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Fish kills in drought affected ponds

During a drought, the phones of Alabama Extension county offices will ring with reports of fish kills. These kills happen for two primary reasons: low water that creates crowding and poor water quality or oxygen depletion from over-fertilization and/or excessive weed growth.

Often, pond owners do not have many options towards managing around such drought impacts. However, there are a few management techniques that can be applied to reduce the chances or impacts of a fish kill during a drought.

Enhance Aeration

If low oxygen levels are suspected, indicated by fish gathering at the pond edge and surface in the early morning hours, or a fish kill has begun to occur, the first and foremost action would be to provide surface aeration. This is best done with commercial paddlewheels or floating surface aerators. The idea is to thrash the surface of the water to incorporate atmospheric oxygen back into the pond. However, bush hogs, outboard motors, and trash pumps can all be rigged to thrash the water surface, but care must be taken to not muddy the pond as this can worsen the issue. The idea here isn’t to save each and every fish in the pond, but rather to create a pocket(s) of oxygenated water to hold over the majority of fish. Know that as the sun sets and photosynthesis ceases, pond oxygen levels decrease and are the lowest right before the sunrises, making this period the most critical time to aerate.

If feasible, it is best to maintain serviceable water levels by pumping from other sources of water, whether it be a well, nearby pond, lake, etc. In terms of prevention, reducing and controlling aquatic weed growth goes a long way towards reducing fish kills, even in a year of normal precipitation. Although aquatic plants provide the water and other aquatic life with oxygen during the day, plants use oxygen from dusk through dawn.

If plant density is too high (greater than 30% of the pond) a fish kill may be at risk, and ever increasing as water levels decrease. The same situation goes for the phytoplankton bloom we purposely create when fertilizing a pond. If over fertilized (that is water clarity is less than 18”) the risk of a fish kill is high.

On top of that, phytoplankton densities continue to increase with heat as do the microscopic zooplankton that feed on them. Add all of this up and the oxygen demand in the evening and early morning is more than what the pond can provide and a fish kill is the result.

Fertilize as Needed

Fertilize carefully and only as water clarity indicates fertilizers are necessary, and, in the case of a drought, halt fertilization all-together.

Also directly related to the topic of crowding discussed above is the impact of poor water quality, high water temperatures, and transfer of bacterial disease, fungus, viruses, and parasites. Illnesses can be magnified as drought crowds fish and results in poor water quality. Heat and low oxygen-related stress can cause fish to eat very little and go into survival mode. In addition, as fish are in close proximity and come in contact with one another the probability of contracting a sickness can increase dramatically, particularly if the population is already in poor health and high stress from prolonged drought.

Symptoms of these issues are often exhibited as blotchy reddish/white sores, degraded fins, pale and discolored gills, and/or extremely skinny fish. To know exactly what your fish are ailing from requires a fresh (preferably live) sample rushed to the Auburn University’s Fish Disease Laboratory or the Alabama Fish Farming Center in Greensboro, AL, whichever location is nearest to you.

Use ANR- 0562 Guidelines for Collecting and Shipping Diseased Fish (referenced below). Know that medicated antibiotic feeds are available but they are only effective on specific bacterial infections and are with specific limitations, rates, and times of effectiveness. Also, medicated feed is not a cure-all and can be a substantial expense that is most feasible for commercial operations with substantial losses at stake. Lastly, severely sick fish may feed very little if at all, limiting the effectiveness of treatment. Rarely do bacterial disease, fungus, viruses, or parasites result in a complete fish kill, as they are often present in fish populations and run their course naturally.

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