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Variation in mitochondrial DNA (mtDNA) was examined among 707 red snapper Lutjanus campechanus representing 16 samples taken during 3 years from localities in the northern and western Gulf of Mexico. Ninety-two composite mtDNA haplotypes were revealed by 13 restriction enzymes (representing 93 inferred restriction sites). Significant heterogeneity (P = 0.042) in mtDNA haplotype frequencies was detected among the 16 samples; however, homogeneity tests of mtDNA haplotype frequencies between or among samples taken in different years at the same locality and among samples at different localities within the same year, were not significant. No phylogeographic structure of haplotypes was evident, nor were rare haplotypes clustered geographically. Spatial autocorrelations did not differ significantly from those expected when no correlation exists. These findings are consistent with the hypothesis that a single breeding population of red snapper inhabits the northern Gulf of Mexico. Intrapopulational mtDNA diversities, however, differed significantly among samples, suggesting that red snapper in the Gulf may not be drawn from a single population. Red snapper in the Gulf of Mexico may possibly include recently derived populations for which there has been insufficient time for accumulation of significant differences in mtDNA haplotype frequencies.

Genetic studies of population or 'stock' structure in exploited marine fishes typically are designed to determine whether geographic boundaries useful for conservation and management planning are identifiable. Implicit in many such studies is the notion that subpopulations or stocks, if they exist, have fixed territories with little or no gene exchange between them. Herein, we review our long-term genetic studies of red drum (Sciaenops ocellatus), an estuarine-dependent sciaenid fish in the Gulf of Mexico and western Atlantic Ocean. Significant differences in frequencies of mitochondrial DNA haplotypes and of alleles at nuclear-encoded microsatellites occur among red drum sampled across the northern Gulf of Mexico. The spatial distribution of the genetic variation, however, follows a pattern of isolation-by-distance consistent with the hypothesis that gene flow occurs among subpopulations and is an inverse (and continuous) function of geographic distance. However, successful reproduction and recruitment of red drum depend on estuarine habitats that have geographically discrete boundaries. We hypothesize that population structure in red drum follows a modified one-dimensional, linear stepping-stone model where gene exchange occurs primarily (but not exclusively) between adjacent bays and estuaries distributed linearly along the coastline. Gene flow does occur among estuaries that are not adjacent but probabilities of gene exchange decrease as a function of geographic distance. Implications of our hypothesis are discussed in terms of inferences drawn from patterns of isolation-by-distance and relative to conservation and management of estuarine-dependent species like red drum. Based on estimates of the ratio of genetic effective population size and census size in red drum, observed patterns of gene flow in red drum may play a significant role in recruitment.

A simple procedure for long‐term storage and preparation offish cells for flow cytometric analysis of genome size or DNA content is described. The method has been used on a broad variety of cell or tissue types, including blood, soft organs, and muscle, and on fish ranging in size from 10 to 15 mm to over 500 mm in total length. No differences in estimated genome sizes or coefficients of variation have been observed with storage times of up to 8 months.