Comebacks. Sports teams and aging rockers have them. Fish rarely do. But in a bit of good news for fisheries scientists, one ecosystem devastated by overfishing off the coast of Nova Scotia is showing early signs of recovery, a new study suggests. Here, Atlantic cod and other predatory fish, whose numbers nosedived in the early 1990s, seem to be struggling back, pointing to the resilience of marine communities, researchers say.
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.
But now that trend may be reversing, Petrie says. In an earlier study, he and his colleagues estimated that the Scotian Shelf was capable of supporting just over 4 million metric tons of scrounging fish. But in 1994 and 1999, capelin, herring, and other foragers jumped to about 10 million metric tons, researchers concluded in the current study after examining two separate annual fish surveys. In yet another seesaw reversal, the small fish starved and their numbers plummeted, opening a window about a decade ago for cod and haddock to recover.
And they seem to be doing just that. Cod combined weights are the highest they've been since the collapse, and the smaller, faster-spreading haddock are doing even better, the team reports online today in Nature.
But the comeback is far from complete, Petrie says. Individual cod and haddock are still about half the size, on average, that they once were. Study co-author Kenneth Frank, an ecologist also with the Bedford Institute, compares rebuilding an ecosystem to the Humpty Dumpty nursery rhyme. "When Humpty Dumpty fell, it broke into many pieces," he says. "Putting him back together again is quite a challenge." And although the new Scotian Shelf is on its way toward rebuilding that egg, it may never look the same again, he adds. Because haddock seem to be recovering faster, they, not cod, could become the shelf's dominant predators in the future.
Regardless, the study paints a hopeful picture for struggling fisheries worldwide, says Daniel Pauly, a fisheries scientist at the University of British Columbia, Vancouver, in Canada, who was not involved in this study. It may be impossible to bring a lost ecosystem back exactly the way it was, but if overfishing stops, marine communities should regain a stable mix of predators and prey alike. "If you give them a break, they bounce back," he says.
George Rose, a fisheries scientist at the Memorial University of Newfoundland in Canada, is more cautious. The Scotian Shelf is a very small ecosystem, he says, and many of Canada's larger and more critical fisheries might not respond to fishing moratoria in the same way. "It is an interesting story and a very fine use of the data available on the [Scotian Shelf], but it does not solve our much bigger problem."