Pelagic Habitat Partitioning of Late-Larval and Juvenile Tunas in the Oceanic Gulf of Mexico

Tunas are ecologically important in pelagic ecosystems, but due to their high economic value, most species are overfished. Declines in fishery landings of large-bodied tuna species in the Gulf of Mexico (GoM) are expected to increase fishing pressures on unmanaged, small-bodied tuna species, whose life history traits are less known. While predicting spawning stocks and recruitment success typically focuses on estimates of larval abundances, juveniles may provide a better estimate of future adult stock sizes, as they are more likely to survive to adulthood. However, distributional studies on juveniles are rare, leading to a gap in our understanding of tuna ecology. In the present study, tuna early life stages were collected across the GoM from December 2010-September 2011 (NOAA-supported Offshore Nekton Sampling and Analysis Program) and in May and August from 2015-2017 (GOMRI-supported DEEPEND Consortium). The size class examined in this study, representing large larvae and small juveniles, is larger than that of previous larval tuna studies in the GoM. In total, 11 of the 16 scombrid species inhabiting the GoM were collected, with small-bodied tuna species (Euthynnus alletteratus and Thunnus atlanticus) dominating the assemblage. Generalized additive models and distributional plots indicated that early life stages of E. alletteratus were associated with productive continental shelf/slope environments (low salinity, high chlorophyll a concentrations, nearer to shelf break), while T. atlanticus juveniles were associated with oligotrophic habitats (high salinity, low chlorophyll a concentrations, further from shelf break). These results demonstrate that over a broad spatiotemporal domain, large larvae and juvenile tunas partition pelagic habitat on the mesoscale in addition to the temporal partitioning of adult spawning. These factors are important for spatially and temporally explicit modeling aimed at predicting tuna stock sizes.