CHARLOTTETOWN - Prince Edward Island investigators are looking at whether pesticide runoff is to blame for the deaths of hundreds of trout in two rivers.

The Environment Department said Monday in a news release that dead fish had appeared over the weekend on the Trout River and Big Pierre Jacques River following sudden downpours of rain.

The release didn't specify a cause, saying investigators were doing soil and water checks.

Rosanne MacFarlane, a biologist with the department working on the scene, said in an interview that a possible pesticide runoff is being investigated.

Cautioning that the investigation was still underway, she added: "We've got an area of interest we're looking at in terms of possible runoff. But we can't say for sure right now."

MacFarlane said so far about 200 dead fish have been collected along a four-kilometre section of Trout River in western P.E.I.

She said it's likely thousands more fish died, but estimates are difficult as predators and river currents quickly take away the remains.

"The amount (of fish) we pick up are only a small portion of what's actually killed," she said.

The Trout River fish kill was reported on Saturday while a second kill was reported Sunday in the Big Pierre Jacques River.

MacFarlane said the second fish kill appears to have affected a smaller area and there are fewer dead fish.

Sharon Labchuk, the leader of the province's Green party, said she believes it's almost certain that pesticides are behind the latest kill.

"Let's get real -- when has it been anything but pesticides?" she said in a telephone interview.

She refers to late July -- a time when farmers apply pesticides each week to thousands of hectares of riverside crops -- as "fish kill season."

Last summer, there was a kill of trout and other fish over a two-kilometre section of the Montrose River after heavy rainfalls.

In July 2007, investigators suggested that pesticides from farmers' fields had killed thousands of fish that were found floating in the Dunk and Tryon rivers in western P.E.I.

"This is routine, there's nothing else that's going to flow into those streams lined by potato fields that is going to kill fish," said Labchuk.

"My biggest concern is that this is now being accepted on Prince Edward Island as normal, and it doesn't raise too much comment anymore.

"It's just another fish kill. Whatever."

The streams of the Island flow into salt water estuaries that are rich fishing grounds, and areas where a growing aquaculture industry is based.

The Green party leader said the fish kills are a warning sign for these estuary industries and are also killing off the image of the Island as home to winding, pristine rivers where tourists can enjoy tranquil angling.

MacFarlane said the government will continue its investigation to attempt to find the cause, but she added it's difficult to precisely blame pesticides from water tests.

Asked why the fish kills are occurring, she noted the province has brought in buffer zone regulations in recent years and restricted the use of some chemicals.

"That has reduced the number of fish kills," she said.

However, she added the latest event is disappointing to those attempting to restock the province's rivers.

"It's always a concern, it's always disappointing to see this happen. The community groups are putting a lot of effort into stream restoration and managing the fishery here," she said.

MacFarlane says it can take five to six years for trout populations to fully recover from a significant pesticide runoff.