Growing numbers of farmed salmon in northern Europe are escaping and mingling with their tastier, sturdier cousins from the wild. Tracking this phenomenon is difficult because the two populations look alike.

But chemical signatures in fish scales may reveal a fish’s origin, British salmon sleuths write in the Marine Ecology Progress Series. Fish scales accumulate tree-ring–like layers that reflect a fish’s diet and the waters it has inhabited over the course of its lifetime. Pellet fish food contains slightly higher levels of manganese than is found in the diet of a wild fish. Clive Trueman of the National Oceanography Centre in Southampton and Elizabeth Adey of the Scottish Association for Marine Science in Oban, both in the United Kingdom, used a mass spectrometer to measure manganese levels in the scales of salmon from several Scottish farms and from the wild. They found that they “could easily distinguish between time a fish had spent at sea and in fresh water,” Trueman says. By comparing the scale chemistry—cheaper than DNA analysis—ecologists can track the presence of intruders, the authors say, and determine where countermeasures are needed.

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