Otolith stable isotope micro-sampling to discriminate poorly studied stocks: Crevalle Jack in the eastern gulf of Mexico

Abstract

Developing conservation and management strategies for species with complex life histories, broad spatial distributions, and long lifespans is notoriously difficult. Too often managers cannot identify critical habitats nor vulnerable life stages because of the sheer scale of migration or uncertainty about connectivity among populations. Advancements in otolith stable isotope analysis, and specifically sampling of discrete otolith layers, have provided opportunities to assess lifetime migrations and connectivity without extensive, long-term field sampling. Here, we compared carbon and oxygen stable isotope values in discrete otolith layers for Crevalle Jack (an unregulated and data-poor species) captured in two isotopically distinct regions (Alabama and the Florida Keys). Our goal was to address vital questions about how broad-scale movement patterns through ontogeny differ between the two regions and whether connectivity occurs throughout the life history. Our results revealed that Crevalle Jack appear to inhabit inshore nursery areas at age-0, before migrating to coastal/offshore habitats between age-1 and age-2. Comparisons between fish collected in northeastern and southeastern Gulf of Mexico regions revealed significant differences in the patterns of stable isotope values throughout the life history and in otolith δ13C values. Despite these differences that suggest a separation of the two populations, individual variability was significant, hindering our ability to determine whether fish from the two regions represent separate, self-recruiting stocks. Our research illustrates the potential of otolith stable isotope micro-sampling as a tool for examining broad-scale movement and migration patterns of fishes, and informing future research and management.