We've been getting a few calls to our office lately from local pond owners experiencing fish kills. The extremely hot weather, combined with little or no wind, that was prevalent over the Big Country over the last month appears to be the culprit. This deadly combination results in reduced oxygen levels in local lakes and ponds.

Like all other animals, fish need to breathe oxygen to survive. They absorb this oxygen from the water through their gills. If oxygen levels in the water become too low, fish will essentially suffocate.

While hot weather and lack of wind are what put the "nail in the coffin" with these fish kills, part of the problem stems back to earlier in the year. Most ponds caught some runoff water earlier this year; many ponds even filled completely. This water was much needed and welcomed; however, many nutrients were also washed into these ponds. Nutrients are good for fish production, but they also drive phytoplankton (microscopic plants) and algal blooms. Phytoplankton and algae produce oxygen when the sun shines, but use it up during the night. Therefore, oxygen-related fish kills occur most often just before sunrise. They can also occur on cloudy days.

Here are some clues to determine the cause of a fish kill:

- If only large fish (of all types) die, but small fish are often still alive, the cause is usually low oxygen levels in the water.

- If only small fish (of all types) die, but large ones are often still alive, the cause is usually toxins, such as pesticides, herbicides, and heavy metals.

- If only one type of fish dies, then the cause is likely disease. If fish die quickly within a day or two, then it is probably a virus. If fish die over a period of a week or more, then the culprit is a bacteria.

- If all types and sizes of fish die, the cause could either be temperature-related (too hot or too cold) or toxins. Often the water becomes very clear if toxins are the cause.

- If fish are sick, but still alive and have fuzzy white patches, then they likely have a bacterial disease with a secondary fungus infection. This can be caused by hot or cold temperatures, breeding stress, parasites, lack of food or any other stressor.

To help prevent oxygen-related fish kills (by far most common in this area), consider adding an aeration system to the pond. However, this is an expensive option, so you may not want to do this unless you are certain you will have oxygen problems. Aeration generally only needs to be done from May through September, depending on water temperature. Also, it usually only needs to be done during the morning hours, generally from 5 a.m. until Noon.

Many pondowners are proud of their investments. Aeration is good insurance for those investments! It is always sad to see a pond experience a fish kill.