Tiny fish ear bones can now reveal criminal activity
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 species is roughly 160 miles away by road.
Fish ear bones, or otoliths, can serve as useful forensic tools because the water fish live in leaves a chemical marker on them. The geology surrounding a lakebed, in particular, leaves a distinct chemical signature in the water that gets absorbed into the otoliths, and analyzing the bones can indicate where a fish has spent its time.
To determine where the Swan Lake walleyes had come from, the scientists worked with colleagues around the state to collect otoliths from walleyes living in 12 other lakes that anglers tend to frequent. They found a matching signature in Lake Helena, located 192 road miles south of Swan Lake.
According to Sam Bourret, a fisheries biologist with Montana Fish, Wildlife, and Parks in Kalispell who led the study and published a paper in August documenting the findings, that distance between the two bodies of water is likely what enticed someone to bring the fish to Swan Lake. “There’s probably an angler who wants to fish for walleyes closer to home,” he says, noting that walleyes are a popular sport fish in the region and make for “really good table fare.”
The ear bone sleuthing by the Montana team is “a pretty interesting use of that approach,” says Rachel Johnson, a fisheries biologist with the National Oceanic and Atmospheric Administration in Santa Cruz, California, who uses otoliths to study migration patterns of salmon. The technique worked well in this case, she says, because each lake has such distinct geochemistry. But, she says, over broader geographic regions where the water chemistry isn’t as varied from lake to lake, finding as clear a match may prove harder.
Another factor working in the Montana researchers’ favor was the fortuitous and improbable fact that they seemed to have found the very individuals that had been introduced, rather than their offspring, says Jake Vander Zanden, an ecologist at the University of Wisconsin-Madison who studies aquatic invasive species.
“Typically when you discover a new population of what might be considered an invasive species, you’re not going to capture the individuals that were themselves transported,” Vander Zanden says. He calls the otolith findings “pretty striking.”
Continue reading: How tiny fish ear bones can reveal criminal activity