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bout six tons of fish cultivated in keramba (net cages) were left floating on the surface of Lake Maninjau on Saturday; they are believed to have died of a lack of oxygen.

Agam regency Fisheries and Maritime Affairs Agency head Ermanto said the fish, weighing on average 4 kilograms each and cultivated in cages in an area adjacent to the villages of Bayur Maninjau and Duo Koto, had died suddenly and en masse on Friday.

'€œThe cage owners were in fact already worried, as there had been no waves on the lake the day before. It'€™s a dangerous sign, because when the waters are calm, it can cause a lack of oxygen among the fish,'€ Ermanto said.

Some farmers removed the fish to inland ponds with sources of running water, but others ignored the warning signs, expecting to harvest the fish just two or three days later.

The fish required waves to bring oxygen, Ermanto went on, as the lake contained no oxygen between depths of 5 and 10 meters.

'€œFarmers have also been warned not to overfeed the surviving fish,'€ he said.

Mass deaths of fish in the lake have been a regular occurrence for 15 years, with overfarming generally blamed. Earlier in January, storms killed two tons of fish and destroyed many keramba, releasing a further 100 tons of fish. Last year, three separate incidents saw between 5 and 80 tons of fish die.

Currently, Ermanto said, there were 20,000 cages on the lake. A cage of 5 x 5 meters houses 4,000 small fish, and can produce 750 kilograms of fish per harvest. One cage of fish requires a ton of food per month.

'€œEvery day, five or 10 new cages are added. Cage numbers are growing rapidly,'€ he said.

However, he added, it was not the absolute numbers of cages that was problematic, but their concentration in a certain part of the lake, while other areas remained empty.

'€œIt'€™s tricky to redistribute the cages. We have to convince the local farmers and the cage owners, who mostly come from outside the area,'€ he explained.

The owners, who hail from North Sumatra, Riau and Bukittinggi in West Sumatra, hire local workers to man their keramba. However, as none of them hold business permits, the regency administration earns nothing from their business. The local authority'€™s attempts to curb the business, however, have been met with resistance from local residents.

Separately, fisheries expert Hafrijal Syandri of Bung Hatta University in Padang said the mass fish deaths were mainly down to pollution in the lake water caused by the remains of years'€™ worth of fish food.

Hafrijal, who frequently studies fish-farming on the lake, said almost 112 tons of food sediment left at the bottom of the lake since the large-scale fish farming began in 2001 had reduced the depth of the lake by an average 16 meters.

'€œThe sediment turns poisonous and kills the fish,'€ he said.

The number of cages in the lake, he added, should ideally be capped at no more than 6,000.