MANILA, Philippines -- Pollution, slums and an invasion of foreign fish species are threatening the Philippines' largest lake, one of the nation's major fish farming regions, according to a study published Tuesday.

Laguna de Bay on the outskirts of eastern Manila covers more than 90,000 hectares (222,300 acres) but since 1965, fish pens and cages have been built out across some 15 percent of the lake.

The study, by the Southeast Asian Fisheries Development Center, said the lake's ecosystem was also being threatened by pollution caused by residential, commercial and industrial development and the encroachment of squatters along its shores.

"The worsening water quality in Laguna de Bay, which is caused mainly by water pollution, leads to water algae that kill fish through oxygen depletion as well as taint the flesh of the surviving fish," the report said.

Foreign fish species, such as the janitor fish, destroyed nets and competed for natural food and living space with the cultured species, it added.

Fishermen are being hemmed into ever smaller fishing areas, while illegal construction of fish pens along navigational lanes is causing friction between them and farming industry operators, the report said.

The study said the silting-up and pollution of the Pasig river connecting Laguna de Bay with the sea on Manila Bay also obstructed the natural backflow of saltwater to nourish aquatic life.