As if overfishing and habitat loss weren't threats enough for wild salmon, the imperiled fish are also vulnerable to climate change, suggests a new study in the 27 October issue of the international journal, Science.
Decomposing salmon carcasses have left behind a 300-year record of oceanic nitrogen in Alaskan lake bottoms, revealing population swings that seem to be related to known climate changes from this period.
Climate's impact on wild salmon has 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 lead Science author 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, according to other studies cited in the Science paper.
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 called cladocerans, which are the main source of food for newly-hatched salmon. Thus, fewer adults returning the lakes probably means even fewer surviving juveniles, Finney and his colleagues think.
"People have speculated since the 1920's that this effect might occur, but now we can verify it with our paleoenvironmental techniques. We can ask, when nutrients increased in the lakes, did that actually do anything?" Finney said.
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.
It also appears that climate has a stronger influence upon salmon numbers in lakes with dense populations (holding as much as 30,000 fish per square kilometer). Lakes with smaller concentrations of fish (ranging down to 5,000 in the same area) haven't varied as much in their nitrogen levels, the researchers found.
The other authors of the Science study are Irene Gregory-Eaves and John P. Smol, of Queen's University, in Kingston, Ontario; Jon Sweetman, of the University of Alaska, in Fairbanks, Alaska; and Marianne S. V. Douglas, of the University of Toronto, in Toronto, Ontario.
This research was supported by the Alaska Sea Grant College Program, NOAA-Auke Bay Lab Ocean Carrying Capacity Program, Alaska Department of Fish and Game, U.S. Fish and Wildlife Service, NSF-NOAA GLOBEC Program, Natural Sciences and Engineering Research Council of Canada, the Northern Studies Training Program, and the Ontario Graduate Scholarship program.
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