A fast-growing carnivorous fish that was originally bred in a laboratory could be invading Hong Kong waters, environmentalists have warned amid reports of juveniles being caught off the city’s coasts.

The Sabah giant grouper, a hybrid species of the tiger and giant grouper, was crossbred to reduce overfishing of wild groupers for food.

But experts said if the fish was allowed to proliferate, it could “pollute the native gene pool” and throw the ecosystem off balance – similar to the disastrous lion fish invasion of the Bahamas in 2004.

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In 2016, scientists from the Universiti Malaysia Sabah (UMS) in Kota Kinabalu – where the first generation of the hybrid grouper species was bred in the mid-2000s – discovered that the fish could spawn in captivity.

“No one can say for sure whether the Sabah giant grouper has successfully invaded Hong Kong waters. But if allowed to lay down roots and breed, it could very well become an invasive species,” said Dr Michelle Cheung Ma-shan, science manager at local research group Eco-education and Resources Centre.

In theory, live specimens of the Sabah grouper can only be found in wet markets, restaurant aquariums or inside offshore sea pens at aquaculture sites around Hong Kong.

While there have been cases of some groupers escaping from farms, the species is also a popular choice for religious groups to use in controversial “mercy releases”. As a result, the grouper is often spotted by scuba divers and caught by recreational anglers.

“There’s a high possibility that this could lead to pollution of the local gene pool. They are also a carnivorous fish that can compete with other native species for food or eat up a lot of smaller ones,” Cheung said.

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There have been multiple reports of recreational fishermen reeling in Sabah groupers, mostly adult specimens around Sai Kung.

According to a study by the centre, 85 per cent of local wet markets in Hong Kong sell the fish and most are between 20cm (7.9 inches) and 40cm in length. About a third of stocks come from Malaysia, a fifth from Hong Kong, 10 per cent from the mainland and another third from “unknown” origins.

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The most alarming discovery came in March last year when a juvenile Sabah grouper, less than 10cm long, was caught at a harbour in North Point. The nearest fish farm is off Lamma Island more than 12km away. This suggested that the species may be breeding in the wild. “This is really worrying,” Cheung said.

Dr Man Chi-sum, chief executive of environmental group Green Power, said: “The juvenile was found in North Point where there are no aquaculture sites at all.

“It could not have been released [by religious groups] as a specimen this small in size is not available in wet markets. If it escaped from a fish farm in Sai Kung and swam all the way to North Point, it would have defied a very high probability of death. That leaves only a few other possibilities.”

Because of the species’ quick growth rate and resilience, it has replaced the brown-marbled grouper as the most dominant species in Hong Kong aquaculture, as well as on restaurant plates, according to Cheng King-man, chairman of the Sai Kung Fishermen Association.

The average fish fry mortality of the Sabah giant grouper is just 10 per cent compared to 40 per cent for the brown-marbled grouper. The Sabah giant grouper also takes just eight months to reach “plate size”, where it is ideal to be served at restaurants.

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