Heavy Rains Can Lead To Sudden Die-Off Of Fish In Ponds

(FRANKFORT, Ky., August 5th, 2004, 11:55 a.m.) -- The Kentucky Department of Fish and Wildlife Resources' (KDFWR) Fisheries Division has received numerous calls from private pond owners seeing dead fish in their ponds this year, with increased reports over the last few weeks.

These sudden "kills" usually occur in fishing ponds or lakes with "wild" (bass, bluegill, catfish, grass carp, etc.) fish populations, and that do not receive mechanical aeration. Weather conditions, especially periods with high rainfall and cloudy days, are the main cause for this type of problem. According to KDFWR District Fishery Biologist Kerry Prather, rains can increase growth of plankton (microscopic plants and animals) in ponds.

Phytoplankton (microscopic plants or algae) are the primary producers of oxygen in the pond. Plankton grows continually, but is short-lived, and it is the decay process of these and other organic material that increases dissolved oxygen demand in the pond.

During cloudy periods, sometimes the demand for dissolved oxygen within the pond exceeds what is being produced by the plankton. This situation, combined with night-time conditions that further deplete the oxygen level, leaves fish with too little oxygen to survive. Unfortunately, the larger fish die first, because they have a greater oxygen need.

Ponds that are high in fertility (more "green") and have good fish populations, may be prone to experience a fish kill.

Other pond-kills may be caused by heavy rains in what is commonly known as a "turnover." When rainfall of two inches or more occurs, it can cause a rapid mixing of the pond. Deeper areas naturally have little dissolved oxygen during the summer. Fish can't live in this portion of the pond at that time. When the "turnover" or mixing happens quickly, such as a sudden, heavy downpour, it results in much less oxygen throughout the entire pond and causes fish to die.

In both cases, the fish kill seems to finally occur overnight when dissolved oxygen levels are lowest. The only way to help prevent reduction of the oxygen level is to continually keep the pond aerated with an aerator or agitator, which provides oxygen top to bottom in the water column, regardless of weather conditions.

Pond owners should monitor their waters closely during cloudy periods or after heavy rains and be prepared to aerate their pond with aerators, fountains, water pumps or agitators. Most pond fish kills do not kill all of the fish, but unfortunately many of the larger fish may be lost. Aquatic vegetation control with herbicides should also be limited during these critical times. This activity can also limit oxygen production beyond normal, and may negatively affect fish, too.