Distribution and community structure of deep-sea demersal fish assemblages across the central Pacific Ocean using ROV data

Abstract

Four Marine National Monuments exist within the Pacific Islands region including the Papahānaumokuākea Marine National Monument, the Marianas Trench, Pacific Remote Islands, and Rose Atoll Marine National Monuments. Together these make up an area of 3,063,223 square kilometers; however significant knowledge gaps remain regarding the distribution and community composition of fishes, especially deeper than 500 meters. With the additional threats of global change, fishing, and deep-sea mining, there is a need to characterize these communities for conservation and management purposes. The remoteness of deep-sea habitats has resulted in deep-sea fishes being poorly sampled globally, with most descriptions of demersal fish communities focused along temperate continental margins. Demersal deep-sea fish communities from islands and seamounts are poorly described, even in the Hawaiian archipelago. Knowledge across all depths, in similar settings, is even more sparse for other archipelagos in the central Pacific. However, recent remotely operated vehicle (ROV) explorations of the central Pacific and archived video from submersible dives conducted by the Hawai`i Undersea Research Laboratory (HURL) provide an opportunity to explore the structure of these communities. Here we describe demersal fish communities from archipelagos across the tropical Pacific, including in multiple Marine National Monuments, and examine the relationship of the assemblages to depth and environmental conditions. We used data collected from 227 dives conducted by underwater vehicles resulting in the identification of 24,837 individuals belonging to 89 families and 175 genera across regions for depths ranging from 250-3000 m. The most frequently occurring genera between 250-500 m were Epigonus, Setarches, Polymixia, and Antigonia, between 500- 1000 m were Chlorophthalmus, Aldrovandia, and Neocyttus, and between 1000-3000 m were Synaphobranchus, Kumba, Halosaurus, Ilyophis, and Ipnops. There are strong changes in the fish communities with depth, and communities become more similar between regions with greater depth. Depth explained the most variance in community composition followed by water mass distributions, seafloor particulate organic carbon flux (a food supply proxy), concentrations of dissolved oxygen, and salinity. The Line Islands and Tokelau Ridge had the highest values of seafloor particulate organic carbon flux for all depth zones investigated (250-3000 m) and the highest abundance of fishes at 250-500 m and 500-1000 m, respectively. Accumulation curves indicated that diversity at the genus level within all regions and depth bins (except 1000-2000 m and 2000-3000 m) had not been reached with the existing sampling effort. However, when combining samples from all regions, diversity generally appeared to decrease with depth. Overall, this study demonstrates that there are significant regional differences in the composition of the deep-sea fish fauna as well as differences across depth. Such distribution patterns suggest that existing Marine National Monuments are not replicates of diversity, but complementary components of the regional diversity. The effectiveness of the Monuments in protecting deep-sea fish communities will depend on the spatial distribution and depths of human-caused disturbances such as climate change and deep-sea mining, therefore we suggest further sampling in the regions to fully characterize the communities and better define boundaries and gradients of faunal change.