Stock Enhancement in Greenlip Abalone Part I: Long-Term Growth and Survival

A cohort of Haliotis laevigata, spawned from wild broodstock, was monitored from settlement at a hatchery until age 8. Animals were released into the wild at 31mm (+/- 4 SD), targeting an enhancement size-class of 135-145-mm shell length. Release densities were tailored to match wild-stock densities using a size-dependent mortality model. A total of 7,500 animals were released into 24 sites, and each site was precisely mapped to control release densities. Environmental and husbandry factors were also quantified. Initial survival rates (six months post-release) differed significantly among sites (range: 11-67%) but not beyond this time period. Legal minimum length (140mm) was achieved, on average, at 5years of age or 3.5years post-release, and the cohort entered the fishery over 3years (age 5-8). Cumulative survival at age 5 varied between 20% at the better sites and 6% at the worst sites, with an average of 13%. Water depth was significantly positively correlated with growth (r = 0.47; p < 0.05), but no other ecological variables influenced growth or survival. Husbandry factors were implicated in sites with poor survival, but this was not confirmed statistically. The cohort successfully entered the fishery and was harvested at an overall fishing mortality (F) of 0.27, but site differences in F were significant and highly correlated with growth.