Fish deaths are a natural part of lake ecosystems. Fish may die from natural causes such as post-spawning stress, old age, starvation, or body injury. When many fish from the same water body die at one time (usually 25 fish or more), this is called a fish kill. While fish kills do occur as a natural part of a lake ecosystem, there has been a noticeable increase in the amount of fish kills lately. This increase in fish kills is in large part due to the unusually warm weather Minnesota has been experiencing this summer.  Warmer waters make it harder for fish to survive due to lower dissolved oxygen levels.

Causes of Fish Kills

Low Oxygen

Low dissolved oxygen levels are one of the most common reasons that fish kills occur. Fish are equipped to handle natural fluctuations in dissolved oxygen levels, but struggle to survive prolonged low oxygen levels or rapidly fluctuating oxygen levels. Mortality events that are caused by low levels of oxygen are called summer kill or winterkill events.

Summer Kill

Summer kills primarily happen because of low dissolved oxygen in the water. The warmer water creates more biological activity. Organisms such as decomposing bacteria and plants use oxygen in the water to respirate. As more plants and bacteria respirate, the more dissolved oxygen they use. A certain level of nutrients and aquatic plants are generally good for water quality. It is an excess of nutrients that causes algae blooms and aquatic plant growth, depleting dissolved oxygen levels and killing fish. 

Winter Kill

Ice and snow limit the amount of sunlight getting to aquatic plants. Without sunlight, those plants produce less oxygen. When the plants start to decompose, they use up the remaining oxygen that is left in the lake. Winter kills can also be caused by an early ice-on or late ice-out. To help prevent winter kills, aeration systems can be placed to help circulate oxygen in a lake over the winter. There are two aeration units on District lakes. The District operates a unit on Rice Marsh Lake, and the City of Chanhassen operates a unit on Lake Susan.

Other Causes

Most fish kills occur from high water temperatures or conditions that reduce photosynthesis of aquatic plants, both of which cause low oxygen levels. However, external factors such as pollutant runoff, chemical spills, or pathogens can also cause fish kills.  Learn more about fish diseases here.

Fish Kills Effects on Carp Management

Fish kills don’t only affect anglers who fish recreationally. They also affect the fisheries and water resources of the waterbody, especially regarding carp management. Bluegill sunfish help keep carp populations in check by consuming carp eggs and fry. If a nursey lake has a winterkill, most native fish, including bluegills, won’t survive. With the predators gone, carp eggs have an opportunity to hatch and grow into adults.

Recent Fish Kills

Below are all the fish kills that have occurred in the District within the past twelve months. This page will be updated once per season with any new fish kills that occur in the District. (Last updated 8/26/2026)

Preventing Fish Kills

Reducing the amount of excess nutrients to our lakes can help prevent fish kills. Excess nutrients are what drive algae blooms and decrease the dissolved oxygen levels in water. Rain and snowmelt can wash excess nutrients into our lakes from sources such lawn fertilizers, leaf litter, and pet waste.  Some ways to reduce nutrients include:

• Avoiding placement of fertilizer on your lawn right before a big storm.
• Planting native plants to soak up pollutants and stormwater.
• Picking up your pet’s waste to prevent nutrient runoff.
• Keeping grass clippings and other organic debris out of your storm drain.
• Installing rain barrels to reduce roof runoff.
• Installing permeable pavers to slow stormwater runoff.
• Disposing of hazardous chemicals and materials responsibly.

These actions help reduce the load of phosphorus and nitrogen to our waterways and help stormwater soak into the ground slowly instead of move quickly across the surface.