Preventing Fish Kill

What are some of the main causes of fish kills in a pond?  This is a question that is often asked of me and there are several answers that need to be considered when dealing with a fish kill.

The first consideration when dealing with a fish kill is the amount of fish killed.  If you have up to a dozen “floaters” per acre in your pond, then it is likely a simple explanation, such as death by old age, disease or physical damage.  It is certainly to be expected that some fish would pass in a natural manner.  However, if your fish kill exceeds twenty to forty fish per acre, then something may be amiss and further investigation should be made.

The second consideration when dealing with a fish kill are the sizes of the fish killed.  If you have all young fish that have died, then that would point to something different than if all the dead fish were large and more mature.  If there are fish of all sizes, then oxygen depletion is very likely and a pond turnover is the leading suspect.

The third consideration is the species of fish killed.  If bottom dwelling fish are the only fish killed, then you know something affected only the bottom conditions of the pond.  If all species are affected, then oxygen depletion is again the leading suspect.

The fourth consideration is the presence/absence of weeds and algae.  If lots of living algae and weeds are present, oxygen depletion is unlikely.  However, if dead weeds and algae are present, then oxygen depletion is again likely.

The fifth consideration is the terrain around the pond.  Inspect all areas that drain into the pond.  These could be yards or ditches or drain pipes that lead to the pond.  Heavy rainfall can leach dangerous chemicals from yards and streets and these can cause fish kills within ponds.

Finally, a sure fire way to prevent approximately 90% of fish kills is the introduction of an aerator unit into a pond.  A properly sized aeration system will produce enough oxygen in the pond to make up for any oxygen depleting event that could normally affect a pond.  Aerators are not fountains; they are units that pump air into a diffuser plate located at the bottom of the pond and bubble up to the surface.  The other approximately 10% of fish kills are usually toxins introduced to the pond and cannot be avoided by using an aerator.