Variation in the structure of the deep-sea fish assemblages on Necker Island, Northwestern Hawaiian Islands

Abstract Evidence is accumulating that within any given seamount, the abundance and diversity of fauna may vary strongly with environmental variability. Necker Island, located in the Northwestern Hawaiian Islands, has not been subject to commercial trawl fisheries and is currently protected from fishing activities as part of the Papahānaumokuākea Marine National Monument. The relatively pristine nature of this seamount makes it an excellent location to assess the abundance and diversity of the deep-sea fish fauna of a seamount and their variability relative to environmental parameters, with minimal confounding of natural patterns by human impacts. Using 51,988 AUV photos that showed 18,478 fishes, 92 species were identified from three study sites on different slopes of Necker Island at depths of 200 to 700 m. The deep-sea fish assemblages were dominated by Stomiiformes, Gadiformes, Myctophiformes, Aulopiformes, and Perciformes. From 250-700 m, relative abundance of fishes was significantly different among study sites, with the NE side having the lowest abundance. Species richness and rarefaction estimates of the expected species richness showed significant differences by study site, depth, and their interaction. The NE slope of the island had the lowest estimated richness. By depth, species richness showed two peaks, one at 350 m and the highest at 500 m, in which diversity was also very high with low dominance. The highest values of dominance were observed at 250 and 700 m. Community structure was significantly different by study site, depth, and their interaction. Variation by depth was observed in the NMDS plot, with three assemblages characterized by different dominant species. Fish assemblage structure was correlated with dissolved oxygen, salinity, percentage of sand, rugosity, slope, POC, and current vectors u and v. These results support significant variability in deep-sea fish abundance, diversity, and assemblage structure on seamounts over relatively narrow depth ranges and among sides of a seamount at the same depth. This variability should be considered in future ecological studies of seamounts as well as in the management and conservation of seamounts.