Size of future fish stocks less predictable as populations decline

(CBC) - Year-to-year fluctuations in fish survival rates are more pronounced as populations decline, a Dalhousie University study has found, adding another layer of uncertainty to the effort to manage depleted stocks.

Tracking fish populations and making accurate predictions on future populations is notoriously difficult because of the amount of year-to-year variation in the survival rates of fish in the early stages of development, according to lead researcher Coilin Minto of Dalhousie's department of biology.

But while most predictive models assume this variability, or "noise," is constant, Minto's study, to be published in the journal Nature, found it was inversely related to population. The change in survival rates of a given fish population fluctuated more widely and became less predictable as the size of the population decreased.

"This has implications for fisheries management," said Minto, who worked on the study with Dalhousie statistician Wade Blanchard and the late Ransom Myers, the biologist whose research brought worldwide attention to the issue of overfishing.

"If variation increases as populations decline, there is an increased potential for incorrect recovery projections and also an increased risk of extinction for extremely low populations," he told CBC News.

Research could improve fish stock estimates

The study looked at data from 147 fish populations accounting for 39 species, focusing on recruitment rates - the chance that juvenile fish would make it to the early stage of adulthood.

An example of extreme variation could be found in pink salmon from Sashin Creek, Alaska, where the highest variation in juvenile survival occurred when the number of spawning females was reduced to below 300.

The findings could be the result of changes in fish spawning practices in response to a decreasing population, Minto suggests. Cod populations, for example, shortened the seasonal duration of spawning in response to dwindling numbers, which in turn might have made the survival of the fish's eggs and larvae more susceptible to environmental conditions, both favourable and unfavourable.

The next step for the research, he said, is to incorporate this new understanding of the relationship between variation and population to existing projections of fish stock recovery.

"It's important for us as stewards of these resources to get the most complete picture of what is happening," Minto said. "Hopefully, this will help improve our estimates."