Many Populations Make for Reliable Fishing

The most valuable fish species in the United States—with sales of nearly $8 billion over the past 6 decades—is sockeye salmon, which is prized for its fatty, red flesh. More than 60% of these fish are caught in Bristol Bay in southwest Alaska, where fishing vessels have netted tens of millions of fish year after year. Now researchers have shown that the dependable productivity of this fishery results from the fact that the bay is home to many hundreds of salmon populations, each originating in one of the many creeks and lakes whose water ends up in the bay. Preserving those many small habitats is crucial to keeping the fishery sustainable, they say.

Experts say that many other species may contain similarly diverse populations, although they will be harder to delineate than those of salmon, which can be easily counted in their native streams. "This work clearly identifies the importance of maintaining the breadth of diversity among populations in order to maintain stability in a fishery," says Jeff Hutchings of Dalhousie University in Halifax, Canada, who was not involved with the research.

Sockeye salmon (Oncorhynchus nerka) are anadromous: They live part of their lives in the ocean and return to spawn in the rivers where they hatched. When they make this return journey, fishing vessels line up to catch them at the mouths of the nine rivers that enter Bristol Bay. Fisheries biologists recognized long ago that in any given year, some of these rivers have more fish than others. "There's always a little bit of suspense whether a certain stream population is going be booming," says Daniel Schindler of the University of Washington, Seattle.

Schindler and his colleagues decided to investigate the impact of this variability on the overall sustainability of the fishery. Relying on 5 decades of data collected by the Alaska Department of Fish and Game, they studied how salmon populations—defined as the fish that spawn in a particular tributary or lake—varied in size from year to year. Each of the nine rivers contains dozens to hundreds of distinct populations, each of which is adapted to a particular set of local conditions. And because salmon always return to the same stream or lake where they hatched, the size of these individual populations can be monitored fairly easily.

The team found that the number of salmon that returned to individual streams varied quite a bit year to year. That's to be expected because population size in a particular year depends on many fluctuating factors, such as the severity of winter floods in that particular stream.

But the big picture is much more stable, the team reports in this week's issue of Nature. In any given year, some salmon might reproduce less successfully in their stream, yielding fewer offspring that migrate to the ocean. But meanwhile, a population in another stream might have a banner year. That evens out the total number of fish that will gather the next year at the mouth of the river—called the stock—to head upstream. This matters because fishing happens at the mouth of the river.

The same compensation is true for Bristol Bay as a whole; if the stock in one or more rivers is smaller than usual, other rivers might be higher than usual—so the total amount of sockeye available to be caught is relatively constant. (To quantify this effect, the researchers used a dimensionless measure called the coefficient of variation. In the Wood River, for example, the coefficient was 95% for the individual populations in streams and just 60% for the entire stock in the river.)

The lesson from Bristol Bay, Schindler says, is that a diverse array of intact habitat help populations cope with environmental change—and provide a much more reliable benefit to humans. To prove that, the team made a hypothetical calculation: if Bristol Bay had only a single population of salmon, it would crash much more often and the fishery would have be closed 10 times more frequently than currently happens. Indeed, along the West Coast from Washington to California, where habitat has been highly degraded, many salmon fisheries are in desperate straits. Even in Alaska, Schindler and others are concerned about potential damage to salmon habitat, such as from proposed mining.

"The result is very clear and very strong," says Esben Moland Olsen, an evolutionary biologist at the Institute of Marine Research in Bergen, Norway. "Fishery managers need to understand local population distribution." Conserving populations in a broad range of habitat will likely be important for improving the stability of many fisheries, he says. But for most species, more research will be needed to identify the habitats used by different populations.