Anatomy of a Fish Kill

By: Mike Pehanich

Cloudy days and hot sultry summer days can spell disaster for your lake.

One of the saddest sights a fisherman will see is that of a sudden mid- to late-summer fish kill.

These catastrophic events can turn a honey hole into an empty barrel! Unfortunately, small waters are the most common victims of these tragedies.

Cooler water at the bottom of a lake that has stratified does not get the benefit of Nature’s oxygen-creating forces. On many lakes, that bottom layer of water will remain oxygen depleted until the lake turns over and returns it to the surface.

When dissolved oxygen levels get depleted, all lake life suffers!

Where does lake oxygen come from?

TV host and bass fishing legend Bill Dance points out that Nature delivers dissolved oxygen to a pond or small lake in one of three ways: 1) current, 2) wind, and 3) the photosynthesis of aquatic plant life. 

“First, if current from a river or creek run through the lake or the pond is spring-fed, that is one way,” explains Dance. “Wind creates oxygen on an open lake. But the best way is the third – sunlight.”

Large reservoirs generally get ample supplies of oxygen from all three sources. Small lakes and ponds, on the other hand, are often sheltered from wind and have no current and little plant life to assist with oxygen production.

Summer heat accelerates both growth and decay in the pond ecosystem.

All of a lake’s plant life, including algae and microscopic forms, depend upon a process called photosynthesis, which allows them to take in energy from sunlight to fuel production of carbohydrates and oxygen, the life-essential gas that they release back into the water. 

Night darkness and cloudy days prevent photosynthesis.

A series of cloudless, windless days can diminish oxygen levels in ponds to dangerously low levels.

“The oxygen that plants create with sunlight during the day is taken away at night,” the host of Bill Dance Outdoorscontinues. “Cloud cover also takes Mother Nature’s oxygen producer – sunlight – away. After several cloudy days in a row, D.O. (dissolved oxygen) levels drop. Every living thing in the lake struggles to survive.”

In the absence of sunlight, plants consume oxygen without creating any! Under these conditions, too much plant life can deplete oxygen levels rather than increase it.

In the absence of sunlight, plants consume oxygen without creating any! Under these conditions, too much plant life can deplete oxygen levels rather than increase it.

KO punch

Such conditions are already stressful for bass and most freshwater species living in small waters during summer. But sometimes Nature delivers a knockout blow!

A cold rain following a series of cloudy days is a recipe for disaster! It can produce a sudden mid-summer lake turnover and unexpected fish kill.

Cold rainwater can sink through the warm upper layer of a lake and cause the oxygen depleted water at the lake bottom to rise to the top, bringing up decayed matter with it. Often the water looks and smells as sick as it is! This sudden upwelling of dead water can leave the fish literally nowhere to breathe, and it can spell disaster for the lake!

Healthy fish are a byproduct of a healthy pond. Denny Bauman enjoyed weather and pond conditions as much as the active bass did this day.

Enter the aerator

One of the advantages of small waters, however, is that lake owners or managers can use mechanical means to maintain healthy oxygen levels.

These artificial aeration systems, which some call “bubblers” due to the surface boils they create, add dissolved oxygen to the system, sometimes from bottom to top. Used properly, artificial aeration opens more of the lake to the fish. More importantly, it can prevent catastrophic summer fish kills.

Don’t wait for a tragic lake kill to take action. If your lake is shallow and shows signs of summer oxygen depletion, investigate your aeration options. 

Save your honey hole!

Rebel Tip

One of the most versatile baits in your tackle box is the original Rebel Minnow.  This trailblazing bait with its long slender baitfish profile has been a mainstay of anglers since it exploded on the freshwater fishing scene. The subtle (rolling) action of this floater-diver mimics the natural swimming motion of an injured baitfish. You can fish it as a “twitch” bait (jerkbait) or crank it over the tops of aquatic vegetation, pausing periodically to keep the bait from diving into the tops of the plants. You can even work it as a topwater lure, using the twitching and darting action of a wounded baitfish.

“Remember that a lot of bass remain in that upper layer of oxygenated water in the heat of summer,” reminds Bill Dance. “That’s often the case even when they have moved away from the shoreline and suspended in open water.”

Small Waters Fishing Summer Tip 

Fishermen tend to think that fish react just like they do during hot summer weather. That is, that summer heat renders them sluggish, inactive and even “off the feed.” 

 Well, sometimes fish do get sluggish in summer, but it’s not always due to hot air or water temperatures!

Fish are cold-blooded creatures, which is to say that their body temperatures are the same as the water they are living in. So-called “warm water species” like largemouth and smallmouth bass and the rest of the sunfish family actually get more active and feed more heavily during summer! Their metabolisms demand it. In warm water, they burn energy rapidly. They require more food to maintain body weight and good health than they need in spring and fall when water temperatures are cooler.

Be proactive with your local pond. Treat it like the treasure it is.

On the other hand, low dissolved oxygen levels will make fish sluggish. They struggle to breathe and become reluctant to increase their own oxygen requirements. 

So if you find that the bass on your favorite pond pound a Rebel Craw one day but seem reluctant to hit any fast-moving bait after several windless overcast days, go to a slower presentation or lures that you can work more slowly or which suspend or descend slowly through the water column.

About Mike Pehanich:

Mike Pehanich is a professional writer, photographer, and videographer for BASS publications: Bassmaster, Bass Times, Fishing Tackle Retailer, Cabela’s Outdoor Journal, Wired2Fish, and Fishhound. He is an industry expert in the sport and business of fishing, outdoor travel, conservation, and fisheries management. Mike is also the founder and owner of Small Waters Outdoors and Mike Pehanich’s Small Waters Fishing.