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People shouldn't assume that small, short-lived fish such as sardines are immune to the impact of fishing, says Malin Pinsky, the lead author of a new study. (Matko Biljak/Reuters)

Small fish like sardines may not necessarily be a more sustainable dinner choice than large predators such as cod, a new study suggests.

"We were really surprised by the results," said Malin Pinsky, lead author of a study published Monday that found stocks or regional populations of smaller, short-lived fish collapse because of overfishing just as frequently as stocks of large, long-lived species such as cod, tuna and sharks.

The findings, which appear in the Proceedings of the National Academy of Sciences, are particularly worrisome because the collapse of a small species may have wider-ranging effects than the collapse of large species, Pinsky said.

How the study was done

Researchers looked at a database of 223 scientific fish stock (regional population) assessments of 120 species compiled by researchers at Dalhousie University in Halifax. Within that database, they looked for collapses — situations where the estimated size of a fish stock fell to less than one-fifth of the level deemed necessary to support fishing at the highest rate considered sustainable.

The researchers also looked at the database of fish catches from 1950 to 2006 maintained by the United Nations Food and Agriculture Organization, which includes 891 fish stocks from 458 species. Within that database, they defined collapses as periods of more than two years where catch levels were less than one-tenth of the average catch during the five years with the biggest catches.

Collapses of individual populations of certain small species had previously been recorded but had not been examined on a larger scale to look for trends.

"It removes a very important food source for all the species that rely on it, so actually, a lot of mammals, birds and in some cases larger fishes can also decline," he told CBC News, adding that larger species recover more slowly.

Advocates of sustainable seafood, such as Taras Grescoe, author of Bottomfeeder: How to Eat Ethically in a World of Vanishing Seafood, have suggested that responsible consumers eat species low on the food chain, such as mackerel, herring and sardines.

Those species are also recommended by groups such as SeaChoice and Seafood Watch, which help consumers make sustainable seafood choices.

One of the criteria such groups use is a species' "inherent vulnerability" to fishing, based on factors such as the rate of reproduction — which is usually higher in smaller species — the age of first maturity and the maximum age, which are usually lower in smaller species.

Pinsky, a PhD candidate in biology at Stanford University, and his U.S. and Canadian collaborators wanted to find out whether such "life history" characteristics of different species could help predict which ones were more likely to collapse under pressure from fishing.

He hypothesized that larger, slow-growing species were more vulnerable than smaller species in the ocean, just as larger species tended to be at higher risk of extinction on land. The hypothesis appears to be incorrect.

Daniel Ricard, a PhD candidate in biology at Dalhousie University, collated some of the data for the study and is one of Pinsky's co-authors.

Place of origin matters

Ricard, who has expertise in assessing fish stocks, said the results show "you cannot be complacent about anything" and all fisheries need to be managed carefully.

They also show that consumers need to pay attention to more than just the species they are eating because many larger species have some sustainable stocks and many smaller species have some stocks that are depleted, he said.

"People really have to educate themselves, and also ask the hard questions when you go to the fishmonger at Sobey's or Superstore or Loblaws: Where is that fish from? Where was it caught?"

Pinsky advises eating throughout the food chain rather than focusing toward the bottom.

"Don't assume just because it's small and short-lived that it's immune to the impact of fishing," he said. "We have enough fishing power to collapse anything if we're not careful."