Abstract

The Gulfred snapper, Lutjanus campechanus, is a highly exploited marine fish in the Gulf of Mexico. Currently, management of Gulf red snapper in U.S. waters is based on a single-stock hypothesis. Here in we report results of a genetic survey of adults (1995 year class) and juveniles (1999 year class) sampled offshore from three geographic localities in the northern Gulf: Port Aransas, Texas; Port Fourchon, Louisiana; and Dauphin Island, Alabama. The study employed 19 microsatellites loci and was designed to test the single-stock hypothesis and estímate effective population size in the Gulf. We also assessed a possible genetic impact of shrimptrawling on red snapper populations by comparing genetic variability and relatedness between juveniles sampled randomly and juveniles sampled in single tows of a shrimp trawler.\Significant genetic heterogeneity was observed among localities in the 1995 cohort, with the major differences occurring between red snapper sampled offshore of Port Aransas, Texas, and the other two localities. On another hand, no significant genetic heterogeneity was observed among samples from the 1999 cohort. These findings, along with the overall very low FST value (0.001), suggest considerable genetic mixing within the northern Gulf, consistent with the single-stock hypothesis. A maximum-likelihood estímate of effective population size (Ne) ofred snapper in the northern Gulf based on the temporal method was 7.075 (95 % Cl: 2.933 >50.000). This estímate of Ne is at least two orders of magnitude lower than current estimates of census size (N).\Levels of genetic variability (gene diversity and allelic richness) between reference (control) and bycatch juveniles were equivalent. Estimates of the variance in genetic relatedness in both reference and bycatch samples were non-significant; however, this variance estímate was positive (0.001) for one of the bycatch samples and close to significance (P = 0.1) which might suggest the presence of related individuals in this sample. Further studies employing larger sample size and additional loci are necessary to assess possible genetic impacts of shrimp trawling on red snapper.

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