental cues cues such such as as temperature, temperature, not not appear appear to to be be affected. affected. This This suggests suggests that that the the timing timing of of spawning spawning may may be be as as speed speed up up the the development development of of new new oocytes oocytes the the major major spawning spawning cycle cycle did did availability availability appears appears to to prolong prolong and and increase increase the the intensity intensity of of spawning spawning as as well well Although Although a a decrease decrease in in intraspecific intraspecific competition competition and and an an increase increase in in food food development development proceeded proceeded more more rapidly rapidly on on treatment treatment sites sites than than control control sites. sites. more more effort effort to to be be put put into into reproductive reproductive output. output. It It may may also also explain explain why why oocyte oocyte to to an an increase increase in in food food resources resources and and thus thus an an increase increase in in energy energy allowing allowing for for time time and and more more intensely intensely than than clams clams on on control control sites. sites. This This may may be be attributable attributable However However clams clams on on treatment treatment sites sites were were found found to to be be spawning spawning over over a a longer longer spawning spawning or or process process of of gametogenesis gametogenesis between between control control and and treatment treatment sites. sites. Borrero, Borrero, 1987). 1987). The The present present study study found found no no major major differences differences in in the the timing timing of of gametogenesis gametogenesis and and spawning spawning in in filter filter feeding feeding bivalves bivalves (Ruiz (Ruiz et et al, al, 1992 1992 and and Food Food availability availability has has been been suggested suggested to to be be important important in in initiating initiating treatment treatment sites. sites. spawning spawning and and the the more more rapid rapid development development of of new new oocytes oocytes in in clams clams on on Chapter Chapter Five: Five: Discussion Discussion 86 86 removed removed from from around around the the inlet. inlet. The The effect effect on on the the population population biology biology of of clams clams as as was was undertaken undertaken in in this this study, study, instead instead smaller smaller amounts amounts are are continually continually Harvesting Harvesting is is not not usually usually carried carried out out in in a a comprehensive, comprehensive, concentrated concentrated effort effort one one year year later. later. Several Several questions questions that that need need further further attention attention have have surfaced. surfaced. months months affected affected density, density, biomass, biomass, length length distribution distribution and and reproductive reproductive cycles cycles This This study study looked looked at at how how one one large large harvesting harvesting event event concentrated concentrated over over four four dynamics dynamics over over an an extended extended time time period. period. consistentiy consistentiy and and to to observe observe directly directly how how harvesting harvesting impacts impacts on on population population harvesting harvesting levels levels but but also also offers offers a a chance chance to to monitor monitor the the population population limits limits suitably suitably altered. altered. A A CAY CAY approach approach not not only only offers offers an an annual annual review review of of biomass biomass estimates estimates and and any any change change in in biomass biomass can can be be reacted reacted to to and and catch catch management management appears appears to to be be sensible sensible as as the the catch catch limit limit would would be be set set on on annual annual biomass. biomass. In In this this case case a a constant constant annual annual yield yield (CAY) (CAY) approach approach to to fisheries fisheries settlement settlement of of juveniles, juveniles, also also found found in in the the present present study, study, will will lead lead to to fluctuating fluctuating danger danger of of being being overfished. overfished. Cryer Cryer (1996) (1996) has has suggested suggested that that the the sporadic sporadic under under a a maximum maximum constant constant yield yield (MCY) (MCY) strategy strategy appears appears to to be be under under little little danger danger of of a a rapid rapid fall fall in in biomass biomass or or density. density. The The fishery fishery currently currently managed managed harvesting harvesting practices practices appear appear not not to to do, do, there there should should not not be be any any immediate immediate actively actively stripped stripped leaving leaving them them unsuitable unsuitable for for larval larval settlement, settlement, which which current current consisting consisting mainly mainly of of clams clams greater greater than than 30mm. 30mm. So So long long as as large large areas areas are are not not clams clams recruiting recruiting to to the the spawning spawning biomass biomass at at 18mm 18mm in in length length and and commercial commercial The The fishery fishery does does have have some some natural natural protection protection to to egg egg over-fishing over-fishing due due to to commercially commercially unharvested. unharvested. accessible accessible to to the the public. public. More More accessible accessible beds beds therefore therefore have have been been left left of of harvesting harvesting however however at at Waitati Waitati Inlet Inlet is is carried carried out out on on beds beds not not easily easily users users prefer prefer larger larger clams clams this this is is not not positive positive to to their their perspective. perspective. The The majority majority present present in in the the populations populations measured. measured. As As most most recreational recreational and and traditional traditional of of the the remaining remaining clams clams and and a a reduction reduction in in the the proportion proportion of of larger larger clams clams commercial commercial harvesting harvesting has has however however resulted resulted in in a a decrease decrease in in the the mean mean size size Inlet Inlet has has not not been been significantly significantly altered. altered. Results Results from from this this study study show show that that (1992) (1992) and and NIWA NIWA (Carbines (Carbines pers pers .. .. comm.) comm.) indicate indicate that that total total biomass biomass of of the the classes classes from from the the area. area. Results Results from from surveys surveys of of the the Inlet Inlet by by Stewart Stewart et et al al commercial commercial harvesting harvesting is is depleting depleting the the biomass biomass and and stripping stripping larger larger size size Recreational Recreational and and traditional traditional users users of of clam clam beds beds have have voiced voiced concerns concerns that that be be a a major major problem. problem. three three study study areas areas were were not not easily easily accessible accessible to to the the public public so so this this should should not not removed removed by by recreational recreational or or customary customary fishers fishers during during the the study. study. Although Although the the Chapter Chapter Five: Five: Discussion Discussion ·~. ·~. 87 87 relatively relatively unexploited unexploited resource. resource. provides provides an an ideal ideal opportunity opportunity to to further further study study the the impact impact of of harvesting harvesting on on a a and and has has not not been been severely severely depleted depleted by by human human exploitation exploitation yet yet and and thus thus to to and and reactions reactions of of A. A. stutchburyi stutchburyi to to harvesting. harvesting. This This fishery fishery is is still still fairly fairly young young a a lot lot of of work work to to be be undertaken undertaken in in order order to to fully fully understand understand the the consequences consequences problems problems discussed discussed along along with with the the lack lack of of growth growth measurements measurements there there is is still still absorbing absorbing and and recovering recovering from from quite quite large large stock stock losses. losses. Considering Considering the the indicate indicate that that stocks stocks of of A. A. stutchburyi stutchburyi are are highly highly resilient resilient and and capable capable of of treatment treatment sites sites did did tend tend to to spawn spawn for for longer longer and and more more intensely. intensely. These These results results onset onset of of gametogenesis gametogenesis and and spawning spawning was was unaffected unaffected although although clams clams on on biomass biomass levels through through levels allowing allowing higher higher settlement settlement and and growth growth rates. rates. The The and and space space resources. resources. This This undoubtedly undoubtedly aided aided in in the the recovery recovery of of density density and and reduced reduced the the level level of of intraspecific intraspecific competition, competition, leading leading to to an an increase increase in in food food The The removal removal of of 60% 60% of of the the initial initial biomass biomass on on treatment treatment sites sites effectively effectively effort. effort. spawning spawning and and how how energy energy is is proportioned proportioned between between growth growth and and reproductive reproductive reproductive reproductive cycle cycle and and if if these these control control the the onset onset of of gametogenesis gametogenesis and and environmental environmental variables variables such such as as salinity salinity and and water water temperature temperature on on the the are are restocked restocked also also needs needs to to be be addressed. addressed. Also Also of of interest interest is is the the affect affect of of not not occur. occur. The The level level of of mortality mortality around around harvested harvested strips strips and and how how these these strips strips and and how how heavily heavily harvested harvested an an area area needs needs to to be be before before new new settlement settlement does does acts acts on on its its own. own. Of Of special special interest interest are the the are factors factors that that stimulate stimulate settlement settlement numbers, numbers, how how they they interact interact together together is is of of interest, interest, and and also also how how each each factor factor and and settlement settlement both both play play an an important important role role in in helping helping to to restock restock population population of of this this type type of of continual continual harvesting harvesting would would be be interesting interesting to to study. study. 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