If you find any large groups of dead fish, the Minnesota DNR would appreciate a call.
While fish die-offs are a common occurrence, with as many as 500 happening in the state every year, the DNR says the recent stretch of high temps may be behind an uptick.
Tom Burri, DNR limnology consultant, said in a news release the agency is "getting widespread reports of dead fish following the recent prolonged stretch of hot weather."
The DNR is also asking people to call the state duty officer (651‐649‐5451 or 800‐422‐0798, available 24/7) if they come across a "large group of dead fish" in a lake or stream. This helps the agency determine whether it needs to investigate, as they can get a timely water sample. Knowing the fish type and general size of the fish is also helpful.
Fish kills – in addition to being generally unpleasant to see and smell – can be a bit of a bother. As the Minnesota Aquatic Invasive Species Research Center explains, there are public health concerns associated with such die-offs, as well as potential clean-ups costs.
Just as importantly, these events can lead to "potentially significant declines in fish populations."
The cause of a fish kill can vary, and include warm water, low oxygen levels, algae blooms and spreading infections. But human activity can exacerbate or trigger these issues. That includes:
- Manure runoff
- Fertilizer or pesticides
- Hot water being discharged into a lake
- Dumping of toxic chemicals
- Storm water runoff in urban and rural areas
- Climate change
The DNR says disease-related die-offs often impact one particular fish species in an area. When a fish kill includes various species and a wide range in specimen sizes, human activity is usually to blame.
One study, published in 2019, found recent die-offs in Wisconsin lakes were linked to the warmer summer temperatures caused by climate change. The number of fish kills could double in the next 50 years as a result of rising temperatures, the researchers said, noting this wouldn't just affect biodiversity, but also opportunities for sport fishing and the economic opportunities that come with it.