Nothing is more shocking to a resident, fisherman, or resource manager, than to discover hundreds (or even thousands) of dead fish in the waters in which the recreate or work. While rare, “fish kills” do happen annually in various ponds, lakes, and rivers across the country. These kills can range from partial (only a few individuals) to complete (an entire population or community removed). Fish kills are caused by a variety of factors including low dissolved oxygen, disease, temperature extremes, human induced pollution, and even natural mortality. The warm summer months are especially prone to such events, however a fish kill can occur at nearly any time and place given the right conditions. These “conditions” play a key role in when a fish kill might occur, according to a group of researchers from the School of Forest Resources and Conservation at the University of Florida. In a 2009 publication to the Journal of Aquatic Plant Management, Mark Hoyer and three of his colleagues addressed fish kills and their connection to various limnological characteristics and weather events in and around Florida’s Lakes.
The group started by investigating past fish kills occurring in over 600 water bodies of Florida. From 1984 to 2002, the group cites 637 fish kill events in Florida’s canals, creeks and rivers, and lakes for which reports were retained by management personnel. The group concluded from those reports that 75% were caused by natural processes, 13% from unknown causes, and 12% were human-induced. As is often seen elsewhere, fish kills in Florida were most abundant from June through September. Coincidentally, this was also a time when increased abundance of rainfall and warmer water temperatures were observed. The group found that all water bodies, whether canals, rivers, or lakes, showed correlations between the average monthly rainfall and frequency of fish kills. Furthermore, in lakes with fish kills where water chemistry data were readily available, the waters tended to be alkaline with mean pH of nearly 8. Hoyer and the others also noticed that lakes in which fish kills occurred, high productivity was observed compared to nearby lakes that did not experience fish kills.
Florida is very well known as a popular freshwater angling destination, especially for black bass. Anglers taking to the water on nearly every morning of the year never expect to find thousands of their favorite fish floating at the surface. These kills, although rare, often spark intense accusations and theories of human induced fish kills. More often than not, these suspicions are incorrect, which is supported by the findings of Hoyer and company where more than 3/4ths of all fish kills in Florida water bodies are actually due to naturally occurring processes. According to Hoyer, these natural processes include rapid fluctuations in temperature brought on by weather events, water chemistry changes, viral or bacterial infections, and others. Rapid response to such events is also cited as important as the conditions which bring on a kill may not be apparent hours after an event. This is echoed in Hoyer’s paper, as 13% of kills were of unknown causes. The group further points out that human-induced fish kills are more easily identified because there is generally a point source or obvious evidence of a spill, reducing the probability that the unknown causes they observed were caused by humans.
The vast majority of Florida fish kills observed in the reports reviewed by Hoyer occurred between June and September when rainfall and water temperature most often become elevated. The group cites that strong thunderstorms and the abundant rainfall associated with these storms can dramatically change water chemistry in a water body. The group concurs that many different mechanisms and relationships can be responsible for a fish kill, and recommend that agencies in charge of fish populations take note of these variables to systematically investigate all fish kills in their realm of responsibility. The group also expresses the importance of good reporting of incidents that can later be merged with available data collected before, during, or after an event. In closing, the scientists stress the importance of all of these activities in concert to determine the causes of fish kills and even potentially predict them in the future.
To read the entire 2009 Hoyer et al. publication entitled “Fish Kills in Florida’s Canals, Creeks/Rivers, and Ponds/Lakes”, click here.
For more on Mark Hoyer, click here.
For more information on fish kills in Florida, visit Florida FWC’s bulletin
For more articles related to aquatic plant management, visit our Journal Online
For more information on the Aquatic Plant Management Society, click here.
Stay tuned for more from the APMS Blog!The APMS Blog is prepared by Dr. Brett Hartis, NC State University