Fertilization can increase fish poundage by as much as three fold in some ponds. A properly managed fertilization program should increase both numbers and average size of fish caught. However, applying fertilizer to a pond already rich in nutrients can be harmful and may even cause a fish kill. Nutrients provided by the addition of fertilizer increases the quantity of phytoplankton (microscopic plants) which form the base of the food pyramid in all aquatic systems. Fertilization can also increase the “catchability” of fish by reducing water clarity, thereby making the fish less “spooky”.
The quantity of phytoplankton produced must be closely monitored for a successful fertilization program. Over fertilization can cause an undesirable buildup of dense phytoplankton. If this occurs, decomposition or respiration of the phytoplankton at night can consume all of the available oxygen in the water and cause fish kills. This problem is made more acute if an unusual number of cloudy, windless days occur in the summer. Once you observe fish “piping” or gasping for air at the surface, it is usually too late to save many fish. Fertilization can cause a pond to turn green and may reduce its aesthetic value. Do not fertilize ponds used for swimming.
Most ponds located in properly managed watersheds have enough natural fertility to produce a well-balanced fish community. Because of the potential hazards associated with an improperly managed fertilization program, you should consider fertilization as an option only if you are interested in intensive management or specifically interested in quality bluegill or trophy largemouth bass management. In general, don’t fertilize a pond that is being fed, since the by products of feeding will supply plenty of nutrients.