Behavior exceeds physical forcing in the diel horizontal migration of the midwater sound-scattering layer in Hawaiian waters

The mesopelagic sound-scattering layer around the Hawaiian Islands undergoes diel vertical migrations. This migration also includes a significant diel horizontal onshore-offshore component. Before this study, the mechanisms underlying the horizontal movement of the mesopelagic layer had not been investigated. We took an interdisciplinary approach to evaluate relationships between the mesopelagic layer and physical oceanographic processes in the water column. The study area extended west of the leeward coast of Oahu, Hawaii. We found that the nightly inshore movement of micronekton occurs very predictably each day; onshore migration brings animals nearshore between 20:00 and 20:13 h and offshore migration commences between 03:30 and 03:45 h during the months of April and May. Simultaneous observations of nearshore physical flow and micronekton distribution indicate that the observed nightly inshore movement of micronekton in the sound-scattering layer is not directly due to physical processes in the nearshore environment. Rather, this nightly inshore movement of micronekton can be attributed primarily to micronekton swimming behavior. This behavior may be cued by changes in sunlight, but that cue is strongly mediated by a factor tied to the lunar phase, potentially moonlight. Horizontal migration velocities exceeded the maximum across-slope physical flow by 5 to 6-fold. Thus, typical across-slope flows likely represent a relatively small additional cost or benefit to onshore-off shore swimming, depending on flow direction. We suggest that energy acquisition via increased prey availability nearshore far exceeds the high energetic cost of onshore and offshore migrations.