The most common cause of fish kills is related to the depletion of oxygen in shallow waterbodies. Oxygen depletion may be caused in various ways. Three of the most common are as follows:

Whenever aquatic organisms die, oxygen is pulled from the water column and is used in the decay process. Oxygen can become critically reduced in this way, especially in waterbodies that have an abundance of algae. For waterbodies in which the concentration of total chlorophyll exceeds 100 ?g/L, (indicating a high algae level), oxygen depletion is a likely cause of a fish kill. This is because the fraction of the algal population that is dying naturally is a big amount of mass and therefore its decomposition can consume significant amounts of oxygen.

Similarly, oxygen depletion can also occur when large amounts of aquatic plants die within a short time. Herbicide applicators commonly treat areas of aquatic plants at different times in order to avoid this situation, or they use herbicides that cause plants to die slowly.

A fish kill due to oxygen depletion can also be triggered by several days of overcast skies, especially during hot weather. This can happen because aquatic plants and algae add oxygen to water only when there is sufficient sunlight for photosynthesis. However, they consume oxygen all the time in their normal biological processes. When overcast skies persist for several days, oxygen levels can become depleted because the plants are using more oxygen than they are producing. Waters are particularly vulnerable when the temperature is high, because warmer water contains less oxygen to start with than cooler water. Fish "gulping" at the surface may be a sign of an oxygen problem.

Some speices of fish die naturally in large numbers after spawning or when they are stressed by unusual or harsh weather conditions. High concentrations of hydrogen sulfide (perhaps from a sulfur-spring water or artesian well) may be noticable.

A Florida search-able fish kill database is now available thanks to the research institute of the Florida Fish and Wildlife Conservation Commission (FWC).

The new database allows one to:

search for all kinds of fish kill data (locations, times, dates, etc.)
submit a fish kill report