If you have ever come upon a body of water with hundreds of dead fish along its banks or floating on the surface, then you know what a sad sight it truly is.
You would probably also be curious as to what caused such a tragedy. Sometimes fish kills are caused by toxins in the water that have spilled over from some industrial accident, but oftentimes the causes are natural and less sinister in nature.
Loss of oxygen in surface water is the most common cause of natural fish kills. Depletion of oxygen in water is usually the result of natural processes with storm events most often involved. This lack of oxygen typically suffocates fish within a few hours. There are a couple of scenarios that can result in a depletion of oxygen.
One scenario is high levels of oxygen-depleting organic materials in the water. Early spring storms sometimes wash organic matter and other soil materials into bodies of water, which raises the biochemical oxygen demand of the water. This creates dangerous, oftentimes deadly anaerobic (without oxygen) conditions. When fish can’t escape this water, they can no longer sustain life.
Another scenario is when heavy storms mix the water. When it is dry and hot, the upper layer of water heats up. The water stratifies, which means there will be warm water on the surface, cool water in the deep areas and a thermocline in the middle. This can cause the deeper levels to become depleted of oxygen. Then wind and rain come and cool the surface layer and mixes the water, which reduces the oxygen levels. This mixing also tends to stir up organic matter from the bottom, which calls for a higher demand for oxygen. This process further reduces oxygen to critical levels.
Yet another way in which oxygen is depleted from the water is through plants. During the day, plants and algae in the water produce oxygen as a byproduct of photosynthesis, which raises the levels of oxygen in the water. Then at night, the oxygen is used by respiration of all the flora and fauna in the water which leaves the oxygen at minimal levels by morning.
On particularly hot days, with high temperatures and declining water levels due to evaporation, there may be little, or no oxygen left in the early-morning hours. This causes oxygen stress in the fish and can lead to death. This is why you will sometimes see catfish farms using turbines to oxygenate the water in early-morning hours.
Fish that are fighting for oxygen display several symptoms. They tend to stop feeding or take to breathing just under the surface of the water. They will also gulp at the surface film or jump out of the water altogether. You may also notice that fish are swimming in a disoriented manner or lying on the bottom and gasping. All of these signs indicate a fish’s decline to death from lack of oxygen.
James L. Cummins is executive director of Wildlife Mississippi, a nonprofit, conservation organization founded to conserve, restore and enhance fish, wildlife and plant resources.
