Determining the cause of a third fish kill in western P.E.I. this week will be difficult, because its discovery comes days after the event.

An angler discovered the dead fish in Mill River Monday night. The province is investigating, but that investigation will face problems.

"It is the timing that's the challenge. The fish that we're seeing have been dead for more than a couple of days, so it is difficult to start looking at the causes at that point," said Department of Environment fisheries biologist Rosanne MacFarlane.

MacFarlane said the timing of this finding will make it difficult to determine the severity and source of the kill. The fish they're finding are already starting to decay, but officials are taking samples for analysis.

The Mill River kill follows two separate incidents on the weekend in Trout River and the Big Pierre Jacques River.

Thousands of fish are believed killed in those rivers, and provincial officials are still trying to determine the cause. Soil, water and fish samples were also taken from those rivers for analysis.

As bad as 1972

Daryl Guignon, a biologist who works for  UPEI, said prior to what he's seen in the last few days, the worst fish kill was in 1972. Back then, chemicals were dumped in the West River and tens of thousands of fish were killed.

"When I look at this one, it seems every bit as bad as that original one in 1972. As a matter of fact, I don't think I've ever seen so may big fish at one place and from one river as there was in Trout River," he said Tuesday.

He said for every dead big fish, it's likely 1,000 younger ones died too.

"If you have something that kills the fish and all you have left are the big ones, it means there are many, many that just didn't show up. They're in the mud or grass, consumed by predators," he said. 

It's expected to take several weeks for test results to come back, but whatever the cause, Guignon said it is the worst in decades or possibly ever. It will take years for the rivers to recover.