Biogeophysical Influence Of Large-Scale Bathymetric Habitat Types On Mesophotic And Upper Bathyal Demersal Fish Assemblages: A Hawaiian Case Study

Seamounts, pinnacles, and crests are abrupt seafloor features that modify physical processes and ecological patterns. Fishers often target these local bathymetric highs, which can have high catch-per-unit-effort. Increases in the abundance of target species has been qualitatively noted around these features and promontories, however, a quantitative evaluation of local highs as preferred habitat for fishes is still lacking. Here, we used an extensive database of fish abundances (N = 2381) from mesophotic and upper bathyal depths (40-300 m) gathered over 8 yr around the Main Hawaiian Islands to evaluate the effects of macro-scale habitat categorized by combining bathymetric position index (BPI) and slope. A numerical model simulating physical ocean processes was used to test the hypothesis that local highs, herein called crests, are distinct and preferred habitat for many species due to their modified flow conditions. We show that crests host a unique, diverse fish assemblage with double the abundance relative to other habitats. Therefore, fishes are concentrated at crests rather than being uniformly distributed along the island shelf at preferred depths. These habitats are characterized by enhanced current amplitudes and convergence zones. Direct correlations between fish abundance, convergence, and current amplitude strongly suggest that flow-enhanced food availability is likely the driver of habitat-related changes in fish assemblage. These results have important implications for conservation and management. We identify a simple and informative method to classify habitat, quantitatively link bathymetry-induced flow alterations to fish abundances, and provide information for refining definitions of essential fish habitat for species that are important commercial targets worldwide.