Dale Cameron of Trout Unlimited says more than 2,000 fish have been scooped out of a three-kilometre stretch of Barclay Brook in Coleman since Thursday.
Cameron says that's more than triple the amount of fish that washed up on the same shores of the brook last July, though the current discovery is concentrated in a smaller area.
Provincial Green party Leader Sharon Labchuk says legislation that requires buffer zones between waterways and farm fields is not working.
She says pesticides should be eliminated from the province's agriculture industry, as pesticide runoff has historically been a major cause of fish kills, though the cause of this fish kill is not yet known.
Gretchen Fitzgerald of Sierra Club Canada says the use of pesticides should at least be reduced and the province should better enforce buffer zone regulations.
Environment Minister Janice Sherry says last year's fish kill prompted the provincial government to draft changes to regulations that she hopes will be introduced in the legislature this fall.