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Home , Akaroa, Charteris Bay, Common cockle, Governors Bay, Lyttelton Harbour, Port Levy

Cockle populations in / Whakaraupō

Report No. R13/49 ISBN 978-1-927257-31-9 (print) ISBN 978-1- 927257-32-6 (web)

Emma K. Woods1 Islay D. Marsden1 Lesley Bolton-Ritchie2

1 School of Biological Science University of Canterbury 2 Environment Canterbury

June 2013

Report No. R13/49 ISBN 978-1--927257-31-9 (print) ISBN 978-1- 927257-32-6 (web)

Environment Canterbury PO Box 345 8140 Phone (03) 365 3828 Fax (03) 365 3194

75 Church Street PO Box 550 Timaru 7940 Phone (03) 687 7800 Fax (03) 687 7808 Website: www.ecan.govt.nz Customer Services Phone 0800 324 636

School of Biological Sciences University of Canterbury Private Bag 4800 Christchurch 8140 [email protected]

Cockle populations in Lyttelton Harbour

Executive summary

Cockle populations were sampled from five bays in Lyttelton Harbour/Whakaraupō. In , Head of the Bay, and cockles were sampled at 3 low- tide and 3 mid-tide sites. In Rāpaki, cockles were sampled at mid-tide and sub-tide sites in different substrates both east and west of the jetty.

At each site the density (number of cockles per m2) of cockles was determined and lengths of 100 individuals were measured. At Rāpaki, cockle density was too low to collect 100 cockles. The condition index of cockles from each bay was determined by measuring, weighing and drying the shell and flesh of a number of individuals.

Mean cockle density ranged from 1 – 518 per m2. The low-tide sites had a higher density of cockles/m2 than the mid-tide sites at Charteris Bay, Governors Bay and Head of the Bay. At Purau and Rāpaki the mid-tide sites had significantly higher cockle densities than the respective low-tide and sub-tide sites. Rāpaki had the lowest cockle densities of all sampling sites.

Cockles were divided into four size classes – recruits, juveniles, reproductive adults and edible sized adults. Of the combined total, 56.5% of individuals were juveniles, 39.6% were reproductive adults, 4.0% were recruits and 1.3% were edible sized cockles. Edible sized cockles were only present at Governors Bay, Rāpaki and Purau. The total dry weight biomass/m2 of cockle flesh at each site ranged from 0.91 – 56.35 g/m2.

The densities and biomass of cockles found is low compared to those from Childrens Bay in Harbour and from a site in the Avon-Heathcote Estuary/Ihutai. In Childrens Bay mean densities up to 1880 per m2 and in the estuary mean densities up to 690 per m2 have been found. The low density and biomass of cockles in Lyttelton Harbour/Whakaraupō may be due to increased catchment erosion following land-use changes which has caused increased sedimentation in the upper harbour. An increased concentration of suspended sediment in the water column has been shown to alter the feeding behaviour and growth capabilities of suspension feeding bivalves.

Environment Canterbury Technical Report i Cockle populations in Lyttelton Harbour

ii Environment Canterbury Technical Report Cockle populations in Lyttelton Harbour

Table of contents

Executive summary ...... i

1 Introduction ...... 1

2 Methods ...... 3 2.1 Study area ...... 3 2.2 Density and abundance sampling ...... 4 2.3 Condition index and dry weight biomass ...... 5 2.4 Data analysis ...... 5

3 Sampling sites ...... 6 3.1 Charteris Bay ...... 6 3.2 Governors Bay ...... 7 3.3 Head of the Bay ...... 8 3.4 Purau ...... 9 3.5 Rāpaki ...... 10

4 Results ...... 12 4.1 Density ...... 12 4.2 Size distribution ...... 15 4.3 Condition index ...... 15 4.4 Biomass ...... 19

5 Discussion ...... 19

6 Acknowledgements ...... 21

7 References ...... 22

Appendix 1: GPS co-ordinates of sampling sites...... 25

Appendix 2: Results of statistical analysis of cockle density ...... 26

Environment Canterbury Technical Report iii Cockle populations in Lyttelton Harbour

List of Figures

Figure 1-1: The variable substrate across Rāpaki, Bay with sand and mud habitat for cockles ...... 1 Figure 2-1: Map of and Lyttelton Harbour/Whakaraupō, (inset) ...... 3 Figure 2-2: Map of Lyttelton Harbour/Whakaraupō showing sampling bays ...... 4 Figure 3-1: Charteris Bay sampling sites ...... 6 Figure 3-2: Governors Bay sampling sites ...... 7 Figure 3-3: Head of the Bay sampling sites ...... 8 Figure 3-4: Purau sampling sites ...... 9 Figure 3-5: Sampling sites at the east and west ends of Rāpaki ...... 11 Figure 4-1: Average cockle density + standard error at low tide and mid tide in each bay ...... 12 Figure 4-2: Size distribution of cockles ...... 16 Figure 4-3: Scatter plots of condition index versus cockle length (mm) ...... 18

List of Tables Table 3-1: Details of the sampling sites in Charteris Bay ...... 6 Table 3-2: Details of the sampling sites in Governors Bay ...... 7 Table 3-3: Details of the sampling sites in Head of the Bay ...... 8 Table 3-4: Details of the sampling sites in Purau ...... 9 Table 3-5: Details of the sampling sites in Rāpaki ...... 10 Table 4-1: Cockle density/m2 at low and mid tide sites in each bay ...... 12 Table 4-2: Cockle density/m2 at sub-tide and mid-tide sites at Rāpaki ...... 14 Table 4-3: Percentage of recruits, juveniles, reproductive and edible cockles per metre square at each sampling bay ...... 17 Table 4-4: Average number of recruits, juveniles, reproductive and edible cockles per metre square at each sampling bay ...... 17 Table 4-5: The dry weight tissue biomass (g) of cockle per square metre at each bay and tide level ...... 19

iv Environment Canterbury Technical Report Cockle populations in Lyttelton Harbour

1 Introduction

Tuangi () is a cockle that is endemic to New Zealand. It is a suspension-feeding bivalve that burrows to shallow depths in soft mud and sand in estuaries and sheltered beaches across New Zealand and the Chatham Islands (Marsden 2004, Stewart 2008). Tuangi are valued for customary and recreational gathering and are commercially harvested as littleneck . Across New Zealand cockle stocks are becoming depleted due to overharvesting and habitat degradation. Increasing interest in the protection and restoration of these populations has driven research into optimal management, restorative techniques, methods for growing cockle spat and regional transplantation (Cummings et al., 2007, Marsden and Adkins 2010). Banks Peninsula in Canterbury is an important example of this, where two mātaitai reserves are in place, one at Koukourarata () and one at Rāpaki in Lyttelton Harbour/Whakaraupō. Mātaitai reserves allow tangata whenua to manage marine resources including cockle beds. The Koukourarata cockle beds were closed to harvesting in 1995. Local residents and the Koukourarata Rūnanga have undertaken surveys to assess the status of these cockle beds over time (Marsden and Adkins 2010). At Rāpaki the mātaitai reserve was established in 1998. The status of the cockle beds in Rāpaki is not well defined, with most evidence being anecdotal.

Figure 1-1: The variable substrate across Rāpaki Bay with sand and mud habitat for cockles

Environment Canterbury Technical Report 1 Cockle populations in Lyttelton Harbour

Lyttelton Harbour is 15.5 m deep at the entrance. From the entrance the sea bed gradually slopes into the extensive intertidal in the upper harbour. Significant land-use changes have occurred over time on the hillsides surrounding the upper harbour which includes Governors Bay, Head of the Bay and Charteris Bay. European settlers cleared the forests off the hillsides and the land was used for farming. This resulted in soil erosion. More recently suburban development has increased catchment erosion resulting in increased sedimentation in the upper harbour (de Vries 2007). According to de Vries the mudflats have increased in height by 91 cm since European settlement with a horizontal advance of 9-12 cm yearly in this time. De Vries expected the mudflats to continue to accrete sediment under current land-use conditions.

Limited research has been undertaken on the seabed sediment composition of the upper harbour and the biological communities present. Two main communities have been identified, a mud /sea pen community (Hemiplax/Virgularia) and an Austrovenus stutchburyi community, which is the only to form significant beds. Hart et al. (2008) documented the distribution and abundance of cockles but did not carry out an in depth analysis of the characteristics and variation in cockles across the upper harbour.

The first aim of this study is to determine the current status of the cockle beds at Rāpaki. The second aim is to investigate the density, abundance, size distribution and condition of cockle populations in Lyttelton Harbour/Whakaraupō intertidal mudflats. This research also provides information on whether there are any areas of the harbour that would be suitable for transplanting cockles. Cockles are available from Otago Harbour (a new channel is to be dredged in Otago port, cockles will be present in the dredged sediment) and if transplantation is possible it could strengthen the current cockle stocks in Lyttelton Harbour/Whakaraupō.

2 Environment Canterbury Technical Report Cockle populations in Lyttelton Harbour

2 Methods

2.1 Study area All bays sampled in this study are located in Lyttelton Harbour/Whakaraupō, Banks Peninsula, , New Zealand (43 37′S, 172 42′E, Figure 2-1). The bays were sampled between November 2012 and January 2013. Cockle populations were sampled at ⁰ ⁰ 3 low-tide and 3 mid-tide sites in Governors Bay, Head of the Bay, Charteris Bay and Purau (Figure 2-2). At Rāpaki, cockles were sampled at 9 mid-tide and 9 sub-tide sites across the bay. In each bay sites were situated along transects that were evenly spread across bay pointing perpendicular from the shore to the low-tide mark. The position of each site was determined using a Garmin GPS. Sites were mapped using Google Earth.

Figure 2-1: Map of New Zealand and Lyttelton Harbour/Whakaraupō, Banks Peninsula (inset)

Environment Canterbury Technical Report 3 Cockle populations in Lyttelton Harbour

2.2 Density and abundance sampling At each site in four of the bays (Governors Bay, Head of the Bay, Charteris Bay and Purau), four 25 cm x 25 cm quadrats were randomly placed and sediment removed to a depth of 5 cm. The sediment was sieved through a 2 mm screen. The number of cockles retained on the screen was recorded as a measure of density. The length of the first 100 cockles at a site was recorded as a measure of size distribution. If 100 cockles were not found in the four quadrats random sampling was carried out to find the remainder of cockles needed. At some sites density was too low to find 100 individuals in the time allowed by the tides.

The bay at Rāpaki is smaller than the other sampling bays and the sediment type varies across the bay. As the intertidal area in this bay is narrow, sub-tide and mid-tide sites were sampled. The 25 cm x 25 cm quadrat was used to collect samples at mid-tide sites while at the sub-tide sites, a corer was used. The corer was circular and collected an area of 17 cm2 to a depth of 10 cm. To obtain enough data to assess cockle density in this bay, 10 randomly placed quadrats and 10 randomly placed cores were sampled. At Rāpaki, cockle density was too low to collect 100 cockles to measure.

Figure 2-2: Map of Lyttelton Harbour/Whakaraupō showing sampling bays

4 Environment Canterbury Technical Report Cockle populations in Lyttelton Harbour

2.3 Condition index and dry weight biomass Condition indices are used to measure the health status of bivalves. In this study the dry weight condition index is used to assess the health of cockles’ populations between study sites. Cockles from each bay were collected to measure the condition index. At Head of the Bay, Governors Bay, Charteris Bay and Purau, 25 cockles were collected from one low-tide and one mid-tide site. At Rāpaki only 10 cockles were taken from mid-tide and subtidal sites due to its status as a mātaitai reserve. In the laboratory cockles were kept in a cool store until analysis. The length (mm) and wet weight (g) of each cockle was measured. The shells of the cockles were opened and the state of the gonads was recorded. The flesh was then removed and both shells and flesh were dried in a drying oven for a minimum of 3 days. Once dried, the dry shell weight (g) and dry tissue weight (g) were measured. The condition index of each individual was calculated as dry flesh weight (g)/dry shell weight (g) x 100.

To calculate the dry weight biomass of cockle/m2 at each site, cockle length (mm) was plotted against dry tissue weight (g) to produce a regression equation. The biomass (g) of one individual from each size class from a site was determined using the regression equation. The biomass for each size class was calculated by multiplying the number of individuals in the size class/m2 by the biomass value. The biomass for each size class were added together to determine the total biomass/m2 of cockle flesh at each site. The biomass/m2 reflects the density/m2 at each site.

2.4 Data analysis For statistical analysis the statistical programs R and Statistica were used. Microsoft Excel was used for datasets and drawing graphs.

Bar graphs were used to show cockle density (number per square metre). Histograms were used to show cockle size distribution at each tide level in each bay.

The condition index was plotted against shell length to investigate relationships between cockle nutrition, health and growth. Dry flesh weight (g) was plotted against shell length (mm) and the relationship between these determined (regression equation). The regression equation was applied to the density and abundance data to calculate the total biomass (g) per metre square at each tidal level in each bay.

Density data were analysed using Two-Way Analysis of Variance (ANOVA) to test the individual and combined effects of tide and transect on cockle density/m2 in each bay. To meet the requirements of Two-Way ANOVA the populations were random and independent with equal sample size. Leven’s test for homogeneity of variance was used to test the critical assumption that variance within samples must be equal.

Environment Canterbury Technical Report 5 Cockle populations in Lyttelton Harbour

3 Sampling sites

See Appendix 1 for GPS coordinates.

3.1 Charteris Bay

Table 3-1: Details of the sampling sites in Charteris Bay

Transect Tide Site Sediment type Organisms present 1 Low CL1 Soft mud, ankle deep (across bay) 1 Mid CM1 (across bay) Mud () 2 Low CL2 Red seaweed (Gracilaria sp.) 2 Mid CM2 Polychaetes (worms) 3 Low CL3 wedge shell (Macomona liliana) 3 Mid CM3 Mud crab (Austrohelice crassa)

Figure 3-1: Charteris Bay sampling sites

6 Environment Canterbury Technical Report Cockle populations in Lyttelton Harbour

3.2 Governors Bay

Table 3-2: Details of the sampling sites in Governors Bay

Transect Tide Site Sediment type Organisms present

1 Low GL1 Soft mud/silt (across bay) 1 Mid GM1 mid-calf depth Polychaetes

2 Low GL2 (across bay) Mud crab

2 Mid GM2 Mud snail

3 Low GL3

3 Mid GM3

Figure 3-2: Governors Bay sampling sites

Environment Canterbury Technical Report 7 Cockle populations in Lyttelton Harbour

3.3 Head of the Bay

Table 3-3: Details of the sampling sites in Head of the Bay

Transect Tide Site Sediment type Organisms present 1 Low HL1 Sand/silt (across bay) 1 Mid HM1 (across bay) Mud snail 2 Low HL2 Polychaetes

2 Mid HM2 Stalk eyed mud crab (Hemiplax hirtipes)

3 Low HL3 Mud crab

3 Mid HM3

Figure 3-3: Head of the Bay sampling sites

8 Environment Canterbury Technical Report Cockle populations in Lyttelton Harbour

3.4 Purau

Table 3-4: Details of the sampling sites in Purau

Transect Tide Site Sediment type Organisms present

1 Low PL1 Small cobbles (across bay)

1 Mid PM1 (across low-tide) Pipi ( australis)

2 Low PL2 Wedge shell

2 Mid PM2 Sand/small cobbles Polychaetes

3 Low PL3 (across mid-tide) Top shell (Diloma subrostrata)

3 Mid PM3 Mud crab, Stalk eyed mud crab

Figure 3-4: Purau sampling sites

Environment Canterbury Technical Report 9 Cockle populations in Lyttelton Harbour

3.5 Rāpaki

Table 3-5: Details of the sampling sites in Rāpaki

Transect Tide Site Sediment type Organisms present 1 Sub RS1 Coarse sand (across bay) 1 Mid RM1 Large cobbles Wedge shell 2 Sub RS2 Small Cobbles Pipi 2 Mid RM2 Small Cobbles Undaria pinnatifida 3 Sub RS3 Small cobbles, sand Macrocystis pyrifera 3 Mid RM3 Small cobbles, sand Polychaetes 4 Sub RS4 Sand, large rocks Mud crab 4 Mid RM4 Coarse sand Stalk eyed mud crab 5 Sub RS5 Thick, black mud 5 Mid RM5 Cobbles 6 Sub RS6 Thick, black mud 6 Mid RM6 Coarse sand 7 Sub RS7 Fine sand and mud 7 Mid RM7 Coarse sand 8 Sub RS8 Thick, black mud 8 Mid RM8 Coarse sand 9 Sub RS9 Coarse sand 9 Mid RM9 Coarse sand

10 Environment Canterbury Technical Report Cockle populations in Lyttelton Harbour

West

East

Figure 3-5: Sampling sites at the east and west ends of Rāpaki

Environment Canterbury Technical Report 11 Cockle populations in Lyttelton Harbour

4 Results

4.1 Density At Charteris Bay, Governors Bay and Head of the Bay the low-tide sites typically had a greater density of cockles/m2 than the mid-tide sites (Table 4-1, Figure 4-1 and 4-2).

Table 4-1: Cockle density/m2 at low and mid tide sites in each bay

Average Average Average Standard Standard Bay Tide density density density Average/m2 deviation error Site 1 Site 2 Site 3 Charteris Bay Low 544 540 472 518.7 129.0 64.5 Governors Bay Low 136 416 516 356 100.7 50.4 Head of the Bay Low 812 300 600 570.7 176 88 Purau Low 0 4 0 1.3 2.7 1.3 Charteris Bay Mid 164 96 376 212 66.1 33.0 Governors Bay Mid 272 92 56 140 32.9 16.4 Head of the Bay Mid 24 180 388 197.3 68.7 34.4 Purau Mid 64 520 596 393.3 177.4 88.7

700 Low-Tide 600 Mid-Tide 2 500

400

300

Average Density/m Average 200

100

0 Charteris Bay Governors Bay Head of Bay Purau Rapaki

Figure 4-1: Average cockle density + standard error at low tide and mid tide in each bay

12 Environment Canterbury Technical Report Cockle populations in Lyttelton Harbour

Head of the Bay had the highest average density of cockles at the low-tide (Table 4-1). At Purau, the mid-tide cockle density was greater than at the low-tide and Purau had the greatest mean mid-tide density out of all the bays sampled. Cockles were only present at one low-tide site at Purau (Table 4-1). At Rāpaki cockle densities were variable, which may reflect the heterogenic nature of this bay (see site description). However, mid-tide sites had higher cockle densities than sub-tide sites (Figure 4-3). Cockles were present at all mid-tide sites in Rāpaki but were only present at six subtidal sites (Table 4-2). Rāpaki had the lowest cockle densities of all sampling sites (Figure 4-1).

Densities at Charteris Bay, Governors Bay, Head of the Bay and Purau data met the requirements for the Two-Way ANOVA test (Appendix 2). Rāpaki data had unequal variance within the tide and transect groups so did not meet the Two-Way ANOVA test requirements. The Charteris Bay, Governors Bay, Head of the Bay and Purau data were analysed using the Two-Way ANOVA test. The Rāpaki data were analysed using a T-Test (Appendix 2).

At Charteris Bay and Purau, there was no significant combined effect of tide and transect on cockle density. At Governors Bay and Head of the Bay there was a significant combined effect of tide and transect on cockle density. This means that the effect of each predictor depends on the other.

At Charteris bay, Governors Bay, Head of the Bay and Purau there was a significant difference in density with tidal level. In Head of the Bay there was a significant difference in density between transects.

1000 Low-Tide 900 Mid-Tide 800 2 - 700 600 500 400

Average Density/m Average 300 200 100 0 C1 C2 C3 G1 G2 G3 H1 H2 H3 P1 P2 P3 Bay and transect

Figure 4-2: Average cockle density + standard error across each site at Charteris Bay, Governors Bay, Head of the Bay and Purau

Environment Canterbury Technical Report 13 Cockle populations in Lyttelton Harbour

In Rāpaki there was a significant difference in density between sub-tide sites. At the mid-tide there was also a significant difference in density between sites. Cockle densities at mid-tide sites was significantly higher than at sub-tide sites; mean of 18/m2 in the mid-tide and mean of 5/m2 in the sub-tide.

40 Sub-Tide 35 Mid-Tide

30 2 25

Density/m 20

15

10

5

0 1 2 3 4 5 6 7 8 9 10 Site

Figure 4-3: Cockle density at Rāpaki mid-tide and sub-tidal sites

Table 4-2: Cockle density/m2 at sub-tide and mid-tide sites at Rāpaki Sites Sub-tide Mid-tide 1 0 21.3 2 0 8.9 3 3.8 17.8 4 3.8 10.7 5 0 24.9 6 0 26.7 7 7.7 19.5 8 7.7 3.5 9 15.3 14.2 10 7.7 33.8 Average/m2 4.6 18.1 Standard deviation 5.1 9.1 Standard error 1.6 2.9

14 Environment Canterbury Technical Report Cockle populations in Lyttelton Harbour

4.2 Size distribution The data from each site were combined to produce size distribution histograms for the mid- tide and low-tide in each sampling bay (Figure 4-4). Due to low cockle densities, there were not enough data from Purau low tide and Rāpaki subtidal for the production of size distribution histograms.

The measured cockles were divided into size classes. These size classes were new recruits (<5 mm), juveniles (5-20 mm), reproductive adults (>20 mm) and edible adults by commercial standards (>35 mm). The percentage of individuals in each size class (Table 4-3) and the average number of individuals per metre square each size class (Table 4-4) were calculated for each sampling bay at mid-tide and low-tide. The most common size class was juveniles with juveniles making up an average of 56.5% of all individuals. Reproductive cockles were the second most size class making up 39.6% of all individuals. Edible cockles were the least common size class and only present at Governors Bay, Rāpaki mid-tide and Purau mid-tide.

4.3 Condition index The condition index (dry tissue weight (g)/dry shell weight (g) x 100) was plotted against shell length (mm) to assess the relationship between these factors (Figure 4-5). There were insufficient cockles at the low-tide in Purau and fewer individuals were collected at Rāpaki than in other bays due to the gathering restrictions of the mātaitai reserve.

At Charteris Bay low-tide (CL1), Charteris Bay mid tide (CM1), Head of the Bay low-tide (HL1), Head of the Bay mid-tide (HM1), Rāpaki mid-tide and Purau mid-tide there was no significant linear correlation between condition index and shell length. At Governors Bay low- tide (GL1), Governors Bay mid-tide (GM1) and Rāpaki sub-tide there was a significant linear correlation (P=0.05) between condition index and shell length. At GL1 r = 0.415, at GM1 r = 0.703 and at Rapaki sub-tide r = 0.653.

Environment Canterbury Technical Report 15 Cockle populations in Lyttelton Harbour

Charteris Bay low-tide Charteris Bay mid-tide 180 180 160 160 140 140

120 120 100 100 80 80 60 60 40 40 Number ofindividuals Numberof individuals 20 20 0 0 5 10 15 20 25 30 35 40 5 10 15 20 25 30 35 40 Shell length (mm) Shell length (mm) Governors Bay low-tide Governors Bay mid-tide 120 140

100 120

100 80 80 60 60 40 40 Number individuals of 20 Number of individuals 20

0 0 5 10 15 20 25 30 35 40 5 10 15 20 25 30 35 40 Shell length (mm) Shell length (mm) Head of Bay low-tide Head of Bay mid-tide 120 120

100 100

80 80

60 60

40 40

Numberof individuals Number of individuals 20 20

0 0 5 10 15 20 25 30 35 40 5 10 15 20 25 30 35 40 Shell length (mm) Shell length (mm)

Purau mid-tide Rapaki mid-tide 100 40

35 80 30

60 25 20

40 15

10

Numberof individuals 20 Numberof individuals 5

0 0 5 10 15 20 25 30 35 40 5 10 15 20 25 30 35 40 Shell length (mm) Shell length (mm) Figure 4-2: Size distribution of cockles

16 Environment Canterbury Technical Report Cockle populations in Lyttelton Harbour

Table 4-3: Percentage of recruits, juveniles, reproductive and edible cockles per metre square at each sampling bay

% Recruits % Juveniles % Reproductive % Edible <5 mm 5 – 20 mm >20 mm >35 mm Charteris Bay Low 2.69 26.2 71.1 0 Charteris Bay Mid 0 77.83 22.17 0 Governors Bay Low 0.28 29.58 70.14 3.1 Governors Bay Mid 0 55 45 0.71 Head of the Bay Low 11.34 85.22 3.44 0 Head of the Bay Mid 6.59 92.39 1.02 0 Purau Mid 1.02 30.46 68.53 1.27 Rāpaki Mid 10 55 35 5

Overall 3.99 56.46 39.55 1.26

Table 4-4: Average number of recruits, juveniles, reproductive and edible cockles per metre square at each sampling bay

Recruits Juveniles Reproductive Edible <5 mm 5 - 20 mm >20 mm >35 mm Charteris Bay Low 14 136 369 0

Charteris Bay Mid 0 165 47 0 Governors Bay Low 1 105 249 11 Governors Bay Mid 0 77 63 1 Head of the Bay Low 66 496 20 0 Head of the Bay Mid 13 182 2 0

Purau Mid 4 120 270 5 Rāpaki Mid 2 11 7 1

Environment Canterbury Technical Report 17 Cockle populations in Lyttelton Harbour

CM1 (n=25) r2=0.0004 CL1 (n=25) r2=0.0049 8 6 6 4 4 2 2

0 Index Condition 0 Condition Index Condition 0 5 10 15 20 25 30 0 5 10 15 20 25 30 Shell Length (mm) Shell Length (mm) GL1 (n=25) r2=0.176 GM1 (n=25) r2=0.49 6 8 5 y = -0.1303x + 7.1016 4 6 3 4 2 2 1 Condition Index Condition 0 0 Condition Index Condition 0 10 20 30 40 0 10 20 30 Shell Length (mm) Shell Length (mm)

HL1 (n=25) r2=0.063 HM1 (n=25) r2=0.14

6 8 5 6 4 3 4 2 2

Cndition Index Cndition 1 Condition Index Condition 0 0 0 5 10 15 20 25 30 0 5 10 15 20 25 Cockle Length (mm) Shell Length RS (n=10) r2=0.42 RM (n=34) r2=0.10 6 5 8 4 6 3 4 2 y = 0.0442x + 2.9852 1 2 Condition Index Condition 0 0 Condition Index Condition 0 10 20 30 40 50 0 5 10 15 20 25 30 Cockle Length (mm) Shell Length (mm) PM1 +PM2 (n=50) r2=0.017 12 10 Figure 4-3: Scatter plots of condition index 8 versus cockle length (mm) 6 4 n = number of individuals

Condition Index Condition 2 r2= strength of the relationship 0 0 10 20 30 40 Cockle Length (mm)

18 Environment Canterbury Technical Report Cockle populations in Lyttelton Harbour

4.4 Biomass The calculated dry weight biomass of cockle flesh at each site is presented in Table 4-5.

Table 4-5: The dry weight tissue biomass (g) of cockle per square metre at each bay and tide level

Bay Tide level g/m2 Charteris Low 35.7 Charteris Mid 8.2 Governors Low 45.54 Governors Mid 14.32 Head of the Bay Low 13.3 Head of the Bay Mid 3.03

Purau Low N/A Purau Mid 56.35

Rāpaki Low N/A Rāpaki Mid 0.91

5 Discussion

The low-tide sites had a higher density of cockle/m2 than the mid-tide sites at Charteris Bay, Governors Bay and Head of the Bay. However, at Purau and Rāpaki the mid-tide sites had higher cockle densities than the respective low-tide and sub-tide sites (Figure 1-1, Figure 2-1). The Purau low-tide was cobbled and the Rāpaki low-tide sites contained high density pipi beds and anoxic sediments which could negatively affect cockle densities. The range of mean cockle density/m2 was 1 - 518/m2 (Table 1-1). We compared this density range to that recorded in Childrens Bay, which is on the southern side of Banks Peninsula and the Avon-Heathcote Estuary/Ihutai to the north of Lyttelton Harbour/Whakaraupō. In Childrens Bay cockle density in 2009 ranged from 300 – 3000/m2 with an average density of 1420/m2; in 2010 density ranged from 150-2030/m2 with an average density of 930/m2 and in 2011 density ranged from 900-3600/m2 with an average density of 1880/m2 (Environment Canterbury unpublished data). In the Avon-Heathcote Estuary/Ihutai at Plover Street cockle density in 2009 ranged from 75-1580 m2 with an average density of 690/m2; in 2010 density ranged from 150-2030/m2 with an average density of 440/m2; in 2011 density ranged from 230-900/m2 with an average density of 496/m2 and in 2012 density ranged from 150-830/m2 with an average density of 460/m2 (Bolton-Ritchie 2010, 2011, 2012 and 2013). The densities are higher than the Lyttelton Harbour/Whakaraupō values which can be considered as low cockle densities. Without

Environment Canterbury Technical Report 19 Cockle populations in Lyttelton Harbour

earlier estimates of cockle bed densities across the harbour we cannot say whether cockle populations are decreasing or if environmental factors are maintaining population densities at these low levels. Research has shown that cockle populations are in decline across New Zealand, due to factors such as habitat loss, overharvesting and pollution (Cummings et al. 2007, Marsden and Adkins 2010). The low densities that this study has found may be evidence that cockle populations in Lyttelton Harbour/Whakaraupō are being similarly affected.

At Charteris Bay, Governors Bay and Head of the Bay cockles were larger at the low tide than the mid-tide (Figure 4-4). At Rāpaki and Purau too few cockle were present at the low- tide sites to allow for size distribution analysis, however the mid-tide sites were the only sites apart from Governors Bay to contain edible size cockles (Figure 4-4, Table 3-2). In combination with the density results, the size distribution results of the cockles indicate less than optimal growth. The largest size class was the juvenile age group (5-20 mm) which made up on average 56.46% of individuals collected. Reproductive adults (>20 mm) made up the second largest proportion of individuals (39.55%) collected. New recruits (3.99%) and edible adults (1.26%) were the least abundant age groups in our sampled populations (Table 3-2). It is characteristic of cockles to be dominated by one or two size groups and it is not uncommon for new recruits less than 5 mm in length and adults greater than 35 mm to be rare in populations (Marsden and Adkins 2010). Whilst these size distributions are relatively normal for cockle populations they are not favourable for recreational and customary gathering as edible adults are uncommon or absent across all sampling sites (Figure 4-4).

Biomass estimates ranged from 56.35g/m2 at the Purau mid-tide to 0.91g/m2 at the Rāpaki mid-tide (Table 3-4). Cockle densities at Purau and Rāpaki low-tide sites were too low to produce biomass estimates. Governors Bay had the highest overall biomass/m2 with 45.54g/m2 at the low-tide and 14.32g/m2 at the mid-tide. In comparison to the Avon- Heathcote Estuary/Ihutai these values are low. Stephenson (1981) found most of the estuary supported a dry weight tissue biomass of less than 500g/m2 with a peak biomass of 1200g/m2. More recent sampling (2008-2009) suggests that values now are less than in the past, with average values less than 100g/m2, and maximal values up to 500g/m2 close to the estuary mouth (I Marsden own data).

Condition indices of cockles from the present study were plotted against shell length (mm) to compare the quality and health of cockle populations across sites. The only sites to have a significant correlation between condition index and shell length (mm) were Governors Bay low-tide (GL1), Governors Bay mid-tide (GM1) and the Rāpaki sub-tide. This means that at these locations the health and potential growth of these populations can be assessed. Cockles from both Governors Bay sites had a significant negative linear correlation with shell length (Figure 4-5). A negative correlation shows that the recruits and juveniles are growing at a fast rate whilst the adults have decreased in tissue weight probably due to loss of gonad tissue following reproduction. Cockles from Rāpaki the sub tidal had a significant positive linear correlation between these variables (Figure 4-5). This indicates that adults have the greatest tissue weight in proportion to shell weight due to the presence of developing gonad tissue (Crosby and Gale 1990).

20 Environment Canterbury Technical Report Cockle populations in Lyttelton Harbour

The observed differences in density and size between low-tide and mid-tide sites within the sampling bays indicate tidal height affects cockles. Tidal height has been found to be the major factor affecting growth rate in cockle, as longer periods of immersion for cockles in the low-tide enables them to feed for longer and acquire more resources for growth (Dobbinson et al., 1989). Cockle at the mid-tide have less time to feed and endure longer exposure times to environmental stressors and predators which reduce survival (Marsden 2004).

The reasons for the low cockle density, size and biomass in this harbour are uncertain. Increased catchment erosion following land-use changes during early settlement and recent suburban development has caused increased sedimentation in the upper harbour (Hart et al. 2008). The increase in height of the upper harbour mudflats by 91 cm over a period of around 160 years and the yearly horizontal advance of the mudflats by 9-12 cm (de Vries 2007) is evidence of this. An elevated concentration of suspended sediment in the water column due to human induced land run-off has been shown to alter the feeding behaviour and growth capabilities of suspension feeding bivalves. This occurs because of a reduction in the quality of the seston (organic food resource) in the water column that they feed on. Increased sediment load can also reduce benthic primary production directly by decreasing light penetration through turbid waters (Norkko et al. 2006). It is not known if the health and growth capacities of the cockles in this harbour are affected by wastewater discharges, storm water discharges and activities within the port (Bolton-Ritchie, 2011).

Overall, cockle density and biomass is low in the upper harbour of Lyttelton/Whakaraupō and the habitat supports few adult cockles of edible size. Without historical data it is unclear whether cockle numbers are in decline or if the habitats are sustaining these low densities at a constant level. Shellfish beds at Rāpaki have been protected from gathering by a mātaitai reserve since 1998 but the cockle numbers here would not support renewed harvesting. Increased sedimentation and the resulting water column suspended sediment concentrations in the upper harbour are likely to be the main limiting factor for cockle survival and growth. Purau and Rāpaki are less affected by this and provide cockles with a mixed sand and mud habitat which the mudflats of Charteris Bay, Governors Bay and Head of the Bay do not. In terms of potential for cockle transplantation Rāpaki would be a more suitable site than Purau as the Purau low-tide is mainly small cobbles with very little mud or sand habitat for cockle. At Rāpaki mid-tide and sub-tide sites provide some suitable habitat and Rāpaki currently supports adults of edible size. The presence of pipi beds and potentially anoxic sediment observed during this study may be a limiting factor. The extent of these may need to be quantified before transplantation could go ahead. Transplantation of cockles to Rāpaki has the potential to aid the recovery of this valued resource and could help to re-establish customary gathering beds there.

6 Acknowledgements

This research was undertaken by one of the authors (Woods) as a student project at the University of Canterbury, financially supported by Environment Canterbury. It has received peer review through the internal University of Canterbury processes and the other two authors of the report (Marsden and Bolton-Ritchie).

Environment Canterbury Technical Report 21 Cockle populations in Lyttelton Harbour

7 References

Bolton-Ritchie, L. 2010. The sediments and biota of the Avon-Heathcote Estuary/Ihutai and tidal reaches of the Avon and Heathcote rivers. Summary report on data collected in 2009. Environment Canterbury Report R10/147

Bolton-Ritchie, L. 2011. Factors influencing water quality of Lyttelton Harbour/Whakaraupō. Environment Canterbury , Report R11/45 104pp.

Bolton-Ritchie, L. 2011. The sediments and biota of the Avon-Heathcote Estuary/Ihutai and tidal reaches of the Avon and Heathcote rivers. Summary report on data collected in 2010. Environment Canterbury Report R11/66

Bolton-Ritchie, L. 2012. The sediments and biota of the Avon-Heathcote Estuary/Ihutai and tidal reaches of the Avon and Heathcote rivers. Summary report on data collected in 2011. Environment Canterbury Report R12/110

Bolton-Ritchie, L. 2013. The sediments and biota of the Avon-Heathcote Estuary/Ihutai and tidal reaches of the Avon and Heathcote rivers. Summary report on data collected in 2013. Environment Canterbury Report R13/In prep

Crosby, M. P. and L. D. Gale. 1990. A review and evaluation of bivalve condition index methodologies with a suggested standard method. Journal of Shellfish Research 9:233-237.

Cummings, V., J. Hewitt, J. Halliday, and G. Mackay. 2007. Optimizing the success of Austrovenus stutchburyi restoration: Preliminary investigations in a New Zealand estuary. Journal of Shellfish Research 26:89-100.

de Vries, W. J. 2007. morphodynamics and sedimentation rates: a case study of the intertidal mudflats at the Head of the Bay, Lyttelton Harbour. Honours Dissertation (Geography) University of Canterbury, 40pp.

Dobbinson SJ, Barker MF, Jillett, JB. 1989. Experimental shore level transplantation of the New Zealand cockle Chione stutchburyi. Journal of Shellfish Research 9 :197-21.

Hart, D. E., I. D. Marsden, D. J. Todd, and W. J. de Vries. 2008. Mapping of the Bathymetry, Soft Sediments, and Biota of the Seabed of Upper Lyttleton Harbour. Ecan Report 08/35, University of Canterbury prepared for Environment Canterbury

Marsden, I. D. 2004. Effects of reduced salinity and seston availability on growth of the New Zealand little-neck clam Austrovenus stutchburyi. Marine Ecology Progress Series 266:157-171.

Marsden, I. D. and S. C. Adkins. 2010. Current status of cockle bed restoration in New Zealand. International 18:83-97.

Norkko, J., J. E. Hewitt, and S. F. Thrush. 2006. Effects of increased sedimentation on the physiology of two estuarine soft-sediment bivalves, Austrovenus stutchburyi and . Journal of Experimental Marine Biology and Ecology 333:12-26.

Stephenson, R. L. 1981. Aspects of the energetics of the cockle Chione (Austrovenus) stutchburyi in the Avon-Heathcote Estuary, Christchurch, New Zealand. PhD Thesis. University of Canterbury, New Zealand.

22 Environment Canterbury Technical Report Cockle populations in Lyttelton Harbour

Stewart, B. 2008. Stock Assessment of (Austrovenus stutchburyi) in Waititi Inlet, Otago,2007. Ryder Consulting Ltd, Dunedin, New Zealand.

Environment Canterbury Technical Report 23 Cockle populations in Lyttelton Harbour

24 Environment Canterbury Technical Report Cockle populations in Lyttelton Harbour

Appendix 1: GPS co-ordinates of sampling sites

Bay Site Latitude Longitude Charteris CL1 43°39'1.14"S 172°41'52.20"E Bay CM1 43°39'8.88"S 172°41'48.30"E CL2 43°38'55.68"S 43°38'55.68"S CM2 43°39'10.79"S 172°41'40.53"E CL3 43°38'54.02"S 172°41'36.07"E CM3 43°38'56.45"S 172°41'29.94"E Governors GL1 43°37'37.08"S 172°39'17.64"E Bay GM1 43°37'43.26"S 172°39'13.52"E GL2 43°37'24.67"S 172°39'12.07"E GM2 43°37'25.17"S 172°39'9.36"E GL3 43°37'56.15"S 172°39'17.36"E GM3 43°37'58.69"S 172°39'8.44"E Head of the HL1 43°38'34.86"S 172°40'2.82"E Bay HM1 43°38'23.34"S 172°39'59.46"E HL2 43°38'50.50"S 172°40'9.64"E HM2 43°39'4.56"S 172°39'51.16"E HL3 43°39'5.49"S 172°40'15.66"E HM3 43°39'15.21"S 172°40'8.02"E Purau PL1 43°38'13.17"S 172°45'0.30"E PM1 43°38'15.22"S 172°45'1.06"E PL2 43°38'12.23"S 172°45'12.09"E PM2 43°38'13.97"S 172°45'12.08"E PL3 43°38'12.18"S 172°45'9.75"E PM3 43°38'13.14"S 172°45'9.53"E Rāpaki RS1 43°36'28.55"S 172°40'52.11"E RM1 43°36'27.98"S 172°40'51.95"E RS2 43°36'28.17"S 172°40'50.84"E RM2 43°36'27.52"S 172°40'50.93"E RS3 43°36'27.69"S 172°40'49.84"E RM3 43°36'26.74"S 172°40'48.17"E RS4 43°36'28.31"S 172°40'54.82"E RM4 43°36'27.62"S 172°40'54.56"E RS5 43°36'27.42"S 172°40'57.58"E RM5 43°36'26.99"S 172°40'57.42"E RS6 43°36'27.72"S 172°40'59.46"E RM6 43°36'26.80"S 172°40'59.75"E RS7 43°36'28.70"S 172°41'3.35"E RM7 43°36'27.98"S 172°41'4.06"E RS8 43°36'28.34"S 172°41'1.64"E RM8 43°36'27.30"S 172°41'2.43"E RS9 43°36'29.13"S 172°41'4.64"E RM9 43°36'28.44"S 172°41'4.99"E

Environment Canterbury Technical Report 25 Cockle populations in Lyttelton Harbour

Appendix 2: Results of statistical analysis of cockle density

Results of Levene's test for equal variance n Df a P F Variance Charteris Bay 18 5 0.01 >0.01 3.5847 equal Governors Bay 18 5 0.001 >0.001 4.4 equal Head of the Bay 18 5 0.05 >0.05 2.11 equal Purau 18 5 0.001 >0.001 5.95 equal Rāpaki 162 17 0.001 <0.001 8.89 not equal

Results of Two-Way ANOVA n Df a P F Interaction Charteris Bay 18 2 0.01 >0.01 5.06 not significant Governors Bay 18 2 0.001 <0.001 17.9 significant Head of the Bay 18 2 0.05 <0.001 13.064 significant Purau 18 2 0.001 >0.001 7.23 not significant

Results of Two-Way ANOVA Interaction Effect of Tide Effect of Transect Charteris Bay not significant Significant not significant Governors Bay significant Significant not significant Head of the Bay significant Significant significant Purau not significant Significant not significant

Results of Rāpaki T-Test for comparison of sub and mid-tide density Df a P T Result Sub vs mid tide 89 0.05 <0.01 4.4638 Mid > sub

Results of Rāpaki O ne Way ANOVA for variance of densit y with tidal height n Df a P F Result significant Sub tide 81 8 0.05 <0.01 3.182 variance significant Mid tide 81 8 0.05 <0.0001 5.938 variance

26 Environment Canterbury Technical Report