Depth and structure as environmental drivers of fish communities across a shallow to mesophotic gradient in the northern US Virgin Islands

The factors that drive fish communities on coral reefs are varied, complex, and often location specific. In the northern United States Virgin Islands, the large areas of mesophotic coral ecosystems across an insular shelf provide a multitude of gradients along which fish communities form. Using a stratified-random visual fish census, we collected comprehensive data on fish communities across a seascape scale. Fish density and diversity showed responses to depth and hard relief, with these factors occasionally interacting in ways that are species specific. Analysis of trophic groups showed that densities of all groups were positively associated with relief, but while herbivores and invertivores decreased with depth, planktivores and piscivores increased, and piscivores additionally showed an interaction. Some commercially important species occurred more frequently and in larger sizes on deeper reefs, suggesting that mesophotic reefs shelter more fisheries resources, although they are less commonly sampled in visual census programs. Understanding how fish communities change across the seascape informs patterns of ecosystem function, such as identifying areas of resilience and vulnerability. This dataset is a valuable contribution to more completely understanding the ecology and conservation of fishes in the Western Atlantic.