The once widespread Atlantic cod—traditionally, the "fish" in "fish and chips"—are now just as famous for their decline as they are for their deep-fried crunchiness. In the 1970s, fishermen harvested about 100,000 metric tons on average of cod, haddock, and other hefty fish each year from the Scotian Shelf. Those years of plenty, however, took their toll on the ecosystem as these big predators disappeared. In 1993, the Canadian government imposed a moratorium on cod and haddock fishing along the shelf after populations of these fish crashed. Despite the ban, the region's entire cod population has remained in the dregs, weighing in at less than 5% of its total precollapse mass.

The reason for the slump lies in which fish flourished when the cod floundered, says study co-author Brian Petrie, an emeritus research scientist at the Bedford Institute of Oceanography in Dartmouth, Canada. When the shelf's large predators disappeared, populations of small, scrounging fish like capelin and Atlantic herring—normally cod food—exploded. "The wolves are gone, and now the rabbits can prosper," Petrie says. But in a twist worthy of Monty Python, those rabbits grew teeth: The small scroungers started eating cod eggs and larvae, he suspects, stamping down the predators' recovery.

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