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Summer 1998 Newsletter


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BVD Virus
Oxygen Depletion
Equine Endometrial Biopsy
Feline Infectious Peritonitis
Herpes Simplex Virus Encephalitis in Rabbits
Organophospate and Carbamate Insecticide Poisoning
Feline Hyper- Thyroidism
Porcine Hemorrhagic Syndrome
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Catastrophic Oxygen Depletion and How to Avoid It

 During the warm spring and summer months, we receive an increased number of phone calls regarding sudden fish kills in ponds. The typical history includes observing a very large number of fish dead in an otherwise normal pond following a rainstorm or summer thunderstorm. Usually the owner is very concerned that the fish may have died due to "run-off" of farm chemicals into the pond. Most of the time, these fish kills are a result of a phenomenon known as "pond stratification." Pond stratification is somewhat of a misnomer, since the stratification can also occur in lakes, creeks and some rivers. The stratification leads to a catastrophic depletion of oxygen which almost always results in a very high mortality of aquatic animal life within 24-48 hours following the "de-stratification."

 The scientific reasoning behind this phenomenon of pond stratification relates to the temperature of the pond. In the early spring, while the temperature of the pond is still relatively low, the dissolved oxygen is uniformly distributed throughout the pond. As the atmospheric temperature increases, the pond begins to stratify, that is, become layered, with the surface water becoming warmer and lighter while the cooler and denser water forms a layer underneath. Circulation of the colder bottom water is prevented because of the difference in densities between the two layers of water. Dissolved oxygen levels decrease in the bottom layer since photosynthesis and contact with the air is reduced. The already low oxygen levels are further reduced through the decomposition of waste products, which settle to the pond bottom. After a rain, or any other event which disrupts the two layers, a "de-stratification" or "turn-over" of the pond occurs. This has the effect of releasing all of the dissolved oxygen from the upper layer of the pond into the atmosphere, hence, a catastrophic oxygen depletion.

 Once stratification of a pond occurs, there is nothing that can be done to alleviate the situation. However, pond stratification can be very easily prevented by the use of supplemental aeration. Aerators come in all sizes and shapes as well as different power sources, i.e., tractor p-t-o, electrical, mechanical, etc. It is important to aerate the pond properly, i.e. match the size of the aerator to the pond, since over-aeration is wasted and may even lead to oxygen supersaturation, known as "gas-bubble" disease and under-aeration will not prevent stratification.

In those cases where we suspect catastrophic oxygen depletion, all other possible pathogens including bacterial, viral, parasitic agents are eleminated from the differential diagnosis list. However, the history of several days to weeks of warm weather followed by a sudden rainstorm are highly suggestive of this condition. If you suspect that you are dealing with a pond turn-over situation following pond stratification, it is imperative to have the pond owner take a water sample and have the dissolved oxygen (DO) concentration evaluated immediately. This water sample should be collected in a clean glass jar or bottle with a screw-top lid and should be completely filled by completely submersing the sample and container and placing the lid on the container while it is still under water.

- by: Tim Muench, DVM, MS

- edited by: Randy White, DVM, PhD



ADDL-West Lafayette:
406 S. University
West Lafayette, IN 47907
Phone: 765-494-7440
Fax: 765-494-9181

11367 E. Purdue Farm Road
Dubois, IN 47527
Phone: (812) 678-3401
Fax: (812) 678-3412

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