A new study claims that seals have contributed to increases in natural cod mortality off Nova Scotia since the late 1980s and have precluded the stock’s recovery since 1993.
In the early 1990s, the cod stock on the Eastern Scotian Shelf collapsed and has since then experienced high natural mortality. Conversely, grey seal numbers in the area have doubled about every seven years since the 1960s.
“We conclude that indeed the seals are responsible for the lack of recovery of the cod”, Mike Sinclair, former director of science at the Bedford Institute of Oceanography and retired Department of Fisheries and Oceans (DFO) scientist, told CBC News.
Fishers have been making the argument for years, and it has finally been substantiated by science.
The results are published in the online edition of Fisheries Research journal.
“Basically, we were not convinced that the studies that were out there really accurately reflected what was going on, so we thought we’d have a look at it ourselves,” Sinclair said.
Cod fishing has been banned off Nova Scotia since 1993, when it was concluded that the stock’s collapse was a result of overfishing.
The study estimates abundance trends of the Sable, Eastern Shore and Gulf of St Lawrence seal herds which forage on the Scotian Shelf to 2020. The Sable herd is projected to stabilize at some 350,000 seals, according to FIS.COM.
Scientists believe that if the seals’ exponential growth continues unfettered, the Eastern Shore and Gulf herds combined could total about 200,000 individuals. On the other hand, density dependent processes may slow population growth of these two herds in the coming decade.
The study estimates the total annual food consumption of the three herds: more than 550,000 tonnes of fish were eaten annually by the Sable and Eastern herd, with the Gulf herd consuming about 138,000 tonnes, in 2010.
“What we are saying is given the size of the seal population, which we know is very large, you are not going to get a recovery”, said Bob O’Boyle, Sinclair’s colleague and also a former director of science at the Bedford Institute and retired DFO scientist.
Predictions by the study’s functional models were found to be inconsistent with estimates of recent increases in abundance of cod in trawl surveys, however.
Present levels of grey seal abundance have not been seen on the Scotian Shelf since the 1800s or earlier, the researchers noted.
A study by the Canadian Science Advisory Secretariat suggested killing 70 per cent of grey seals in the southern Gulf of St Lawrence to determine whether cod stocks would rebound, but acknowledged that more research is needed. The report also concluded that a cull on Sable Island of 50,000 seals over five years would be relatively fruitless.