THE IMPORTANCE OF APPROPRIATE ESTIMATION OF CULTURED CONTRIBUTION IN STOCK ENHANCEMENT: CASE STUDIES FROM SOUTH CAROLINA

Stock enhancement goals can vary significantly by species, but understanding the life history and fishery of the target species is essential to determining stocking strategies and their success.  One effective method for gauging the success of a stock enhancement program and its impact on wild populations is to regularly sample a population and calculate the hatchery contribution on a cohort-by-cohort basis.  In South Carolina, research focuses specifically on three species: red drum, cobia and spotted seatrout.  The genetic population structure and life history parameters of each candidate species needs to be considered prior to releasing fish, and calculating their contribution to a particular cohort can range from simple to complex.  Stock enhancement research on red drum, a long-lived species, focuses primarily on the subadult population found in the estuary.  Red drum have a short distinct spawning season and fast growth rates, but long subadult stage (3-5 years).  Fish are sampled by SCDNR's fisheries independent trammel net survey and estimation of hatchery contribution to a cohort by size is relatively simple.  Similarly, cobia, a coastal migratory pelagic species with short spawning season and fast growth rate, can also be reliably aged in the first two years so that stocking contribution can be assigned accurately for young fish.  For adult cobia, we rely on cooperating anglers, freezer programs, and otolith aging to estimate hatchery contribution of stocked fish.  The life history characteristics of spotted seatrout require considerable effort for estimating stocked contribution because the species is genetically isolated by distance, spawns over a six-month period, and exhibits sexually dimorphic growth.  For spotted seatrout, subsamples are taken for otolith aging and predictive statistics are used to estimate a cohort for each individual before estimating a hatchery contribution.  This research compares and contrasts three disparate species with unique life histories and addresses caveats to understanding stock enhancement impacts and designing appropriate stocking strategies.