Diversified portfolio yields benefit for salmon stocks

Local diversity helps keep sockeye from going bust every few years

As Dr. Seuss noted, it’s good to have both red fish and blue fish. A new study of a salmon fishery expands on this concept, showing the importance of having lots of different red fish.

Sockeye salmon that migrate through Bristol Bay are shown pausing in Hidden Creek before swimming into a lake to spawn. A new study shows that diversity is key to maintaining the fish populations. Michael Webster

The millions of sockeye salmon that spawn in the watershed of Alaska’s Bristol Bay each year represent several hundred distinct populations of the same species, says ecologist Daniel Schindler of the University of Washington in Seattle.

Drawing on some 50 years of records showing the annual variation in numbers of salmon there, he and his colleagues have now calculated how much the diversity of the bay’s sockeye populations matters to the long-term stability of the bay’s fishery.

Without the current diversity, total sockeye numbers would boom and bust, possibly dipping every two or three years so low that managers would have to ban fishing for the season, the researchers say. Instead, some of Bristol Bay’s diverse populations go bust in a given year, but others boom. Overall, sockeye numbers stabilize and fisheries are expected to close only once every 20 or 30 years, the researchers report in the June 3 Nature.

“What is surprising is the magnitude” of the diversity benefit, Schindler says.

In fish stocks as in financial stocks, diversifying a portfolio improves stability, said study coauthor Ray Hilborn, also of the University of Washington, in a press conference. “Anyone who put all their money on the hot stock, be it Enron or the Florida real estate market, learned that lesson.”

The researchers borrow a financial term, the “portfolio effect,” to describe how a mixture of salmon populations adds up to a robust whole that can weather environmental challenges.

“We’re the first to calculate how strong the portfolio effect is within a fish species,” Schindler says. Much of the concern over the world’s shrinking biodiversity focuses on preserving a mix of different species, he says, but diversity within a single species deserves just as much attention. In the Bristol Bay watershed, for example, sockeye in streams not even a mile apart show detectable genetic differences.

Preserving diverse habitats is key to preserving that precious within-species diversity, Schindler says. Each sockeye population adapts to a particular stretch of stream or a certain lake in the region, where adults return to spawn.

Peculiarities of diverse spawning spots can ensure that some habitats will be good for fish even in particularly wet or dry years. Also, fish from each spot vary considerably in their approaches to life, spending different amounts of time in freshwater areas and at sea. “All of this complexity just builds up and compounds,” Schindler says.

Bristol Bay provides just such a variety of habitat types today. In contrast, he says, many watersheds to the south of the bay have lost a lot of their quirky spawning grounds and thus have fewer distinct populations. And youngsters grown en masse in man-made hatcheries, which are not common in Bristol Bay, push other salmon populations toward genetic sameness.

The Bristol Bay analysis parallels work in other organisms showing how within-species diversity favors stability, says Jay Stachowicz of the University of California, Davis. His own research found that patches of an eelgrass species with greater genetic diversity recovered more quickly from disaster than less diverse patches did.

Importantly, the new study documents the stabilizing effects of biodiversity “out in the big, messy real world,” says marine ecologist Emmett Duffy of the Virginia Institute of Marine Science in Gloucester Point, who wasn’t part of sockeye group. He also welcomes the work for linking an ecological effect “directly to human well-being.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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