February 11, 2020 By Katie Roundtree

Fish Kills typically occur in South Florida when water temperatures drop as a result of cold weather. For exotic and non-native fish, initially the cold itself causes the fish to die.   In addition, the protective mucus that coats each fish is weakened by the cold and may totally disappear. This could leave fish vulnerable to bacteria and other adverse elements in the environment causing further harm. The longer the cold temperatures remain, the greater the potential for mortality and secondary effects.

Another way fish are killed by cold occurs in stratified lakes that have a warm layer of water on top of a colder layer. Lakes are typically stratified into a warmer, less dense, oxygen rich layer on top and a cooler, denser, oxygen poor layer on the bottom. The difference in density does not allow these layers to mix. The point where you have the greatest temperature difference is called the “thermocline” and it marks the transition from warm, oxygen rich water to cooler, anaerobic water (lacking oxygen). In a stratified pond, the water close to the pond bottom and the associated sediment layer is devoid of oxygen and can only support anaerobic bacteria.  These bacteria are inefficient at breaking down the organic material that settles on the pond bottom (leaves, grass, algae, aquatic vegetation, etc.).  As a result, a thick layer of decaying “muck” accumulates over time.  High levels of methane and sulfur gases concentrate in this layer.  Due to this poor water quality, fish are not able to utilize that portion of the pond below the thermocline.

When the temperature drops or a sudden cooler rain happens, the oxygen rich water on the top becomes much cooler and dense and sinks to the bottom. The cooler denser, oxygen depleted (anerobic) layer will then mix with the rest of the lake, causing the oxygen level to drop. This is a process called “flipping” or “turnover”. As a result, the upper layer now has the lower oxygen level versus the lower layer. The fish living in this upper layer now have less oxygen and may die as a result. In addition, the bottom sediments are stirred and anaerobic conditions are mixed throughout the entire pond.  As this happens, it brings up mass quantities of sulfur and methane gas which is released into the atmosphere. This can also lead to fish kills.

Lake flipping is critical to lakes in colder regions since the oxygen rich layer flips to the bottom allowing for the fish to live in deeper water during the winter. When the lakes are a uniform temperature and density, it takes relatively little wind energy to mix water deep into the lake. Wind moves highly oxygenated surface water to the lake bottom, forcing low oxygen water from the lake bottom up to the surface where it becomes saturated with oxygen. This is critical for aquatic organisms, as once the lakes freeze over for the winter, no new oxygen gets mixed into the lake from the atmosphere, and what is in the lake must last until ice melts in the spring.