A new study led by the University of Southampton has found that a drastic change in climate conditions in the North Atlantic around 800 years ago led to a marked decline in Atlantic salmon populations returning to rivers. According to the experts, the transition from a warm to a cold climate initiating what is known as the Little Ice Age (1300-1850) caused large-scale changes in marine habitats corresponding with a decline in salmon in the River Spey in Scotland. Over the following centuries, increased human exploitation played a significant role in further diminishing these populations.
Atlantic salmon lay their eggs in the gravels of headwater streams, where their offspring live for about a year or two before migrating out to sea. After spending their adulthood in the ocean, they finally return to the rivers to spawn, and eventually die. Since their sperm, eggs, and carcasses are rich in marine nutrients which can be detected in sediment even centuries later, scientists can now map the historic population levels of salmon.
By collecting and measuring nutrients left by salmon in the River Spey, the researchers found that larger salmon populations in the past declined during a cooling climate – a period that coincided with an intensification in human exploitation. These findings also suggest that previous theories arguing that beavers have significantly impacted salmon populations are unfounded: larger salmon populations in the past occurred at a time when rivers were also inhabited by beavers, showing that the two animal species could peacefully coexist.
“These results can help us understand some of the controls on salmon populations prior to and during major human exploitation,” said study lead author David Sear, a professor of Geography and Environmental Science at Southampton.
“Our study shows that historically, beavers – common in Scotland hundreds of years ago – do not appear to have significantly impacted salmon numbers. This is very relevant today, as the animals are being reintroduced to UK rivers and a debate continues about their potential impact on migratory species like salmon.”
Since migratory fish, such as salmon, bring marine nutrients into nutrient poor upland rivers, population declines negatively affect river ecosystems today. Further research is needed to untangle the complex relationships between salmon and other aquatic animals in order to properly asses the larger impact that diminishing salmon populations have on a variety of ecosystems.
The study is published in the journal The Holocene.