The fish were found in the Laxey River
About 400 fish have been killed by pollution in an Isle of Man river.
Dead salmon and sea trout were found in the Laxey River between the Woollen Mills and the Shore Hotel last week.
Fisheries staff found the source of the pollution and removed the poisoned fish, and the authorities are now seeking to prosecute those responsible.
Environmental pollution officer at the Department of the Environment, Dr Phil Styles, said it could have an impact on fish stocks.
"[It affected] a significant number, hundreds in fact, of large, mature salmon and trout, and smaller fish and eels," he said.
"Obviously any loss of fish of that age, mature salmon, is significant.
"What can be done about it at this stage I don't really know, but it's a great shame because obviously these fish could have spawned later in the year."