Habitat associations and assemblage structure of demersal deep-sea fishes on the eastern Flemish Cap and Orphan Seamount

Advancements in remote-sampling and optical technologies have considerably improved our understanding of fish-habitat relationships and assemblage structure in the deep ocean through direct observations. The composition and complexity of seafloor habitats can strongly influence species diversity and distributions, but the relative importance of different microhabitats - both abiotic and biotic - is poorly understood. We examined differences in fish species composition and relative abundance between different physical (sediment type and boulder density) and biological (coral and sponge densities) habitat categories through in-situ observations from remotely-operated vehicle surveys of unglaciated deep-sea features off the coast of Newfoundland, Canada. Fish-habitat relationships were observed across 61 km of seafloor and spanning depths of 875-3003 m at five dive locations, with additional quantification of fish behaviour and assemblage patterns. Distinct assemblages occurred among depth zones (ANOSIM, Global R = 0.47, p = 0.001), although global tests of overall biological and physical habitats were not significant. Significant pairwise differences in assemblages were only observed in more complex physical habitats (e.g. boulder fields, and outcrops) and complex biological habitats (e.g. dense corals) compared to less complex areas of fine-grain sediment or locations with no or few corals and sponges present. Our results suggest overall structural complexity of physical and biogenic habitat features may be particularly important to some deep-sea fishes. Until further details of these relationships can be explored, conservation efforts should strive to protect a wide-range of microhabitats to preserve valuable fish habitats in the deep-ocean environment.