Gulf Menhaden Brevoortia patronus is one of the most intensively fished organisms in the Gulf of Mexico. Fishery managers and industry operators have historically worked towards a sustainable fishery and have cooperated on stock assessments to estimate feasible reference points for management. These stock assessments are necessarily rooted in a strong definition of the fishery stock, including the estimation of the number of populations that are exploited. Here, genetic population structure and variability were examined in B. patronus, using genetic markers specifically designed for the species. We observed genetic variability that indicates a relatively high effective population size for a marine finfish (Ne ≥ 1,200), and two analytical approaches implied a single genetic population of B. patronus. We compared the latter finding with the population structure in the closely related Yellowfin Menhaden B. smithi, for which two distinct populations from Florida were identified using the same genetic loci (Fst = 0.015, P = 0.027). These contrasting patterns of population structure between sympatric congeneric species are likely driven by differences in distribution and census size and may relate to factors that originally drove speciation in North American Brevoortia species. The finding of a single Gulf-wide population of B. patronus suggests that there is an extensive migration throughout the species range and supports the notion of a single genetic stock.
