Gre seals would be responsible for cod slow recovery, not forage fish. (Photo: FIS/NOAA)

Cod recovery hindered by grey seals

CANADA
Monday, May 07, 2012, 06:20 (GMT + 9)

Researchers have found that the delayed recovery of Atlantic cod (Gadus morhua) on the eastern Scotian Shelf could be due to higher predation by grey seals or other governing factors -- not the effect of forage fish, as previously believed.

Biologist Douglas Swain of the Gulf Fisheries Centre and emeritus scientist Robert Mohn at the Bedford Institute of Oceanography published their findings in 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," said Rolf D Vinebrooke, Co-editor of the Canadian Journal of Fisheries and Aquatic Sciences. 

"They report a lack of a direct relationship between forage fish biomass and cod recruitment rates, suggesting that density-dependent effects of competition and predation by forage fish cannot therefore account for lack of recovery by cod stocks,” he added. 

Most importantly, he said, their data demonstrate that the stock’s lack of resilience stems from unidentified factors pushing high death rates among adult cod.

The stock of Atlantic cod on the eastern Scotian Shelf collapsed in the early 1990s and showed no sign of recovery during a 15-year fishing moratorium, although it has recently grown in abundance. Both the prolonged failure to recover and the recent improvement have been blamed on changes in the biomass of forage fish through effects of predation and competition by these fish on early life stages of cod.

An examination of the relationships between forage fish biomass and the population dynamics of Scotian Shelf cod, however, did not back up that hypothesis. Instead, cod recruitment rate was unrelated to forage fish biomass.

The main factor delaying recovery was found to be high natural mortality of adult cod. The recent improvement in the stock is due to the strong 2004 year class and a decline in mortality, which cannot be attributed to an effect of forage fish.

Last year, a study published by Department of Fisheries and Oceans (DFO) researchers stressed a promising rebound in the species and attributed the delayed recovery to an explosion in the number of forage fish and their consumption of cod larvae. That study, published in Nature, concluded that a "predator-prey reversal" meant fish typically preyed on by adult cod become so populous they began to eradicate cod larvae and hinder the recovery of cod, Postmedia News reports.

The authors conclude that further investigation is required to ensure proper understanding and appropriate management to improve the chances for recovery of Atlantic cod stocks.

By Natalia Real
editorial@fis.com
www.fis.com

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