UPDATE: It appears pollution may not have been the reason for the fish deaths at Swanpool, but instead a dramatic change in oxygen levels could be responsible.

A spokesman for the Environment Agency said today: "Shortly after our statement, we received more reports of dead fish and, after observing the behaviour of the fish, a theory was developed about the unusual mortality rate. We were at the site last night to see if it was correct.

"By day, algae in the lake is super-oxygenating the shallow warm water to a level of 180 per cent. But by night, the oxygen level crashes as the algae consume the oxygen.

"This phenomenon, combined with the stress a mullet experiences when it acclimatises from the saline water of the stream to the brackish water of the lagoon, appears to be responsible for the mortality rate.

"A full suite of water sampling was taken and sent for analysis. While we can’t rule out pollution until the results return, it seems the fish are subject to varying oxygen levels caused by temperature combined with adapting to different waters."

A suspected water contamination caused the deaths of dozens of fish which were found floating in the lake at Swanpool in Falmouth at the start of this week.

The Environment Agency and the leaseholders of the leisure facilities at Swanpool Beach are urging homeowners and businesses to be careful what they dispose of near the Tregoniggie Stream, as it is believed pollutants from the stream entered the lake, which is a nature reserve and site of special scientific interest.

Tests carried out on Monday showed the water to be normal for the site, and it seems that whatever killed the fish has already dispersed.

One eyewitness reported seeing "a black bag full" of dead fish recovered from the lake, and workers from Swanpool Beach recovered at least 15 from the water on Tuesday morning, while more could still be seen floating lifeless on Tuesday afternoon.

It is believed the contaminants entered the water course some time over the weekend, when the first dead fish were spotted, and by the time Environment Agency workers came to check the water it was back to normal. However it may have taken some time for the effects on the fish to become clear.

Esther Lochrie of Swanpool Beach said: "Sadly we’ve found some dead fish in Swanpool over the last few days. We contacted the Environment Agency immediately who came to test the water in the pool and in the sea yesterday. Both of which were absolutely fine. "The Environment Agency believe that one of the tributary streams that feed into the pool had been polluted in some way a few days before and this has killed the fish.

"We’d like to highlight the fact that whilst the water is now completely fine, everyone is responsible for what goes into storm drains and feeds into our streams – anything that goes in has a follow on effect. This is a tragedy in that the fish that have died (bass and mullet) are slow growing species and the population will take some time to recover. If there can be one take away from this unfortunate event it’s that people should think twice about what they are disposing of and where."

An Environment Agency spokesperson confirmed that an officer had found eight dead mullet "but the cause of death is not known," and "the fish were decomposed so had been dead for some time," while tests showed normal dissolved oxygen levels in the lagoon with no signs of ammonia, pollution or algal bloom.

The spokesperson added: "Mullet are marine/estuarine fish which enter the lagoon from the sea. A number of live, active fish were found at the point that the freshwater stream enters the lagoon.

“It is possible that an undetected surface water pollution entered the stream at some point, impacting fish before diluting. There is no conclusive proof as there are no reports of any pollution events on the stream."

Lee Warren, one of the lease holders at Swanpool Beach, said: "We don't want it to happen again. There's nothing we can do, we're completely at the mercy of that water course."

He added that there had been anecdotal stories about contaminants getting into the water before, and said: "We want to raise awareness that it's a stream that feeds a nature reserve. Be careful what you put in it, and into the storm drains around it."

Swanpool is a brackish lagoon which contains a mix of freshwater and saltwater in clearly defined layers, making it a rare ecosystem, and is also home to the trembling sea mat, a collection of small animals which together look like a plant and which is found nowhere else in Britain.