Spatio-temporal and size-dependent variation in the success of releasing cultured sea cucumbers in the wild

Large-scale releases of cultured "sandfish," Holothuria scabra, were used to examine size-and density-dependent effects on survival among sites. Juveniles were marked by fluorochromes in 3 size classes and released into open 500-m2 sea pens. A preliminary trial involved the release of 4,000 juveniles at two sites. In a subsequent large-scale experiment, we released 9,000 juveniles at 0.5, 1, or 3 individuals m- 2 at 4 sites. Growth and survival up to 2 years post-release were estimated from successive recapture surveys and marker verification. Most of the surviving animals attained the size at first maturity (180 g) within 12 months in the preliminary trial but grew slower in the second experiment. Growth was density dependent, with carrying capacity at one site of 200-250 g sandfish m-2. Survival varied greatly among sites, explained in part by microhabitat features, but site suitability was ephemeral; previous success at sites did not guarantee success later. Juvenile size at release significantly affected long-term survival, but survival was density-independent within the experimental range. Juveniles should be released at a minimum size of 3 g and at multiple sites and occasions to mitigate spatio-temporal variation in survival. We predict that 7-20% of sandfish released at a size of 3-10 g in optimum habitat could survive to market size, which gives qualified support for restocking. Our results also help to assess the viability of sea ranching, which will depend on sale price, harvest efficiency, and reduced costs of producing juveniles.