Fine-scale diel migration dynamics of an island-associated sound-scattering layer

Abstract

The Hawaiian mesopelagic boundary community, an island-associated, midwater sound-scattering layer, undergoes diel vertical and horizontal migrations. To understand the dynamics of the community’s migration at small temporal scales and large spatial scales, five bottom-mounted, 200-kHz active-acoustic mooring that transmitted ten signals every 15 min, from dusk until dawn for 5 days. Two layers within the boundary community were observed to undergo simultaneous diel vertical and horizontal migration. Vertical migration rates were measured at 0–1.7 m/min, while the horizontal rate averaged 1.67 km/h, swamping the vertical movement. The vertex of the migration pattern was observed 45 min before the midpoint between sunset and sunrise. Until the vertex, animal density increased relatively constantly as the animals migrated towards shore, with the highest animal densities found in the shallowest areas at midnight. Animal abundance estimates at the leading and trailing edge of the layer support the hypothesis that increased animal densities near shore 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 near-shore and oceanic systems.