Federal scientists disagree on why Atlantic cod struggled to recover

By Randy Boswell
May 3, 2012

Two teams of federal fisheries scientists are clashing over their interpretations of why Atlantic cod struggled to recover following the population’s devastating collapse in the early 1990s, raising fresh questions about the best way to sustain the resource in the future.

Less than a year ago, a high-profile study published by Department of Fisheries and Oceans researchers highlighted a promising rebound in the species — for generations an economic mainstay for Atlantic Canada — and blamed the delayed recovery on an explosion in the number of forage fish (prey normally eaten by adult cod) and their ravenous consumption of cod larvae.

That study, published in the journal Nature and co-authored by DFO scientists Kenneth Frank and Brian Petrie and biologists William Leggett and Jon Fisher from Queen’s University in Kingston, Ont., concluded that a “predator-prey reversal” had seen fish typically preyed on by adult cod — such as herring and capelin — become so numerous they began to decimate cod larvae and retard the overall recovery of the valuable groundfish, protected by a fishing moratorium since 1993.

But research published by another team of DFO scientists this week discounts that factor altogether and again raises the contentious possibility that grey seals — now targeted for a proposed cull aimed at bolstering cod stocks in the Gulf of St. Lawrence — may well have postponed the cod’s comeback in other Atlantic waters.

The new findings by co-authors Douglas Swain and Robert Mohn “suggest the delay in recovery of Atlantic cod on the eastern Scotian Shelf could be attributed to increased predation by grey seals or other governing factors and not the effect of forage fish as previously thought,” according to study overview released by the Canadian Journal of Fisheries and Aquatic Sciences.

“Swain and Mohn provide provocative scientific evidence challenging the commonly held belief that species interactions with forage fish have impaired the recovery of Atlantic cod on the eastern Scotian Shelf during the 15-year fishing moratorium,” noted journal editor Rolf Vinebrooke.

“Understanding the causes of these severe declines in productivity,” Swain and Mohn state in the journal article itself, “is a critical first step in determining what additional management actions, if any, would be appropriate to promote the recovery of these once important resources.”

Referring to the earlier DFO research identifying forage fish as the culprit, Swain and Mohn state bluntly in the new study that their findings “provided no support for this hypothesis” and that changes in the cod population “cannot be attributed to an effect of forage fishes.”

The co-authors are not conclusive about the cause of the cod’s slow recovery or its recent rebound. And they point out that “most modelling studies have concluded that predation by grey seals is not an important component” of high cod mortality.

But they do point to another recent study in the journal Fisheries Research — by a third team of DFO researchers, Halifax-based scientists Robert O’Boyle and Michael Sinclair — that applied a different analysis to show that grey seals likely were responsible for keeping cod numbers so low for the past 15 years.

Swain and Mohn further speculate that the recent bump in cod numbers may result from changes in seals’ feeding activity, or changes in cod behaviour to avoid being eaten.

Frank told Postmedia News that he and his collaborators on last year’s Nature study “welcome and encourage” challenges to their hypothesis from fellow researchers such as Swain and Mohn.

And he noted that, “it is good to see the authors agree with our previous findings that cod are starting to recover on the eastern Scotian Shelf. This had been a highly contentious issue which now seems to be shifting toward a consensus view of early stages of cod recovery.”

But Frank disputed Swain and Mohn’s conclusion that forage fish played no role in the cod’s slow recovery and said the latest study used an “invalid index” to measure trends in cod reproduction.

“We are concerned the authors have overstated their case and fail to acknowledge a critical assumption” underlying their interpretation of the slow cod recovery, said Frank, adding that key data contained in the new study actually “provides support for our hypothesis.”

In September, the Halifax-based Fisheries Resource Conservation Council controversially recommended in a report to federal Fisheries Minister Keith Ashfield that the government should cull up to 70 per cent of the grey seals inhabiting the southern Gulf of St. Lawrence “in order to test the hypothesis that predation by grey seals is the major factor preventing recovery of groundfish stocks in that area.”

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