Cod resurgence in Canadian waters

"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, and an expert in the dynamics of large marine ecosystems.

This study is the first to show evidence of a recovery at the multi-species level and delve into the underlying ecological mechanisms facilitating the recovery.

The researchers hypothesize that the 20-year delay in recovery was caused by a reversal of fish predator and prey roles. When large-bodied fish like cod were dominant, they fed on smaller forage fish species. However, the overfishing of cod and other groundfish populations meant that these smaller forage fish had the opportunity to prey on large-bodied fish species in their earliest life-stages, preventing these populations from recovering their numbers.

This absence of large-bodied fish predators resulted in the forage fish population ballooning by 900 per cent and ultimately outstripping its food sources. The subsequent decline in forage fish numbers has been vital for the recovery of the cod and haddock populations.

While this recovery is positive and bodes well for other collapsed fisheries, Dr. Leggett and research colleague Jonathan Fisher are quick to note that the process is not straightforward. Cod populations are now at around 34 per cent of the level that was typical when commercial fishing was flourishing in the 1970s and 1980s. Haddock, on the other hand, now exceeds its historical levels and is playing a more dominant role.

"It's difficult to say if this switch may have any long-term implications," explains Dr. 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."

This research was conducted in collaboration with Kenneth Frank and Brian Petrie from the Bedford Institute of Oceanography and was funded by the Natural Sciences and Engineering Research Council of Canada.