AGU Ocean Science Press Release and Images

January 2004

FOR IMMEDIATE RELEASE

Using Acoustics to Study the Ecology of Hawaii's Mesopelagic Zone

Portland - Researchers at the University of Hawaii are using acoustic techniques to study the ecology of the midwater zone above the slopes of the Hawaiian Islands.

The research involves monitoring the movement and distribution patters for both midwater fish and shrimp and the resulting distribution pattern for predator fish and dolphins. Results from the acoustic study find that the prey move much closer to the islands and into much shallower water than previously known. "We didn't know how much biomass or how high the density of these animals was, and now we have a much clearer picture of the midwater zone" says Kelly Benoit-Bird, one of the researchers. "The reason that we didn't know about this movement before is because we weren't using acoustics."

Acoustic data from shipboard echosounders, active sonar moorings, broadband high-resolution sonar water column profiles, multibeam sonar, and passive listening were used to examine the prey and predators in the study. Previous attempts at measuring distribution proved unreliable for these subjects. "These fish are good at avoiding the nets," explains Benoit-Bird. "And the highest density of these fish is in the shallow regions, regions where you can't trawl."

The spatial and temporal dynamics of the scattering layers around Hawaii have significant impacts on their interactions with other animals, including their predators. Spinner dolphins, animals that forage exclusively on these layers in Hawaii, were previously assumed to forage offshore for the entire night. Echosounder surveys provided synoptic data on dolphins and the scattering layer which revealed that spinner dolphins follow both the vertical and horizontal components of the layers' migration, moving inshore around midnight. In addition, spinner dolphins were found in the densest regions within their prey.

Acoustic techniques have played a critical role in our developing understanding of the mesopelagic zone around the Hawaiian Islands. "By using this technique, we were able to be in more than one place at a time, and for longer deployments", explains Benoit-Bird.

The dynamics of the scattering layers in this zone have significant impacts on their predators like spinner dolphins, tunas, and deepwater snappers by changing the costs and benefits of foraging. These dynamics are also likely to have important consequences on other components of the ecosystem because of the large amount of biomass that moves energy as well as nutrients between pelagic and neritic systems each day.

For more information, contact Kelly Benoit-Bird

benoit@hawaii.edu, 808-247-5063, FAX: 808-247-5831

Hawaii Institute of Marine Biology, University of Hawaii P.O. Box 1106,Kailua, HI, 96734

PDF version of this press release

The boundary community is not evenly distributed around the islands. This heterogeneity, called patchiness is evident off all the islands surveyed. Interestingly, the boundary community off the coasts of the Big Island and parts of Oahu is distributed in distinct, relatively low prey density patches. The boundary community off Lanai and other areas of Oahu is distributed in relatively continuous, high density layers. This is shown in the figures below, where the brightness of the color purple is proportional to the density of mesopelagic animals. These differences in prey distribution are likely to affect the tactics predators use to forage on the boundary community.

Image credit: Kelly Benoit-Bird.

To observe the migration patterns of the boundary layer with more temporal resolution than is possible with a single-platform survey, a series of bottom-mounted, sonar moorings was used. Mesopelagic animals are profiled in the water column in five locations up the slope of the island in five locations at the same time.

Image credit: Kelly Benoit- Bird.

(Image credit )

For more information, contact Tara Hicks.

Last Updated Wed January 21, 2004. Maintained by Tara Hicks.

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