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As if overfishing and habitat loss weren't threats enough for salmon, the imperiled fish are also vulnerable to climate change according to a study conducted in Alaska. The impact of climate change on wild salmon had previously been uncertain, because existing population records were relatively short and complicated by the effects of fishing, dam-building, and other human activities.

"Traditionally, fisheries managers assumed the environment was constant when they calculated the maximum number of salmon that fishermen could harvest. But our study suggests a need for new, flexible management policies that take climate and lake nutrient levels into account," said Bruce Finney of the University of Alaska-Fairbanks.

Finney and his colleagues delved beyond recent human influences by studying historical levels of the isotope nitrogen-15 in sediment core from lakes on Alaska's Kodiak island and near Bristol Bay. In the ocean, salmon incorporate high levels of nitrogen-15 into their tissues. The isotope is only abundant in Alaska's coastal lakes if it's released from dead salmon that returned to the lake at the end of their lives to spawn. Lakes blocked off by waterfalls, for example, have lower levels of nitrogen-15.

Finney's team found several major fluctuations in nitrogen-15 levels over the last 300 years. These swings implied drops in the salmon population during the early 18th and 19th centuries, periods that were cooler than average. The biggest salmon decline occurred in the 20th century, as human activities started to take effect.

The researchers also discovered a positive feedback effect that may be further inhibiting salmon recovery. It appears that the adult carcasses actually "fertilize" the lake ecosystem that provides food for the juvenile fish. Overharvesting salmon disrupts this loop, according to Finney.

The microfossils in the lake core show that algae are very sensitive to the nutrients released to the water from the adult salmon carcasses. The algae are then eaten by tiny invertebrates which are the main source of food for newly-hatched salmon. Thus, fewer adults returning the lakes probably means even fewer surviving juveniles.

The nature of the climate changes underlying the salmon trends are unclear right now. Finney said many scientists believe the process occurs in the ocean, rather than in individual lakes, and may involve changes in salmon food supply. This effect might be focused in coastal waters, which young salmon inhabit during the most precarious stage of their development.

"I could see a situation in which, if you knew that in a few years things would be bad for the fish in the oceans, you'd adjust other aspects of your management approaches on land," Finney said.

See also story on global warming affecting coral reefs