About 1,000 dead fish were scooped up along a section of the North River in central P.E.I. on Sunday.
Someone discovered the fish on Saturday, then alerted the province.
Volunteers were out Sunday to pick up the dead brook trout, rainbow trout and Atlantic salmon along a 3.5-kilometre section of the river.
Federal and provincial environment officers have collected water and fish samples to try to determine the cause.
Rosie MacFarlane, a provincial fisheries biologist, suspects heavy rain earlier this week played a role.
"We have seen fish kill events in recent years related to rainfall and runoff from agricultural land," she said.
"(I'm) not saying that's what caused this one, but there's usually a connection. And we don't normally see massive numbers of dead fish without a cause."
This is the first fish kill of the year on the island.
In each of the past few years, thousands of fish have died in western P.E.I. rivers.
MacFarlane said she'll have a better idea of how many fish died this time once the cleanup is done.
The cleanup is expected to continue on Monday.