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Cod (Courtesy: NOAA)

The collapse of the East Coast cod fishery appears to have caused the region's marine ecosystem to restructure itself, researchers say.

Kenneth Frank of the Department of Fisheries and Oceans in Dartmouth, N.S., and his colleagues studied more than 40 years of data on the food chain.

The study focused on the northwest Atlantic ecosystem off Nova Scotia, an area that was dominated by cod for centuries.

Starting in the mid-1980s, several commercially exploited bottom-dwelling species including cod, haddock and pollock declined.

Once these top predators in the food chain were gone, populations of smaller fish and invertebrates like northern snow crab and northern shrimp increased.

Meanwhile, zooplankton and algae were consumed faster as more of the smaller fish species dined on the base of the food chain.

Ecologists have long suspected that if top predators are removed then prey will increase.

Until now, they've haven't been able to trace the effects of removing a top predator down through all levels of the food chain – what Frank calls a "cascading effect."

Although crab and shrimp are now worth more than the original cod fishery, the study's authors caution the importance of biodiversity can't be ignored.

"One must acknowledge the ecological risks inherent in 'fishing down the food web,' as is currently occurring on the Scotian Shelf, or the ramifications associated with indirect effects reverberating across levels throughout the food web, such as altered primary production and nutrient cycling," the team wrote in Friday's issue of the journal Science.

They note several management measures meant to reverse the collapse of cod stocks have failed.

Species like seals, though, have benefited from not having to compete against cod for food, the researchers said.