The following is an article in the local paper that may be of interest.

July 1, 2002

Oxygen loss in ponds can cause fish die-off

Each year, a number of pond owners experience a partial or total loss of the farm pond fish population due to an oxygen depletion. Oxygen depletion is not the only cause for a fish die-off but it is the most common.

Pond owners should be aware of the causes, signs and corrective measures to lessen the likelihood of experiencing an oxygen depletion.

Several factors may contribute to an oxygen depletion, some of which cannot be controlled by the pond owner.

The most common factors which contribute to oxygen depletion follow:

� Season - Oxygen depletion is most likely to occur in warm weather, usually June to Sept. Warm water holds less oxygen than cool water.

In addition, fish experience a faster metabolic rate as water temperatures increase, which means a greater requirement for oxygen.

This increased requirement of oxygen, at a time when it is less available, increases stress in the fish population.

� Weather Conditions - Sunlight is necessary for phytoplankton to produce oxygen through photosynthesis.

Several cloudy days in a row limits photosynthesis resulting in less oxygen produced. Cloud cover along with hot, still weather worsens conditions.

Oxygen demand by fish and plants may exceed the rate of oxygen production, thus creating an oxygen depletion.

Another weather condition conducive to oxygen depletion is cold wind and/or rain during the summer.

Usually, ponds are stratified during the summer season with a warm layer of water near the surface and a cooler layer on the bottom.

Cold wind and/or rain causes these two layers to mix rapidly, resulting in the breakdown of organic material present on the bottom which is evidenced by a foul, musty smell.

This process removes oxygen that is normally available for fish to utilize and it is a process that is commonly referred to as a pond turning over.

� Exceeding Carrying Cap-acity - This simply means too many fish in too little water.

A good rule of thumb is to never exceed 1000 pounds of fish per surface acre of water during warm weather.

� Improper Aquatic Weed Control - Most pond owners wait until aquatic weeds become too mature and dense before they decide to use an aquatic herbicide.

Herbicides should be used when new growth begins in the spring.

Best control is obtained at this time compared to treating once plants mature.

Treated plants sink to the bottom to decay and this decomposition process removes oxygen which is already at a marginal level during hot weather.

If a heavy enough growth of weeds is killed in a single application during hot weather, oxygen depletion is likely to occur.

A pond that must be treated in late summer should be treated in three or four different sections, some seven to ten days apart.

Select herbicides formulated for sectional or spot treatment as mentioned on the label.

Never use a herbicide that does not have an aquatic use label as this may further complicate matters.

� Overfeeding/Over Fert-ilization - Occasionally ponds become too rich due to an over abundance of nutrients.

Sources of nutrients include waste products from the fish, decaying uneaten feed and over fertilization.

A deep green or blue-green water color may develop as a result. Feed fish 3 percent of their body weight per feeding.

If this cannot be calculated, feed what they will clean up in 15 minutes (not to exceed 15-18 pounds of feed per surface acre). A floating ration is best for observing fish during feeding.

A fertilizer that is high in nitrogen and phosphorus, such as a 20-20-5, should be applied at the rate of 100 pounds per surface acre in the spring once water temperature reaches 65 degrees F, followed by 50 pound applications at 30 day intervals until the water temperature drops to 70 degrees F in the fall.

More or less fertilizer may be required depending on plankton bloom.

Small ponds managed intensively for catfish are the most susceptible to oxygen depletions, but larger sport fish ponds are not exempt.

If fish appear at or near the surface and swim sluggishly, this may be a sign that oxygen depletion is occurring.

Oxygen is normally at its lowest level at daylight, therefore pond owners are advised to observe fish early in the morning.

A change in water color from a greenish hue to a brown or coffee color may also signal an oxygen loss due to a phytoplankton die-off.

Aeration of the pond to increase oxygen content is the best technique for relieving an oxygen depletion.

To increase oxygen levels, pond water must be brought in contact with air.

Pulling water from near the surface and spraying it back over the pond will increase oxygen content.

Outboard motors can also help increase oxygen levels by running in a fixed position.

The idea with a pump or motor is to create a pronounced circulation pattern in the pond.

These corrective measures are usually only temporary and help little if the cause of oxygen depletion is not corrected.

While some causes may not be controlled, others may be controlled according to pond owner management decisions.

An awareness of the causes, signs and corrective measures of oxygen depletion allows the pond owner an opportunity to reduce the risks of fish losses.

� C. Jack Hunter is a Texas Cooperative Extension agent in Brazos County specializing in agriculture and natural resources.

Bill