Spatial Patterns of Distribution of Deep-Sea Fish Assemblages Inhabiting Seamounts in the Papahānaumokuākea Marine National Monument

Seamounts are considered biodiversity hotspots with a range of physical, chemical, and geological parameters that enrich their habitat heterogeneity compared to the surrounding environment. Evidence for the spatial variation of fauna within a given seamount and between seamounts is increasing around the world, but there are still gaps in certain areas and for some taxonomic groups. Deep-sea fish assemblages and their variability on seamounts, for example, are poorly understood. The Northwestern Hawaiian Islands (NWHI) is a seamount chain that includes seamounts protected from fishing activities as part of the Papahānaumokuākea Marine National Monument. These seamounts had not been studied below 200 m depth but are of interest as their relatively unimpacted deep-sea ecosystems as well as because of their great variability in physiographic and oceanographic characteristics. To generate valuable information on the deep-sea fish communities inhabiting seamounts in the NWHI, and their spatial variability within a given seamount and between seamounts, Necker Island, Pioneer Bank, and French Frigate Shoals (FFS) were studied. The fish fauna and their habitat on all seamounts were characterized using AUV photographs and environmental parameters measured with a CTD, extracted from databases, and described from photos. Necker Island was studied from replicate transects at depths of 200 to 700 m, while Pioneer Bank and FFS were studied at 300, 450, and 600 m depths. Significant variability in the abundance, diversity, and assemblage structure of deep-sea fishes was observed on Necker Island across narrow depth ranges (50 m) and between sides at a given depth. Similarly, spatial variability was observed at Pioneer Bank both with depth and over short horizontal distances (31 km) within a given depth zone. Finally, the study of fish assemblages on the three seamounts at three different depths showed significant horizontal variability on larger scales. These studies demonstrate the vertical and horizontal variability of fish assemblages within a single seamount and the horizontal variability between three seamounts in a marine protected area. These results constitute an ecological baseline to increase our understanding of the fish fauna that inhabit seamounts in the NWHI. Evidence of spatial variability of deep-sea fish assemblages on seamounts that are protected from fisheries should be considered for the management and conservation of the Monument and other areas.