Climate change and commercial fishing are reducing salmon numbers

The researchers studied the nitrogen-15 content of sediment cores from five Alaskan lakes, allowing comparisons of the amount of fish having returned to the lake at the end of their lives to spawn, their carcasses then sinking to the bottom of the lake. Incorporating high levels of nitrogen-15 into their tissues in the ocean, the dead salmon also release phosphorus and other nutrients which enter the food chain via uptake by algae. In turn, the algae is eaten by zooplankton, an important prey of the juvenile salmon. According to the researchers, a decrease in the number of adult fish dying in the lake could disrupt this cycle.

“The lakes we studied on Kodiak Island and near Bristol Bay often had similar patterns of salmon abundance that corresponded to climate changes over the last three centuries,” said lead Science author Bruce Finney of the University of Alaska Fairbanks. “That’s consistent with the belief that climatic and oceanographic changes are driving the sockeye populations up and down in sync over decades-long timescales.”

The researchers found that salmon numbers were low during the early 1700s and again in the early 1800s. “These periods were particularly cold, some of the coldest in the past several centuries, and we found that salmon runs were also quite low,” said Finney. Salmon numbers then increased during warm periods, such as the late 1700s, mid 1800s, and early 1900s. However, during recent decades, increases in numbers have been hampered by commercial fisheries, according to the researchers.

“Even though the catch went up in the late 1970s, and reached records in the 1990s, we don’t see that in the core samples,” said Finney. “That’s because the management philosophy is to manage for a constant, optimal number of spawning salmon in each system. So, essentially they harvest the excess salmon, and the nitrogen levels in the sediment remained relatively constant.”

“But our study suggests a need for new, flexible management policies that take climate and lake nutrient levels into account,” said Finney.

The nature of the climate changes underlying the salmon trends are unclear, say the researchers. According to Finney, many scientists believe the process occurs in the ocean, and may involve changes in salmon food supply, possibly focused in coastal waters inhabited by young salmon. The climate effect also appears to be most pronounced in the most densely populated lakes with up to 30,000 fish per square kilometre.

“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,” said Finney.