How & Why Lake and Pond Fish Die

One of the recommended services that Lake & Wetland Management, Inc. provides involves stocking lakes with native sports fish. Why is native fish stocking so important to the management of lakes and ponds? Because fish (specifically, fish species that are native to Florida) are a stabilizing part of Florida’s aquatic ecology. Just as significant, too, is the role they can play in conservation and local economics (e.g. the need to hire personnel to feed and maintain the fish), as well as in recreational activities.

With this significance in mind, it is important to realize that fish stocking also serves to replenish and restore local fish habitats that are dying out. Lake and pond fish can die due to a variety of reasons, which may include:

  • Old age or natural injuries
  • Natural predation
  • Starvation
  • Suffocation
  • Pollution
  • Diseases or parasites
  • Toxic algae & invasive plant species
  • Severe weather

What The Death of Fish Means

If your property, park or housing development has a lake or pond within it and you see dead fish floating to the surface, should you be alarmed? Not necessarily. As implied above, the cause of death can be completely natural. On the other hand, however, causes such as pollution or invasive species are wholly unnatural.

Your cause for concern should also be scaled to the degree of fish that are dying. A few dead fish is likely natural, but large quantities dying at a high frequency and rapid rate is indeed worrisome.

What Is Fish Kill

This kind of death on a massive scale is known as a “fish kill,” and it certainly poses a threat to the health of both lakes and the communities that contain them. When confronted with a fish kill, it is important to first diagnose the catalyst for it.

What Causes Fish To Die

Different causes call for different solutions, and what follows is an overview of the top reasons behind fish kills and how they can be dealt with.

Oxygen Depletion

Low levels of dissolved oxygen (DO) is a primary indicator of lakes’ and ponds’ water quality, and as such, it is also directly related to the health of that water’s ecological community. In fact, it is most often the #1 reason why fish kills occur.

Unlike the Earth’s atmosphere (which is roughly 20% oxygen), water contains only a fraction of a percent of O2 that is dissolved. This oxygen is needed in order to sustain wildlife and deliver nutrients to aquatic flora, but insufficient oxygen saturation can be deadly to a body of water’s fish. Here are a few reasons why DO can be reduced in bodies of water:

  • Water Temperature: Along with atmospheric pressure and salinity, temperature is a main factor in the amount of DO that water can be saturated with. Inversely related to DO levels, a rising water temperature will lead to less oxygen within that water. As a result, all organisms that depend on oxygen to survive (from fish to algae and bacteria) will compete over this diminishing resource.
  • Plants: Regardless of water temperature, fish also compete with plants for sunlight, oxygen, and natural resources. Thus, the state of a lake’s plantlife will also have an effect on DO levels. Invasive plants can lead to oxygen depletion, as will excess algae caused by nitrogen and/or phosphorus.

What can be done about oxygen depletion? In general, it may be worth installing an aeration system that will at least maintain consistent DO levels, if not improve them over time. If the root of the problem is algae overgrowth vis-a-vis phosphorus enrichment, then the solution will likely be more effective waste management, irrigation and/or storm runoff.


algae caused by pollution that leads to fish killWastewater treatment discharges, agricultural runoff and surface runoff can indirectly cause fish kills by increasing phosphorus levels in bodies of freshwater, thereby causing more algae growth. However, they can also be a cause for fish kills in and of themselves. A few of the most offending chemicals include:

  • Herbicides
  • Pesticides
  • Ammonia
  • Aluminum compound
  • Cyanide
  • Chlorine

Diseases & Parasites

Fish are naturally susceptible to a wide range of various diseases, pathogens and parasites, which range from bacteria and fungi to flukes and worms. Normally, these occurrences are natural and benign to the population at large. However, infections can spread rapidly among large fish populations such as those that are farmed or overstocked.

Diseases and parasites are relatively easy to diagnose (compared to other causes of fish kills), and can be identified by:

  • Discoloration
  • Open sores or missing scales
  • Strange growths, swollen areas or abnormal shapes
  • Abnormal, often erratic behavior
  • Lack of appetite


Finally, the natural change in weather can also lead to fish kills. Water temperatures begin to rise as the climate gets warmer in early spring. This not only leads to lower levels of DO, but also facilitates the population growth of bacteria and parasites.

One weather-related phenomenon that is often linked with fish kills is turnover. This occurs during the spring and autumn when the more oxygenated surface water mixes with the less oxygenated water at the lake or pond floor. This can cause buildups of CO2 and H2S (Hydrogen sulfide), gases that can be deadly to fish but usually remain innocuous at the bottom of a lake. During turnover, they can spread quickly throughout a lake and, during times of strong winds or rain, rapidly kill large numbers of fish.

What You Should Know About Fish Kills

How to Spot Fish Kills

Notwithstanding the peculiarities of the cause for a fish kill, one can usually be distinguished by the large numbers of fish that come to the surface in a noticeably nervous or erratic fashion. This is especially prevalent if the cause is oxygen depletion, in which fish come to the surface in attempts to gasp for the oxygen that is unavailable underwater.

Fish Kills Can Occur Anytime

Although certain factors such as oxygen depletion and turnover are indeed weather-dependent, fish kills can occur anytime. So, if you manage or maintain a lake, it is important to remain vigilant even in between the dry seasons and times when fish kills might “normally” occur.

How to Handle a Fish Kill

Neglected or otherwise unnoticed, the affected fish of a kill will decay and their bodies will usually be consumed by other fish and wildlife such as turtles and alligators. With their minerals and nutrients consumed, water fertility will naturally re-balance itself over time. However, in the interim, the lake or pond will suffer from a destabilized ecology. Fish will be unavailable for recreational sport, and invasive plants and algae will consume nutrients unimpeded by competing fish species.

Once a fish kill occurs, there is little that can be done to halt its progress or save the fish that have already been affected. However, fish kills can be prevented with proper lake maintenance. Aerating the water and regularly testing the water quality will both serve to ensure that fish kills are avoided as much as possible. In addition, it is imperative that farmers properly irrigate their crops and control their runoff, that both private and municipal facilities dispose of waste appropriately, and that water pollution is avoided at all costs.

The Florida Fish and Wildlife Conservation Commission provides an official procedure for reporting fish kills on its website. If you come across a fish kill, please call their reporting hotline ASAP. To help prevent fish kills from occurring in the first place, be sure to have your lake, pond, or wetland maintained by industry experts at all times. And for that, you can always contact Lake & Wetland Management, Inc. for a free quote — call us at 855-888-LAKE TODAY!