A Guernsey Water spokesperson said: ‘Guernsey Water is currently investigating the death of fish in a stream at the reservoir.
‘The stream was isolated from St Saviour’s Reservoir at the time and there are no concerns over the safety or wholesomeness of water supplies from this or our other water treatment works.’
Guernsey Biological Records Centre manager Liz Sweet said: ‘There are many reasons fish can die: overpopulation, food chain collapse, a pollution event, heat, extreme temperature changes, age. A cause can’t be certain for now.
‘Stream water may have evaporated to the point it can’t sustain populations. Laws are in place to prevent pollution and pesticides entering water sources.
She said that in extreme heat algae can grow.
‘Or you can get phytoplankton blooms, and pesticides can kill all the bacteria and remove all the oxygen from the water, which can be disastrous.’
Guernsey Water has done a huge amount in terms of biodiversity sensitivity and management, she said.
‘As a company in Guernsey they really are practising what they preach. They will be the best bet to find the cause.’
Updates on the investigation will be revealed once more evidence is found.
In 2004, reservoir carp specimens were sent for analysis after about 30 had died – the cause was found to be stress from over-population.
Warnings were issued to Guernsey Trout Society members in 2011 to limit reservoir catch consumption after minor amounts of PFOS was discovered in the water, a chemical used in firefighting foam and pesticides.