Bad weather has damaged a "fortress pen", allowing 20,000 yellowtail kingfish to escape into the local ecosystem off the coast of Port Stephens, in NSW, Australia.

There are fears thousands of "ravenous" kingfish that escaped a government jointly run fish farm in Australia will devastate a marine park's wild fish population.

Up to 17,000 predatory yellowtail kingfish, used to being fed automatically, are now hunting in marine park waters off Port Stephens, NSW. Last week, 20,000 of the fish escaped from a fish-farm sea cage, described as a "fortress pen", that was destroyed in rough seas. About 3000 fish have been recaptured.

SAM NORRIS The Huon Aquaculture and NSW Department of Primary Industry commercial-size yellowtail kingfish trial site off Port Stephens, in Australia. (File photo)

The future of the controversial joint NSW government and Tasmania-based Huon Aquaculture project, which is 18 months into a five-year research trial, is under a cloud following the loss of almost half its stock with a retail value of more than A$2 million (NZ$2.2 million).

Conservation groups and local tourism operators described the multimillion-dollar project as a "disaster" threatening the pristine marine park's delicate ecosystem.

READ MORE:

* When catching waves turns into catching fish

* Escape of octopus Inky makes a splash worldwide

* Manager 'devastated' by illegal release of 6000 salmon

Marine Parks' Association chairman and whale watching tour operator Frank Future said fisheries staff "repeatedly assured" the community the pens could handle waves up to 15 metres.

According to Huon, the "fortress pens" were designed to withstand "high energy, exposed sites, frequently receiving storms swells and gale-force winds".

"The pen that had the release was mangled and now we have thousands of mature kingfish released into the wild, nothing will be safe from them," Future said.

"They are voracious feeders and from what I understand they are ravenous. Once they realise they won't get any food in the form of pellets they'll be eating anything they can find. I don't want to think about the impact on wild species."

The commercial-scale kingfish trial at Providence Bay - the result of an existing offshore research lease being boosted to 62 hectares - includes five pens, each about 60 metres across, two that were stocked with 20,000 fish each. There is capacity for 12 sea pens in the trial.

Word of the bounty spread quickly prior to the long weekend, via social media, pricking the ears of recreational and commercial fishers who flocked to the area.

With the kingfish - selling for up to A$32 (NZ$35) per kilogram - churning up the waters 7km off Hawks Nest, fishers set to work scooping them from the sea in any way they could.

Recreational fisherman Jeff Thompson was on his way to Broughton Island, off Port Stephens, on Saturday, January 20, the morning after the pen was damaged, when kingfish started gathering around his boat.

Thompson said he respected the bag limit of five and the fish he caught were all legal size, between 70 and 75 centimetres.

"I've never seen anything like it in more than 40 years of fishing," he said.

"I think it was the sound of the motor that attracted them and anything you threw at them they took, even just a bare hook.

"There's no doubt 20,000 kingfish would have a big impact on the ecology of the area, I think it's better if people catch them and get them out of the area. The next day there were dozens of boats out there."

About 2 tons of the restaurant-quality fish made its way through the Commercial Fisherman's Co-operative before authorities closed down the area to fishing. The ban remains in place until February 7.

A spokeswoman for Huon Aquaculture said the nearest wave buoy to the farm recorded wave heights over 11 metres when the pen was damaged, but others questioned the accuracy of the recording.

Weatherwatch senior meteorologist Don White said given the known weather conditions at the time, he doubted there would have been 11-metre swell in the area.

Huon's spokeswoman declined to comment on the pen's failure before the release of a review into the incident.

She said the farmed kingfish were of the same genetic stock as wild populations and the company was "researching the behavioural and feeding responses of the escaped fish" as it continues to try and recapture them.

"Early indications suggest that equipment design is not the lead cause of equipment failure," she said.

"Whilst we are disappointed with the recent fish escape following a weather incident, it is an important learning experience and we remain committed to ensuring that there isn't a repeat of the incident."

'PLAGUE PROPORTIONS'

Fishers reported the kingfish in "plague proportions" off Broughton Island and schooling at Little Beach, Shoal Bay.

Professor Bronwyn Gillanders, of the School of Biological Sciences at the University of Adelaide, said the key concern was the yellowtail kingfish's predatory nature.

She described the escape as a "natural experiment" and said it was "hard to know" what impact the farmed fish would have on wild stock.

"They haven't been brought up to feed on wild fish," she said. "But they are predatory fish and they would normally eat other fish. Whether they have the impact claimed, it's difficult to tell."

A spokesman for the NSW Department of Primary Industries said 17,000 kingfish was not "significant" in terms of the total wild yellowtail kingfish population in the area.

"The farmed yellowtail kingfish are of the same genetic stock as wild populations with broodfish being sourced locally," he said.

"The farmed fish are from local parent stock and are health checked on a routine basis, so they are not considered a biosecurity risk."

It is the second large-scale fish farm operation in Port Stephens to suffer huge stock losses due to storm damage.