Last Thursday's mass fish deaths, including those farmed in sea cages off Tanjung Kupang, are believed to have been caused by the 'red tide' phenomenon.

The Department of Environment (DOE), in a statement here Sunday, said the phenomenon was caused by excessive plankton resulting in reduced oxygen in the water.

It said DOE which conducted preliminary investigation on the day of the incident, found no contaminant or oil spill near the venue.

"A similar incident also occurred on Dec 28, 2009. Nevertheless, DOE will continue to monitor and take enforcement measures on the premises in Gelang Patah," said the statement.

It also said the Johor Fisheries Department had been referred to identify the exact cause of the fish deaths through the analysis of fish samples.

Meanwhile, Kumpulan Prasarana Rakyat Johor (KPRJ) denied that offshore land reclamation work conducted by the company had caused the death of an estimated four tonnes of fish.

Its executive deputy chairman, Datuk Md Yusof Othman said the reported fish deaths were not just happening in the reclaimed areas but also in Singapore.

According to him, the reclamation work began on Jan 22, involving a 0.4 hectare area.

"If the fish were dying due to reclamation work, why are they only dying now and not a day or two after we started work?," he asked.

The incident has affected 10 fish farms and 250 traditional fishermen.