In the Hawaiian Islands, a distinct, resident community of micronekton is distributed along a narrow band where the upper underwater slopes of the islands meet the oceanic mesopelagic environment. The animals in this midwater sound-scattering layer serve as an important food resource for many animals while being important planktivores. The objectives of this work were to investigate the spatial and temporal dynamics of the mesopelagic boundary community and their impact on the foraging behavior of its predators. To accomplish these objectives, a variety of sonar techniques were utilized, complimented with trawling and optical techniques.

The ex-situ acoustic scattering characteristics of live myctophid fishes, and midwater shrimp and squid were measured as a function of animal size, tilt and roll angle, and biomass measures to permit acoustic assessment of the boundary layer. Target strength measures facilitated the use of echo-energy integration techniques and allowed conversion of acoustic backscattering measures to estimates of caloric content.

Results from echosounder surveys off the leeward coasts of three Hawaiian Islands showed that mesopelagic animals were heterogeneously distributed, in time and space. The Hawaiian mesopelagic boundary community fits the hierarchical model of patch structure with patches within patches that are part of a larger scale matrix of patches. Differences in the distribution patterns of the boundary layer were related to animal density, suggesting biological mechanisms are important in structuring the layer. I found that the boundary layer undergoes diel horizontal migrations in addition to its vertical migrations. At night organisms were within 1 km of shore, in waters much shallower than their daytime habitat. The highest densities of mesopelagic animals, 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 mesopelagic boundary community provides between nearshore and oceanic systems.

To understand how temporal and spatial heterogeneity of prey affected the movement patterns and behavior of a pelagic predator, an echosounder was used to simultaneously measure the abundance of spinner dolphins and the boundary community. Spinner dolphin abundance closely matched the abundance patterns in the boundary community both horizontally and vertically. Spinner dolphin abundance patterns showed that they follow the diel horizontal migration of their prey, rather than feeding offshore the entire night as previously believed. Spinner dolphins also followed the vertical migrations of their prey and exploited the vertical areas within the layer with the highest density. Characteristics of prey patches containing dolphins indicate that large groups of dolphins alter patches through cooperative foraging. 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, suggesting that the collection of synoptic data may alter previous conclusions about pelagic predator-prey interactions. 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.
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