The influence of a Hawaiian seamount on mesopelagic micronekton

Abstract

The distribution of mesopelagic micronekton (small fishes, crustaceans, and cephalopods) is not uniform throughout the oceans. Seamounts are a feature that may influence the abundance, biomass, diversity, and taxonomic composition of a community of mesopelagic micronekton by introducing a hard substrate and benthic predators into a realm normally devoid of these. Cross Seamount, located roughly 295 km south of the island of Oahu, Hawaii, has a summit that is 330 m below sea surface and has a diameter of approximately 8 km. Using a large, modified Cobb trawl, samples were taken both directly over and away from the summit of Cross Seamount to sample the deep scattering layer during the day and the shallow scattering layer during the night. Trawls were conducted during two cruises in the spring of 2005 and 2007. All organisms collected were identified to the lowest taxonomic level possible resulting in a description of the local assemblage of mesopelagic micronekton over and around Cross Seamount. Results indicate that there is a significant decrease in total abundance of organisms and an absence of certain diel vertically migrating taxa directly over the summit as opposed to away. While predation might be partly the cause, the taxa that are absent from the summit all have daytime depths that are deeper than the depth of the summit indicating that avoidance may be a major reason for the low abundance. The overall taxonomic composition of the community over the summit is dominated numerically by epipelagic juvenile fishes and stomatopod larvae. This is in opposition to that found away from the summit where the community is dominated numerically by mesopelagic fishes, mostly myctophid fishes, with the epipelagic juvenile fishes and stomatopod larvae contributing little to the overall taxonomic composition. The community over the summit also contains two species that appear to be found in higher abundance over the summit as opposed to away and may be considered as seamount-associated species. These are a cranchiid squid, Liocranchia reinhardti, and a myctophid fish, Benthosema fibulatum. This seamount is known to impact the mesopelagic micronekton community and tuna community, but the mechanisms behind these impacts are largely unknown at this time.