URI oceanographers: Gravel habitats on Georges Bank need 10 years to recover from bottom fishing

Researchers recommend long-term closure of some areas

NARRAGANSETT, R.I. — November 9, 2005 — The gravel pavement on the northern edge of Georges Bank in the northwest Atlantic Ocean – habitat for herring eggs and juvenile cod and haddock – is fished with scallop dredges and bottom trawls. While much research has been conducted on the effects of this fishing gear on the gravel habitat, a recent long-term study reveals for the first time that it may take as long as 10 years for this habitat to recover from these impacts.

In an article in the recently published book Effects of Fishing on Benthic Habitats, University of Rhode Island oceanographers Jeremy Collie and Jerome Hermsen, along with colleagues from the U.S. Geological Survey and the Northeast Fisheries Science Center, assessed the effects of disturbance to organisms living in the gravel sediment on Georges Bank and the rate of recovery in an area closed to bottom fishing.

From 1994 to 2000 the scientific team went on eight expeditions to the northern edge of Georges Bank to collect dredge samples and take photographs of the sea floor at sites between 40 and 85 meters deep and with varying degrees of fishing disturbance. The fishing history of each site was determined from data on the spatial distributions of otter trawling and scallop dredging. In 1995 a large portion of the area of Georges Bank near the U.S./Canadian boundary was closed to all bottom fishing to reduce fishing mortality of cod, haddock, flounder.

The scientists found a significantly higher abundance and biomass of marine organisms in the sediments at undisturbed gravel habitat sites, and they observed that the community composition is significantly different from that of disturbed sites. The study suggests that bottom fishing reduces the abundance of prey species that are important in the diets of bottom-dwelling fish species.

“The differences in production between disturbed and undisturbed sites are substantial when viewed in the context of the Georges Bank food web,” said Collie. “Our results indicate that bottom fishing alters the flow of energy through continental shelf ecosystems.”

The recovery time of gravel habitats is clearly longer than for soft-sediment communities. “Our results so far,” continued Collie, “suggest that the recovery time of the gravel habitats is on the order of 10 years, but continued sampling is required to validate this prediction. Similar recovery rates were observed during 10 years of sampling a gravelly habitat off the Isle of Man following closure to scallop dredging.”

Collie concluded that “permanently closed areas of gravel habitat are preferred over temporary or rotating closures to mitigate the effects of fishing on benthic communities.” Short-term closures could maintain these areas in a perpetually disturbed state.

Collie’s research was funded with grants from the NOAA/URI Cooperative Marine Education and Research Program, the U.S. Geological Survey, the National Undersea Research Center and the National Sea Grant College Program.