TAIPEI - Up to 15 tonnes of dead fish have surfaced at the Pacific Ocean mouth of a river near Taipei, leaving local officials puzzled.

The migratory snakehead fish, which are native to the area where they died, were discovered on Thursday in piles along several kilometres of the banks of the Tamshui River.

Officials said the fish may have died due to a lack of water-borne oxygen caused by Typhoon Saomai as it passed over northern Taiwan on its way to southeastern China.

"So many fish dying is a signal for the ecosystem. It means there's something wrong with the water quality," said Lin Hsen-cheng, co-chair of the Green Party of Taiwan. "We should check for the reason as soon as possible."

Taipei County officials said the fish posed no public health danger, but the stack had begun to stink.

The county would find a way to scoop up the fish and might take some to a laboratory to see why they died, said Yang Zhi-hong, vice director of the county environmental protection department.

Lin of the Green Party said he suspected that unusually warm water brought by the typhoon had upset the oxygen balance in the water and killed the fish.

A local newspaper said other theories for the death of the fish included industrial waste or changes in the water content caused by the storm.