Many of you have heard about or seen fish dying in Florida ponds, lakes, and streams. On Friday, the Florida Fish and Wildlife Commission stated that there had been 241 reports of dead fish/ fish kills in Duval, St John’s, and Hillsborough counties related to cold weather.
When the water or air temperatures fall below a critical level (scientifically termed their lower lethal temperature) for a particular species, they will die. Humans, for example, can die due to hypothermia, when their body core temperature falls below a critical level. As a result of Florida having a sub-tropical climate and several years of mild winters, quite a few exotic tropical fish species have become established or extended their range further into North Florida. Some of our native fish, such as the common snook, have also expanded their range further north due to mild winters. This year, we have record–setting cold temperatures. Our nighttime low temperatures have been very low (temperatures in the teens and twenties), and our daily high temperatures have also been low. When this occurs, our water temperatures rapidly fall, often below the lethal temperature for many species. This combination of a recent history of mild winters combined with an unusually cold winter has resulted in large die offs of tropical fish such as blue tilapia (http://myfwc.com/wildlifehabitats/profiles/freshwater/nonnatives/blue-tilapia/).
In addition to exotic fish species, some of our native (indigenous) aquatic species are also prone to cold-weather-induced kills. Gizzard and thread fin shad often die during cold weather. In Florida, we also have the Florida subspecies of the largemouth bass, which have evolved in Florida’s subtropical climate. Florida largemouth bass will often die due to low water temperatures, while the ‘Northern’ largemouth bass survives. Marine species, such as our common snook, are also currently dying in north Florida, especially if they are located in shallow areas that experience rapid drops in water temperature.
Even if fish don’t immediately die due to the cold, they will often become stressed, which can make them more susceptible to future illnesses (such as bacterial and fungal infections). Some of these fish may later die, if their illnesses are severe. Even if they don’t die, many may have temporary sores on the exterior parts of their bodies. See Stress – Its role in fish disease (http://edis.ifas.ufl.edu/fa005).
There are other reasons as to why fish die. To learn more about cold-induced and other types of fish kills, see our Florida LakeWatch Extension Circular #107 (A Beginner’s Guide to Water Management – Fish Kills), which can be found at http://edis.ifas.ufl.edu/pdffiles/FA/FA10400.pdf.