Deep pelagic ecosystems are some of the largest on Earth but are amongst the least understood. As human impacts on the deep oceans continue to increase, there is an urgent need to understand the processes that influence pelagic fauna, particularly in deep waters. One of the most globally-important taxa are the myctophid fishes, which are a ubiquitous component of the deep-pelagic micronekton and play key roles in the vertical and horizontal transfer of energy between ecosystems. In the present study, we analysed quantitative, depth-stratified trawl data to assess the distributions of the dominant myctophid species in relation to physical and chemical environmental variables in the northern Gulf of Mexico (GoM) during summer 2011. The data were collected through the NOAA-supported Offshore Nekton Sampling and Analysis Program. Strong vertical changes in myctophid distributions occurred over relatively short spatial scales (hundreds of metres), linked to their diel vertical migration behaviours. Correlations with other environmental variables were generally weak, suggesting limited horizontal structuring across the study region (>500 km). These results suggest that other processes such as migration, random dispersal, population growth or interactions between species may be more important in structuring myctophid communities in the northern GoM than local environmental conditions.

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