Consistent effects of the Loop Current and associated mesoscale eddies on the vertical distributions of deep-pelagic fishes in a subtropical ocean basin

Mesoscale eddies are important sources of environmental heterogeneity for oceanic fauna, with ecological effects from primary producers to top predators. In the Gulf of Mexico basin (the Gulf), Loop Current Origin Water (LCOW) brings warm, low-productivity waters into the Gulf from the Caribbean and can give rise to associated eddies. Here, we present an analysis of the effects of LCOW on deep-living myctophid and stomiiform fishes in the Gulf from 0 – 1500 m, and show effects of LCOW on the community structure, vertical distributions, and migration rates throughout this depth range. Where LCOW had an effect, it was associated with fauna moving to deeper daytime depths, and/or inhibition of diel vertical migration into the epipelagial at night. These effects were clearest in upper mesopelagic depths at night, which is where the greatest differences in assemblage composition and species diversity were observed. Some influence of water mass was still detected into bathypelagic depths, with the abundant bathypelagic species Cyclothone obscura moving deeper under LCOW to 1500 m, suggesting a deeper-reaching effect of LCOW than has previously been recognized. Overall, our results suggest that a driver of meso- and bathypelagic fish distribution is individual responses to changing local conditions on the fine scale that then governs assemblage structure on the mesoscale. Our study highlights the importance for future studies to consider taxon-specific and individualistic (e.g., trait-based) approaches to understanding the drivers of assemblage structure of deep-living pelagic fishes and their predictability in the face of future environmental change, as well as their long-term stability and continued ability to produce ecosystem services as key intermediate trophic groups.