Genetic Effective Size in Populations of Hatchery-Raised Red Drum Released for Stock Enhancement

Abstract

Genetic analysis of progeny from 13 spawning events occurring over a 2-week period in a Texas Parks and Wildlife Department (TPWD) hatchery for red drum Sciaenops ocellatus during the spring of 2002 and hatchery spawning and release records over the 2003 spawning season were used to estimate the average genetic effective size of an average spawn and an average hatchery-released population. The purpose of this study was to assess the potential for a Ryman-Laikre effect in the TPWD red drum stock enhancement program. Genetic analysis revealed that 16 of 27 dams (59.2%) and 16 of 18 sires (88.9%) spawned at least once. The average effective size (N_e ) for a single spawn was 2.59, approximately 43% less than the maximum N_e (4.55) predicted if all possible mating (dam × sire) combinations had occurred and family size per mating combination had been equivalent. The reduction in N_e stemming from the actual number of mating combinations was approximately 34% and appeared to be due primarily to nonspawning dams; the reduction in N_e generated by the actual variation in family size was approximately 9%. Spawning and release records at the TPWD hatchery indicate that in 2003 the number of released populations per bay or estuary ranged from 7 to 27. Using the average effective size (N_e ) estimate for a single spawn (2.59), the estimated average effective size of all released fish per bay or estuary (N_eR ) in 2003 ranged from about 28.5 to about 46.6. These values of N_eR are less than the averages estimates of about 272 and 263 for the long-term (N_eI ) and contemporaneous (N_eV ) effective size, respectively, of red drum in bays and estuaries in the northern Gulf of Mexico and indicate a reasonable potential for a Ryman-Laikre effect. Approaches that might be employed to increase the N_eR of TPWD-released fish and decrease the probability of a Ryman-Laikre effect are discussed.