Cockle populations in Lyttelton Harbour/ Whakaraupō Report No. R13/49 ISBN 978-1-927257-31-9 (print) ISBN 978-1- 927257-32-6 (web) Emma K. Woods1 1 Islay D. Marsden 2 Lesley Bolton-Ritchie 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 Christchurch 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 Governors Bay, Head of the Bay, Charteris Bay and Purau 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 Akaroa 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 New Zealand and Lyttelton Harbour/Whakaraupō, Banks Peninsula (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 (Austrovenus stutchburyi) 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 clam. 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