Transplanting cockles in Raglan Harbour

In New Zealand, the cockle is an important recreational fishery and a small but growing commercial industry. There is pressure on the resource through estuarine degradation and overharvesting, which has prompted interest in ecosystem improvement and population enhancement through transplanting adults or hatchery reared cockles to depleted beds. Field transplant studies were carried out in summer and winter, with a range of cockles sizes, at four sites of contrasting sediment characteristics within Raglan harbour. Shell hash was applied to additional plots at all sites and survival and growth of transplanted cockles compared to control plots. Laboratory flme experiments were used to dtermine the burial ability of a range of cockles sizes in contrastings sediment types. Maximum cockle mortality was 14% and showed no consistent trend with site exposure or season. Predation accounted for less than 22% of the total mortality in most cases. Shell material was found to increase retention, survival, growth, recruitment of small cockles and reduce predation by as much as 100%. Results from this study suggest transplanting small hatchery reared cockles in the winter will allow grwoth before summer when a larger size is beneficial to retention. Site characteristics were found to have a large influence on growth and survival of transplants. Purpose: To investigate the success of transplanting cockles Austrovenus stutchburyi in Raglan Harbour: specifically to determine how site, season and cockle size affect the survival and growth fo transplanted cockles, whether shell material applied to sediment enhances survival and growth, and to determine the effect of sediment type and cockle size on establishment/burial ability of the cockles.

Data and assessment of transplant experiments with cockles. See details in abstract and methods. Response variables of establishment analysed using a fixed factor ANOVA. Analysed site, cockle size and treatment (shell).

Raglan Harbour - four sites (Cliff St, Patikirau, Okete, and Bridges).

1999 Summer (January - May) and Winter (June-October) experiments.

Two cockle sizes transplanted at four sites. 30-40 replicate 0.25m2 plots placed around mid tide at each site. 30 cockles added to each site. Shell hash added to half plots. Frequency of collection: Okete and Bridges sites sampled after four months. Patikirau and Cliff St sites sampled every two weeks.

Gaps in collection: Data could only be collected two weekly at two sites due to time constraints. At other two sites, data only collected at beginning and end of study.

Data reported in printed thesis.

Sensitivity/Confidentiality: No confidentiality.

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