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July 16, 2020

Or is it?

An Explanation of Summer Fish Kills

By: Joseph Berg
Aquatic Field Biologist, Fisheries Specialist

We know that many changes occur from winter to summer with our water in the Midwest. For instance, most fishermen are quite familiar with “turnover”. We also see our lakes, ponds, and rivers become frozen over winter and warm in summer to allow plenty of recreation. However, many people do not give much thought to what actually happens to the water as it warms in summer.

 Despite summer being the largest time for use in most lakes, ponds, and rivers in Wisconsin, this is when we see some of the most negative water quality factors. Although there are various human and pet impacts that are highly covered by media outlets, let’s focus on what happens to some fisheries as the water warms.

 Why do fish kills happen in the summer?

Warm Water

There is a common theme with fish in Wisconsin and neighboring regions; warmer water may mean some good fishing, but not all fish like warm water. Imagine being in a sauna. Not so bad, right? Similar to humans in a sauna, the longer the fish (that aren’t meant to be in very warm water) stay in the warm water, the more they want and need to escape it. Warmer weather equals a lot of extra stress on fish. For many species, warm water temperatures alone are enough to cause a slowed diet, stress, or even death.

Low Dissolved Oxygen

A large percentage of summer fish kills are a direct result of low oxygen levels in the water. Dissolved oxygen in water is required for fish to survive. Some fish species need more (like trout) than others (such as carp or bullheads). As water temperatures climb the amount of oxygen present tends to drop as cold water holds more oxygen than warm water. As oxygen levels dip, fish stress or loss can occur.

Algae Growth (as well as plant growth)

The public is becoming more aware of the impacts some algae blooms have on human and animal health. Yes, these algae blooms can also impact fish. Toxic compounds can be released into waterbodies by harmful algae species that have these dangerous effects. However, most effects we still see on fish will be caused by excessive growth. Excessive algae and vegetation in lakes and ponds produce oxygen during the day through photosynthesis. However, this photosynthesis stops at night. All these lake and pond organisms will still need to consume oxygen. The result can be oxygen crashes overnight.  Simply put, too much growth can be a bad thing.

What can you do to help prevent a summer fish kill in your waterbody?

Species Selection

Choose the right species for your waterbody if you are stocking a lake or pond. Not all lakes and ponds can hold all types of fish. If your water body warms fast or tends to have higher levels of algae or vegetation growth, think of species that do better in those conditions. If your waterbody has high oxygen levels and deeper and cooler water, you are able to have a larger range of fish species.


Aeration will not “cure” all fish losses. However, you can expect to see a drop in summer (and winter) fish kills from natural low oxygen levels and/or heavy growth waterbodies that are prone to oxygen crashes.

Lake and Pond Management

Take care of your waterbody. This can be done from a broader, watershed approach by controlling runoff, better shoreline practices, and controlling inputs into the waterbody. Direct management can be also be performed to the waterbody. Not eliminating, but controlling levels of growth can greatly reduce the likelihood of a fish kill by oxygen crashes (natural or plant/algae driven).

Contact us if you are interested in learning more about protecting your waterbody during the warmest months of the year!