Diel migration dynamics of an island-associated sound-scattering layer

Abstract

The Hawaiian mesopelagic boundary community, consisting of island-associated, midwater sound-scattering layers, undergoes diel migrations with both vertical and horizontal components. To understand the dynamics of the community's migration at fine temporal scales, we utilized a bottom-mounted, 200-kHz active-acoustic mooring that transmitted 10 signals every 15 min, from dusk until dawn for 5 days. Five moorings were deployed 1.0–3.0 km from the leeward coast of Oahu in 0.5 km intervals. Two layers within the boundary community were observed to undergo simultaneous diel vertical and horizontal migration. The shallow layer came within 10 m of the surface and 1 km of the shoreline. The deeper layer remained 90 m from the surface and 2.5 km of the shoreline. Vertical migration rates were measured at 0–1.7 m min –1 while the horizontal rate averaged 1.7 km h –1, swamping the vertical movement. The turning point of the migration pattern was observed 45 min before the midpoint between sunset and sunrise. Until the migration's turning point, scattering strength increased relatively constantly as the animals migrated towards shore, with the highest scattering densities found in the shallowest areas at midnight. Total scattering strength measured at the leading and trailing edge of the layer support the hypothesis that increased animal densities nearshore are related to packing as mesopelagic animals avoid the surface and the bottom. We observed high levels of biomass moving rapidly, over a great distance, into shallow waters very close to shore providing insight into the significant link the mesopelagic boundary community provides between nearshore and oceanic systems.