THEY could challenge the South American piranha for viciousness and they can make sharks look positively docile.

A small, innocuous-looking fish called a chinaman leatherjacket is swarming in Sydney waters, attacking and eating anything that moves, from bare fishing hooks to large marlin.

From 10cm to about 40cm long, the yellow and black striped leatherjackets look nothing like the popular perception of a marine predator, having tiny beak-like mouths a few centimetres wide.

But inside is a set of teeth like razor-sharp chisels. Anglers complain of leatherjackets biting straight through fishing hooks.

Geoff Somerville, who operates Ibex deep sea fishing charters out of Botany Bay, said that the chinaman leatherjackets had returned to Sydney waters in the past few weeks after several months of making life hell for fishermen on the South Coast.

"They are back in plague proportions, the numbers are unbelievable," Mr Somerville said.

"They have been stripping the bottom of the ocean of food in some places. The desirable fish like snapper leave because there's nothing left for them to eat.

"If you do manage to hook a desirable fish and don't bring it to the boat quickly enough, all you'll reel in is a head or a skeleton.

"The jackets just strip the fish bare of flesh. They'll follow a fish up to the boat and then swarms of them will just sit under the boat and it becomes impossible to fish."

The leatherjackets' aggression and gang attack behaviour has been revealed in a stunning video shot at the scallop beds in Jervis Bay a few weeks ago, which showed leatherjackets attacking a large octopus.

The Daily Telegraph columnist and Modern Fishing magazine writer Al McGlashan said that on some days the fish were so aggressive he feared what would happen if someone fell in the water.



"I've caught and seen mako sharks, tiger sharks, all manner of sharks and fish like barracuda - but leatherjackets are the most vicious fish I've ever seen," he said.



"Sharks can actually be quite shy and wary, but they [the leatherjackets] will eat anything and they aren't afraid of anything."

Mick Collins from Otto's Tackle World at Drummoyne said there were reports a pack of 40cm leatherjackets had attacked a 2.5m marlin as it was being brought in to a boat during a tournament at Port Stephens last month.

"We are getting guys who have lost hundreds of dollars worth of tackle in a day because the jackets bite off anything put in the water. They are losing $30 lures one after another," he said.

"Smaller ones of 10cm to 15cm long have come into the Harbour in the past few weeks and it can be actually impossible to fish."

Industry and Investment NSW Researcher Marcus Miller said the earliest known catch record in NSW is from 1883, when an annual report to NSW Fisheries identified leatherjackets as "troublesome to schnapper fishers".

Mr Miller said that they are one of the most important species in the NSW commercial trap and trawl fisheries.