‘Summerkill’ season can be lethal to fish

AKRON – As we enter into the “dog days” of summer, conditions in many lakes and ponds can become life-threatening to fish, according to the Ohio Department of Natural Resources (ODNR) Division of Wildlife.
Water temperatures across the state are now averaging more than 80 degrees and the likelihood of the occurrence of a condition known as summerkill is increasing.
The term “summerkill” is used to describe fish kills that result from critically low levels of dissolved oxygen in the water.
Depending upon the severity, summerkills can result in total or partial death of a fish population.
Suffocation. Death of the fish is actually caused by suffocation and since larger fish require more oxygen, they are typically the ones to die first.
Andy Burt, fisheries biologist in northeastern Ohio, points out that low oxygen is not the only factor in summerkills because some species are more sensitive than others.
“Cool water fish such as walleye and muskellunge stop feeding when water temperature exceeds 77 degrees Fahrenheit. So that combined with low oxygen levels and 80 to 90 degree water puts a considerable amount of stress on the fish, often resulting in death.”
Combination. The ingredients which lead to summerkills are a combination of weather and water conditions that combine to slowly reduce the amount of dissolved oxygen available to fish.
Oxygen in ponds and lakes comes primarily from two sources – absorption from the air through the water’s surface, and production by aquatic plants.
During most of the year, the oxygen produced by plants and absorbed through the water’s surface is sufficient for all aquatic life in the pond.
However, during the heat of summer, cloudy weather and little to no wind can combine to reduce the amount of oxygen generated during the day.
At night, no oxygen is produced by plants, and much of what is produced during the day is used up.
Successive hot, cloudy, calm days, which are common during Ohio summers, may cause pond oxygen concentrations to decline to dangerously low levels.
At this point, the stage is set for a summerkill.
Pea soup. Lakes and ponds that receive excessive amounts of nutrients from the surrounding watershed can produce dense blooms of microscopic algae (tiny plants).
These blooms can give the water the appearance of pea soup.
A sudden die-off of these microscopic plants and their resulting decay can reduce oxygen to levels that are lethal to fish.
The real cause. A common misconception is that a large quantity of dead fish is related to chemical spills or some type of poisoning when suffocation due to the lack of oxygen is usually the real cause.
Unfortunately, little can be done to stop a summerkill, particularly on larger lakes.
Aeration devices can decrease the likelihood of summerkills in ponds. However, adding oxygen by means of aeration in large lakes is not practical because of the number of aerators required to be effective and the expense.
Fortunately, most summerkills run their course within a few days and the majority of fish survive, so restocking is not necessary.