Human encroachment into wildlife habitat, especially streams and small rivers, is not good news for turtles. But one group of turtles may actually be making the best of a sticky situation: Eastern snapping turtles (Chelydra serpentina), a species of freshwater turtles commonly found across North America, have found a way to use culverts to their advantage. They use these tight spaces to ambush passing migratory fish.

A recent study tracked the predation habits of snapping turtles in and around culverts constructed near the Herring River in Massachusetts. The area sees seasonal migrations of river herring. Like salmon, river herring are born in freshwater, live their lives in the ocean, and return to freshwater to spawn. All this back and forth movement means that the herring have to cross many culverts, which act as bottlenecks and slow down the passage of fish. 

At the Herring River culverts, researchers used video cameras to track the movements of fish and the predation habits of turtles. Along with the migratory herring, they also tracked the resident fish that live in these rivers all year round and are familiar with the culverts. 

The scientists found that resident fish avoided the center of the tunnels in culverts where snapping turtles lay in wait, successfully avoiding becoming a turtle's meal. The herring, on the other hand, did not learn to avoid the turtles and were caught more often. The turtles, in turn, preferred to target the herring, which tended to swim lower and were easier for them to catch.

The study showed how the snapping turtles used the culverts and quickly learned to target the naïve herring over the experienced resident fish. The turtles also seemed to slow down the overall movement of herring as these fish eventually started avoiding culverts that had turtles. Although the culverts might allow good eating for the turtles, the researchers caution that this could potentially delay fish migration in the long term if the herring are scared off of culverts altogether.