Anglers from northern Europe are using live kittens for bait while fishing in the Po river for the gigantic sheat-fish, according to police.
Giuseppe Lagana, a police officer in Mantua in the Po valley, said that the first case was detected in May. But Mr Lagana, who doubles as a commander with Ampana, Italy's voluntary nature protection force, said the incident was not isolated.
"We've received a lot of indications that there are other such cases," he said. "During one of our last nocturnal patrols, one boat managed to evade our checks. It ignored warnings and cleared off with its lights extinguished. We suspect that this was also one of these fishermen without a conscience. Using live kittens to catch sheat-fish is an unheard-of cruelty."
The sheat-fish is a freshwater catfish that can grow to a length of two metres or more. It can weigh up to 660lb (300kg). It has decimated the traditional fish of the Po, Italy's longest river, since its introduction some years ago, and has colonised all the waterways around Mantua. The fish, which has a jutting lower jaw and the long curling whiskers of the catfish, is omnivorous, and when it has exhausted local supplies of carp and eel is happy to snap up ducks and rats. The largest specimens think nothing of charging anglers' boats. They also eat each other.
Traditionally, Italians have been sniffy about sheat-fish, holding that its meat is not worth eating.
Sometimes the fish is passed off to credulous restaurateurs as sturgeon. But the big-game challenges of catching the brute have proved irresistible.
Anglers descend on the Po, pile into rented houseboats and set out to do battle, with or without a secret stash of kittens.
Mr Lagana said that another cruel method of catching the fish was to spear an eel with a harpoon and dangle it in the water, where its agonised thrashings were intolerably tempting for the big beast of the deeps.
Ugly though these practices are, Mr Lagana holds out little hope of stopping them.
"Last year, the voluntary guards in the region covered 15,000 kilometres [9,300 miles]," he said. "But to catch these squalid people in the act is very difficult, even using co-ordinated land and water night patrols."