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Field Notes

Covering the natural wonders of the Pacific Northwest

January 24, 2013 at 7:00 AM

Predicting salmon returns: food supply is a good indicator

Every year fish managers attempt to predict how many Pacific salmon will migrate back to their home rivers. It’s a key exercise that drives everything from harvest limits to hatchery policy.

Alan's Salmon.jpg

A chinook salmon makes its way home to the Issaquah hatchery

Photo by Alan Berner, Seattle Times staff photographer

A team of scientists from NOAA and Oregon State University have found clues are better than others for predicting fish abundance.

The most reliable predictor turned out to be the dinner bell: When there was abundant prey in the ocean, salmon runs were more robust.

But as always with salmon, the picture was more complicated: What proved to be a good test for some species was not as effective for others.

The ocean is a black box of imponderables, with complexities interacting to make predictions difficult.

But in a paper published this month in the scientific journal PLoS One, researchers reported when they combined 31 indicators ranging from sea surface temperatures to the amount of salmon prey to help predict adult spring Chinook salmon returns to the Columbia last year, the best predictor by far was the presence of food in the ocean.

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Local physical indicators, including water temperature, were not as important.

A computer model based on the findings has had promising results, accurately predicting 221,000 fish would return in 2011, and nearly nailing the number of fish coming back in 2012, with 180,000 fish predicted, close to the 203,000 that actually came back.

The model may prove useful for fish managers in their annual task of predicting salmon returns to the Columbia, made months before the season actually starts.

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