Like other areas off Canada's east coast, the Scotian Shelf suffered a collapse of its fisheries during the 1990s. Haddock and cod were caught in an unsustainable fashion, eventually leading to a tremendous decline in their numbers that prompted the government to shut the fisheries. Despite this drastic action, stocks of fish like cod and haddock have remained low for years, raising the prospect that these ecosystems had shifted into an entirely new structure, one in which the former top predators would only occupy a tiny niche. Now, however, researchers are reporting the first signs that this alternate structure itself may be collapsing, raising the prospect that the Scotia Shelf may be on the verge of returning to its former self.

Although the Scotia Shelf was closed to commercial fishing, researchers were able to undertake annual surveys of the ecosystem's health using a standard bottom trawl, a practice that had started well in advance of the fishery's collapse. These provided data on the numbers and health of different species of fish; other data tracked plankton and other organisms at the base of the food web.

This data tracks a dramatic decline in the number of top predators that bottomed out just after 1990. With the predators gone, the population of smaller foraging fish (capelin, herring, and the sand lance) exploded, increasing by 900 percent in the post-collapse years. These foragers decimated the large-bodied plankton, causing their numbers to plunge as well. Worse still, these fish started feeding on the immature predators in such large numbers that they kept the population from recovering long after the maturation of young fish should have restored any temporary upset. In short, the ecosystem appeared to have shifted from one stable state, dominated by large predators, to a different one, in which smaller foragers existed in such large numbers that they kept most predators from making it to adulthood. The authors term this a predator-prey reversal.

The boom in forager population, however, followed a pattern seen in other ecosystems that have lost their top predators: the foragers outstripped their food supply, and the boom was followed by a population crash. The authors estimate that the area could support somewhere in the area of 4.3 million metric tons; the total population of foragers peaked in the mid-'90s at over 10 million tons. But these fish were individually smaller than they had been in previous years, suggesting that food had in fact become a constraint on the population. At the present, their population has dropped dramatically, down to 3 million tons.

With the forager population shrinking, the larger zooplankton species have started to increase in population, and the predatory species are starting to come back. After hovering at five percent of their previous numbers, cod have returned to a level not seen since the early 1990s. Haddock have come back even stronger, and have displaced cod as the most significant predator.

It has only been the during the last two years that the top predators have shown signs of recovery, so it's too early to conclude that the predator-prey reversal has itself been reversed. And the authors aren't sure whether the ecosystem will ever return to its former state, where cod is the dominant species. Nevertheless, they seem optimistic that this may be the case, and view the recent changes as an indication that the switch between different states of this ecosystem need not be permanent.

Any recovery is also currently fragile, as the authors note that other areas where fisheries have collapsed have been kept from recovery by a variety of factors: booms in jellyfish populations, the arrival of invasive species, and eutrophication from fertilizer runoff. So far, at least, none of these has prevented what may be a nascent recovery. There's still the chance that the predators will undergo a boom and bust of their own before the ecosystem finds a new stable point, and Canadian authorities might find themselves under pressure to reopen the area to fishing before it's ready. If and when it does reopen, however, we can hope that everyone involved has learned the importance of managing it for sustainability.

Nature, 2011. DOI: 10.1038/nature10285 (About DOIs).