Abstract
The nighttime mid-water biomass occurring near six banks in the Northwestern Hawaiian Islands was investigated using 38- and 120-kHz EK60 echosounders. Locations investigated included: French Frigate Shoals, Maro Reef, Lisianksi Island/Neva Shoals, Pearl and Hermes Atoll, Kure Atoll and Midway Atoll. Surveys were designed to sample transect lines parallel and normal to shore between 10 p.m. and 7 a.m. and once during daylight hours. A diurnal trend was found in the occurrence of mid-water sound scattering organisms at all six locations. Communities of organisms accumulate at night on the edges of each island between the 20- and 100-fathom isobaths. The highest densities of organisms restrict their horizontal movements to depths of 20 fathoms or deeper, but increases in biomass were also observed at shallower depths. The northern islands of Kure, Midway, and Pearl and Hermes Atolls exhibited patchier distributions than the southern islands. The composition of the biomass is presently unclear but resembles the mesopelagic boundary community found near the Main Hawaiian Islands. Simultaneous observations with the TOAD camera system revealed clouds of zooplankton mixed with small fish and other micronekton. The nightly influx of these organisms is likely a significant, though poorly understood, component of these islands ecosystems.
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