Diel variation of zooplankton distributions in Hawaiian waters favors horizontal diel migration by midwater micronekton

Micronekton in deep-scattering layers around the Hawaiian Islands undergo diel migrations with both vertical and horizontal components. We sought to determine whether resource availability provides an adaptive explanation for this migration. We simultaneously measured the spatio-temporal patterns of micronekton, using acoustics and imaging optics, and of their potential zooplankton prey, using net tows, acoustics, and optics. Zooplankton biomass, density, and total abundance were higher at night than prior to sunset at nearshore sites, whereas relatively little diel variation was observed offshore. All measures of zooplankton availability were 5 to 6 times higher nearshore than offshore during nighttime hours when migrating micronekton species were nearshore. There was no significant nearshore-offshore gradient in zooplankton prior to sunset, leading to 2 possible explanations for the day-night patterns in zooplankton: benthic emergence and vertical migration coupled with horizontal motion. Analysis of taxonomic patterns from net tows did not support the benthic emergence hypothesis. All 3 zooplankton assessment techniques supported the conclusion that zooplankton distribution could favor horizontal migration by micronekton given the pressures for micronketon to be in deep water during daylight to avoid predators. Recently published work has shown that small animals (2 to 10 cm in length) in scattering layers comprised of micronekton travel distances of at least 11 km roundtrip each night, often against currents, to obtain these increased food resources. The length and likely cost of the journey provides some insight about the importance of the potential feeding gains.