Our long-term investigations of mortality and predation in the large, economically significant gastropod Strombus gigas (queen conch) indicate that instantaneous rates of natural mortality (M) in nursery grounds in the Bahamas and the Florida Keys are often higher and
more variable than recognized earlier. Wide divergence from a simple size-specific curve of natural mortality was caused by seasonal, interannual, and site-specific sources of variation. In the Florida Keys, mortality rates for 200-mm conch were highest in summer and most stable in the fall,
but summer values of M varied interannually from <2.0 to 12.0. Mortality rates for 100-mm conch, often considered ideal in size for field releases, varied from 0.5 to 12.0, depending on site, season, conch density, and deficits in behavior and shell form associated with hatchery
rearing. Unlike many marine invertebrates, juvenile queen conch have an inverse density dependence in mortality. Because of the logarithmic relationship between mortality rate and the cost to enhance a stock, it is critical for those involved in stock enhancement to have a realistic understanding
of mortality processes and effects and to optimize culture and release strategies for highest possible survival. Specific recommendations are made for queen conch releases.
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