“A dead fish is a dead fish.” That’s the key finding of a recent study on the scavenging habits of our native crayfish, led by Calum MacNeil, a freshwater and invasion ecologist at Cawthron Institute.
Kōura detect predators, such as eels, in the water through chemical cues. They can also tell when a predator is dead—which, to kōura, means food. The freshwater crays are far better at picking up on the cues of native predators than they are introduced ones, though. MacNeil thought they might be wary about approaching the carcass of something like a trout or a catfish.
Catfish populations have exploded in the central North Island, and kōura have crashed as a result. In a bid to control the problem, more than 180,000 catfish have been removed from lakes in the Rotorua district alone. The fish were mostly buried, which to MacNeil seemed like “a lot of protein… going to waste”.
Perhaps, he thought, kōura could benefit from this carnage. He and his colleagues experimented with kōura in tanks to see if they would eat a dead catfish as readily as they would a native eel. Kōura, it turns out, aren’t fussy, or fearful. “They’re not like, ‘What’s that fish that’s in my tank?’” says MacNeil. “It’s like, ‘That’s a dead fish. I’m going to eat that dead fish.’ There wasn’t any hesitation.”
The findings, MacNeil says, could be useful in the nascent kōura farming industry. Perhaps we could be getting rid of catfish and growing fat kōura at the same time. “It’s one way the native can use the invader.”
