No 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 3 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.

Heritability of juvenile growth rate was estimated for red drum (Sciaenops ocellatus), an economically important sciaenid fish in the southern USA. Thirty‐eight families were generated via ‘natural’ spawning of multiple sets of five breeders (three dams × two sires) in individual brood tanks. Offspring were individually tagged with Passive Integrated Transponder (PIT) tags and mixed for grow‐out in replicate ‘common‐garden’ tanks. Juvenile growth was followed from 166.4±18.6 to 254.0±27.0 mm (total length). Offspring were assigned a posteriori to individual brood fish (dam and sire) based on genotypes at nuclear‐encoded microsatellites. Heritability (h2) of a thermal growth coefficient was estimated using an animal‐additive model and a restricted maximum‐likelihood algorithm. Estimates of h2 were 0.33±0.08 and 0.31±0.08 for thermal growth coefficient based on length and weight respectively. These results indicate a significant genetic component in juvenile growth rate in red drum. Estimates of h2 for condition coefficient (K) at various measurement dates averaged 0.38, suggesting a genetic component to shape in juvenile red drum.

Heritability (h 2) of cold tolerance was estimated for red drum Sciaenops ocellatus, an economically important sciaenid fish in the southern USA. Nineteen families were generated via environmentally induced spawning of multiple sets of five broodfish (three dams × two sires) and were mixed in three common‐garden replicate tanks for cold tolerance challenge. All offspring were assigned postmortem to parents based on genotypes at nuclear‐encoded microsatellites. The cold tolerance trial was initiated when offspring were 230–251 d old (total length mean ± SD = 182 ± 26 mm). Temperature was decreased progressively from 25°C to 3°C over a 30‐d period and was maintained at an average of 3.1°C until all fish expired. Mortality began when temperature reached 5°C. Cold tolerance of individual fish was quantified based on survival time and a cooling‐degree‐hours (CDH) index. The h 2of cold tolerance was estimated using an animal mixed model and a restricted maximum likelihood algorithm. The h 2 (mean ± SE) estimate was 0.32 ± 0.12 and indicated a significant genetic component to cold tolerance in red drum. Phenotypic correlations between cold tolerance and body weight or length were positive but low (0.18 and 0.23, respectively), suggesting that selective breeding for increased growth rate in red drum would not have a negative impact on cold tolerance in the selected strain.

Genetic e¡ects on cold-stress tolerance were assessed for red drum (Sciaenops ocellatus), an economically important sciaenid ¢sh in the southern USA. Thirty-¢ve families were generated via 'natural'spawning of multiple sets of ¢ve breeders (three dams  two sires) in individual brood tanks. O¡spring from the 35 families were transferred abruptly from an acclimation temperature of $24 1C to 5.7 1C and maintained subsequently at an average temperature of 6.5 1C. O¡spring were assigned a posteriori to individual brood¢sh (dam and sire) based on genotypes at nuclear-encoded microsatellites. Heritability of the survival^time probability function was estimated using a proportional hazard approach and an animaladditive model. The estimated heritability was 0.20 (95% CI: 0.07^0.40), indicating a signi¢cant genetic component to acute cold-stress tolerance in red drum.