Population Structure of Sciaenops ocellatus and Cynoscion nebulosus (Pisces: Sciaenidae): Biochemical Variation, Genetic Subdivision and Dispersal

Abstract

Genetic variation was examined in red drum (Sciaenops ocellatus) and spotted seatrout (Cynoscion nebulosus), sciaenid marine fishes that differ in life-history components such as age and size at maturity, longevity, and migratory behavior. Survey of 22 enzymes and seven structural proteins (40 loci) revealed that measures of genic variability and F-statistics were comparable to those for other sciaenids. Differentiation into subpopulations was only weakly evident for either red drum or spotted seatrout (F,, = 0.019 and 0.032). Spotted seatrout had lower heterozygosity (<1%), spatial clumping of rare alleles, and a regional cluster of localities in the eastern Gulf of Mexico. Red drum had higher heterozygosity (3%), a mosaic geographic pattern for protein variation and for a meristic polymorphism (number of tailspots), and one slightly divergent population on Florida's Atlantic Coast. As suggested from life-history components and supported by allelic dispersion, regional differentiation, and autocorrelation analysis, the less mobile and more rapidly-maturing species, spotted seatrout, displays isolation-by-distance. Less divergence of red drum in bays is contrary to expectation based on relative inshore abundance of the two species; adult reproduction by the larger and longer-lived species, red drum, produces an offshore effective population size which may equal or exceed colonies of spotted seatrout in bays. Annual exchange of spotted seatrout between bays, derived from a one-dimensional stepping stone model and estimated to be 5% of the breeding population, is compatible with previous movement data. Red drum may show Gulf-wide panmixia, but too little is known about the migratory adults to propose an appropriate population model.