New Canadian study finds fish populations dip, rise dramatically as stock sizes dwindle

Tor-Eddie Fossbakk A press release at the end of last week stated that 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. 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 tons in 1965 from 800,000 tons in the 1950s, leaving it at only five per cent of its former, healthy size. 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. 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. 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.