Fish Populations Dip, Rise Dramatically As Stock Sizes Dwindle: Study

March 19, 2008

Release from: Canadian Press

HALIFAX — Fish populations dip and rise more dramatically as the size of their young spawning stock dwindles, according to new scientific study that sheds light on the fragility of dozens of species.

Researchers at Dalhousie University in Halifax looked at juvenile fish in close to 150 stocks off northern Europe and Canada, finding they can fluctuate wildly from year to year and swing close to extinction or rebound to healthier levels.

Coilin Minto, lead author of the report to be published Thursday in the journal Nature, said the findings add urgency to the need to bring stocks back to more stable levels.

"It's like a wobbly bike at low speeds that's going from side to side and it becomes less stable" he said from his office in Halifax.

"The benefits of keeping a population at a large size would be that you would achieve more stable populations and achieve more realistic projections for recovery."

The scientists examined data related to 147 different populations of roughly 40 species, including herring, mackerel and Atlantic salmon, and found that fish survival rates become greatly varied as the stock becomes depleted.

When they looked at historical data for North Sea herring, they saw that the population dropped to 44,000 tonnes in 1965 from 800,000 tonnes in the 1950s, leaving it at only five per cent of its former, healthy size.

"We observed in our analysis that the fluctuations in survival of these young fish dramatically increased," he said.

Cod stocks in the Gulf of St. Lawrence and off northeast Newfoundland also showed sharp swings in juvenile survival rates, whereas populations off southern Newfoundland haven't shown the same fluctuations.

The researchers, including renowned fisheries biologist Ransom Myers, who died last year, said it's hard to know what's causing the dramatic variances, but said it could be due to stresses in the habitat, predation or lack of food.

Fluctuations in cod populations might be due in part to the fact that cod have begun spawning at younger ages in response to fishing pressures, Minto said.

The study included saltwater and freshwater fish species, such as salmon, finding that "the overall results indicate this is quite a general phenomenon."

The findings could help fisheries managers more accurately predict the viability of stocks, something that has proven difficult in the past since there was little known about what happens to certain species in their early stages and what role stock size plays in their survival.

Minto said the information could be used by managers to better protect stocks that are in danger of collapse and add new insight into population theory.