The Philippine fisheries chief on Sunday said he had ordered a study into a foreign species called the "knife fish" that was posing a threat to the local fishing industry at the country's largest lake.

The knife-shaped fish are reported to be multiplying in Laguna Lake where they are displacing the native species, said Asis Perez, head of the Bureau of Fisheries and Aquatic Resources.

"It is carnivorous. It will compete with our existing natural fish. We have yet to get a full appreciation of the damage caused by this fish," he told AFP.

The fish, which can measure about 51 centimetres (20 inches), was reportedly being bred locally as an ornamental species for aquariums but somehow got into the lake, which is located on the edge of Manila.

Fishermen's groups have complained that their catches of native fish are shrinking, with their income threatened because few people want to buy the knife fish to eat.

Perez said this appeared to be the latest case of an alien invasive species harming the local environment.

In previous years, the suckermouth catfish, known locally as the "janitor fish", was also introduced to local waters by ornamental fish breeders, where it displaced native species and damaged nets.

"This could be like the janitor fish. They were both introduced in the Philippines as ornamental fish. They could have escaped or they could have been intentionally freed," said Perez.

The Philippines has only recently stepped up its concern over the introduction of alien species to native habitats, which can drive drive local species to extinction.

The country's numerous porous ports have made it difficult to prevent the introduction of such species. In December, five people were arrested after they were caught with dozens of banned, carnivorous piranha fish they had brought into the country.