As the “Fishing Capital of the World,” Florida boasts more than 3 million anglers. At the Florida Fish and Wildlife Conservation Commission (FWC), we appreciate feedback from these responsible conservationists. This time of year concerned anglers and citizens begin to see sporadic fish kills across the state. Most are on a small scale but, depending on circumstances, can seem quite significant.
A recent example occurred in Lake Weir, a 5,685-acre lake in Marion County. Several people took time to call the FWC Fish Kill Hotline at 1-800-636-0511 or went online at http://research.myfwc.com/fishkill/ to report the incident directly to the FWC’s Fish and Wildlife Health Group.
The FWC dispatched freshwater fisheries biologists Andrew Schaefer and Dustin Everitt to investigate. Following standardized procedures established by the American Fisheries Society, they counted and identified dead fish in random zones throughout the lake. This allows an estimate to be made of how many fish died. At the same time, they collected water samples and dissolved oxygen (DO) measurements.
At Lake Weir, it seemed like a case of nature taking its course. Heavy rainfalls likely flushed dead leaves and other plant material into the lake. This organic matter began to decompose, resulting in a low-DO fish kill. As is true in most such cases, the die-off did not kill all of the fish; biologists observed numerous surviving fish.
“Fortunately, Weir is one of our deeper lakes with spots as deep as 20 feet, which provide habitat diversity and refuges that can help at times like this,” Schaeffer said. “Lake Weir was already on a list for habitat restoration considerations, dealing primarily with aquatic plants, and features numerous fish attractors. We expect the lake to recover quickly and completely.”
Low-DO kills are common in summer. A number of factors can occur simultaneously to deplete dissolved oxygen in the water. Fish need oxygen from the water, in the same way people need air. A healthy lake or river has approximately 7 to 9 parts per million (ppm) of oxygen. When temperatures rise, DO levels naturally drop due to reduced solubility. If the level drops below 5 ppm, fish may start showing adverse impacts, and below 2 ppm it can be lethal, if the condition lasts too long. Some species of fish are better adapted to low DO conditions, such as bowfin or gar, but most of our freshwater sport fish (e.g., bass, bream and crappie) are less tolerant. These were the ones affected at Lake Weir.
Aquatic plants produce most of the oxygen in lakes through photosynthesis. Additional oxygen enters the water from the atmosphere by diffusion through wind and wave action. At night or on cloudy, rainy days, plants use oxygen and give off carbon dioxide. In addition, a major demand on oxygen comes from decomposition of dead plant and animal tissue.
At Lake Weir, bacterial populations blossomed to feast on and break down organic material washed into the lake. The bacteria required large amounts of oxygen, reducing what was available to fish. Moreover, it occurred at the end of spawning season, when many already-stressed adult sunfish were affected by this kill.
The FWC also collaborated with both the Florida departments of Health and Environmental Protection. There was no visual evidence of chemical pollution or signs of disease among dead fish. However, scientists observed a blue-green algae bloom. Such algae may or may not produce toxins. So to be safe, local Department of Health staff issued a temporary advisory on fish consumption and swimming for the lake. They lifted the advisory when they found water samples were free of toxins. Normal fish consumption and swimming are allowed now.
Given that relatively few fish died at Lake Weir, biologists do not anticipate any measurable impacts to the fishery. FWC biologists will monitor the lake and consider various management options, if the need arises.
Although low DO is the most common cause of fish kills, especially in summer, other factors may contribute and occur in combination, such as:
- Spawning stress – Spawning stress usually involves adults of a few species, and sores are often noticeable, resulting from spawning.
- Diseases and parasites – Viruses, bacteria and fungi may cause fish kills, typically of one species at a time. Fish parasites rarely kill fish but can make them susceptible.
- Algal blooms – Some algae are toxic, but most algae-related fish kills result indirectly from low DO. Algal blooms usually appear as a green-brown scum on the water.
- Human induced – Pesticides, herbicides or fertilizer applied on land can flush into the water and result in a kill.
- Low temperatures – Cold seldom kills native Florida fish. However, most exotic fishes came from tropical climates, so temperatures may drop enough to cause a die-off.
- Finally, occasionally, a few dead fish are observed, typically around a boat ramp that came from either commercial or recreational anglers inappropriately disposing of their catch.
To learn more, visit MyFWC.com and search “fish kills.” Also check online at Research.MyFWC.com/Fishkill, to see if a kill has been reported in your area.
Ending on a positive note, Florida has 7,700 lakes and 12,000 miles of fishable rivers and canals comprising 3 million acres of freshwater fishing opportunities that are open year round. There are plenty of healthy, enjoyable angling opportunities for everyone within 45 minutes, wherever you are in Florida and whenever the urge strikes. Enjoy it.
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