Scientists say conventional fishing practices put some fish at risk, even though their stocks appear healthy.  The researchers say the finding poses a double threat to some fish populations.

Fishing crew hauls aboard a small catch of pink and chum salmon  outside Eliza Harbor and Admiralty Island in Alaska (file photo)

The study, published in the journal Nature, analyzed data from a more than 50-year survey of fish in the waters off the Pacific coast of North America, particularly the U.S. state of California.

The researchers found variability in the numbers of fish from month to month, and over time. 

The paper's senior author, Professor George Sugihara of Scripps Institute of Oceanography in San Diego, says the reason for the variability in populations is that the fishing industry tends to prefer more mature fish.

"Fishermen like big fish.  And their gear is set up to have a bigger impact on the bigger individuals, removing the bigger ones from the population," he explained.  "So, that leaves the population of younger, maybe near juveniles, that are less able to provide continuity to the population, when there are small environmental pulses like El Ninos or minor alterations in ocean currents that maybe alter temperatures that, in turn alter survivorship and metabolism, things like that."

Sugihara says variability makes it difficult to predict the size of a fish population during a prime season.

He says these subtle fluctuations and overfishing in populations of targeted fish put an added strain on some species.

"So, the fact that those two things occur together really does represent a double jeopardy," he added.

George Leonard is science manager for Seafood Watch, the conservation arm of the Monterey Bay Aquarium in California.

Leonard believes fishery managers now have a duel task of guarding against major declines in fish populations, as well as watching out for subtle variations in fish populations.

"And that precautionary management is going to have to, perhaps, be stronger to ensure that those potentially negative impacts on otherwise healthy populations are reduced, so that we can ensure that there is an abundance of seafood for all of us to consume in the future," he said.

Although the study was conducted in California, experts say the threat to fish populations caused by overfishing and subtle variations in their numbers is a global problem.