And Juvenile Chinook Salmon (Oncorhynchus Tsha Wytscha Walbaum) in an Estuarine Marsh
INTERACTIONS BETWEEN THREESPINE STICKLEBACK (GASTEROSTEUS ACULEATUS LINNAEUS) AND JUVENILE CHINOOK SALMON (ONCORHYNCHUS TSHA WYTSCHA WALBAUM) IN AN ESTUARINE MARSH
by
ROBERT JOSEPH SAMBROOK B.Sc, University College of North Wales, Bangor, 1985
A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE
in THE FACULTY OF GRADUATE STUDIES (Department of Zoology)
We accept this thesis as conforming to the required standard
THE UNIVERSITY OF BRITISH COLUMBIA DECEMBER, 1990 © Robert Joseph Sambrook, 1990 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission.
Department of ~Z~OOUO>C,"i The University of British Columbia Vancouver, Canada
Date P^C Lf™ mO
DE-6 (2/88) ii
ABSTRACT
Threespine stickleback (Gasterosteus aculeatus) and juvenile chinook salmon (Oncorhynchus tshazvytscha) co-occur during high tide in tidal channels of the Fraser River estuary. Given the high density of resident stickleback, there is the potential for strong interactions within and between the two species. Inter- and intra-specific interactions were tested by means of laboratory experiments, with support from field studies.
Laboratory experiments placed stickleback and chinook in mixed and
single species groups. The levels of aggressiveness were quantified, along
with prey choice between surface (Drosophila), midwater (Artemia), and benthic (Tubifex) prey; microdistribution was also recorded. The experiments
demonstrated that stickleback were highly aggressive towards chinook, and
would drive them away from optimal feeding territories. Chinook consumed surface prey only when tested with stickleback, exploiting benthic and midwater prey when feeding alone. Stickleback demonstrated no significant difference in diet between single and mixed species trials, which is consistent with the supposition of strongly asymmetrical competition for food and space.
Field data lend further support to this premise; a marked difference observed in diet suggests microhabitat partitioning between the two species, with stickleback feeding on benthos and chinook largely consuming surface prey.
This thesis proposes that interactive segregation is an important process between sympatric stickleback and juvenile chinook in estuarine tidal channels and might have important implications for Fraser chinook stocks. iii
TABLE OF CONTENTS Page ABSTRACT ...... ii TABLE OF CONTENTS iii LIST OF FIGURES v LIST OF APPENDICES vii LIST OF TABLES viii ACKNOWLEDGEMENTS . ix CHAPTER 1: Introduction 1 CHAPTER 2: Study Area 6 CHAPTER 3: Spatial and Temporal Distribution Patterns 14 Introduction 14 Methods 15 i) Stickleback population structure. ii) Chinook population structure. Results 18 i) Stickleback Residency. ii) Chinook Residency. iii) Estuarine Growth. CHAPTER 4: Estuarine Dietary Patterns 29 Introduction 29 Methods 30 i) Sampling. ii) Stomach content analysis. Results 33 CHAPTER 5: Laboratory Experiments 45 Introduction 45 Methods 46 i) Collection of Fish. ii) Holding Facilities. iii) Collection and Maintenance of Prey. iv) Selection of Prey Densities v) Experimental Arenas. vi) Procedures. vii) Statistical Analysis iv
Page Results 55 i) General. ii) MiCTodistribution. iii) Aggressive Behaviour. iv) Size Effects. v) Prey Choice Experiments. DISCUSSION 71 LITERATURE CITED 77 APPENDICES 83 V
LIST OF FIGURES
Figure Page
1. Study site location. 7
2. Locations of sampling areas on Ladner Marsh. 8
3. Mean monthly discharge of the Fraser River, recorded at the Port Mann Pumping Station (20 km upstream from the study site on the Fraser River mainstem) for 1986, 87 and 88 (Water Survey Canada 1986,1987,1988). 9 4. Water temperature during morning and afternoon high tides in Ladner Marsh side channels (1987). 11
5. Typical turbidity (Nephelometric Turbidity Units) of side channel water over a tidal cycle (April 23 1990 - mean values from channels 1 and 3). 12
6a. Stickleback densities in side channels; estimates based on Peterson single mark-recapture in channels 1 and 3 (with 95% confidence limits). 19
6b. Seasonal change in numbers per 100m2 of stickleback and prickly sculpin in side channel habitat, South Arm Fraser River (from Dunford 1975); estimates based on total captures using a pole seine. 19
7. Schematic representation of chinook migration in and out of side channels (based on the data in table 4). 26
8. Seasonal variation in mean fork length of stickleback and juvenile chinook in Ladner Marsh side channels. 27
9. Diet of stickleback and juvenile chinook during high tide in channels 1 and 3 (% frequency occurrence). 34
10 Diet of stickleback and juvenile chinook during high tide in channels 1 and 3 (% numbers). 35
11. A comparison of the relative proportions (% numbers) of prey in the environment (sweep and benthic samples) and the diets of stickleback and juvenile chinook. March 29 1987. 36
12. A comparison of the relative proportions (% numbers) of prey in the environment (sweep and benthic samples) and the diets of stickleback and juvenile chinook. April 14 1987. 37 Figure
13. A comparison of the relative proportions (% numbers) of prey in the environment (sweep and benthic samples) and the diets of stickleback and juvenile chinook. April 28 1987.
14. A comparison of the relative proportions (% numbers) of prey in the environment (sweep and benthic samples) and the diets of stickleback and juvenile chinook. May 20 1987
15. Diet of stickleback and juvenile chinook during high tide in channels 1 and 3, with prey items pooled by microhabitat (% frequency occurrence).
16. Diet of stickleback and juvenile chinook during high tide in channels 1 and 3, with prey items pooled by microhabitat (% numbers).
17. Per-capita frequency of inter- and intra-specific aggression, during four-minute intervals, by stickleback and chinook (experiment 1).
18. Per-capita frequency of inter- and intra-specific aggression, during four-minute intervals, by stickleback and chinook (experiment 2).
19. Per-capita frequency of inter- and intra-specific aggression by stickleback and chinook (experiment 4).
20. Frequency of aggression as a function of relative size of stickleback ancfjuvenile chinook (experiment 3).
21. Prey choice (mean per individual, ±95% C.L.) of stickleback ana chinook feeding together (experiment 2).
22. Prey choice (mean per individual, ±95% C.L.) of stickleback alone (experiment 2).
23. Prey choice (mean per individual, ±95% C.L.) of chinook alone (experiment 2). vii
LIST OF APPENDICES
Appendix Page
LA. length-frequency distributions of threespine stickleback from channels 1,3 and 5 in Ladner Marsh - March to June 1988. 83
IB. Length-frequency distributions of threespine stickleback from channels 1,3 and 5 in Ladner Marsh - June and July 1988 and April and May 1989. 84
2. Length-frequency distributions of juvenile chinook salmon taken from channels 1,3 and 5 (Ladner Marsh). 85
3. Scale diagram of channel 1, showing maximum depths (measured from the channel bank) at 2 m intervals. Areas enclosed by dashed lines are shallow (30-70 cm) banks exposed only at low tide. 86
4. Scale diagram of channel 3, showing maximum depths (measured from the channel bank) at 2 m intervals. Areas enclosed by dashed lines are shallow (30-70 cm) banks exposed only at low tide. 87 viii
LIST OF TABLES
Table Page
1. Stickleback population estimates from Ladner Marsh side channels (using single mark-recapture) with 95% confidence limits. 20
2. Lateral plate morph frequencies (percentage) of stickleback from Ladner Marsh side channels (channels 1,3,5 and 6). ' 22
3. Body length: depth ratios - mean (and variance) - in the three stickleback morphs (n=15). 23
4. Chinook residency determinations: The total number of chinook (and chum) captured in channels 1 and 3 when the stop seine was raised before, during and after high slack tide (April 29 to May 5 1988). 25
5. Contingency table analysis of diet choice in chinook and stickleback - %2 values (for percent frequency and percent numbers combined). 42
6. Summary of aggressive interactions between stickleback and juvenile chinook (experiment 1). 59
7. Summary of aggressive interactions between stickleback and juvenile chinook (experiment 2). 61
8. Summary of aggressive interactions between stickleback and juvenile chinook (experiment 4). 63
9. Summary of laboratory prey choice data (experiment 2). 69 ix ACKNOWLEDGEMENTS
I would like to thank my supervisor, Dr. Tom Northcote for his support, encouragement and candour throughout the development of this thesis. A number of colleagues provided invaluable assistance in the field, most notably Dana Atagi and
Gerry Simpson as well as Tom Suzuki, Lisa Serinewald and Paul Grindlay, who also helped reduce the interminable hours in the laboratory with a mixture of industry and humour. My wife, Natasha has kept me on track and provided me with three years of unselfish support and a beautiful child. 1
CHAPTER 1: Introduction
Interspecific competition is an important structuring force in many ecological communities (Diamond 1978; Schoener 1982), particularly between
closely related species that are similar in structure and habits.
Competition theory was given mathematical support early this century by Lotka and Volterra - see Krebs (1978) for a review. The early models were
followed by a series of tightly controlled laboratory experiments (Gause 1932;
Park 1948), leading to the development of the "competitive exclusion principle" (Hardin 1960). This principle predicts that for closely related species to coexist they must differ in resource use and, in doing so, partition
the available resources among themselves (Schoener 1974).
An extension of this concept was advanced by MacArthur and Pianka
(1966) who described mathematically the response of two competing species
as they first begin to overlap spatially. The model predicts that there will be a restriction or shift in the realized niche of one or both of the species, each species utilizing the niche space to which it is best adapted. This theory was labelled the "compression hypothesis". Schoener (1974) argued that the most likely niche dimension to be affected would be microhabitat. The addition of one or more competitors to a given community might cause a reduction in
certain food types in the short term; however the residents should not drop
any food items from their diet because, once found, a food item worth eating
in the absence of competition is worth eating in its presence (assuming prey is patchy and encounter rates are low relative to handling times). Either or both 2
the resident and intruder species would most probably diverge in microhabitat use, each depleting the food in their preferred patch type such
that the other species would not find it profitable to feed there. The result,
therefore would be microhabitat partitioning which may be accompanied by a
concomitant shift in prey, as a consequence of differing food availability in the
two habitats. The preponderance of this pattern is borne out in a review of
studies on niche compression in which 55% of the time the most important
dimension was microhabitat (Schoener 1974). The compression hypothesis
has also been elegantly supported in a series of 'natural' pond experiments
with three species of centrarchid fishes (Werner and Hall 1977,1979) which
demonstrated dramatic niche shifts along the microhabitat dimension in
response to experimental manipulations. As a corollary of the compression
hypothesis, one would intuitively expect that a reduction in numbers of one
competing species should cause niche expansion in the other because
intraspedfic competition would become relatively more predominant
(Schoener 1986). This pattern has been demonstrated in a number of
experiments, where the removal of one species allows a competitor to expand
its realized niche, exploiting a broader range of habitat and consequently
increasing its consumption and growth rates. Noteable studies on fish are
Andrusak and Northcote (1971), Hume and Northcote(1985) and Persson
(1986).
Interactions between and within fish species are often complicated by the
marked ontogenetic niche shifts that they frequently display. Because fish
tend to grow continuously throughout their life, they may undergo a number
of niche shifts, particularly in the dimension of food size and type. It is often
the case that two fish species can be competitors as juveniles but predator and 3
prey as adults (Werner 1984). Ontogenetic niche shifts can therefore result in some very interesting and complex community dynamics. My study sidesteps these complexities by examining two fish species that share the same habitat for a brief, albeit crucial stage of their life-cycle.
Niche shifts and resource partitioning have been demonstrated in many guilds of fishes, particularly salmonids (Nilsson 1960; Hartman 1965; Schutz and Northcote 1972). The processes leading to resource partitioning may involve direct aggression by one species towards another (interference competition) or merely the more efficient use of resources by one of the two species (exploitative competition). Notwithstanding this distinction, the segregation may be one of two types (Nilsson 1960).
i) Interactive segregation - refers to an immediate behavioural response when two allopatric species come into sympatry, leading to diet or microhabitat changes in one or more of the competing species (Werner and
Hall 1977; Paszkowski 1985).
ii) Selective segregation - involves a longer term, evolutionary (genotypic) divergence of characters of two coexisting species. Direct proof that current differences in coexisting species are due to competition in the past has generally proved elusive. Although Connell (1980) ostensibly failed to make a convincing case for the existence of 'the ghost of competition past' (Wiens
1984), the concept of selective segregation has been supported by a few studies, particularly on island communities (Roughgarden et al. 1983) but also in aquatic systems (Crowder 1986).
A few studies have looked at interactive segregation between salmonids and non-salmonids. McCabe et al. (1983) implied interactive segregation, based on high diet overlap, between threespine stickleback (Gasterosteus 4
aculeatus) and juvenile chinook salmon (Oncorhynchus tshawytscha).
Gaudreault et al. (1986) showed that ninespine stickleback (Pungitius pungitius) were aggressive towards juvenile brook charr (Salvelinus fontinalis) both in the laboratory and field and proposed that competition may be important at certain times of the year. Interactions between adult creek chub
(Semotilus atromaculatus) and brook charr led to niche divergence in both habitat and diet (Magnan and Fitzgerald 1982): The mechanism appeared to be aggression by charr (Magnan and Fitzgerald 1984). Further incidences of interspecific interactions involving stickleback are cited in Wootton (1984).
Threespine stickleback (Gasterosteus aculeatus Linnaeus) and juvenile chinook salmon (Oncorhynchus tshawytscha Walbaum) coexist in the Ladner
Marsh region of the Fraser River estuary during spring and early summer.
Ladner Marsh consists of a series of large drainage channels or 'sloughs' and a network of smaller 'side channels' (or 'tidal channels'). When the side channels dewater on the ebbing tide, small pools (1 to 4 m long and 0.5 to 1.5 m wide) remain at intervals along them. These isolated pools support high densities of stickleback which remain in the vicinity when the channels are flooded on the incoming tide (Levy et al. 1979). From March to June each year, juvenile chinook salmon (fork length 40-70 mm) are present in the marsh. The chinook move into the side channels during the flood tide and leave during the ebb (Levy and Northcote 1982). For a period of up to seven hours over each tidal cycle the salmon and stickleback cohabit at moderate to high densities in the flooded tidal channels. Resource limitation could become important in shaping the trophic ecology and behaviour of the two species.
Dunford (1975) found evidence of spatial segregation (possibly as a result of 5
competition for food) between chinook and chum salmon that co-occurred in this Fraser River marsh complex. Significant dietary overlap has been demonstrated for stickleback and juvenile chinook in the Columbia River estuary (McCabe et al. 1983) and the high spatial overlap at high tide suggests that competition may be important in Ladner Marsh.
The main objective of this study is to determine whether interactive segregation occurs when salmon and stickleback are in sympatry during the period of high tide that the channels are flooded and if so, to elucidate the mechanisms behind the segregation.
The primary hypothesis tested was that chinook salmon and stickleback segregate spatially and as a consequence tend to diverge trophically when co- occurring in the side channels. This hypothesis was tested in the field and laboratory.
It is difficult to determine whether any spatial segregation observed in the field is interactive or selective (i.e. fixed) because chinook feed in different environments at low tide (sloughs) and high tide (side channels) so there is no indication of their niche in the absence of stickleback. Laboratory experiments in aquaria with the two species kept alone and together address this question by examining prey choice between surface, midwater and benthic prey.
Changes in prey selection suggest, by implication, changes in microhabitat use. Behavioural observations in the laboratory should help elucidate the mechanisms behind any resource partitioning that is apparent.
Experimental variation of food densities was not practical because of the wide range of prey types available, difficulties with the accurate enumeration of prey in the field and the highly fluctuating nature of the resource. 6
CHAPTER 2: Study Area
The study was conducted in side channel habitats of Ladner Marsh
(49°06'N, 123°05'W) in the Main Arm of the Fraser River, approximately 8 km from the river mouth (Fig. 1). The marsh consists of silt, clay and detrital deposits with vegetation dominated by sedge (Carex lyngbyei), bulrush (Scirpus americanus) and cattail (Typha latifolia) (Hoos and Packman 1974). The area may be almost completely flooded (up to a depth of 25 cm in places) during peak freshet and high spring tide, while on the lower tides or reduced river discharge, water is confined to broad channels ('sloughs') and narrow side channels which ramify throughout the marsh (Fig. 2).1 The side channels are between 0.5 and 3 m wide, up to 1.5 m deep and of variable and indeterminate length (Appendices 3 and 4).
Aquatic macrophytes are entirely absent from the side channels throughout April and May, although some marginal cover is provided by vegetation overhanging the channel banks. From mid June aquatic macrophytes appear in the more open areas of the channels and growth of cattail and sedge provides marginal cover during high tides.
Fraser River discharge increases steadily from February through May, reaching its peak in June (Fig. 3). Notwithstanding this marked freshwater influence, the Fraser is a highly stratified estuary, so the less dense fresh water flows over the surface of the saline intrusion from the Strait of Georgia (Hoos and Packman 1974; Northcote et al. 1976). This ensures that the shallow
1 During the period of study (April to June), the side channels would have overtopped on 42% of the high tides (those above 4.3m) for a duration of 1-2 h. U.S.A.
urel: Study site location. 8 9
12
Jan Fee Mar Apr May Jun Jul Aug Sep Oct Nov Dec Month
Year
1986 1987 -±- 1988
Figure 3: Mean monthly discharge of the Fraser River, recorded at the Port Mann Pumping Station (20 km upstream from the study site on the Fraser River mainstem) tor 1986,87 and 88 (Water Survey Canada 1986,1987,1988). 10
sloughs and side channels consistently have very low salinity levels, but particularly so during the period of study (Kistritz 1978).
Marked diel fluctuations in water temperature in the side channels result from the combined influence of periodic innundation with cool mainstem water at high tide and the warming effect of ambient air temperature on isolated pools at low tide. Notwithstanding this daily variability, the mean high tide water temperature increased (in 1987) from 9C in late March to more than 20C by August (Fig. 4); (also see Dunford 1975).
Turbidity also varies considerably over the tidal cycle. During low tide the water in isolated pools is relatively clear, but innundation by the incoming tide brings in turbid water from the Fraser River, causing a substantial decrease in transparency (Fig. 5). In addition to this tidal fluctuation, there is marked seasonal variation in turbidity. High tide turbidity levels increase rapidly in late March and early April, peaking in mid April and May and then decreasing gradually through June and July (Dunford 1975; R. S. Gregory,
University of British Columbia, pers. comm.). As one might expect, turbidity levels more or less track the seasonal variation in Fraser River discharge.
Fish species commonly found in the slough habitat between March and
July are threespine stickleback (Gasterosteus aculeatus), prickly sculpin (Cottus asper), peamouth chub (Mylocheilus caurinus), staghorn sculpin (Leptocottus armatus), starry flounder (Platichthys stellatus), chinook salmon {Oncorhynchus tshawytscha) and chum salmon (O. keta). Of these, only stickleback were consistently abundant in side channels; prickly sculpin were present in limited numbers (<20/100m2; Dunford 1975) and the two salmon species were abundant during high tides. The only potential fish predators sampled in the side channels were small (presumably young-of-the-year) prickly sculpins 11
25
a.
« 5
Ql i i ! 1 1 April May June July August
Figure 4: Water temperature during morning and afternoon high tides in Ladner Marsh side channels (1987). 12
10:00 12:00 14:00 16:C0 18:00 TIME cnannei aewatsrec cnannei floocaa
Figure 5: Typical turbidity (Nephelometric Turbidity Units) of side channel water over a tidal cycle (April 23 1990 - mean values from channels 1 and 3). 13
(Dunford 1975; this study) none was large enough to prey on adult stickleback or juvenile chinook. The great blue heron (Ardea herodias) is the only avian predator present in large numbers. It occurs throughout the year in the Fraser
Estuary and it feeds mostly on frogs, sculpins and small flatfish (Hoos and
Packman 1974). Although I have observed it only in the slough and mainstem areas, it is quite conceivable that it also feeds in the side channels. A number of merganser species are seasonally abundant but tend to feed in the deeper water of the mainstem Fraser (Munro and Clemens 1939). 14
CHAPTER 3: Spatial and Temporal Distribution Patterns
Introduction
Exploratory sampling has shown that stickleback occur in high densities in the side channels at low and high tides. Levy and Northcote (1982) have demonstrated that chinook salmon migrate into the side channels with the flooding tide, returning to the main sloughs on the ebb. On a number of high tides the channels overflow and the marsh surface becomes flooded (for about
90 min) to a depth of up to 25 cm. It seems unlikely however that fish would venture from the safety of the side channels because of the attendant risks of avian predation and stranding. In addition to this, most of the food on the marsh surface will be of terrestrial origin, having been swept from the vegetation. Surface feeding fish would be safer remaining in the side channels where their food is brought to them on the ebb. For the purposes of this study it is therefore assumed that fish do not move out over the overtopped marsh surface.
In order to test the main hypotheses of this study in sufficient depth, more accurate information was required on fish densities throughout the season, their size-frequency distributions and their precise temporal and spatial utilization of the channels: This section addresses these issues. 15
Methods
i) Stickleback Population Structure
Stickleback occur in the side channels throughout the tidal cycle and are easily captured during low tide using a dip-net. An accurate population estimate can be obtained using a simple Peterson mark-recapture technique
(Robson and Regier 1971). Mark-recapture was performed regularly
throughout the period March through July 1988 in two separate and
morphologically similar channels (channels 1 and 3 - see Appendices 3 and 4).
At low tide stickleback were captured and marked with dorsal spine and
fin clips. Fish were recaptured during the same low tide, examined for marks
and measured (fork length)1 to the nearest mm. All of the assumptions of
mark-recapture were met (specifically that mortality due to predation was
minimal2 and the population was entirely closed at low tide3) and the
estimate generally had a fairly high degree of accuracy because a large
percentage of the population was marked4.
On one occasion (in April 1988) an extension of the basic mark-recapture
was used to determine whether stickleback remained in the channels at high
tide. A mark-recapture population estimate was taken during an early
1 Fork length (FL) is measured from the anteriormost extremity of the fish to the tip of the median rays of the tail.
2 Birds were the main predators in these side channels and my uninterrupted presence in the vicinity of the channels prevented any predation from this source.
3 At low tide the side channels consisted of discrete pools joined by shallow rivulets. During mark-recapture the channel mouth could be fenced off with a small mesh screen.
4 Stickleback could be captured in large numbers because of their confinement in small pools 16
morning low tide. At high slack tide a mesh screen (5m x 4m; 4mm stretched mesh) was set across the channel mouth, preventing any further movement of fish in or out of the channel.5 On the subsequent low tide a second mark- recapture was performed. Any decrease in numbers found by the second mark-recapture should be largely due to migration of stickleback out of the channel during the incoming tide. Minimal handling mortality was assumed because the second population estimate was carried out so soon after the first.
ii) Chinook Salmon Population Structure
Chinook salmon are known to migrate out of the side channels at low tide and return on the flood tide (Levy and Northcote 1982). This study requires that a more precise determination be made of the amount of time spent by chinook in the side channels. The channels begin to flood approximately 3 h before high tide. Salmon could be migrating in to the channels any time during this period. Similarly, the time period between high tide and the point at which the channels are effectively dewatered on the ebb is approximately 3 h. Again, the precise timing of the outward migration is uncertain.
Two channels similar in depth profile and length (ch. 1 and 3) were chosen as replicates. At low tide a mesh screen was placed across the channel mouth with the lead line pushed firmly into the substrate to form a tight seal.
During the flood tide the floating line of the net was kept fully submerged by
3 The high tide on which this population estimate was performed was low enough to ensure that water was confined to the side channels. 17
an anchor. This allowed migrating salmon unimpeded access to the channel.
In order to determine residency patterns of juvenile salmon more precisely, three 'treatments' were applied to each of the two replicate channels on three consecutive days. In all cases, once the screen was lifted it was left up until the following low tide. Any salmon remaining in the channel at low tide were concentrated in the small pool just upstream of the screen and could be captured easily using a dip-net or pole seine. All fish captured were enumerated and released.
- Treatment 1:- The screen was lifted IV2 h before high tide. Any salmon remaining in the channel represent fish which migrated in early on the flood tide.
- Treatment 2:- The screen was lifted at high slack tide. Any salmon remaining in the channel represent the total number of fish migrating in on the flooding tide.
1 - Treatment 3:- The screen was lifted l /2 h after high tide. Any salmon remaining in the channel represent fish which migrate out towards the end of the ebbing tide.
Comparison of these three treatments (specifically treatments 1 and 3 against treatment 2) should yield a detailed picture of the temporal utilization of these channels by the salmon as well as providing density estimates. 18
Results
i) Stickleback Resident
The density of stickleback in the two channels that were sampled
consistently (ch. 1 and 3 - see Appendices 3 and 4) was approximately 20 -
25/m2 through March and April, but declined in May and June (Fig. 6a).
Although it follows the same trend, this is a considerably higher density than
that recorded by Dunford (1975). Dunford's enumeration (Fig. 6b) was however based on sampling with a pole seine in partially flooded channels -
a method which consistently yielded poor capture rates in this study. The
discrepancy is therefore not surprising.
The residency determination executed in April 1988 yielded a similar
population size on sequential low tides (Table 1). This clearly demonstrates
that there is not a significant migration of fish out of the channels on the
incoming tide, suggesting that stickleback populations are largely resident in a
given side channel, throughout the tidal cycle. This finding is supported by
Levy et al. (1979), who fished tidal channels in the Fraser estuary for five days in succession. This sustained fishing effort had a dramatic effect on stickleback populations: After the third day there was a 90% decline in
numbers and no recovery on the following day, which implies some degree of
site specificity in stickleback.
The existence of three distinct morphs of stickleback based on lateral
plate number is well established (Wootton 1976). The low plated morph
typically has 3-7 bony plates on each side of the body, while the partially 19
Figure 6a: Stickleback densities in side channels; estimates based on Peterson single mark-recapture in channels 1 and 3 (with 95% confidence limits).
Figure 6b: Seasonal change in numbers per 100m2 of stickleback and pricklv scuipin in side channel haoitat. South Arm Fraser River (from Dunford 1975); estimates based on total captures using a pole seine. 20
Table 1: Stickleback population estimates from Ladner Marsh side channels (using single mark-recapture) with 95% confidence limits.
Date Channelt Number Number 95% Confidence
per channel per m2 Limits
12 May 87 1 505 5.3 1.57 22 May 87 3 2570 26.5 16.8 18 Mar 88 1999 20.8 12.3 < N < 35.6 20 Mar 88 6180 64.3 23.8 < N < 235.5 13 Apr 88 2010 20.9 11.9 < N < 39.3 28 Apr 88 3 (am) 2565 26.5 19.8 < N < 36.7 3 (pm) 2108 21.8 14.4 < N < 35.7 19 Jun 88 1 289 3.0 2.0 < N < 4.8 3 111 1.2 0.7 < N < 2.4 t Wetted surface areas of side channels at high tide: Channel 1 - 96.15 m2 Channel 2 - 96.85 m2 21 plated form has between 8 and 29, and the fully plated form between 30 and 35 (Hagen 1967; Wootton 1976). The stickleback population in Ladner Marsh side channels is polymorphic for lateral plate morph. All three forms (low, partially and fully plated) are present throughout the sampling period: The low plated morph is the most abundant, comprising greater than 70% of the population; the fully plated morph is quite uncommon, making up less than 8% of the total; the remaining 20% are partially plated (Table 2). There is not a significant difference in mean body length to depth ratios1 between the three forms (Table 3); it can, therefore be regarded as a stable polymorphism (J.D. McPhail, University of British Columbia, pers. comm.) rather than a hybrid population of the two "ecological morphs", anadromous and freshwater (Bakker and Sevenster 1988). This has important implications in studies of stickleback feeding ecology because the ecological morphs have marked differences in trophic apparatus and feeding behaviour (Wootton 1984). On the other hand, morphs in a population with a stable lateral plate polymorphism are morphologically and behaviourally uniform, therefore such populations can be treated as relatively homogeneous in trophic habits. 1 Hagen (1967) demonstrated that the three "ecological morphs", 'trachurus', 'semiarmatus' and 'leiurus' differed significantly in their body length:depth ratios, the trachurus (or plated, anadromous) tending to be much more slender than the unplated, freshwater leiurus. 22 Table 2: Lateral plate morph frequencies (percentage) of stickleback from Ladner Marsh side channels (channels 1,3,5 and 6). low plated semi-plated fully plated April 14 70.4 21.7 7.9 n=161 May 22 78.8 16.9 4.4 n=105 June 15 71.25 22.5 6.3 n=118 Table 3: Body length:depth ratios - mean (and variance) - in the three stickleback morphs (n=L5). (Ratios are not significantly different: Analysis of Covariance; p=0.07).1 low plated semi-plated fully plated 4.39 (0.21) 4.56 (0.25) 4.57 (0.24) 1 %2 test against a theoretical normal distribution confirms that distributions satisfy the assumptions of normality 24 ii) Chinook Resident Chinook residency determinations were carried out between April 29 and May 5 1988; their density in the side channels increases through the flood and early part of the ebb tide (Table 4). Thus the data show that chinook enter the channels steadily over the full duration of the flooding tide and remain there until very close to the end of the ebb (Fig. 7). The data, although by no means conclusive, do lend support to the observations of Levy and Northcote (1982) that chinook have a long residency period in the side channels at high tide. The length of time that large numbers of chinook and stickleback co- occur in the side channels is around 4 to 5 h, straddling high tide. iii) Estuarine Growth Stickleback grow steadily throughout the season (March to July), although reproduction within the side channels results in a switch from a unimodal to a strongly bimodal length frequency distribution in June and July (Appendices IA and IB). Chinook increase gradually in size between the end of March and the middle of May (by about 4 to 5 mm) but subsequently undergo a rapid increase of about 10 mm in late May and early June (Fig. 8). Notwithstanding this pattern, over the months March through June, the mean lengths of stickleback and chinook remain within 10 mm of each other. A similar picture has also been presented by Dunford (1975) and Levy and Northcote (1982). Table 4: Chinook residency determinations: The total number of chinook (and chum) captured in channels 1 and 3 when the stop seine was raised before, during and after high slack tide (April 29 to May 5 1988). Channel 1: Chum Chinook Before (Apr 29) 2 7 High Tide (May 4) 4 11 After (May 5) 2 15 Channel 3: Chum Chinook Before (May 5) 9 29 High Tide (Apr 29) 15 69 After (May 4) 9 94 26 Figure 7: Schematic representation, of chinook migration in and out of side channels (based on the data in table 4). 27 Figure 8: Seasonal variation in mean fork length of stickleback and juvenile chinook in Ladner Marsh side channels. juverme t 28 Although the evidence is entirely circumstantial, competition theory could explain this pattern of chinook growth based on the seasonal population decline of stickleback (Fig. 6); (Dunford 1975). If competition for food was indeed occurring, dense populations of stickleback would depress feeding and growth rates of chinook. The dramatic decline in stickleback numbers in May would subsequently reduce competition, allowing chinook to grow rapidly. 29 CHAPTER 4: Estuarine Dietary Patterns Introduction An indirect, though entirely circumstantial way of demonstrating competition in the field is to look at differences in the degree of diet overlap in sympatric and allopatric populations of two species (Werner 1986). At high tide, stickleback and juvenile chinook are sympatric in the side channels, while at low tide in allopatry they feed in distinctly different habitats; stickleback in shallow water in the side channels and chinook in the deeper, more open slough habitats (Dunford 1975; Levy et al. 1979). During low tide the two species are therefore exposed to different habitats and prey spectra, which invalidates the comparison of niches between allopatry and sympatry for the purpose of demonstrating competition. Notwithstanding this shortcoming, it is still necessary to demonstrate minimal dietary overlap in the field in order to allude to the existence of interspecific competition. This part of the study is intended to ascertain the dietary patterns of stickleback and juvenile chinook in the side channels and determine the extent of overlap in this niche dimension. 30 Methods i) Sampling At approximately two week intervals (from March 29 to July 6 1988), samples of fish and food organisms were taken from two channels (ch. 1 and 3) on two occasions during the tidal cycle. Stickleback were sampled (n=15) using a dip-net and pole seine towards the beginning of the flood tide, then chinook and stickleback at the end of the ebb. The former sample represents stickleback feeding during the 6h of low tide; the latter samples represent fish feeding during the period (6h) that the channel was flooded. This sampling regime therefore assumes that all food items were consumed during the 6h period before collection. This assumption is supported by a number of laboratory studies on gut evacuation rates in related fish species. In juvenile coho salmon (103 mm mean fork length), feeding on sockeye fry, 80% evacuation took 6h at 10C and 5h at 13C (Ruggerone 1989a). Complete evacuation (100%) of Daphnia took 7h at 15C in (0.3-0.6g) tenspine stickleback, Pungitius pungitius (Cameron and Kostoris 1973). Gut evacuation rates are highly temperature dependent (Brett and Higgs 1970) and channel water temperatures were fairly high between March and June, rising steadily from 10 to 20C (Fig. 4). Evacuation rates in stickleback and juvenile chinook of 6h or less are therefore likely within this temperature range, particularly as 31 continuously feeding fish seem to accelerate the rate of gut emptying1 (Ruggerone 1989b). Fish were killed immediately in 90% ethanol and preserved in 5% formaldehyde; no regurgitation was apparent. Benthos and drift samples (n=3) were taken from two channels at both low and high slack tide. Benthos samples were taken - to a depth of 4 cm - using a core sampler (4.60 cm diameter); benthos was only sampled in areas that were permanently submerged. Drift samples were taken using a 125 |im mesh plankton net with a 22 x 11 cm aperture.2 The net was attached to a wooden pole and was pulled through the water 5 - 10 cm below the surface for 4 m at a constant velocity of approximately 60 - 80 cm s*1. Benthos and drift samples were stained with Rose Bengal for approximately lh and then preserved in 5% formaldehyde. Turbidity and temperature readings were also taken at low and high tides. ii) Stomach Content Analysis Stomach contents of stickleback and chinook were defined as those food items lying between the pyloric sphincter and the junction of the oesophagus. Individual fish were weighed (blotted wet mass) and measured (FL) and their stomach contents dissected out and examined under a binocular microscope (x25). Prey items were enumerated to genus or family. 1 The studies cited all involved single meals fed to starved fish. 2 Sampling sites (for both benthos and drift) were selected randomly using a numbered grid on scale diagrams of the channels. 32 In the analysis and presentation of data, the following measures of importance of prey in the diet were used: i) Percent frequency of occurrence - The proportion of stomachs containing one or more of a particular food item expressed as a percentage. ii) Average percent numbers - The total number of a given food item in all stomachs expressed as a percentage of the total number of all food items in all stomachs. A Spearman Rank Correlation (Manzer 1976) and Contingency Table Analysis (Gaudreault et al. 1986; Behrents-Hartney 1989) were used to quantify diet overlap between the two species; p values of less than 0.05 were accepted as significant. Contingency Table Analysis has the advantage (over other non-parametric tests of multiple comparison) that each cell in the table may be checked for its level of significance: Because of this it is possible to determine the prey types which showed the greatest contribution to diet differences. 33 Results On March 29 the diets of stickleback and chinook showed a close correlation (Spearman Rank Correlation; p<0.01). This was the case whether quantified as frequency occurrence or percent numbers. Notwithstanding this finding, Figures 9 and 10 demonstrate that chinook consumed mostly adult diptera, Collembola and Corophium, while stickleback fed on copepods, ostracods and some other epibenthic Crustacea such as Gnorisphaeroma and Anisogammarus. This general picture is repeated throughout the subsequent sampling dates; furthermore, the correlation disappears on these three dates (April 14, April 28 and May 20) - Spearman Rank Correlation (P>0.05), suggesting a divergence in diet of chinook and stickleback. On April 14 and 28 the diet of chinook was dominated by surface prey and Corophium, while stickleback consumed smaller epibenthos. By May 20, Corophium were no longer present in the diets of chinook or stickleback and surface prey constituted an even greater proportion of chinook diet. Stickleback continued to consume a broad spectrum of benthic, epibenthic and midwater prey. Comparison of stomach samples with the availability of prey in the environment provides information on the electivity (or prey preference) of the two fish species (Figs. 11,12,13 and 14). On March 29, chinook had a strong tendency to consume Corophium, Collembola and dipteran adults, while avoiding abundant midwater and benthic prey (Fig. 11). Chironomids, ostracods and harpacticoids were taken more or less in proportion to their relative adundance in the channels. This pattern was repeated on April 14 (Fig. 12) but by April 28 it was planktonic copepods that were being selected in proportion to their relative abundance in the sweep samples (surface 34 March 29 1988 April 14 1988 STICKLEBACK + •§3 20 40 0 20 40 80 % frequency occurrence % frequency occurrence April 28 1988 May 20 1988 20 0 20 40 60 20 0 20 40 60 % frequency ooourrenoe % frequency occurrence Cor • Coroptiium sp. Chi • cfilronomld larvae DipL - dlpteran larvae ben • other benthos Gam - Anlsogammarvs ap. Gno - Gnorlsphaerorna oregonensls Har • harpactlcold copepods Cop - other copepoda Ost - ostracoda mid • midwater DIpP • dlpteran pupae DipA • dlpteran adults Col • Collembola ap. sur • other surface orev Figure 9: Diet of stickleback and juvenile chinook during high tide in channels 1 and 3 (% frequency occurrence). 35 March 29 1988 April 14 1988 Cor CM CHINOOK ft STICKLEBACK •lOL + •on + Gam Qno + Har Cog Cor Oat mia + DlpP + •IDA Col sur 40 80 80 100 100 SO 100 % numoers 100 80 30 *0 20 20 April 28 1988 May 20 1988 Cor Cor + cm CHINOOK fi STICKLEBACK Oil CHINOOK STICKLEBACK OloL + 0I0L fl Sen I- •en -9 Cam + Sam -a Gno > Gno + Har -9 Har Coo Coo Oat -3 Cat mid •* mio + 01 oP + DloP ft- OlOA •- 01 OA Col • Col sur , t , 3ur + 100 80 60 -tO 20 0 20 40 80 80 100 100 80 80 40 20 0 20 40 SO 80 100 % rrumoera % numbara Cor - Caroonium so. Chi • chironomid larvae DIpL - dloteran larvae ben • otner bentnoa Gam - Antsogammarus so. Gno • Gnorisohaeroma oregonensls Har • harpactlcoid copepods Cop • other copepods Oat • ostracoda mid - midwater DlpP - dloteran pupae Dip A - dlpteran adults . • Col • Collembola so. sur • other surface orev Figure 10: Diet of stickleback and juvenile chinook during high tide in channels 1 and 3 (% numbers). 36 CHINOOK March 29th Sweep sample EH3 Benthic sample I Chinook diet 80 40 120 % Numhers STICKLEBACK March 29th Istlcklsoack diet Wi Sweeo samols EH*3aentnic aamoie 80 40 120 % Numbers Figure 11: A comparison of the relative proportions (% numbers) of prey in the environment (sweep and benthic samples) and the diets of stickleback and juvenile chinook. March 29 1987. 37 CHINOOK April Hth Chinook diet 120 80 40 0 40 30 120 % Numbers STICKLEBACK April Hth IStlcKleoacK diet SSSSSweeo samole Eiii3 Bentnic sample 80 40 120 % Numbers Figure 12: A comparison of the relative proportions (% numbers) of prey in the environment Osweep and benthic samples) and the diets of stickleback and juvenile chinook. April 141987. u 38 CHINOOK April 28th Sweep sample ES3 Benthic sample I Chinook diet Cor Chi DIpL ben Gam Gno Har COD Ost mid 01 pP OlOA Col sur 120 30 40 40 80 % Numoers 120 STICKLEBACK April 28th I Stickleback diet I Sweep sample EiiS Sentnic sample Cor Chi DIPL ben Gam Gno Har "1 Cop Ost mid DlDP •IpA Col sur 120 80 40 40 30 120 % Numbers Figure 13: A comparison of the relative proportions (% numbers) of prey in the environment (sweep and benthic samples) and the diets of stickleback and juvenile cxtinook. April 28 1987. 39 CHINOOK May 20th Sweep sample HI Benthic sample I Chinook diet 80 40 120 % Numbers STICKLEBACK May 20th Istlckleoack diet SaSBSweeo sample iiiiilii Bentntc aampla Cor Chi DIOL ben Gam Gno Har Coo Oat mid DloP •I PA Col sur 120 80 40 40 80 120 % Numbers Figure 14: A comparison of the relative proportions (% numbers) of prey in the environment (sweep and benthic samples) and the diets of stickleback and juvenile chinook. May 201987 40 waters), while the numbers of Collembola, Corophium and dipteran adults in chinook stomach samples were reduced correspondingly (Fig. 13). On May 20, chinook were highly selective, avoiding the dominant prey groups present in benthos and sweep samples (namely copepods, ostracods and other midwater and benthic prey) and tending to concentrate on surface prey such as Collembola and dipteran pupae and adults (Fig. 14). In accordance with their catholic dietary preferences (Wootton 1984), stickleback appeared to select prey more or less in proportion to its relative abundance in the environment. On March 29 and April 14, harpacticoid copepods, planktonic copepods and ostacods were the preferred prey items, all of which were well represented in samples from the side channels (Figs. 11 and 12). Collembola were also consumed, but not to the extent that they were by chinook. By April 28, planktonic copepods had become numerically dominant in the diet of stickleback with minimal contributions from the still abundant chironomid larvae, harpacticoids, ostracods and Collembola (Fig. 13). Towards the end of May, the diet of stickleback had broadened again, to show a pattern somewhat similar to that in late March: Collembola, ostracods, harpacticoids, planktonic copepods and chironomid larvae were all equally important prey (Fig. 14.). In general there seemed to be a greater tendency for stickleback to track the availability of food than chinook. In order to simplify data interpretation and satisfy the assumptions of Contingency Table Analysis1, prey types were combined into one of three categories, benthic, midwater and surface. The Benthic/Epibentic category included Corophium, chironomid larvae, dipteran larvae, nematodes, 1 Fewer than 20% of the cells should have an expected frequency of less than five and no cell should have an expected frequency of less than one (Siegel 1956). 41 oligochaetes, Anisogammarus, Gnorisphaeroma, harpacticoid copepods and ostracods. Midwater consisted of nauplii, corixids and 'other' copepods, and Surface prey - dipteran adults and pupae and Collembola. Contingency Table Analysis, using the %2 statistic, yielded significant differences on all sampling dates. In all cases the largest contributor to these differences was a preference of chinook for surface prey (see Table 5). In all but one, the second most significant differences were due to a preference for benthic prey by stickleback. In general, stickleback selected benthic prey, chinook selected surface prey and midwater prey was taken by both (Figs. 15 and 16). 42 Table 5: Contingency table analysis of diet choice in chinook and stickleback, expressed as indices of relative importance (percent frequency occurrence + percent numbers / 2) - %2 values. March 29 benthi c / epibenthic midwater surface stickleback 3.22 0.76 11.1 chinook 3.07 0.73 10.6 Total %2=29.5 (p < 0.001) April 14 benthi c / epibenthic midwater surface stickleback 4.66 16.9 24.9 chinook 4.73 17.3 25.2 Total %2=93.7 (p < 0.001) April 28 benthic / epibenthic midwater surface stickleback 1.2 0.042 4.27 chinook 1.26 0.043 4.35 Total %2=11.2 (p<0.01) May 20 benthic/epibenthic midwater surface stickleback 19.7 7.6 19.9 chinook 19.7 7.6 19.9 Total %2=94.4 (p < 0.001) 43 March 29 1988 100 100 0J ntnic/Eomanimo Midwanr Surtaea aonime/EoiiMninie Miowatsr Surtaea 28 April 1988 20 May 1988 100 100 1001 1100 - 73 75 -SO 50 - - 28 25 itnie/EolBantnie Mdwnar Surfaca SanlUMci /Eaoentnie Midwatar Surface I Chinook ftm Stlckiedack Di5^°f sti?lebac!c ™d Juvenile chinook during high tide in rcc^ce) Prey ^ 1X3016(1 by ™CTohabita<^ rVequenq 44 March 29 1988 14 April 1988 too 100 100 100 aantnia/EoiDamnto Mlowaior Surface aanime/Eoioantnio Miowatar 3ur<«ci 28 April 1988 20 May 1988 100 100) i100 76 - OJ .a 80 - 28 " 3—niiarf awning SxOMafaibamnia IHwv 3W>*M I Chinook CZ3 Stlckleoack Figure 16: Diet of stickleback and juvenile chinook during high tide in channels 1 and 3, with prey items pooled by microhabitat (% numbers). 45 CHAPTER 5: Laboratory Experiments Introduction The marsh constitutes a highly variable and logistically difficult environment, which makes adequately controlled and replicated field experiments impractical. For this reason the possible mechanisms for chinook - stickleback interactions were explored in the laboratory. A number of questions were addressed: i) Is niche divergence interactive? (i.e. would either species' food niche demonstrate an expansion or shift in response to the removal of its potential competitor?). ii) Is the mechanism of competition interference or exploitation? iii) How might interactions vary seasonally (with the effects of changing densities and sizes)? The first two of these questions were addressed in food choice experiments using various densities of the two fish species alone and together. These experiments also went some way towards elucidating the mechanisms involved in the interactions. They will be referred to as experiments 1 and 2. Experiment 2 is a refined version of experiment 1 with fewer fish density combinations and decreased quantities of prey. The third problem (regarding seasonal variations in interaction) was investigated in a third experiment which looked at interaction between species pairs over a range of size combinations. The mechanisms of interaction were further tested in a larger experimental arena in order to reduce the effects of small aquaria on behaviour (experiment 4). 46 Methods i) Collection of Fish Both chinook and stickleback were collected from Ladner Marsh in mid- April 1987. Chinook were captured in the main slough using a beach seine - 9.5 m long x 1.2 m deep, with a stretched mesh size of 11 mm (ends) and 4 mm (middle); stickleback were caught using a dip net and pole seine (3.1m x 1.35 m, 4 mm stretched mesh). The fish were transported to the laboratory in 32 L plastic containers. Oxygen and ice were supplied in transit to minimize mortality and handling stress. Stickleback and salmon were transferred to separate holding tanks and after two days recovery were treated with methylene blue to inhibit fungal and bacterial infection. ii) Holding Facilities Chinook were kept in a 200 L fibreglass tank, while stickleback were housed in a 150 L glass fronted, wood aquarium. Light for both was supplied by diffuse overhead fluorescent lighting with an approximately natural photoperiod. Both tanks were vigourously aerated and supplied with a continuous flow of freshwater at a rate of 1.5 L/ min. Water temperature was maintained between 10 and 12C. 47 Stickleback were fed once a day with live Tubifex, supplemented with Daphnia fed ad libitum. Chinook were fed just over 1 /10tn their body weight per day in Oregon Moist Pellets. Seven days prior to being used in experiments, fish were moved to smaller troughs and fed liberally with Artemia, Daphnia, Tubifex and Drosophila, in order to condition them to these unfamiliar prey items. The fish were then starved for 24 h before the experiment. iii) Collection and Maintenance of Prey The three prey species used in all experiments were Tubifex, Artemia and Drosophila. Daphnia was also used in the first prey choice experiment but was subsequently dropped because of problems of supply, maintenance and enumeration. Tubifex and Artemia were supplied by an aquarium retailer, and therefore were freely and consistently available. Tubifex could be kept for up to a month in a small aquarium with continuous water flow, keeping them cool and supplied with oxygen. Artemia survived for 3 to 4 days in vigourously aerated saline solution. Drosophila were cultured in 2 L glass jars, supplied with a medium of commercially available Drosophila food (formula 4-24 instant Drosophila medium; Carolina Biological Supply Co.). The stock for the culture was vestigial winged flies, so that all progeny would be flightless and therefore easy to handle. 48 iv) Selection of Prey Densities Experiments on competition require that the resource being studied is limiting. Accordingly, a series of preliminary feeding trials was performed in order to determine the satiation levels of 45 mm chinook and 35 mm stickleback. Individual fish (starved for 24 h) were fed ad libitum with one prey type for 1 h. The mean satiation levels for both stickleback and chinook (n=10) were 10 Artemia or Drosophila, 100 Daphnia and 0.05g Tubifex. Limitation of each prey type during experiments could therefore be achieved by providing fish with less than these amounts. v) Experimental Arenas Experiments 1 and 2 Experiments were run in 5 replicate aquaria measuring 50 x 30 x 30 cm. The substrate (4 mm deep) was sediment from the Ladner Marsh area which had been sieved to remove the smallest particles (<125 um); the remaining fraction consisted of silt particles which would come out of suspension within about 2 h (so that changing of the water between trials was practical). Water temperature was recorded throughout the experiment and remained between 12 and 14C. The water turbidity was increased to 15 (±3) NTU (Nephelometric Turbidity Units)1 by the addition of fine particles of mud. This level of turbidity is somewhat lower than levels in the Ladner Marsh side channels (during high tide, from March to June) but was the highest that 1 Turbidity was measured using a DRT-150 Nephelometric Turbidimeter (H.F. Instruments). 49 would still allow the fish to be observed. The water was aerated by a small airstone and light was supplied by two 122 cm (40 Watt) fluorescent tubes, 30 cm above the tanks. The aquaria were separated by white plastic dividers placed at the back and sides with the front left open to allow observation of the fish. The whole setup was enclosed in black plastic sheeting. Observations could be made through small slits in the plastic. Experiment 3 Experiments were run in 5 all-glass aquaria, measuring 40 x 25 x 30 cm. A 4 mm deep silt substrate was provided and turbidity was increased to 15 FTU (±3). Temperature was held between 14 and 16C throughout the experiment. Light was supplied by two 122 cm (40 Watt) fluorescent tubes and the water was aerated by a small airstone. Aquaria were separated by white plastic dividers at the back and sides. Experiment 4 In order to observe behaviours in a more open setting, a large, 120 x 45 x 35 cm, glass fronted, wood aquarium was used. A fine silt substrate was provided and the water turbidity was not increased because recognition of individual fish would not be possible in a 45 cm wide tank at 15 NTU. Light was supplied by two 122 cm fluorescent tubes and aeration by a small airstone at either end of the tank. During the pre-experimental periods of stickleback acclimatization the tank was divided in half (across its long axis) by an opaque plastic partition in order to simulate the reduced water volume of low tides. 50 vi) Procedures Experiment 1 This experiment was designed to test whether either species of fish would switch its preferred prey type (i.e. display niche divergence) when exposed to various densities of the other species in small tightly controlled experimental arenas. The prey species presented were chosen to fill three microhabitat niches; surface (Drosophila), midwater (Artemia and Daphnia) and benthic (Tubifex). Also during the course of the experiment behavioural observations were made on aggressive interactions within and between the two species and the depth distribution was noted every 10 to 15 min. Ten different density combinations (fish per tank) of stickleback and chinook were used in the experiment.1 The average densities of stickleback (four per tank) broadly correspond to field densities in March and April, provided the greater water depth in the field is accounted for. Chinook experimental densities are somewhat inflated in order to simplify experimental procedure and analysis. STICKLEBACK: 2468000224 CHINOOK: ' 0020248624 Mean sizes of fish (fork length and 95% confidence limits) used were 38.23 (±0.85) mm for stickleback and 45.36 (±1.04) mm for chinook.2 1 In the text and tables density treatments will be written in a standard abbreviated format, with the number of stickleback followed by the number of chinook (e.g. 2S/0C denotes two stickleback and no chinook). 2 This corresponds to the maximum size difference between the two species in mid to late May. (In reality chinook are, on average, a little smaller and stickleback larger). 51 Ten minutes prior to the introduction of fish, 60 Artemia, 600 Daphnia, 60 Drosophila and 0.3g Tubifex (wet weight) were introduced into the tanks. Each quantity corresponds to the average consumption to satiation by 6 fish of both species (such that food may be limiting for eight fish treatments but not for four). The fish were left to feed for 1 h, after which they were killed in 95% ethanol and preserved in 10% formaldehyde. A dissecting microscope (x25) was used for the enumeration of stomach contents. Behavioural observations were made over the course of 1 h. Each tank was observed for three, 4 min periods. Four fish were chosen randomly for observations of behaviour in single species treatments. Two fish of each species were observed in the mixed species treatments. Individual fish could be recognized by colouration patterns. The following behaviours were recorded: 1) Nip - A fish makes a motion as if to bite an opponent, although contact is not necessarily made. 2) Chase - A fish swims towards an opponent and the opponent flees. 3) Retreat - The response of a subordinate fish to any aggressive act (nipping or chasing). Nipping and chasing were considered exclusive and independent behaviours, i.e. when a nip was followed by a chase it was counted as two aggressive acts. Similarly, when a subordinate fish responded to a nip and chase, even in quick succession, it was regarded as two retreats. Frequencies of these behaviours were recorded on an OS-3 Event Recorder or on data sheets with the help of a plankton counter. All density treatments were replicated five times. Between each trial the water was changed and the substrate sieved to remove any remaining prey. 52 The front and rear glass of the aquaria were marked with four horizontal lines at 7 cm intervals. During the experiments the location of all fish with respect to depth was noted every 10 min. Experiment 2 After running experiment 1, it was clear that prey density was too high and all fish could feed to satiation on the midwater prey (Daphnia and Artemia). Experiment 2 is a refined version where, in order to increase the intensity of competition, the prey density was greatly reduced to 20 Artemia, 20 Drosophila and O.lg Tubifex (making food limiting to all but two-fish treatments); in addition to this, five different fish density treatments were used rather than ten. These were as follows: STICKLEBACK: 2 4 2 0 0 CHINOOK: 0 0 2 2 4 This allowed more efficiency, greater replication and simpler analysis. Mean fork lengths of fish used were 35.67 mm (±0.69) for stickleback and 45.28 mm (±0.68) for chinook. Experiment 3: The Effect of Relative Size on Interactions The relative sizes of two species could be important in determining the intensity of any interactions. This experiment matched species pairs over a range of size combinations and examined the aggressive interactions. The sizes of fish used were as follows: Stickleback - 35,40,45,50 and 55 mm; chinook - 45,50,55,60 and 65 mm. One chinook and one stickleback 53 were used in each aquarium and treatments were replicated eight times with different fish. Five Artemia, five Drosophila and 0.05g Tubifex were introduced into each aquarium approximately 10 minutes before the fish. Observations on aggressive behaviour were made over the course of 1 h (three observations of 4 min each per tank). At the end of the experiment stickleback were killed for sexing. Chinook were not sexed. Experiment 4 A larger tank was used for further experiments in order to determine whether aggression in experiments 1,2 and 3 was merely an artifact resulting from excessive confinement. The fish density combination used was eight stickleback and four chinook. For each of four replications the eight stickleback were introduced into one half of the divided tank (randomly assigned), 5 d prior to the start of the experiment. They were allowed to acclimate to the aquarium and set up a clear dominance hierarchy. After this five day period, groups of 4 chinook salmon were introduced into the tank for 2 h, twice a day (8 h apart) over a period of 4 d. Before each introduction the partition was removed and the whole tank was available for all fish throughout the 2 h experiment.1 Following each trial the chinook were removed and the stickleback herded back into their half of the tank and again confined with the partition. A different group of chinook was used for each of the eight trials. 1 The simultaneous increase in available water volume and introduction of chinook to a resident group of stickleback was intended loosely to model the situation in the estuarine tidal channels. 54 The timing of chinook introductions was designed to correspond as closely as possible to the natural cycle of chinook migration into the side channels, while the stickleback remained in the aquarium throughout the four day period. The food organisms provided (at the beginning of each trial) were 60 Artemia, 60 Drosophila and 0.3g Tubifex. Because it was impossible to remove all prey remaining after each trial (except Drosophila, which were skimmed from the surface) without disturbing the stickleback, they were left for consumption between trials. Prey choice was not recorded in these experiments. Behaviour was quantified in the same manner as in the previous experiments. Also, every 10 min, the number of each species of fish in each half of the aquarium was recorded. This was in order to determine microdistribution in response to territorial resident stickleback in one half of the tank. The stickleback used measured 38.47mm (±0.75) and the chinook measured 46.39mm (±1.04) fork length. Each four day experimental treatment was replicated four times. vii) Statistical Analysis Fish behaviour and diet choice in the laboratory were characterized by a great deal of inter-individual variability. As the data were not normally distributed and had a high proportion of zero values, non-parametric statistics were used. With the exception of the microdistribution data, all comparions between two samples were analysed using the Mann-Whitney U Test. The 55 Kruskall Wallis Test was used when three samples were being compared. Microdistibution was analysed with the y} test. P values less than 0.05 were accepted as significant. Results i) General In general the laboratory experiments were characterised by a great deal of overt aggression by stickleback towards both conspecifics and chinook. Inter-individual variability in aggressiveness was high, with one or two fish usually dominating most encounters. In many replicates, particularly those with lower densities, no interaction was apparent; conversely, if a particularly aggressive fish was present the number of aggressive acts in one hour could be very high. ii) Microdistribution In experiments 1 and 2, no patterns in depth distribution of the two species could be determined. Any tendency that the fish may have had to segregate spatially might have been disrupted by the small size of the enclosures and the brevity of experiments. In general both species tended to restrict their movements to the lower half of the tank. 56 Spatial segregation was, however more apparent in the larger enclosure (experiment 4). Chinook tended to avoid the half of the aquarium where stickleback had set up territories (i.e. the section in which they had been confined prior to the experiment) (%2 p=0.01). From the data, stickleback appeared to be evenly distributed - four in each half (%2, p=0.5) but in most cases this was a manifestation of two or three dominant fish holding territories in one half of the tank and the occasional forays of members of the remaining (subordinate) seven or eight into that half. iii) Aggressive Behaviour The total frequencies of nips and chases were approximately the same (p=0.48) so for further analysis the two behaviours were combined to give an overall measure of aggression. In both of the small enclosure experiments (1 and 2), chinook demonstrated no aggression towards stickleback and low levels of aggression towards conspecifics (Figs. 17 and 18). On the other hand, stickleback were highly aggressive towards both conspecifics and chinook at all density treatments (Fig 17, Table 6). Furthermore, the level of intraspecific aggression was directly related to density (8S/0C > 4S/0C > 2S/0C, p=0.0025). This was also the case for stickleback aggression towards chinook, although the level of significance was not so striking (4S/4C > 2S/2C, p=0.048). The cause of these density effects is not entirely clear. It is possible that increased density heightens the overall aggressiveness of each fish per se, but the raw data suggest that an experimental artifact is at work. There is high inter-individual variability in stickleback aggression, to the extent that about one in four fish is 57 Stickleback towards chinook, ' 1 1 1 1 i 1 1 23/OC 2S/2C *S/OC *S/«C SS/OC 5S/2C 23/8C 23/2C AS/AC 8S/2C 2S/0C Nio (atlckisoacKi ESJ Chase Otlcxlaoacxl E mm =iatrsat (fr stickle) I i Nio (cninooKl 98 Cnase (cninookl Uli Retreat (fr enlnooK) Chinook towara3 cninooK towards atlckleoack 0 2 4 e 8 10 .12 0S/2C 23/2C 03/'C *8/*C 03/8C 23/SC 03/2C 2S/2C «S/4C 2S/SC S3/2C I 1 Nio (oningoMI Chase (cninooKl GZ3 Retreat (fr cnmooKl K3 NIO (aiicHieoacK) E3 Cnsse (SIICKISOSCK) Retreat (tr sticxiel Figure 17: Per-capita frequency of inter- and intra-specific aggression, during four-minute intervals, by stickleback and chinook (experiment 1). 58 Stickleback cowards stlckleoack towards chinook o 2 n t_ | I*™ <_ 1 2S/2C 2S/0C 4S/0C 2S/2C 1X3 Nio (stlckleoack) CHO Chase (stlckleoack) >B Retreat (tr.stickle) I I Nio (chinook) BBS Chase (chinook) (Z2 Retreat (fr chinook) Chinook towards Chinook towards stickleback o 2 £= i i 3 o- 0 * 1 u_ 2 • 3 2S/2C 0S/2C 0S/4C 2S/2C I 1 Nip (cninook) BSS Chase (Chinook) fZ2 Retreat (fr chinook) i\N Nip (stlckleoack) WB. Chase (stlckleoack) M Retreat (fr stickle) Figure 18: Per-capita frequency of inter- and intra-specific aggression, during four-minute intervals, by stickleback and chinook (experiment 2). 59 Table 6: Summary of aggressive interactions between stickleback and juvenile chinook (experiment 1). Treatment(s) Comparison compared and outcome Pt 2S/0C vs. 0S/2C Svs. S > Cvs. C 0.028* 4S/OC vs. 0S/4C S vs. S > C vs. C <0.001*** 2S/2C vs. 2S/2S S vs. S > C vs. C <0.001*** 2S/2C vs. 2S/2C S vs. C > C vs. S <0.001*** 4S/4C vs. 4S/4S Svs. S > Cvs. C <0.001*** 4S/4C vs. 4S/4C Svs. C > Cvs. S <0.001*** 8S/0C vs. 4S/0C S(8) vs. S > S(4) vs. S 0.01** vs. 2S/0C > S(2) vs. S (density effects) 4S/4C vs. 2S/2C S(4) vs. S > S(2) vs. S 0.048* (density effects) t Mann-Whitney U Test > denotes "is more aggressive than" * p<0.05 ** p<0.01 *** p<0.001 60 openly aggressive and involved in the majority of confrontations. For this reason alone the overall level of aggression will tend to increase with stickleback density because there would be a greater chance of having aggressive fish present in four and eight fish treatments than in the two fish treatment (in which a number of the replicates yielded no interactions at all). The results from experiment 1, with ten density combinations, were more or less mirrored by experiment two, in which just five density combinations were tested (Fig. 18, Table 7). In single species treatments of both two and four fish, stickleback were significantly more aggressive towards conspecifics than were chinook (2S/0C > 0C/2S, p=0.007; 4S/0C > 0S/4C, p<0.001). This pattern was much weaker in the mixed species treatment (2S/2C stickleback > 2S/2C chinook, p=0.139). Interspecific aggression was highly asymmetrical, with stickleback dominating most encounters; stickleback in mixed species treatments (2S/2C) were more aggressive than chinook (2S/2C) (p<0.001). The findings of experiments 1 and 2 were substantiated in experiment 4, which yielded the same patterns of interaction - high levels of inter- and intra- specific aggression by stickleback (Fig. 19, Table 8). iv) The Effect of Relative Size on Interactions Stickleback tend to be most aggressive when matched against chinook of a similar size (Fig. 20). Both males and females were found to be aggressive, so sex differences were not further explored. The highest levels of aggression were recorded when both stickleback and chinook measured 45 mm (FL). 61 Table 7: Summary of aggressive interactions between stickleback and juvenile chinook (experiment 2) Treatments Comparison compared and outcome pt 2S/0C vs. 0S/2C S vs. S > C vs. C 0.007** 4S/0C vs. 0S/4C S vs. S > C vs. C <0.001*** 2S/2C vs. 2S/2C S vs. S > C vs. C 0.139 S vs. C > C vs. S <0.001*** 4S/0C vs.2S/0C S(4) vs S > S(2) vs S <0.001*** (density effects) tMann-Whitney U Test > denotes "more aggressive than.." * p<0.05 ** p<0.01 *** p<0.001 62 ... •>' Stickleback a. o >> u 3 or aj s- v3. stickleoack vs. cninook i Nip Chase l l Retreat Chinook 20' 3 o s_ a. o 20-- ra oe aj 80 - - 3 cr cu 80 vs. stlckleoack vs. chinook I Nio Chase I I Retreat Figure 19: Per-capita frequency of inter- and intra-specific aggression by stickleback and cninook (experiment 4). 63 Table 8: Summary of aggressive interactions between stickleback and juvenile chinook (experiment 4). Type of Comparison interaction and outcome pt Nip S vs. S > C vs. C 0.014* Chase S vs. S > C vs. C 0.01** Retreat S vs. S < C vs. C 0.29* Nip S vs. C > C vs. S 0.029* Chase S vs. C > C vs. S 0.029* Retreat S vs. C < C vs. S 0.029* tMann-Whitney U Test. > denotes "more aggressive than.." < denotes "retreats more frequently from.." * p<0.05 ** p<0.01 *** p<0.001 64 4-0 stickleback FL (mm) 55 60 65 Chinook FL (mm) Figure 20: Frequency of aggression as a function of relative size of stickleback and juvenile cninook (experiment 3). 65 Nonetheless, stickleback were aggressive towards chinook that were up to 10 mm larger than themselves, although at this size discrepancy the number of encounters was reduced. When the size discrepancy was greater than 10 mm, stickleback ceased to be aggressive towards chinook. Isolated incidences of aggression by chinook towards stickleback were observed. Although there was no clear pattern, the largest size of chinook tested (75 mm, FL) did demonstrate low levels of aggression towards 35 mm stickleback. A size discrepancy as large as 40 mm is however uncommon in the study area (see Fig. 8). v) Prey Choice Experiments Controlled feeding experiments showed that stickleback ate large quantities of Artemia and Tubifex whether kept alone or with chinook (4S/0C=2S/2C, p=0.355 for Tubifex and p=0.064 for Artemia). In effect, the presence of chinook did not cause stickleback to switch from their preferred prey types (Figs. 21 and 22, Table 9). Chinook kept alone ate mostly Artemia (Fig. 23) but when stickleback were present they switched to Drosophila, with significantly reduced consumption of Artemia (p<0.001 that 2S/2C>0S/4C for Drosophila and p=0.013 that 0S/4C>2S/2C for Artemia). It would be fair to assume that Artemia represents the most profitable prey item (in being free swimming, relatively slow moving, large and conspicuous). Because stickleback consume Artemia in large numbers whether 66 2 Si:ickleback/2 Chincck Figure 21: Prey choice (mean per individuai +95% c T }*te*i n i. t chinook feeding together (experiment 2) ' L) °fstlckiefaack 67 2 Stickleback 0.012 Stlcxlaoacx Artemia EaMDrasoonila CZiTuoifex 4 Stickleback 0.012 Stlckleoack • Artemia G^si Croaoonila l l Tudifex Figure 22: Prey choice (mean per individual, ±95% CD of stickleback alone (experiment 2). 68 2 Chinook 0.012 Chinook Artemia Drosophila t I Tub!fox 4 Chinook 0.012 Chinook Artemia Drosophila I I Tubifex Figure 23: Prey choice (mean per individual, ±95% C.L.) of chinook alone (experiment 2). 69 Table 9: Summary of laboratory prey choice data (experiment 2). mean no. consumed/fish pt Treatment(s) Food item compared Stickleback Chinook 2S/2C Artemia 5.71 2.36 <0.001 Drosophila 0.34 7.21 <0.00l" Tubifex 0.0036g 0.00021g <0.00l" 2S/0C Artemia 3.81 5.43 0.095 vs. 0S/2C Drosophila 0 2.07 . 0.009* Tubifex 0.0071g 0.0027g 0.041* 4S/0C Artemia 4.375 4.36 0.31 vs. 0S/4C Drosophila 0.22 1.18 0.082 Tubifex 0.0053g 0.0036g 0.064 4S/0C 2S/2C Stickleback Artemia 4.375 5.71 0.064 Tubifex 0.0053g 0.0036g 0.355 0S/4C 2S/2C Chinook Artemia 4.36 2.36 0.013 Drosophila 1.18 7.21 0.0001 J Mann-Whitney U Test denotes a significant difference (p<0.05). 70 in single or mixed species groups this would suggest a dominance of stickleback in the feeding hierarchy. In single species groups of two and four the amounts of Artemia and Tubifex consumed per fish did not differ significantly between the two species; this also holds true for groups of four fish, regarding consumption of Drosophila. Only in single species groups of two fish did chinook consume significantly more Drosophila than did stickleback (0S/2C > 2S/0C for Drosophila, p=0.009). In mixed species groups there was a shift in prey choice, with stickleback consuming significantly more Artemia and Tubifex than did chinook (2S/2C stickleback > 2S/2C chinook, p=0.001) and with chinook selecting Drosophila (2S/2C chinook > 2S/2C stickleback, p=0.001). 71 DISCUSSION There is minimal overlap in the diets of chinook and stickleback during sympatry in the tidal channels. Surface prey dominate in the diet of chinook, while stickleback tend to feed on benthic and epibenthic forms. Diet of chinook in the side channels broadly corresponds with that recorded by Levy et al. (1979) for slough habitats in the Fraser estuary, where insect adults, larvae and pupae predominated. This pattern was also found in various other British Columbia estuaries in studies cited in Healey (1981). Other investigations on the estuarine diet of chinook have stressed the importance of Corophium sp. - Sasaki (1966) for the Sacramento-San Joaquim, Reimers (1973) for the Sixes River (Oregon) and McCabe et al. (1983) for the Columbia River. All of these studies took their samples from open estuarine habitats where surface prey would be comparatively rare; therefore the difference is explainable because in Ladner Marsh, conditions of low flow and high terrestrial input provide a great deal of surface food, such that terrestrial prey might surpass Corophium as the major contributor to the diet of chinook. One of the few studies examining the diet of threespine stickleback in estuaries (Henning and Zander 1981) demonstrates that, in the Elbe River (W. Germany) between April and June, copepods, chironomids and oligochaetes are the preferred prey items. Other studies cited in Wootton (1976) also established the importance of benthic organisms in the diet of stickleback. Diet of both chinook and stickleback seems therefore, to be rather consistent from region to region. A number of studies have demonstrated dietary overlap between chinook and stickleback in estuaries (Dunford 1975; Levy et al. 1979; McCabe 72 et al. 1983). In my study, significant dietary overlap has been found only on the first sampling date, at the end of March. This pattern is not surprising because the low temperatures (and therefore, food requirements) coupled with low overall fish densities are unlikely to provide the conditions necessary for intense competition. On all subsequent sampling dates, diet overlap was low. These field data provide the foundation for a series of laboratory experiments that give credence to the existence of competition. Although the ecological differences between the two species manifest themselves trophically, the dimension that is actually involved is microhabitat, as is further demonstrated in laboratory experiments. This supports conventional theory that when two species first begin to overlap spatially they will tend first to partition microhabitat because prey specialization is costly in expenditure of time and energy moving between acceptable prey types (Schoener 1986). This spatial and trophic divergence is presumably due to phenotypic plasticity in behaviour; character displacement per se (Connell 1980) is unlikely in these populations because the time period of spatial overlap is probably too brief to cause fixed morphological divergence in response to competition. Taken in isolation, this pattern of food partitioning observed in the field clearly offers neither support for nor refutation of competition between these species but it does add to a body of experimental evidence presented here that leads to the conclusion that interference competition may affect trophic and spatial patterns of chinook and stickleback in estuarine tidal channels. Laboratory experiments provide the evidence for the existence of interspecific competition, as well as illuminating a.possible mechanism for the process. 73 In single species groups both chinook and stickleback fed predominantly on Artemia and, to a lesser degree, on Tubifex. When placed together, stickleback maintained this pattern, while chinook switched to Drosophila. This pattern is presumably due to asymmetrical interspecific competition favouring stickleback over chinook because the niche shift in chinook is not accompanied by a concomitant shift by stickleback (Morse 1974). The most striking feature of interactions between chinook and stickleback was the high level of inter- and intraspecific aggression demonstrated by the latter. The outcome of this aggressiveness was invariably a spatial divergence of the dominants and subordinates, which was demonstrated statistically in the fourth (large tank) experiment, where chinook tended to avoid territorial stickleback. It is therefore interspecific interference competition that drives chinook out of favoured feeding patches and leads to the observed divergence in diet in the laboratory. There are also a number of facets of the field situation which point towards interspecific territoriality being important in nature. Firstly, stickleback are apparently breeding in the side channels, as evidenced by the large numbers of fry appearing in June and the frequent discovery of eggs in stomach samples. This in itself will cause territoriality among stickleback. Secondly, dietary evidence from the field suggests that chinook are confined to surface waters which, particularly given the preponderance of avian over fish predators, is probably a suboptimal habitat. In addition to this, the increased growth rate of chinook in late May and early June coincides with the decline in stickleback numbers. Finally, the field densities of stickleback are high enough to ensure that effective territoriality is feasible. This corresponds with the predictions of Case and Gilpin (1974) who argued that unilateral interference competition 74 may result in abutting resource use distributions, whereupon the subordinate species would become restricted to habitats where interference is unprofitable for the dominant. Interference competition should also be favoured only when its cost is small, its effect is substantial and resource overlap high (Case and Gilpin 1974). Interspecific aggression by stickleback in the tidal channels seems to fit this model well. Stickleback are admirably preadapted to interspecific aggression because of their well developed intraspecific territoriality; its cost is therefore probably small. Laboratory experiments suggest that aggression is very effective at securing preferred food items for stickleback and the confined space in the side channels ensures that there must be some degree of niche overlap. Finally, in order for interference competition to be favoured, the individuals that are aggressive towards the intruding species must also reap the benefits of increased resource availability. This is quite conceivable in stickleback populations because they are also highly aggressive towards conspecifics. There are numerous examples of interspecific territorially, among both sticklebacks and other related species. Threespine stickleback (Gasterosteus aculeatus) dominate encounters with the smaller G. wheatlandi (Rowland 1983a) and with Apletes quadracus (Rowland 1983b), causing them to congregate in the top half of experimental aquaria. The same pattern has been observed in yellow perch (Percaflavescens), which exclude mudminnows (Umbra limi) from prime foraging areas (Pazsowski 1985), in ninespine stickleback (Pungitius pungitius) which drive juvenile brook charr (Salvelinus fontinalis) from benthic feeding territories (Gaudreault et al. 1986) and in brook charr that aggressively displace creek chub (Semotilus atromaculatus) in oligotrophic lakes (Magnan and Fitzgerald 1984). In all of these cases the aggressor was the larger fish. 75 However, in interactions between threespine stickleback and juvenile chinook the aggressor is considerably smaller (on average by about 10 mm) than the subordinate. This appears to be a rather uncommon occurrence in fish interactions (Morse 1974). Interestingly, intraspecific aggression and, to a certain extent, niche breadth in stickleback are density dependent, so intraspecific competition is probably as pervasive as interspecific competition. This could have important implications for the population dynamics of stickleback in nature by acting as a regulatory mechanism within the population, thereby reducing their competitive influence on chinook. On the other hand, it can be argued that intraspecific aggression is a necessary prerequisite to the appearance or evolution (based on individual selection) of interspecific aggression (Case and Gilpin 1974). If this were not the case then intraspecifically aggressive individuals would expend energy defending feeding patches against other species, only to have conspecifics reaping the benefits at no cost. Effective intra- and interspecific aggression has been demonstrated among yellow perch (Perca flavescens) in sympatry with roach (Rutilus rutilus) (Persson 1986). Perch populations were limited by intraspecific aggression while still maintaining a strong negative effect on roach. Because of the high degree of environmental variability and the transitory coexistence of these two fish species, it is clearly necessary to adopt the non-equilibrium viewpoint, espoused by Wiens (1977,1984) and others, in order to explain the dynamics of the Ladner Marsh side channel system. However, although competition is traditionally found in the equilibrium camp, none of the arguments presented here is at odds with the non- equilibrium framework. Specifically, interference competition bypasses the 76 assumption that the populations have to be at, or even close to, their resource defined carrying capacities, particularly if the costs involved are low. This assumption seems highly plausible for stickleback. Also, because I am examining interactions over a brief time period and only implying short term behavioural niche shifts, the importance of past history or the need for continuous and intense selection can be disregarded. It is totally unnecessary for two species to be at any kind of conceptual equilibrium in order to demonstrate clear, short term interactive niche shifts. Although further field experimentation is required to address the question fully, laboratory experiments and field evidence point towards the existence of interspecific aggression by stickleback towards chinook. This could exclude chinook from safe and productive feeding areas in the benthic region of the tidal channels and drive them into suboptimal open water and surface microhabitats. Severe competitive pressure is probably kept in check because of strong intraspecific competition within populations of stickleback that limits their numbers and, in turn, ensures that there are some feeding refuges for chinook in the productive side channels. Juvenile chinook appear to rely on estuarine marshes for crucial pre- smolting feeding and growth (Levy and Northcote 1982) which enhances survival at sea (Reimers 1973). Highly asymmetrical interference competition favouring stickleback might increase their risk of predation and compromise growth rates. This can have dramatic effects on early marine survival as predation pressure is invariably highly size dependent (stronger on smaller fish). 77 LITERATURE CITED Andrusak, H. and T.G. Northcote. 1971. Segregation between adult cutthroat trout (Salmo clarki) and dolly varden (Salvelinus malma) in small, coastal B.C. lakes. J. Fish. Res. Bd. Canada. 28:1259-1268. Bakker, Th.C.M. and P. Sevenster. 1988. 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London. 265pp. 83 March 18 1988 March 29 1988 23- 25 20- 20 15 0J 10 OJ lillllllmllh , 5 0 [J.llll ••••••• 25 30 35 40 46 50 55 iLihiib 26 30 35 40 45 50 55 Fork Length (mm) Fork Length (mm) n-S5 n-93 April 12 1988 April 28 1988 25 r 25 r 20 • 20 • O 15- 15 0J 10 3 10- OJ 5 £ 5- JUL 0 jurfll 25 30 35 40 46 50 56 25 30 35 40 46 50 S5 Fork Length (mm) Fork Length (mm) n-118 n-150 May 20 1988 June 8 1988 25 r 26 20 • 20 <_>>) £= 15- 16 0J 3 O-10 • 10 OJ s_ 6 U- 5 • 0 . 1.1 ll lllll 23 30 35 40 43 50 53 26 30 35 40 46 50 66 Fork Length (mm) Fork Lengtnh (mm) n-130 n-173 APPENDIX IA: Length-frequency distributions of threespine stickleback from channels 1,3 and 5 in Ladner Marsh - March to June 1988. 84 June 18 1988 July 6 1988 25- 25 20 - 20 u 15 • 15 • c OJ 10 • 10 • cr 5 5 - 0 0 il-i-l-i-ii- 14 16 35 40 45 50 55 25 30 35 40 46 50 55 Fork Length (mm) Fork Length (mm) n-60 n-35 April 20 1989 May 2 1989 25 25 20 20 >» 15 15 | 10 10 CT S_ 5 5 MM i M Jl 0 •II—I- - 25 30 35 40 45 50 55 25 30 35 40 46 50 55 Fork Length (mm) Fork Length (mm) n-102 n-34 1 ^SPPV * j^S^-frequency distributions of threespine stickleback from channels 1,3 and 5 in Ladner Marsh - June and July 1988 and April and May 85 APPENDIX 2; Length-frequency distributions of juvenile chinook salmon taken from channels 1,3 and 5 (Ladner Marsh). 86 Hu. Jmil Uwca (cat APPENDIX 3: Scale diagram of channel 1, showing maximum depths (measured from the channel bank) at 2 m intervals. Areas enclosed by dashed lines are shallow (30-70 cm) banks exposed only at low tide. APPENDIX 4: Scale diagram of channel 3, showing maximum depths (measured from the channel bank) at 2 m intervals. Areas enclosed by dashed lines are shallow (30-70 cm) banks exposed only at low tide.