136 



Abstract — We assayed allelic varia- 

 tion at 19 nuclear-encoded microsat- 

 ellites among 1622 Gulf red snapper 

 iLutjanus campechanus) sampled 

 from the 1995 and 1997 cohorts at 

 each of three offshore localities in 

 the northern Gulf of Mexico (Gulf). 

 Localities represented western, cen- 

 tral, and eastern subregions within 

 the northern Gulf. Number of alleles 

 per microsatellite per sample ranged 

 from four to 23, and gene diversity 

 ranged from 0.170 to 0.917. Tests of 

 conformity to Hardy-Weinberg equi- 

 librium expectations and of genotypic 

 equilibrium between pairs of micro- 

 satellites were generally nonsignifi- 

 cant following Bonferroni correction. 

 Significant genie or genotypic het- 

 erogeneity (or both) among samples 

 was detected at four microsatellites 

 and over all microsatellites. Levels 

 of divergence among samples were 

 low (Fj,.;. sO.OOl). Pairwise exact tests 

 revealed that six of seven "significant" 

 comparisons involved temporal rather 

 than spatial heterogeneity. Contem- 

 poraneous or variance effective size 

 (N^y) was estimated from the tempo- 

 ral variance in allele frequencies by 

 using a maximum-likelihood method. 

 Estimates of N^,y ranged between 1098 

 and >75,000 and differed significantly 

 among localities; the N^.y estimate for 

 the sample from the northcentral Gulf 

 was >60 times as large as the esti- 

 mates for the other two localities. The 

 differences in variance effective size 

 could reflect differences in number 

 of individuals successfully repro- 

 ducing, differences in patterns and 

 intensity of immigration, or both, and 

 are consistent with the hypothesis, 

 supported by life-history data, that 

 different "demographic stocks" of red 

 snapper are found in the northern 

 Gulf. Estimates of N^,y for red snap- 

 per in the northern Gulf were at least 

 three orders of magnitude lower than 

 current estimates of census size (A^). 

 The ratio of effective to census size 

 {N^,/N) is far below that expected in an 

 ideal population and may reflect high 

 variance in individual reproductive 

 success, high temporal and spatial 

 variance in productivity among subre- 

 gions or a combination of the two. 



Manuscript submitted 17 July 2004 

 to the Scientific Editor's Office. 



Manuscript approved for publication 

 25 July 2005 by the Scientific Editor. 



Fish. Bull. 104:136-148 (2006). 



Population structure and variance effective size 

 of red snapper (Lutjanus campechanus) 

 in the northern Gulf of Mexico* 



Eric Saillant 



John R. Gold 



Center for B)osystematics and Biodiversity 

 Texas A&M University, TAMU-2258 

 College Station, Texas 77843-2258 

 E-mail address (for E Saillant) esaillantiStamu edu 



Red snapper (Lutjanus campechanus) 

 is a highly exploited marine fish found 

 primarily on the continental shelf of 

 the Gulf of Mexico (Hoese and Moore, 

 1977). Red snapper abundance in the 

 northern Gulf of Mexico (hereafter. 

 Gulf) has decreased by almost 90% 

 in the past two decades (Goodyear 

 and Phares') owing to overexploita- 

 tion by commercial and recreational 

 fishermen, high juvenile mortality due 

 to the shrimp-trawl fishery, and habi- 

 tat change (Christman-; Gallaway et 

 al., 1999). An important question for 

 management and conservation of red 

 snapper resources regards delineation 

 of geographic stock structure. Should 

 separate stocks exist, management of 

 the fishery, including assessment and 

 allocation, could be subdivided to avoid 

 subregional over-exploitation and to 

 maintain potentially adaptive genetic 

 variation (Carvalho and Hauser, 1995; 

 Hauser and Ward, 1998). A second 

 important question for management 

 is whether sufficient genetic resources 

 exist to ensure long-term integrity of 

 red snapper stocks. Preliminary esti- 

 mates of the number of red snapper 

 adults in the northern Gulf range from 

 7.8 to 11.7 million (Cowan-'; Porch""), 

 which may indicate a priori that suf- 

 ficient genetic resources are available. 

 However, recent studies in other, com- 

 mercially exploited marine fishes have 

 shown that genetic effective size iN^.) 

 can be three-five orders of magnitude 

 smaller than estimates of census size 

 or N (Hauser et al., 2002; Turner et 

 al., 2002). Briefly, A^,, is defined as the 

 number of individuals in an "ideal" 



population that would experience the 

 same magnitude of genetic drift as the 

 actual population (Hartl and Clark, 

 1989). N^. is an important biological 

 parameter, in part because it reflects 

 the relative effects of genetic drift and 

 selection on nonneutral loci, and in 

 part because it can indicate long-term 

 risk of extinction from genetic factors 

 (Turner et al., 2002). As long-term 

 sustainability requires maintenance of 

 sufficient genetic resources (Allendorf 

 and Waples, 1996), populations (or 

 stocks) with small N, potentially may 



* Contribution 133 from the Center for 

 Biosystematics and Biodiversity, Texas 

 A&M University, College Station, Texas 

 77843-2258. 



1 Goodyear, C. P., and P. Phares. 1990. 

 Status of red snapper stocks of the Gulf 

 of Mexico: report for 1990, 72 p. Na- 

 tional Marine Fisheries Service, South- 

 east Fisheries Centre, Miami Laboratory, 

 CRD 89/90-05, 75 Virginia Beach Drive, 

 Miami, FL 33149-1099. 



- Christman, M. C. 1997. Peer review 

 of red snapper (Lutjanus caii^pechanus) 

 research and management in the Gulf 

 of Mexico: statistics review, 51 p. Of- 

 fice of Science and Technology, National 

 Oceanic and Atmospheric Administra- 

 tion, National Marine Fisheries Service, 

 1315 East-West Highway 9th Floor F/CS, 

 Silver Spring, MD 20910. 



■^ Cowan, J. H. 2004. Personal commun. 

 Department of Oceanography and 

 Coastal Sciences, Coastal Fisheries 

 Institute, Louisiana State University, 

 Baton Rouge, LA 70803. 



■• Porch, C. E. 2004. Personal commun. 

 National Marine Fisheries Center, 

 Southeast Fisheries Science Center, 

 75 Virginia Beach Drive, Miami, FL 

 33149. 



