Seyoum et al : Genetic population structure in Saaenops ocellatus 



135 



Red drum is a pelagic marine fish with these demo- 

 graphic and Hfe history characteristics. However, 

 although some genetic exchange may occur between 

 red drum from distant locations, populations differ 

 from the expectation of genetic homogeneity. Hierar- 

 chical analysis of the structuring of genetic variance 

 supports the hypothesis that the species is weakly 

 subdivided between the Atlantic Ocean and the Gulf 

 of Mexico. The existence of the subdivision is also 

 supported by cluster analysis of sequence divergence 

 values. The geographic coverage of our samples, par- 

 ticularly the inclusion of samples from Florida's east- 

 ern seaboard, allowed us to infer that the genetic 

 break separating these two populations occurs in 

 south Florida. Another genetic discontinuity appar- 

 ently occurs in Gulf waters off northwest Florida. 



The Atlantic-Gulf subdivision in red drum may 

 result from a combination of extrinsic and behavioral 

 factors. Gold et al. ( 1993, 1994) summarized a number 

 of potentially important oceanographic and geogi-aphic 

 factors. Because the southernmost portion of the east 

 Florida shelf is extremely nairow and provides little of 

 the neritic habitat (Jones et al., 1985) generally occu- 

 pied by adult red drum, it may represent a significant 

 barrier to adult migration. Biotic factors largely pre- 

 clude large-scale passive dispersal of eggs and larvae 

 (Peters and McMichael, 1987), and widespread disper- 

 sal at the juvenile stage is rare (Murphy and Taylor^). 

 If partitioning of genetic variation in red drum results 



from adult migration or vagrant movement between 

 spawning locations, it is not evident from studies of 

 fish movement. Although very few tagged adult red 

 drum have been recaptured after spending significant 

 periods at large, the evidence suggests that some are 

 highly mobile and may disperse to distances of up to 

 320 km (Woodward and Nicholson, 1997; Crabtree^). 

 However, movement of red drum between the Atlan- 

 tic and Gulf regions has not been documented. In 

 apparent contrast to the mark-recapture data, recent 

 ultrasonic tracking studies of adults provide limited 

 evidence for spawning fidelity to certain Atlantic estu- 

 aries over two-to-three-year periods, and potentially 

 longer (Nicholson and Jordan'"). Overall, the available 

 movement data for adult red drum are not adequate 

 to draw conclusions relating to regional recruitment 

 processes and patterns of dispersal. Nevertheless, it is 

 clear from the genetic data that reproductive exchange 

 between spawning populations in the Atlantic and 

 Gulf regions is limited. 



Because red drum from Mosquito Lagoon were 

 used to produce hatchery populations for at least 

 two stock enhancement programs along the Atlan- 

 tic seaboard, it was important to determine whether 

 the spawning aggregation within that system repre- 

 sented a self-contained, genetically divergent popu- 



*• Murphy, M. D., and R. G. Taylor 1989. Tag/recapture and 

 age validation of red drum in Florida. Final report, NOAA 

 grant NA86-WC-H-06136, National Marine Fisheries Service, 

 Pascagoula, MS, 27 p. 



■'Crabtree, R. 1997. Florida Marme Research Institute. 



Department of Environmental Protection, St. Petersburg, 



FL. Unpublished data. 

 '"Nicholson, N., and S. R. Jordan. 1994. Biotelemetry study 



of red drum in Georgia, November 1989^June 1993. Coastal 



Resources Division, Georgia Department of Natural Resources, 



Brunswick, GA, 65 p. 



