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New research has revealed that an abrupt change in climate conditions in the North Atlantic around 800 years ago played a role in a decline in Atlantic salmon populations returning to rivers. Human exploitation reduced their populations still further.

Using state-of-the-art geochemistry, a team of scientists has discovered that large-scale changes in the marine habitat, brought on by a transition from a warm to a cold climate and what is now known as the Little Ice Age (approximately 1300 to 1850 CE), corresponded with a decline in salmon in the River Spey, Scotland. The study, published in the international journal The Holocene, was led by the University of Southampton, working with ���ճԹ�.

These results can help us understand some of the controls on salmon populations prior to and during major human exploitation.

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.

Prof David Sear, professor of geography and environmental science at the University of Southampton and lead author of the study.

This research benefited from state-of-the-art geochemistry which enabled us to fingerprint salmon abundance over hundreds of years. We show that climate has been an important influence of salmon numbers, which is very relevant today due to the speed of climate change.

Prof Melanie Leng, ���ճԹ�, co-author.

Atlantic salmon lay their eggs in the gravels of headwater streams, where their young live for a year or two before migrating out to sea. Here, they feed and grow into adults, eventually returning to the river to spawn, where many then die. The sperm, eggs and carcasses are rich in marine nutrients, which can be detected in sediment hundreds of years later.

Using core samples from Loch Insh on the River Spey, the scientists collected and measured marine-derived nutrients (MDNs), which give an understanding of the historic population levels of salmon. The team also examined a 150-year record of net-catch data from the lower Spey to help calibrate the MDN record.

The scientists were able to construct a 2000-year record of both salmon-derived nutrients and variations in climate conditions.

The findings show:

• bigger salmon populations (inferred from changes in MDNs) in the past reduced during a cooling climate at around the same time humans began to exploit them, leading to a major decline in the fish over the last 800 years
• larger salmon populations in the past occurred at a time when rivers were also inhabited by beavers, which suggests migratory fish are capable of co-existing with beavers; this is an important concern of anglers around current beaver reintroductions
• migratory fish, such as salmon, bring marine nutrients into our nutrient-poor upland rivers and probably represented a major boost to aquatic and wetland ecosystems in the past, with a decline in nutrients negatively affecting these ecosystems today

It is the first study to use MDNs to measure Atlantic salmon, although the method has previously been used for Pacific salmon in north-west USA and Canada.