Haddock and redfish collected during DFO survey of the Canadian Scotian Shelf. (Photo: Queen's University)
Cod numbers on the rise: study
Thursday, July 28, 2011, 23:50 (GMT + 9)
Cod, haddock and other groundfish stocks are showing encouraging signs of recovery for the first time in 20 years, after their populations off the east coast of Canada collapsed in the early 1990s, according to research published in Nature.
“This early-stage recovery represents a long ecological transition for an ecosystem that was pushed out of balance and that is gradually moving back into balance,” says William Leggett, a professor in the Department of Biology, former principal at Queen’s University, and an expert in the dynamics of large marine ecosystems.
This is the first study to offer evidence of an upturn at the multi-species level and probe into the core ecological mechanisms facilitating the recovery.
"The answer to the critical question of whether or not such profound changes in the dynamics of large marine ecosystems are reversible seems to be 'yes'," the team reported.
The researchers hypothesize that the 20-year delay in recovery since the 1990s is attributable to a reversal of fish predator and prey roles.
When large-bodied species like cod were dominant, they hunted smaller forage fish species. The overfishing of cod and other groundfish populations, however, allowed these smaller fish to prey on large-bodied fish in their earliest life stages, which precluded these populations from bouncing back.
This absence of large-bodied fish predators led to a 900 per cent-boom in the forage fish population and ultimately outstripped its food sources. Since then, forage fish numbers have dropped and given room for cod and haddock populations to recuperate.
Notably, while some species are burgeoning, fish are generally much smaller than they used to be, explained co-author Brian Petrie of the federal Bedford Institute of Oceanography in Nova Scotia.
Five-year-old cod are just 60 per cent the size they were before the 1990s, and five-year-old haddock are 40 per cent of their original size, The Vancouver Sun reports.
Petrie said this might be because overfishing eradicated the big fish and left behind fish small enough to escape through the nets, which left them genetically predisposed to be small.
This recovery is positive and portentous for other collapsed fisheries. Even so, Leggett and research colleague Jonathan Fisher reminded that the process is not straightforward.
Cod stocks now hover around 34 per cent of the level typical when commercial fishing was prospering in the 1970s and 1980s. Conversely, haddock now exceeds its historical levels and its more dominant role.
“It’s difficult to say if this switch may have any long-term implications,” explained Fisher, a postdoctoral fellow in the Department of Biology. “This system may return to its historical character, but there’s also the possibility that it won’t and that another species will dominate. Only time will tell.”
The study was done in collaborationwith Kenneth Frank and Brian Petrie from the Bedford Institute of Oceanography and was funded by the Natural Sciences and Engineering Research Council of Canada.
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By Natalia Real