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 31 mm (± 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 (140 mm) was achieved, on average, at 5 years of age or 3.5 years post-release, and the cohort entered the fishery over 3 years (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.

A series of stock enhancement experiments were carried out on Haliotis laevigata populations. Methodologies included a large-scale BACI (before, after, control, impact) experiment (42 sites); a carrying capacity experiment, which involved a high-density release at two sites; and a detailed survey of abalone populations and ecological parameters. Increased densities were detected for most age classes, although fishing mortality began obscuring the effect by age 5+. Age-4+ animals showed the clearest result, with no difference between enhanced and control sites at 6, 12, and 18 months post-release, and then a 300% increase at enhanced sites at 30 months post-release. Overall, a single release of age-1+ animals in May 2006 had doubled the total density by November 2008. In the carrying capacity experiment, densities initially increased rapidly (by up to 800%) but had stabilized at a 400% increase after 2.5 years, at around 8 per m 2 . This was the predicted carrying capacity, with the enhanced cohort representing 50% of the population. A PERMANOVA (permutational multivariate analysis of variance) analysis of ecological similarity detected no effect of enhancement, although changes in algal percent of coverage were detected at both control and enhanced sites. Overall, this study suggests that as long as release densities are controlled within natural limits, successful stock enhancement can be attained for this species with minimal ecological impacts.

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