which killed hundreds of Bermudian fish.

Director of Fisheries John Barnes said experts at top US universities were still "scratching their heads'' two months after the dead pinfish were washed up on the shores of Harrington Sound.

But he said marine biologists had practically ruled out pollution as the cause of the deaths.

They now believe natural chemicals, possibly caused by the El Nino weather system, may have caused an increase in toxic plankton in the Sound.

The plankton could have released poisonous acids which wiped out the fish in September.

Mr. Barnes said: "The phenomenon is over now and the tests are still being carried out at the Bermuda Biological Station and American universities.

"Tests have so far proved inconclusive and it could even take years before we find out exactly what killed the fish,'' he said. "In fact, it may have to happen again before we can be really sure.'' He said water from the Sound had been tested at the bio station and fish samples had been sent to Lousiana State University, where most of the tests are being done.

Mr. Barnes added: "We believe it was a natural occurrence -- a flash in the pan.

"We certainly don't believe it was caused by pollution. We had some evidence that the recent El Nino may have been one reason for a chemical imbalance in the water.

"Water chemistry systems are constantly in flux with each other and certain organisms can only thrive in certain conditions. We know that the conditions in Harrington Sound in September were unique.

"But even scientists are pulling their hair out and scratching their heads over this. It's a bit of a baffler.'' The fish died during a week-long period in mid-September, when there was unusually high rainfall and a drop in seawater temperatures.

Experts are still working on a theory that microbes called "red tide'' caused gill infections which killed the fish.

Scientists have already discounted a connection between the Harrington Sound fish deaths and major fish kills in Maryland and Delaware, thought to have been caused by killer poisons around sewage pipes.