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The mix of DNA floating in rivers and lakes will finally be used to monitor the state of aquatic ecosystems, after years of tests to show that the technique works.

Conventionally, aquatic life is monitored by capturing organisms, either by using nets or scraping under boulders, for examination. These techniques are time-consuming, can harm species and require skilled ecologists. Monitoring fish typically involves using electricity to stun them, which can sometimes prove fatal.

But these techniques could be replaced by simply taking a water sample and analysing the DNA in it. This environmental DNA (eDNA) comes from the cells, waste and…