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
Contact: Ginger Pinholster
American Association for the Advancement of Science