Sounding Life at Sea

Charles H. Greene
Ocean Resources and Ecosystems Program
Cornell Unviversity

The profitable commercial cod and haddock fishery on Georges Bank, located approximately 150 kilometers east of Cape Cod, collapsed in the early 1990s. As a result, the government has restricted the harvest of these groundfish, by closing portions of the Georges Bank indefinitely and requiring fisherman on the remainder of the Bank to use nets with a larger mesh. Fisheries managers hope that this will allow larval and young fish to escape capture and to replenish the stock. Still, the government has begun buying out fisherman--retiring their licenses and scrapping their boats.

Causes for the fishery's collapse are unclear, but they probably include overfishing and environmental changes that have altered the complex biological and physical processes at play within this highly productive ocean ecosystem. Unfortunately, this system is not yet well enough understood to isolate the contibuting factors, let alone solve the problem.

Oceanographers Charles H. Greene and Jacqueline M. Popp of the Ocean Resources and Ecosystems Program at Cornell University, with collaborators Peter H. Wiebe and Mark C. Benfield of Woods Hole Oceanographic Institution, are looking at some of the feeding interactions in this ecosystem. In particular, they have studied the availability and distribution of the zooplankton, accumulations of minute sea animals, that represent a critical food source for the larval fish.

The team used sound to focus on cloud-like patches of the tiny creatures drifting along with the current. They conducted a shipboard field study, systematically sampling the waters to a depth of 80 meters on Georges Bank using bioacoustic remote sensing techniques, and then processed these data to create images.

For this particular data set, it appears that the ship steamed directly over a large patch of zooplankton, oriented southwest to northeast, on at least three different legs of the cruise. Working with visualization producer Chris Pelkie at the Cornell Theory Center (CTC), the research team was able to peer into the dark ocean and get a three-dimensional view of this important link in the food chain.

The next step in the project is to analyze Video Plankton Recorder (VPR) images, collected simultaneously with the acoustic backscattering data and from the same surveyed volume of ocean, in order to document the zooplankton composition of the 3D patch. These visual data will also give them an estimate of the density of the fish larvae in these patches and of the ratio of predators to prey--a key to understanding food availability.

Greene's team is gathering fundamental information about this ocean ecosystem. Ultimately, the aim is to understand the relationship between the springtime thermal layering of the water column and the distribution of the fish larvae and their food as they drift along on their way to a critical point in their seasonal journey around the Bank.

Credits

Statistical Tuning
On Course
Life at Sea

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