Fish Eat Mammals on the Regular

A new study indicates that in some ecosystems, mammals are a fairly normal foodstuff for fish

rainbow trout
Michael Durham/Minden Pictures/Corbis

As a rule, with the exception of notorious predators like barracudas and sharks, we see fish as pretty low on the food chain. They swim around the sea or lake or stream nibbling on plants, insects or smaller fish. Trout, in particular, are generally seen as a threat only to favored insects and insect larvae. But it turns out that rainbow trout and other fish in Alaska sometimes add small mammals to their diet.

A new study published in Ecology of Freshwater Fish looked at the stomach contents of rainbow trout and arctic grayling over the course of 13 years and found that the fish were regularly adding shrews to their diet, usually every two to three years when the shrew populations in Alaska are at their peak.

Matt Milller, writing for the Nature Conservancy, explains:

Shrews are insectivorous mammals that are known to have boom and bust cycles. The researchers speculate that in years of peak abundance, trout and grayling eat them in significant numbers.

“Fish are good at selecting rich prey sources,” says Lisi. “Fly fishers know this well. During a mayfly hatch, if you don’t have the right fly, you’re not going to catch anything, because trout are all focused on a very specific insect.”

Co-author of the study Peter Lisi told Miller that the research started off as a side project after the researchers found fish with shrews in their stomach and wanted to figure out how often it happened. 

Scientists have observed other fish species with unexpected prey, including Africa's tigerfish, which has been caught on video chomping birds out of mid-airCatfish are known to devour pigeons that stray too close to the waters edge.

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