A fish kill in a southeastern Minnesota trout stream last summer was likely caused by a combination of conditions following heavy rain on July 28, rather than any single chemical, biological or environmental cause, according to a state report released today. Within a few hours of an angler’s report of dead fish in the South Branch Whitewater River July 30, state officials from three agencies were on the scene to investigate what killed the fish in the river northeast of Rochester. An estimated 9,000 to 10,000 fish died, including brown and rainbow trout, dace, white suckers and sculpin.
Over the next several months, technical experts from the Minnesota Department of Natural Resources, Minnesota Department of Agriculture, and the Minnesota Pollution Control Agency examined tissue from dead fish, analyzed water and soil samples, and conducted other tests to determine what caused the die-off.
Investigators also interviewed local farmers, landowners and pesticide applicators. The investigation was unable to pinpoint a cause. Water samples showed no toxic levels of ammonia, pesticides or metals. No insecticides were detected, and only extremely low levels of fungicides. There was no evidence of illegal dumping, unusual wastewater discharges or inappropriate applications of manure.
Instead of a single cause, investigators speculate the kill may have resulted from a combination of biological, chemical, and environmental conditions related to the July 28 storm, during which 1 to 2.5 inches of rain fell in the Whitewater watershed over three hours. In such situations, fish may die quickly and there is often little or no accumulation of toxic compounds in their organs and tissue.
Investigators don’t know which biological or chemical properties may have been involved or how they might have a greater impact in combination than alone. Fish kills are not uncommon, and they can result from natural causes such as disease, environmental stress and temperature changes, as well as from industrial or agricultural spills. Those that occur in streams are especially difficult to investigate because even a short delay between an incident and it being reported can allow for any toxic substances or conditions to dissipate and be diluted by additional runoff as the water moves downstream.
In the case of the fish kill on the South Branch Whitewater, two days elapsed between the heavy rain associated with the lethal conditions and the first report of dead fish. While any fish kill is a matter of concern, the July incident on the South Branch Whitewater did not eliminate all fish, according to recent DNR surveys. The stream’s populations of brown trout and other species are expected to bounce back without additional stocking.
DNR fisheries managers have extensive data on the South Branch Whitewater, both before and after the recent incident. That information will provide them with valuable insights into how a stream recovers from a fish kill, a topic on which there has been little research. Because evidence as to the cause of fish kills may rapidly disappear, state officials urge anglers and others to report such incidents immediately by contacting the state duty officer at 800-422-0798, the nearest DNR fisheries office or local law enforcement.
The full report documenting the investigation and its findings can be found on the Lanesboro area fisheries page.