Michigan DNR warns of more fish kills as summer temperatures heat up

Fish kills, like this one at Strawberry Lake, could become more common as the summer weather heats up in Michigan. Michigan DNR

Officials with Michigan Department of Natural Resources are warning those who spend time in or near water this summer to not be alarmed if they see an increase of dead fish in the coming weeks. With the weather heating up and oxygen supplies in the water decreasing, the DNR says it won't be uncommon for people to find fish and other creatures such as turtles, frogs, toads and crayfish dead in the water.

"The majority of summer fish kills are due to low oxygen in the water, a natural phenomenon associated with weather," said Jeremiah Blaauw, a DNR fisheries biologist. "However, some other factors such as chemical treatments of aquatic plants and algae can increase the risk of a fish kill."

Blaauw went on to explain that while fish need oxygen to survive, they absorb dissolved oxygen -- oxygen gasses that are absorbed in the water -- directly into their bloodstream. But when the weather gets warmer, there are less oxygen gasses present in nutrient-rich lakes. That lack of oxygen combined with the creatures increased need for oxygen during warm months can cause death.

Spawning can also cause stress for the fish, which also increases the number of fish that die during these summer months. An example provided by the DNR said that every year in southern Michigan, bluegills become stressed from the act of spawning. That typically happens around the time the weather gets warmer and when an herbicide is applied to control algae and invasive plant species growth in the water.

In turn, the bluegill's immune system depresses and bacteria that are already in the water can then infect the fish, causing it to break out in columnaris disease.

"‘Nutrient-rich’ sounds like a good thing, but that isn't necessarily true when it comes to water quality," Blaauw said. "Our lakes often have artificially high concentrations of nutrients due to human-connected sources like wastewater or fertilizer runoff. The nutrients support dense growth of plants and algae. While plants and algae produce oxygen during the day through photosynthesis, they consume oxygen at night via their own respiration. This situation can lead to oxygen crashes during warm summer nights."

For more information on fish kills in Michigan, visit Michigan.gov/FishHealth

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