A sudden shift in climatic conditions in the North Atlantic approximately 800 years ago had a part in a drop in Atlantic salmon populations returning to rivers, according to research headed by the University of Southampton.
Salmon stocks were further depleted as a result of subsequent human exploitation.
Climate change and human exploitation affect the population of Atlantic salmon
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The findings hinted at large-scale changes in the marine ecosystem caused by a shift from a warm to a cold climate and what is now known as the Little Ice Age (about 1300-1850), which coincided with a fall in salmon in Scotland's River Spey.
These findings can help us understand some of the constraints on salmon populations before and after considerable human exploitation, according to lead author Professor David Sear of Geography and Environmental Science at the University of Southampton.
Professor Melanie Leng of the British Geological Survey, one of the study's co-authors, stated that the research benefitted from state-of-the-art geochemistry, which enabled us to fingerprint salmon abundance through hundreds of years.
They demonstrated that climate has had a significant impact on salmon populations, which is especially pertinent now given the rapid pace of climate change.
Atlantic salmon deposit their eggs in the gravels of headwater streams and raise their young for a year or two before going to sea.
They eat and mature here before returning to the river to spawn when many of them perish.
The sperm, eggs, and corpses are high in marine nutrients that can be found hundreds of years later in sediment.
The findings demonstrated that during the previous 50 years, the number of Atlantic salmon returning to Scottish coastal waters has decreased, as per Marine Scotland.
This is not linked to a decrease in salmon leaving rivers and appears to be caused by higher at-sea mortality.
Previously, losses in coastal returns were offset by reductions in commercial net fisheries and the adoption of catch-and-release fishing practices by rod fisheries.
However, this buffering capacity has now been entirely utilized, and the expected quantity of salmon spawning in Scottish rivers has decreased since 2011.
These decreases have been worsened by a drop in the size of returning salmon, and therefore egg production.
The entire stock is below its conservation limit, with rivers on Scotland's east coast faring better than rivers elsewhere.
Although reported spatial disparities may be the consequence of local variables, the major driver of changes in Atlantic salmon populations is occurring in the maritime environment.
According to the best information available, the total sea trout stock is at its lowest point since 1952.
Over the last 20 years, there has been evidence of general deterioration.
A closer look revealed that decreases are occurring across the country.
Larger salmon populations (inferred from MDN shifts) were diminished in the past during a cooling environment at the same time people began exploiting them, resulting in a substantial reduction in the fish during the previous 800 years;
Salmon populations were higher in the past when rivers were also home to beaver, implying that migratory fish can coexist with beaver, which is a major worry of fishermen for contemporary beaver reintroductions.
Migratory fish, such as salmon, transport marine nutrients into our nutrient-depleted upland rivers, providing a significant boost to aquatic and wetland ecosystems in the past, with nutrient declines now significantly harming these ecosystems.
