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Taking Stock in Diversity: Species with a Varied Population "Portfolio" Thrive

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A study of sockeye salmon in Alaska's Bristol Bay suggests why fish and fishermen are doing so much better there than in the Columbia and Sacramento rivers

By Dawn Stover

For at least 50 years Alaska's Bristol Bay has been one of the most valuable fisheries in the U.S. On average, fishermen net about 25 million sockeye salmon annually in the bay's chilly waters. In 2009 the catch was worth more than $120 million.

Scientists at the University of Washington in Seattle's (U.W.) School of Aquatic and Fishery Sciences think they know why Bristol Bay is so productive year after year: Several hundred discrete populations of sockeye salmon inhabit the network of rivers and lakes that empty into the bay, and this tremendous population diversity buffers the entire fishery against the vicissitudes of the environment.

Diversity within the species creates what the scientists call a "portfolio effect," named for its resemblance to a diversified investment portfolio: Some salmon stocks do better under certain conditions, whereas others thrive under different constraints, but the fishery as a whole remains stable.

Scientists have long known that biological diversity stabilizes ecosystems. But as the U.W. scientists report in a paper on the portfolio effect published June 3 in Nature, population diversity usually gets far less attention than species diversity, even though populations may be going extinct at rates 1,000 times higher than those for species. "The measurement we use for assessing biodiversity is focused almost entirely on loss of species," says ecologist Daniel E. Schindler, lead author of the paper. (Scientific American is part of Nature Publishing Group.)

Schindler and his colleagues analyzed five decades of data from Bristol Bay, including counts, ages and timing of salmon returning to spawn in the area's lakes, rivers and tributaries. Each fish returns to its birthplace, and the diversity of these nurseries is what produces variance within the species. For example, male sockeye coming home to a deep lake grow humped backs to attract females, becoming so round they look like dinner plates. But male sockeye returning to a shallow stream would be easy targets for grizzly bears if they had humps protruding from the water. Instead, these fish are shaped like torpedoes.

Timing also contributes to diversity: Some Bristol Bay sockeye spend a year in freshwater before migrating to the ocean, whereas others stay for two years. Some remain at sea longer—and in different locations—than others do; some return to spawn earlier in the season. Not all of these life-history strategies pay off every year, Schindler says, but "there are enough winners to make up for the losers."

The portfolio effect is beneficial for fishermen, too. Schindler and his colleagues predict that without this adaptation Bristol Bay's fisheries would be closed once every two to three years because of poor fish returns, rather than once every 25 years. In places where population diversity is much lower—such as the Sacramento River in California and the once-mighty Columbia in Washington State—salmon fisheries have declined precipitously and go through frequent boom and bust cycles.


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