62 



Fishery Bulletin 92(1), 1994 



in (presumed) nuclear-coding genes is not surpris- 

 ing, given that mtDNA is expected to be at least four 

 times more sensitive to population substructuring 

 (Birky et al., 1983; Templeton, 1987). Because pre- 

 vious studies (Gold et al., 1993, in press) found no 

 evidence of genetic heterogeneity among red drum 

 from eleven estuaries or bays in the northern Gulf 

 or among red drum from five estuaries or bays along 

 the Carolina coast, red drum from Mosquito Lagoon 

 are unusual in representing a genetically distinct 

 red drum subpopulation existing within a single bay 

 or estuary. 



Campton (1992) 4 examined red 

 drum from Mosquito Lagoon for 

 allelic variation at several 

 allozyme loci and found genetic 

 homogeneity among red drum 

 from Mosquito Lagoon, the north- 

 ern Gulf, and the Carolina coast. 

 He suggested that our initial 

 study (Bohlmeyer and Gold, 1991) 

 of allozyme variation among 

 northern Gulf and Carolina coast 

 red drum did not account for tem- 

 poral variation among samples 

 within localities. Our subsequent 

 studies (and this one), however, 

 have included temporal sampling 

 of variation in both allozymes and 

 mtDNA and have demonstrated 

 that weak (but significant) genetic 

 heterogeneity exists (Gold et al., 

 1993, in press). Sampling error 

 associated with specimen procure- 

 ment in varying time and space 

 may account for the different results ob- 

 tained in Campton's (1992) 4 study and 

 this one. However, in Campton's (1992) 4 

 study, the total G-statistic, obtained by 

 summing individual G-values and their 

 associated degrees of freedom, was signifi- 

 cant at the 0.01 level. This suggests the 

 existence of spatial or temporal genetic 

 heterogeneity, or both, among the locali- 

 ties sampled. 



Genetic differentiation of red drum in 

 Mosquito Lagoon is consistent with the 

 hypothesis that red drum in Mosquito 

 Lagoon represent a self-contained, at 

 least partially isolated subpopulation. 

 Three lines of evidence support this hy- 

 pothesis. First, genetic differences be- 

 tween red drum from Mosquito Lagoon 

 and red drum sampled elsewhere involve 

 frequencies of alleles at two or three pu- 

 tative nuclear-gene loci and frequencies of at least 

 six mtDNA haplotypes. Differentiation of several, 

 presumably independent and selectively-neutral, 

 genetic markers suggests a genome-wide effect re- 

 lated to at least partial isolation and reduced gene 

 flow (Wright, 1978; Hartl and Clark, 1989). Second, 

 inferred nuclear-gene alleles present in low fre- 

 quency in red drum sampled outside of Mosquito 



Campton, D. E. 1992. Gene flow estimation and population struc- 

 ture of red drum iSaaenops ocellatus) in Florida. Final Rep. Coop. 

 Agrmt. No. 14-16-009-1522, U.S. Fish & Wildl. Serv, Natl. Fish. 

 Res. Cntr., 7920 N.W. 71st St., Gainesville, FL. 



