A balanced ecosystem is key to a thriving, long-lasting, fish stocking pond. But what exactly does that entail? There are many variables that affect pond ecology – and fish mortality can occur for many reasons. Some of the more common reasons fish will die in a pond environment include dissolved oxygen depletion, pesticide toxicity and damage caused to the environment by parasites and disease. Proper management will prevent the more serious damage, but fish kill may happen in even the best and most professionally managed ponds. Not all incidents are preventable.
Some of the unexpected occurrences include:
Turnover
All ponds stratify to some extent during the summer, and the lower layer of water can have little dissolved oxygen. If there is a sudden cold front or a heavy rain, the pond water can be mixed unexpectedly, and it might result in levels of dissolved oxygen that are too low to sustain fish life. Even strong winds can contribute to oxygen depletion in a shallow pond.
Over Fertilization
Dense phytoplankton blooms or algae problems, even though they produce oxygen when the sun shines, can consume a high amount of oxygen at night, leaving too little to support fish respiration. A related problem can occur if an infestation of aquatic weeds or phytoplankton bloom is treated with an herbicide. As plant material decomposes, it consumes additional oxygen and can lead to an extremely low oxygen level that is lethal to fish. If you notice fish gulping for air at the surface of the pond in the morning, it can be a sign of dangerously low levels of oxygen in the water. Large fish will be the first to die, and the only effective antidote is to aerate the water, either through mechanical means such as aerators or pumps, or by exchanging existing pond water with water from a new source. “Stirring” the water will only serve to further dilute any existing oxygen, and is not a viable solution to the problem
Pesticide or Toxic Pollutant Exposure
Most common in areas that are intensively farmed, this typically happens when a heavy rain occurs soon after a pesticide application to surrounding fields. The runoff will introduce some of the chemicals into the pond and may be a source of pollution that is harmful to the fish population. Careless aerial spraying can also be problematic in agricultural regions. Such damaging runoff may be preventable with a berm or runoff ditch located adjacent to the stock pond, but if a toxic substance enters the water, there is little than can be done to counteract it.
In all cases where toxicity is suspected, quick action may minimize the impact, and time can be critical in assessing the scope and extent of damage.
Parasites and Diseases
In warm, southern locations, parasites and fish diseases are more common than they are in colder, northern waters. Such infestations normally kill only a few fishes at a time and will not typically affect the entire fish population of a small private pond. If you have a commercial operation, some sort of treatment may be advisable and regional fisheries biologists can be instrumental in identifying and responding to the problem. Yet, in most cases, such parasite and disease situations are usually rectified with little or no action over a relatively short period of time.
Muddy Water Turbidity
Cloudy water, that prevents light penetration to the lower depths of the pond, can prevent the growth of beneficial phytoplankton which they are the base of the pond food chain. Although fish tolerate muddy water, their growth can be compromised when the water quality affects the production of food that the fish depend upon. Turbidity is usually caused by runoff from surrounding land; such runoff can be extremely damaging if it includes draining from highways or construction sites and the like. If it is simply surrounding cropland, it is less concerning, except when it contains pesticides and toxic pollutants, as noted above.
It is important to identify the source of the problem; excess runoff can often be minimized by constructing a low berm at the pond perimeter, or by installing a dike to eliminate the source of the intrusive runoff. Sometimes, the rate of runoff entering a stock pond can be controlled by perimeter vegetation. In other cases, excess turbidity can be removed with an application of aluminum sulfate at the rate of 50 pounds per acre-foot of water. The alum, dissolved in water, should be applied by a sprayer to the entire surface of the pond. There are other methods that have proven successful in treating muddy pond water, and owners are urged to contact local experts for additional information. The best treatment, though, is to identify the primary reason for period or chronic turbidity, and to deal with the root cause.
Finally, it may be helpful to understand that too much clarity is not desirable in a stock pond. Remember that this is not a swimming hole. If the water is extremely clear, predator fish may have to expend more energy than necessary to capture prey, and their growth will suffer as a result. With too little clarity, the systemic productivity suffers. Ideal pond clarity has a greenish tint that is the result of beneficial algae growth that provides fish nourishment as well as some protection for both prey and predator fish, while it acts to kick-start the bottom of the food chain and promote healthy fish growth.
How to Assure Pond Balance
Pond balance is the term used by fisheries biologists to describe the ideal relationship between predator and prey species in a pond, but because fish populations are not truly ever stable, it can be a bit misleading to speak of a “balanced” stock pond. What it means, generally, is that there are always fish of catchable (or harvestable) size, that the desired annual reproduction rate is achieved, and that a combination of fishes of different size and species always exists in the pond.
The two primary ways to determine whether a pond is balanced are known as the Angler Method and the Seining Method. They are exactly what they sound like:
If the typical fish caught by anglers’ average 1–2-pounds for bass and six inch or larger bluegill, then the pond is likely in healthy balance. When bass are skinny or in overall poor condition and bluegill weigh more than a third of a pound, the bass may be overcrowded. On the other hand, bluegill may exist in disproportionate numbers if few bass are caught, or if the majority are larger than two pounds, and the average bluegill size is only 3-5 inches.
Similar principles apply when you sample your pond fish by seining. Details should be available from your state’s fish and wildlife agency, or from state university extension departments, so that you can determine when, how, and how frequently to seine for fish to determine pond balance.
