Abstract

Releasing hatchery-reared queen conch (Strombus gigas) has been widely advocated as a practical approach to restore depleted populations; however, the method has not been studied with any rigor. We conducted a five-year study designed to investigate the variables that limit survival of hatchery-reared conch outplants after release, to optimize these variables to achieve maximum survival, and, ultimately, to develop a cost per outplant surviving to approach reproductive size. We examined season of release, moon phase, siphonal length, and stocking density in replicated experiments conducted in the Florida Keys. For the density experiment, we tethered individuals and replaced conch that were missing. For all other experiments, we used underwater metal detectors to efficiently recover tagged, free-ranging outplants at regular time intervals and estimated emigration versus mortality for missing individuals. The results were analyzed using survival analysis. The best survival was for ~9-cm conch released in the fall on an upcoming full moon at a density of 1·5 m-2. We estimated that fewer than 1% of released individuals would survive. This is not surprising; studies of wild conch populations suggest that mortality (M) for 9-cm conch equals approximately 4 for the first year and 1.5 for the second year resulting in an estimated overall survival of 0.4% for the two years required to attain sexual maturity. When cost of production per 9-cm individual was coupled with survival after release, we estimated that each conch surviving to 14-cm would cost a minimum of $9.00 (US) within a restoration program designed to produce a sufficient number of conch so that 10,000 would survive. We conclude that releasing hatchery-reared conch to restore depleted stocks should only be considered if other methods (e.g., translocating wild conch) are not available.

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