Environmental officials are investigating why dozens of dead fish are washing up on the banks of two western P.E.I. rivers.

The first dead fish began washing up on the banks of the Trout River on Friday, following heavy rains that morning.  About a dozen were found that same day, but officials and volunteers with the local watershed group have since found more than 100.

The nearby Mill River also experienced a fish kill, with the first dead fish reported being washed up on Monday.

Officials don't know yet what caused these events, but this is the third fish kill on the Trout River in three years. The last two were caused by pesticide runoff washed into the river by heavy rain, according to the province.

Dale Cameron, a member of Trout Unlimited Prince County Chapter, spent three hours pulling more than 20 kilograms of fish — mostly trout, but some small salmon — out of the Mill River on Monday.

"[The population] can't stand up to repeated pressure, it's going to end up … instead of it recovering in five or six years, if you keep getting it year after year after year then we could be looking at 20 years down the road before it ever gets back to where it was," he said.

Cameron was part of a committee that examined ways to protect the Trout River Watershed against further fish kills. The committee made 18 recommendations — many centred on soil conservation and buffer zones.

It's unclear how many of those recommendations were implemented. It's also unclear if soil runoff was the cause of the latest kill.

"The frustration is that we're still having events like this," said Cameron.

"[The] government's had tons of recommendations, lots of reports over the years, it’s time to take them out and dust them off and do more than give it lip service."

Provincial freshwater fisheries biologist Rosie MacFarlane said this fish kill is a huge setback to the recovery of Trout River, which has been closed to sport fishermen for two years.

"Typically we say five to seven years before a river recovers from a fish kill, but that's a [single] fish kill. I'm not sure what this is going to mean in the recovery. It's going to set it back, you can't keep knocking out your brood stock and expect a fairly quick recovery. I mean, there is a length of stream that's unaffected and hopefully those areas will help to bring the population back," said MacFarlane.

Could be weeks before cause is known

Both the P.E.I. Department of Environment and Environment Canada are investigating the fish  kill, but it's still too early to pinpoint the source or to say whether this incident was caused by heavy rain washing soil from the nearby fields into rivers.

"This fish kill, we don’t know. I mean anything is possible at this point," said MacFarlane.

The causes of these incidents will have to wait until test results come back in a weeks.

Greg Donald, with the P.E.I. Potato Board, was a part of the committee set up last year to study and prevent fish kills with Cameron.

He said one of their recommendations is now in practice — agricultural workers are helping in the early stage of the investigation.

"And that's a positive thing, because as much as can be learned about the situation as possible, to avoid this from something happening in the future," he said.

None of that is likely to make a difference, according to Daryl Guignion.

He has studied nearly every fish kill on P.E.I. since 1974.

"In reality, we have to have sound soil conservation practices in every farm on Prince Edward Island — and that just does not occur, and the recommendations are not followed," said Guignion.