Fish carry a trove of information about their life history and habitats inside their heads—more specifically, inside their ear bones. Now, Montana state biologists have used these flat, oval structures to suss out the source of an illegally introduced fish. It’s the broadest-scale forensics case to date using fish ear bones.

State biologists discovered two illegally introduced walleyes, a carnivorous species that grows to about two feet long, in Swan Lake, in northwestern Montana. The scientists had been netting for lake trout, another invasive species, when they serendipitously caught the walleyes and took them back to the lab for analysis.

The presence of walleyes in Swan Lake came as a surprise, because the closest body of water known to contain this