This summers lingering heat-waves and drought is to blame for several fish kills in shallow ponds and lakes throughout the Midwest.

It typically occurs when ponds are not deep enough for fish to retreat to cooler and oxygen-rich waters.

Shallow ponds get warmer than deeper ponds, and with little rain, area ponds are becoming shallower by the day. Evaporation rates are up to 11 inches per month in these conditions, according to the Missouri Conservation Department.

When there is less water in pond, nutrients often concentrated  in smaller areas and lead to increased plant growth.  Too many plants can cause wild fluctuations in oxygen levels.  Sunlight fuels the growth of aquatic plants and algae to produce oxygen.  The plants will use the oxygen at night and clouds during the day can limit the production of oxygen by plants.

Once a fish kill begins, fish will be at the pond surface gulping for air, especially early in the morning.  The water will change color to a dark tea color and the largest fish will die first.  Largemouth bass, grass carp and large catfish will be accompanied by small bluegill because they require the most oxygen.

There’s no reason to remove the dead fish.  Leaving to decay is the only real option. Fish will decay rapidly in these temperatures and disappear, probably in about five days.

Prevention is difficult, and must be done at the right time as well.

First, the best prevention is to construct the pond properly.  Ponds that have water depths eight feet or greater over at least 25 percent of the pond’s acreage are much more likely to keep fish alive during these hot and dry conditions.

Controlling excessive aquatic vegetation must begin before water temperatures reach 80 degrees.  It’s too late now to try and kill excess aquatic plants.  Killing plants now will lead to more of an oxygen demand in the pond and will dramatically increase the likelihood of a fish kill.

Trying to maintain water depth in your pond by pumping water into it from either a well or city water supply is not feasible.  It takes 325,000 gallons to add a foot of water to a one acre pond.  City water also contains chlorine that could cause a fish kill.

Aeration systems can help but only if used before high temperatures cause low oxygen levels in ponds. Aerators must be the type that draws cooler water from near a pond’s deepest bottom and sprays it into the air to add oxygen. But it’s late to do this as well, because these systems will cause the pond to “turnover.” They will mix water from the very bottom of the pond that is devoid of oxygen with the upper layer of water which is already oxygen-starved.

Low oxygen fish kills usually don’t kill every fish in the pond.  Restocking the pond can be done.  But if the pond conditions are not changed, a fish kill could happen again.   If your pond does have a fish kill, it may be time to deepen it and make it more hospitable for fish in extreme weather.