Medieval water mills may have harmed European salmon populations

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Human activity has been impacting the environment for as long as there have been humans. From our prehistoric ancestors hunting species to extinction, to modern carbon levels, pretty much everything we’ve done has had an impact. An example of this is shown by recent research on salmon stocks in the Netherlands.

According to archival research performed by ecologists from Radboud University in the Netherlands, salmon numbers depleted quickly after the introduction of water mills in the 10th Century.

“Water mills were also built in Scotland, but there were fewer of them and they had a different design that had a far lesser impact on the riverbeds,” says Rob Lenders, leader of the research team.

Previously, researchers had thought that overfishing could have been to blame, but new evidence shows that such fishing couldn’t have impacted salmon stocks the way that water mills did. The water mills required dams to be built so that there would be enough water to turn the wheel. As a result, silt flowing downstream would be stopped by the dam and subsequently build up in the riverbed, covering the gravel in which salmon lay their eggs.

As salmon returned to rivers where mills had been built in order to spawn, they would find nowhere to do so, significantly reducing their numbers.

In addition, this massive reduction in salmon numbers also had other ecological impacts. Bears, wolves, and eagles all began to decline as salmon populations decreased and they stopped bringing important nutrients from the ocean to the rivers and streams where they once lived.

The result was a reduction in ecological variety that would take centuries to overcome.

The vast ecological impact that reduction in salmon stock had shows how changing even one aspect of an ecosystem can have unforeseen consequences that reach far into the future.

The findings of the Radboud University research support the need to better understand ecosystems, and to engage in thorough studies of them before making any changes.