CHAPTER 5 GENETIC IDENTIFICATION OF FISH HARVESTED FROM OFFSHORE AQUACULTURE: AN EXAMPLE INVOLVING RED DRUM, SCIAENOPS OCELLATUS, FROM THE NORTHERN GULF OF MEXICO

A total of 31 nuclear-encoded microsatellites and an ~ 370 base pair fragment from the ‘control’ region of mitochondrial (mt)DNA were employed to resolve potential forensic issues relating to legal sale of red drum (Sciaenops ocellatus) harvested from offshore aquaculture operations in the northern Gulf of Mexico. Exclusion analyses demonstrated that only 16 microsatellites (13 if mtDNA was employed) were necessary to exclude a sample of 101 ‘wild ’ red drum from Biloxi Bay, Mississippi, as having been produced by broodfish in a hatchery near Corpus Christi, Texas; the probability of incorrectly assigning a ‘wild ’ fish as having been produced by the broodfish ranged from 2.58 x 10–19 to 1.33 x 10–27. Probabilities that the most common, hatchery-produced ‘composite ’ genotype would occur in the sample from Biloxi Bay ranged from 1.38 x 10–27 to 2.98 x 10–42. All probability values were several orders of magnitude smaller than the reciprocal of the total number of adult red drum (106–107) estimated to occur in the northern Gulf of Mexico. Comparison of results with and without mtDNA indicated that it would be more cost effective to first sequence individuals for the mtDNA fragment and then determine the number of individuals that needed to be assayed for microsatellite genotypes. The study demonstrated that unequivocal-ly distinguishing red drum spawned from broodstock obtained offshore of Corpus Christi, Texas, from the ‘wild ’ stock in Biloxi Bay, Mississippi, is fairly straightforward, given (i) a sufficient number of polymorphic (variable), independent genetic markers, (ii) the genotypes of the brood-fish, and (iii) a survey of allelic variation at the genetic markers among representatives of the ‘wild ’ stock. The three ‘requirements ’ essentially would be the same for any offshore aquaculture operation where legal sale of the cultured species could be an issue