Extreme diel horizontal migrations by a tropical nearshore resident micronekton community

The mesopelagic boundary community around the Hawaiian Islands is a land-associated, sound-scattering layer that undergoes diel migrations with both a vertical and horizontal component. To understand horizontal migration, it is important to understand the full cycle of migration and to determine which animals undertake these movements and which do not. We developed a video camera system to quan- titatively examine the numerical density, size, and tax- onomic composition of micronekton. We combined the camera system with vessel-mounted echosounders to document the full migration range of micronekton and describe the changes in composition and density throughout their diel migration. Onshore horizontal movement is limited to waters deeper than 23 m. Off- shore, horizontal migration seems to cease between 9 and 11 km from the shoreline. Some animals in the mesopelagic boundary community undertake horizon- tal migrations of at least 11 km roundtrip each night. This impressive migration indicates that there must be a substantial benefit to horizontal migration. The migrating animals split into multiple, distinct layers at night with differences in micronekton density, compo- sition, and size. Myctophid fishes are the most abun- dant component of the horizontally migrating layers. Large myctophids are the first to migrate into and last to leave the shallow water areas. During the day, the layers merge into a single layer at 400 m depth or deeper. At night, there is evidence of strong partition- ing of habitat by these animals in space and time, which—along with the high densities of animals— suggests that competition is important in driving the behavior of, and structuring, the community.