Are otters threatening amphibian populations due to the scarcity of fish?

An image of an otter to illustrate the question of whether otters are affecting amphibians
© iStock/erniedecker

A new study has reviewed amphibian populations which are in global decline and are being eaten by Eurasian otters.

Typically, the Eurasian otter eats fish. However, amphibians are also part of its diet, particularly when there is a scarcity of fish. In the new study in Mammal Review, researchers identified the bones of amphibians in otter faeces from southern Italy.

A major food source for otters

The corresponding author Dr. Alessandro Balestrieri, from the University of Milan, Italy, said: “We knew that amphibians may represent a major food for otters in the Mediterranean area, but I admit we were amazed and impressed to discover how great the diversity of this resource could be.”

They studied the bones to determine which types of amphibians are usually eaten, and also reviewed sixty four existing studies on otter diets.

Reviewing the diets of otters

In the sixty four studies, an average of twelve percent of prey for otters were amphibians. The predation of amphibians increased with longitude and was highest in winter and spring in the Alpine biogeographical region.

Investigating amphibian populations

The study states: “Amphibians form a major component of the diet of the otter Lutra lutra in several areas of its wide geographic range. Yet, amphibian remains are rarely identified to species level and therefore information on the diversity of this food resource is generally scarce.”

The investigators studied 355 individuals belonging to at least seven amphibian taxa. They concluded that when feeding on frogs on toads, otters are more likely to choose noisy males rather than the quieter females as their prey.

The authors added: “We conclude that the contribution of amphibians to the richness of the otter’s prey community is far higher than commonly perceived, and that osteological analyses allow the detailed investigation of the feeding behaviour of this top predator of freshwater habitats.”

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