Florida Fish Busters: Anatomy of a Fish Kill

Recently, north Florida anglers and Florida Fish and Wildlife Conservation Commission (FWC) fishery biologists were heartbroken to see the white bellies of numerous fish floating in Lake Victor. Lake Victor is a popular, 130-acre FWC-managed impoundment in Holmes County. Since the impoundment was constructed, the FWC has stocked fish, managed vegetation, provided boating access, created fish attractors and spawning beds and otherwise strove to create quality fishing.

So, what happened, and could it happen to a lake near you?

Unfortunately, it seems like a case of nature taking its course and, yes, it happens throughout the state each summer. In this case, it appears that heavy rainfalls flushed organic matter into the lake and the organic matter began to decompose, resulting in a low dissolved-oxygen (DO) fish kill. As is true in most such cases, the die-off did not kill all of the fish and when the water cleared and the sun came out, oxygen levels recovered and biologists observed numerous surviving fish.

This type of fish kill is pretty much a natural occurrence. From long before the days of Osceola and his warriors or even before the conquistadors arrived 500 years ago, low dissolved-oxygen fish kills have naturally occurred. They are especially prevalent in summer when a number of factors can come together to deplete the oxygen that is dissolved in the water. Fish absorb this oxygen from the water using their gills, in much the same way we obtain oxygen from the air with our lungs.

Florida Fish Busters: Anatomy of a Fish Kill

A healthy lake or river normally has approximately 7 to 9 parts per million (ppm) of oxygen, which is an ideal level for most fish. When the water temperature is colder, more oxygen dissolves into it. When temperatures rise, DO levels naturally drop due to reduced solubility. If the level drops below 5 ppm, many fish start showing adverse impacts, and below 2 ppm it can be lethal, if it lasts very long. Some species of fish are better adapted to low DO conditions, such as bowfin or gar, but most of our freshwater sunfishes (bass, bream, and crappie) are less tolerant.

In an event such as occurred at Lake Victor, the sport fishery tends to be hard hit.

Florida Fish Busters: Anatomy of a Fish Kill

Aquatic plants produce most of the oxygen in lakes through photosynthesis, which occurs when green (chlorophyll) cells convert light into energy. However, at night or when there is inadequate light, plants use oxygen and give off carbon dioxide. Additional oxygen enters the water from the atmosphere by diffusion. In addition to fish using oxygen and plants removing it at night, a major demand on oxygen comes from decomposition of dead plant and animal tissue (organic matter).

In the case of Lake Victor, bacterial populations blossomed to feast on and break down the organic material washed into the lake. The bacteria use large amounts of oxygen, reducing what was available to fish.

Area residents first contacted the FWC about the fish kill in Lake Victor on April 17, after seeing dying fish floating. Fisheries biologists responded to the lake immediately and determined the DO level in much of the lake was 0.0 to 2.0 ppm, enough to cause the kill, and that the water was a pea-green color. Often descriptions of lakes being pea-green relate to algal blooms, which are another insidious response to nutrients (decaying plant material, or organic fertilizers flushed from yards or crops by rain). These blooms block light to rooted plants so they do not produce as much oxygen. When algae die, they further increase the bacterial bloom.

Biologists visited daily for several weeks, documenting the number of dead fish and concluded that few adult game fish survived. However, they observed some schools of fingerling bass and bream from the spring spawn and a few bass on beds.

“The fish kill resulted from a depletion of dissolved oxygen in the water column, but there are some things that we don’t know and may never know,” said Chris Paxton, the FWC’s freshwater fisheries administrator for northwest Florida. “We know there was over 19-inches of rain in the area from January to April, and the lake got its share of rain and run-off.

He said the FWC was in constant contact with the Florida Department of Health and the Department of Environmental Protection and that FWC staff accompanied investigators from the Department of Agriculture to inspect the lake during the fish kill. There was no visual evidence of chemical pollution, toxic algae blooms or signs of disease among the dead fish.“This is something all of us, including the residents around the lake and our own fisheries management staff, wish never happened but in time the lake will recover,” Paxton said. Once oxygen levels build back up, the FWC will restock the lake with fingerling sport fish.Given the efforts the FWC has made in the past to maintain quality habitat in the lake and establish a quality recreational fishery there, it remains committed to rebuilding the fishery. Bream and crappie populations can respond within one to two years but it may take three to four years for significant bass populations to rebuild. FWC biologists will monitor the lake and consider all fisheries management tools that may quicken the lake’s recovery.

The good news is there is no evidence of any form of contamination that would prevent the lake from recovering. In addition, reduced competition often allows young sport fish to grow rapidly after kills of this type.

Although low DO kills are the most common, especially in the summer, other events can also lead to fish kills. Some of these are describe briefly below.

  • Low temperatures – Cold seldom kills native Florida fish. However, more than 30 freshwater exotic fish species from other countries are reproducing in Florida. Most of these came from tropical climates and during cold weather, water temperatures may drop low enough to cause a die-off.
  • Spawning stress – This is common in spring when most fish reproduce. A fish kill resulting from spawning stress usually involves adults of only one species. In addition, sores are noticeable.
  • Diseases and parasites – Viruses, bacteria and fungi may cause fish kills, and, rarely, fish parasites including protozoans, crustaceans, flukes and worms may kill some fish. Disease-related fish kills often involve only one species.
  • Algae blooms – Some algae are actually toxic, but most algae-related fish kills result indirectly from low DO caused when the algae rapidly dies off due to lack of sunlight and begins decomposing. Algae blooms usually appear as a green or brown scum on the water’s surface.
  • Human induced – Pesticides, herbicides or fertilizer applied on land prior to heavy rain can flush into the water and result in a kill. Chemical spills can cause fish kills too, but are rare. In these cases, there may be other dead animals present (birds, turtles, frogs, etc.).

To learn more about fish kills and what you can do to help prevent them, visit MyFWC.com and search “fish kills.” Although most summer fish kills relate to natural processes, the FWC requests your support in reporting fish kills to the FWC hotline: 1-800-636-0511.

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