Summer heat, low water increasing fish kills

KANSAS CITY, Mo -- Scorching heat and drought are making survival difficult for fish in shallow ponds and lakes all around the state. Several fish kills have been reported to the Missouri Department of Conservation (MDC).

“The hot, dry weather isn’t just hard on humans, it’s hard on fish, too,” said MDC Fisheries Biologist Jake Allman.

"Most problems occur in ponds that are not deep enough for fish to retreat to cooler and more oxygen-rich water," Allman said. "Hot water holds less oxygen than cool water. 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."

He added that less water in a pond also concentrates nutrients and can lead to increased aquatic plant growth.

"Too many plants or too much algae causes wild daily fluctuations in oxygen levels," he said. "Sunlight during the day fuels aquatic plants and algae to produce oxygen through photosynthesis. At night the same plants use this oxygen at a high rate for respiration."

He added that even clouds can be bad news for ponds now, even if they bring much-needed rain.

"An overcast day with little sunlight can prevent plants from producing oxygen during the day," Allman explained. "That can cause oxygen levels at night, when plants are using the oxygen, to dip below levels needed to sustain fish."

Preventing summer fish kills is difficult. They are actually best prevented by constructing 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 a 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.

The signs of a fish kill due to low oxygen are obvious. 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.

What to do with all those dead fish?

"Leaving them to decay is the only real option," Allman said. "Fish will decay rapidly in these temperatures and disappear, probably in about five days. Removing the dead fish isn’t necessary."

Low oxygen fish kills usually don’t kill every fish in the pond. Restocking the pond can be done. But pond owners should realize that fish kills may happen again if these conditions return.

"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," Allman said.