Occasionally, a fish kill occurs in farm ponds because of water quality problems, infectious disease, swarming fire ants (in the spring), or misused agricultural chemicals (pesticides). In some cases, the losses may be enough to affect the balance of the fish population. Get professional help to evaluate the fish population balance after a fish kill. In many cases, a phone call will provide enough information.
By far the most frequent cause of fish kills in farm ponds is low oxygen. Low oxygen can be the result of two separate phenomena in ponds. The first is simple oxygen depletion, which usually occurs July through September in the time of highest water temperature. Dieoffs caused by low dissolved oxygen levels result from natural biological processes, and preventive measures are rarely efficient except for running an expensive aerator every night.
Following are factors that can contribute to low oxygen levels:
Another condition, often called "pond turnover," can occur after heavy cold rains in late spring to early fall when temperatures drop suddenly. During calm, hot days, the pond develops temperature layers called "stratification." The layer of water at the surface is exposed to the sun and warms quickly. This warm layer weighs less than the cool water below, so these layers do not mix.
Surface layers contain high levels of oxygen produced by the phytoplankton. The cooler bottom layers are cut off from the surface layers and their sources of oxygen, so oxygen levels drop over time because of normal biological processes. In fact, these deep waters can actually develop an "oxygen demand," which is like having negative oxygen levels. When a heavy, cold rain enters the pond, or when there are sustained high winds, it mixes the two layers of water. When this occurs, oxygen levels throughout the pond may drop too low for fish to survive.
A severe mixing event can kill nearly every fish larger than an inch or two in one night. It is not uncommon to find large dead fish on dry land in the watershed above the pond following a turnover. These fish swam up the incoming rain waters seeking oxygen. Adult fish die first, and intermediate-sized fish follow, if the low oxygen levels are too low or if low oxygen conditions continue for many days.
Usually, by the time you recognize there is an oxygen problem, it is too late to save your fish. But an early symptom of a low-dissolved oxygen level is fish at the surface of the pond at sunrise. Fish appear to be "gasping for air." If you discover the low oxygen event early enough, you may be able to save some fish by using emergency aeration. A powerhousetype aerator works great, but most people don't have access to aquaculture equipment. You can back a boat with an outboard motor halfway into the pond and tilt the motor at a 45-degree angle to the water surface. Run the motor at high speed to move a "rooster tail" of water into the air and across the pond. Any technique that mixes water and air can help provide an oxygen refuge for fish.
Following a severe fish kill, some fingerling fish usually survive, but overcrowding bream tends to follow. After a severe fish kill, contact a fisheries biologist to assess the status of your fish population.
Poor water chemistry is the second leading cause of fish death in Mississippi ponds. Fish in acidic water with low alkalinity and hardness are more likely to get sick, especially during times of stress, such as spawning season or periods of rapid temperature change. A few fish, usually of different species (although catfish are especially sensitive), die every day, and many may have sores or lesions. If this is the case, have your pond water alkalinity measured to determine if agricultural limestone is needed. Liming increases the dissolved minerals in the water, which reduces stress on the fish. See the section on Liming Ponds for more information.
Bream and bass generally do not have significant problems with infectious diseases in well-balanced ponds, although you may see an occasional sore on individual fish during spawning season or after an injury. This is normal, and these external sores do not pose any health hazard to humans.
The one known exception is Largemouth Bass Virus (LMBV), which is not common in Mississippi ponds. This virus becomes evident during the hot summer, when largemouth bass are seen sick or dead on the surface and around the pond. A few bass die every day during warm weather, and larger fish seem to be more affected. When water temperatures cool in September/October, no more fish die from the virus, but the virus persists in the pond. The best way to avoid LMBV and other health problems is to follow stocking recommendations and do not stock fish from other natural systems.
Occasionally, bass and bream have small white or yellowish grubs imbedded in the flesh. These grubs, although not pleasant to look at, pose no threat to humans. You can trim away the affected area, and the rest of the fish is safe to eat if properly cooked.
Infectious diseases and parasites of channel catfish are common problems in catfish ponds. Overstocking, inconsistent feeding, and poor water quality contribute to this in recreational ponds. Disease and parasite problems of catfish rarely occur when you use low stocking densities (100 to 150 per acre). Stress from handling may cause die-offs of fish within two weeks of stocking new or established ponds.
If you choose to stock catfish at rates higher than recommended (100 to 150 per acre), plan to cope with problems that may occur. Your county agent can help you ship catfish samples to Mississippi State University diagnostic laboratories. You must arrange for someone to receive your catfish before you ship them. Do not send fish samples to the Mississippi Department of Wildlife, Fisheries and Parks or to any agency other than the Mississippi State University College of Veterinary Medicine.
Fire ants are often wash into ponds or fly in during breeding swarms, and small and intermediate-sized bream may die from eating these insects. Bass are rarely affected. This generally does not hurt the population balance.
If you experience a fish kill and suspect that infectious disease or parasites may be the cause, you can send fish samples to the Mississippi State University College of Veterinary Medicine (for recreational ponds) or the Thad Cochran National Warmwater Aquaculture Center (for commercial aquaculture ponds) for analysis. Please contact them beforehand at 662-325-3432 (CVM) or 662-686-3302 (TCNWAC) to make arrangements for sending or transporting fish.
If possible, send this information and the fish sample to the disease specialist:
Arrange for shipping and delivery. Samples should arrive at the lab within 12 to 18 hours. Call the lab and provide details on your case and the anticipated arrival time. Mississippi State University operates two labs, one on campus (662-325-3432) and one at the Delta Research and Extension Center (662-686-9311).
Place live fish in a plastic bag with no water and seal. If you are sending catfish, clip the spines to prevent them from puncturing the bag in transit. Then place the bag in an ice chest containing crushed ice.
If the fish are to be hauled for a short distance, you may place them in a container or ice chest containing well-oxygenated water. Add a few chunks of ice to keep the water cool.
You can freeze fish for transport to the lab when there is no other way to keep them from spoiling. Frozen samples are hard to work with. Avoid them whenever possible. Frozen samples are acceptable if they are for pesticide analysis.
Immediately ice down all dead fish that are still acceptable for examination (freshly dead with gills still red) to slow further tissue breakdown.