Experimental evaluation of the use of hatchery-reared juveniles to enhance stocks of the topshell Trochus niloticus in Australia, Indonesia and Vanuatu

Stock enhancement is an attractive option for the management of fisheries but needs to be carefully evaluated before large-scale implementation. We advocate an experimental approach to test the hypothesis that reseeded stocks will yield greater harvests than unseeded controls. We tested the effectiveness of releasing hatchery-reared juveniles to enhance stocks of the topshell Trochus niloticus in Australia, Indonesia and Vanuatu using a rigorous design enabling direct comparisons among countries. Trochus were released at different sizes (6–12, 16–27 mm) and densities (2–8 m2) at replicated sites in each region. Populations at those sites were subsequently compared with those at unseeded controls. Temporal variation in success was also assessed by repeating the experiment in Indonesia. Initial survival was monitored by tagging the juveniles with aluminium foil and recapturing them using underwater metal detectors. The majority of juveniles released in Australia and Indonesia died or disappeared within a month of release and stocks were not enhanced relative to controls. Predation, especially by fish, was considered likely to be the most important cause of loss, but further investigation is required. Losses in Vanuatu were also substantial, but up to 4.4% of 10–27 mm juveniles were recaptured alive after 12 months. Results from Australia and Vanuatu suggested that survival may be improved by releasing juveniles in greater densities and at greater sizes. We discuss additional strategies for improving survival of free-released juveniles in Vanuatu and suggest alternative approaches for Australia and Indonesia.