ASSESSING THE GENETIC IMPACTS OF STOCK ENHANCEMENT OF RED DRUM Sciaenops ocellatus ON THE WILD POPULATION IN SOUTH CAROLINA USING ESTIMATES OF GENETIC DIVERSITY.

Stock enhancement is the deliberate release of aquaculture-produced animals widely used by states and governments around the world to restore threatened and endangered species, augment recreational and commercially important species, and alleviate restrictive fishing regulations. Fishery scientists hypothesize that these releases could negatively impact the wild population through reduced genetic diversity and greater levels of inbreeding. Between 1999 and 2011, ~6 million juveniles (20-270 mm TL) were stocked by the South Carolina Department of Natural Resources into the Charleston Harbor estuary. The estuary was systematically sampled each year following stocking events and samples were genotyped using eight microsatellite loci. The proportion of cultured fish in age 1 samples (total N=3,361) were as high as 90%.  Red drum were also sampled when they were sexually mature and recruited to the spatially separate offshore adult population (5-40 years old; total N=796).  This study evaluated the genetic impacts of stocking on the wild adult population using a suite of genetic diversity metrics: allele count, rarefied allelic richness, effective number of alleles, allele size range, observed heterozygosity, expected heterozygosity, the inbreeding coefficient (F_IS), effective number of breeders per year-class (N_b) based on age 1 collections, and the effective population size (N_e) based on mixed-age adult collections.

Greater proportions of cultured 1 year old fish caught in the estuary is strongly correlated with lower estimates of N_b, which signifies a potential for elevated levels of inbreeding (Figure 1: Adjusted R²=0.829, p<0.001). However, no significant differences were found between collection years within adult samples in any of the other diversity metrics (Table 1), which suggests that stocking aquaculture-produced red drum as small juveniles, despite exhibiting high year-class contributions in the estuary, did not negatively impact the genetic diversity among wild adults. The results from this study reveal that responsible stocking of large numbers of aquaculture-produced fish has little to no effect on the adult population for a long-lived species with overlapping generations.