Invasive blackchin tilapia fish have been found in Bangkok’s Makkasan swamp, which connects with the Chao Phraya River via canals, according to Bangkok governor Chadchart Sittipunt.
The governor said on Tuesday that blackchin tilapia were among the many dead fish left stranded when local officials drained the 72,000-square-metre swamp in Ratchathewi district in preparation for wet season flooding, and a pumping station was turned off for maintenance.
Blackchin tilapia made up about 20% of the dead fish found on Monday he said during a visit to the area. The rest were common tilapia.
“Makkasan swamp connects with the Chao Phraya River via the Sam Sen, Saen Saep and Khlong Tan canals. This is quite a cause for concern,” Mr Chadchart said.
He said City Hall was closely monitoring the movement of the predatory fish species, which is native to west Africa.
The governor said quick action was needed to contain the population of blackchin tilapia. Finding a use for them might help. Technology could also be applied to sterilise the fish, and the Department of Fisheries was working on this.
On Monday many people rushed to swamp to scoop up the stranded fish after learning about it on social media.
Two thousand blackchin tilapia were originally imported from Ghana in 2010 by CP Foods, part of the Charoen Pokphand agribusiness conglomerate, according to Isra News Agency. The company had a permit to study the fish for breeding at its research centre in Samut Songkhram province.
The company subsequently told the Department of Fisheries the fish had died within three weeks of being brought to Thailand and had been buried. But they later started showing up in local waterways and as their population grew, native fish species started decreasing in number.
Blackchin tilapia found in the waters of 13 Thai provinces were all found to have come from the same parent stock, based on DNA testing by the Department of Fisheries.
Authorities say the species is highly adaptive and can thrive in a wide range of water temperatures.
The fish have been found breeding in Samut Songkhram, Samut Prakan, Phetchaburi, Bangkok, Chanthaburi, Rayong, Ratchaburi, Prachuap Khiri Khan, Chumphon, Surat Thani, Nakhon Si Thammarat, Songkhla and Samut Sakhon.
According to Thanaporn Jiansuk, president of the Khlong Dan Fishery Association in Samut Prakan, blackchin tilapia breed very quickly - with only 22 days from the egg-laying stage to hatching. As many as 99% of their eggs hatched, she said.
The fish could repeat their reproduction cycle right after laying eggs, without a break, she said.
Agriculture Minister Thamanat Prompow said on Tuesday the elimination of blackchin tilapia would be put on the national agenda. A study into effective sterilisation was already underway.
He had also ordered the Rubber Authority of Thailand to allocate funds for the purchase of the fish for use in fertiliser production.
Mr Thamanat said the fish had first appeared in Thailand many years ago and efforts to eliminate it had begun in Samut Prakan, Samut Sakhon and Samut Songkhram provinces.
"They have reached Phetchaburi and Prachuap Khiri Khan. They can live in rivers and tributaries and survive in freshwater, brackish water and seawater,” he said.
The minister said he was worried that the fish invasion had now reached Bangkok. However, there was no clear evidence who was responsible for it.