Dynamics of the Hawaiʻian mesopelagic boundary community and their effects on predator foraging behavior

Abstract

In the Hawaiian Islands, a distinct, resident community of micronekton is distributed over the underwater slopes of the islands. Despite the importance of the mesopelagic boundary community to both nearshore and oceanic ecosystems, it has not been well studied. This work investigated the spatial and temporal dynamics of the mesopelagic boundary community and their impact the foraging behavior of its predators. Several active acoustic (sonar) techniques were utilized, complimented by trawling and optical techniques. Target strength measurements of live animals from the boundary community facilitated the use of echo-energy integration techniques and allowed conversion of acoustic backscattering measures to estimates of caloric content. Using these measures, ship-based echosounder surveys showed that mesopelagic animals are heterogeneously distributed, in both time and space. The Hawaiian mesopelagic boundary community fits the hierarchical patch structure model with patches within patches that are part of a larger scale matrix of patches. Surveys also revealed that the boundary layer undergoes diel horizontal migrations in addition to its vertical migrations. At night organisms were found within 1 km of shore, in waters much shallower than their daytime habitat. Because of this horizontal movement, the highest densities of mesopelagic animals, reaching up to 1800 animals/m^3, were observed nearshore. A series of bottom-mounted, sonar moorings measured vertical migration rates of 0-1.7 m/min and average horizontal rates of 1.67 km/hr. High levels of biomass were observed moving rapidly, over a great distance, into shallow waters very close to shore, providing insight into the significant link the boundary community provides between nearshore and oceanic systems. To understand how temporal and spatial heterogeneity of prey affected a pelagic predator, an echosounder was used to simultaneously measure the abundance of spinner dolphins and the boundary community. Spinner dolphins followed the migration patterns of their prey both horizontally and vertically. Overlap was observed between spinner dolphins and their prey from several minutes to an entire night, at spatial scales of 20 meters to several kilometers. An understanding of the spatial and temporal dynamics of the mesopelagic boundary community permitted the accurate prediction of predator movement patterns and an increased understanding of their behavior.