URI Fisheries Oceanographers Study the Effects of Predator-Prey Interactions

in the Georges Bank Fish Community

Narragansett, RI — December 4, 2001 — Fisheries oceanographers Tien-Shui Tsou and Jeremy S. Collie at the University of Rhode Island’s Graduate School of Oceanography are estimating fishing and predation mortalities by year and age group of predator and prey species on Georges Bank using Multispecies Virtual Population Analysis (MSVPA). This method of analysis integrates virtual population analysis with data on the food consumption rates and diet composition of the major predator and prey species.

In a recent study, reported in the Canadian Journal of Fisheries and Aquatic Sciences, Tsou and Collie tested the reliability of MSVPA by applying the method to the highly productive and long exploited Georges Bank fish community. Over the past several decades the structure of the Georges Bank fish community has changed significantly, with increases in sharks, rays, herring, and mackerel and decreases in groundfish and flounders.

As part of NOAA’s Coastal Ocean Program, Georges Bank Predation Study, Tsou and Collie constructed a nine-species MSVPA of the fish community on Georges Bank to investigate the feeding dynamics between predator and prey. The species included cod, haddock, herring, yellowtail flounder, silver hake, mackerel, sandlance, spiny dogfish, and winter skate. To construct their model, they used catch and diet data gathered by the Northeast Fisheries Science Center from 1978-1992. The diet data come from thousands of stomach samples from marine fish that have been collected along the northeast coast of North America.

The results of the MSVPA agree with previous conclusions that predation is a dominant source of mortality in the Georges Bank fish community. Cod and silver hake were the most important predators, but have been replaced by sharks and rays as the dominant fish eaters in more recent years. The predator diet composition shifted to herring and mackerel as these prey species became more abundant in the late 1990s, but the predation pressure was insufficient to prevent these species from increasing to record levels of abundance.

Predation by sharks and rays does not appear to be a significant source of mortality for cod and haddock. Most of the predation of these species is inflicted by other groundfish species and includes cannibalism. Cod, haddock, shark, and skates compete for mackerel, herring, and sandlance. Yellowtail flounder constituted a small proportion of the spiny dogfish diet, yet there was substantial predation mortality because dogfish far outnumbered the flounder.

In a companion paper, published in the October 2000 issue of the ICES Journal of Marine Science, Tsou and Collie examined the impact of predation on the recruitment of young fish to the Georges Bank fish community. The level of predation mortality varies from year to year and can therefore affect the recruitment of haddock, herring, and silver hake. Cannibalism was the dominant source of predation on cod and silver hake; this predation, therefore, compensates for changes in year-class size.

The authors concluded that “the levels of predation mortality should be considered when making medium- to long-term recruitment forecasts.”

The URI Graduate School of Oceanography is one of the countrys largest marine science education programs, and one of the worlds foremost marine research institutions. Founded in 1961 in Narragansett, RI, GSO serves a community of scientists who are researching the causes of and solutions to such problems as acid rain, global warming, air and water pollution, oil spills, overfishing, and coastal erosion. GSO is home to the Coastal Institute, the Coastal Resources Center, Rhode Island Sea Grant, the Ocean Technology Center, and the National Sea Grant Library.

Contact: Lisa Cugini, (401) 874-6642, lcugini@gso.uri.edu