The fish that have bellies full of mice – but we don’t know how

Jouan & Rius/NaturePL

By Robin Wylie

It’s a cat-and-mouse tale with a difference. The lesser salmon catfish has been found feasting on mice. But how does it catch them?

Some catfish are known to ambush unwary pigeons at the water’s edge, giving them the nickname “freshwater killer whales”. But the lesser salmon catfish might just be an opportunist, gobbling up animals when they drown.

A survey of 18 lesser salmon catfish (Neoarius graeffei) from Ashburton river in northern Australia, suggests the fish can consume large quantities of small land animals when given the chance — almost half of the catfish had mice in their bellies.

“That is a lot, and a rare finding,” says Peter Lisi, an aquatic ecologist at the University of Wisconsin-Madison.

The stomachs of some catfish contained as much as 95 per cent small mammals, with two fish having three animals each in their stomachs.

Lesser salmon catfish can grow to half a metre long and weigh up to 1.5 kilograms. They are a common species in dryland rivers of north-western Australia,  so their diet is important to understanding the local ecosystems.

They were thought to feed mainly on aquatic invertebrates and plants, with the occasional addition of fruit and terrestrial insects, especially during the floods in the wet season.

And though a few freshwater fish species are known to dine on land vertebrates — African tigerfish have been filmedplucking a swallow out of thin air, for example — it is rare for them to eat so many.

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