MADISON, Wis. -- The state Department of Natural Resources is cautioning owners of pets and hunting dogs that the blue-green algae toxin has been fatal.

DNR officials said that three dogs have died in late September and early October, the start of duck hunting season. They happened in Beaver Dam, Eau Claire County after hunting in Minnesota, and another unspecified location in the state.

DNR water management specialist James Vennie said that the toxin is released when the algae die. It then turns colors, including green and purple. He warns if the water is pea soup green, don't go in.

The DNR said that Wisconsin attracts an estimated 85,000 waterfowl hunters, second highest in the country behind Texas. About 60 percent use dogs to retrieve their kill.