Genetic Effective Size Is Three Orders of Magnitude Smaller Than Adult Census Size in an Abundant, Estuarine-Dependent Marine Fish (Sciaenops ocellatus)

Corresponding author: Thomas F. Turner, University of New Mexico, Albuquerque, NM 87131., turnert{at}unm.edu (E-mail)

Communicating editor: M. K. UYENOYAMA

Using eight microsatellite loci and a variety of analytical methods, we estimated genetic effective size (N_e) of an abundant and long-lived marine fish species, the red drum (Sciaenops ocellatus), in the northern Gulf of Mexico (Gulf). The ratio N_e/N, where short-term variance N_e was estimated via the temporal method from shifts in allele-frequency data over four cohorts and where N reflected a current estimate of adult census size in the northern Gulf, was 0.001. In an idealized population, this ratio should approximate unity. The extraordinarily low value of N_e/N appears to arise from high variance in individual reproductive success and perhaps more importantly from variance in productivity of critical spawning and nursery habitats located in spatially discrete bays and estuaries throughout the northern Gulf. An estimate of N_e based on a coalescent approach, which measures long-term, inbreeding effective size, was four orders of magnitude lower than the estimate of current census size, suggesting that factors presently driving N_e/N to low values among red drum in the northern Gulf may have operated similarly in the past. Models that predict N_e/N exclusively from demographic and life-history features will seriously overestimate N_e if variance in reproductive success and variance in productivity among spatially discrete demes is underestimated. Our results indicate that these variances, especially variance in productivity among demes, must be large for red drum. Moreover, our study indicates that vertebrate populations with enormous adult census numbers may still be at risk relative to decline and extinction from genetic factors.

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