The Fisheries Ministry launched a nationwide campaign by hosting a fishing competition at a reservoir in central Kurunegala district, where more than 1,000 anglers were instructed to reel in only the introduced predators.

Ministry secretary Kolitha Kamal Jinadasa said the import, sale and transportation of live redline giant snakehead, knife fish, alligator gar and piranha was banned from Saturday.

People who already keep the scarily named breeds in their homes or in private aquariums will also be given three months to register them with authorities.

"It is not easy to catch them with a net, because they are very aggressive and their teeth are very sharp," Jinadasa told hundreds of anglers during the competition targeting snakeheads at Deduru Oya reservoir.

"In one day, we can remove a large number of fish from the natural environment, and then we can control their population," he said.

Jinadasa labelled the day a success, although only 22 snakeheads of between two and four kilograms (4.5 to nine pounds) were weighed in the best angler competition.

Recreational fisherman N. A. V. Sandaruwan, 37, won the top prize of 20,000 rupees ($66) and a rod and reel.

"I nearly had another big snakehead, but it managed to get away," he said.

Officials also encouraged competitors to take their catch home and cook them, although it is not usually a species consumed by locals.

Two anglers from India, Dinesh Kumar and Raj Thilak, joined the competition but neither was able to bag a snakehead.

"Some days you get one, some days you don't, but that's fishing," Kumar told AFP.

Jinadasa hoped there might even be a tourism spinoff in the campaign to rid Sri Lankan waters of the invasive species.

He said snakeheads, which can grow to more than a metre (3.3 feet), were multiplying rapidly in the Deduru Oya reservoir, threatening smaller native species.