Summer Months Stress Water Quality And Fish

Ponds, especially small ones, are entering most stressful months of the season. Carefully monitor changes in water color or odor. Closely watch fish behavior such as gulping at the surface. Quick, corrective action could prevent harmful effects to fish.

Professor Billy J. Higginbotham and Asst. Professor Todd Sink with the Texas A&M AGRILIFE Extension Service have found small ponds intensively managed for catfish are most susceptible to die-offs. Other common causes are large quantities of vegetation, heavily fed ponds, or sudden die-off of dense phytoplankton. Here are questions they ask to help frantic pond owners:

1) When did fish start dying and how long have they been dying?

This helps determine if the incident was rapid or prolonged. Rapid fish loss typically occurs from oxygen depletion, lasts a few hours, and mortality ends. Chronic or prolonged events span days or weeks and stem from disease or parasite symptoms. Exposure to pesticides or herbicides may cause rapid or chronic mortality, depending on the dose of chemical in the water column.

2) How many fish have died and what size are they?

Loss of a single or small number of fish is not considered a die-off. Biologists define a fish-kill as mass mortality involving 10-percent or more of the entire population of a single or multiple species. It’s disappointing to lose a big bass or catfish, but some occur from natural causes or old age. This may occur during stressful times such as spawning or low oxygen levels during hot summer months. Small fish are less mobile and more susceptible to localized, rapidly changing environmental conditions. Heavy rain runoff may alter pH or cause a turnover. Although they require less oxygen, small fish are more sensitive to depletions.

3) How many different species are dying?

If one or more species is dying, you probably are faced with a water quality problem. If only one species is affected, it may be a disease. If there is only one species in the pond and you experience a die-off, you’ll need more information for a diagnosis.

4) Have pesticides or herbicides been used or introduced into the pond?

Pasture spraying could wash-in with rain. Cattle treated with insecticides could wade into the pond. If these sources are suspected, determine if such products had been applied within 250 to 500 feet of the pond four to seven days prior to the last rain. If you recently treated a large area of aquatic vegetation, oxygen depletion from plant decomposition could be the culprit.

5) How big is the pond?

Learn the surface acreage of your pond. It’s valuable information for many management projects. Measuring tools are available on Google Earth and similar sites. The professors point out, excessive depth does not make up for a small surface area when it comes to fish production. In fact, excessive depth can cause a fish kill. Deep lakes stratify with distinct temperature zones during summer months. Warm, upper areas contain oxygen. Cold, deep areas are void of oxygen. If a rapid destratification or turnover is triggered by specific weather events or heavy, cold rain, a fish kill may occur.

6) Was there a water color change?

A change in color and oxygen depletion is common. Ponds can go from light green or clear to a brown, coffee color. Such action signifies phytoplankton loss and corresponding drop in oxygen. This often occurs when extended periods of cloud cover prevent direct sunlight from reaching the pond.

Oxygen Depletion

Professors Higginbotham and Sink remind when a pond exceeds its carrying capacity for fish during summer months, the stage is set for a die-off. Why in summer months? Because warm water cannot hold as much oxygen as cool water. Fish require higher oxygen this time of year because their metabolism increases as water temperatures rise. Oxygen levels show lowest readings at daylight. This is the best time to check for piping or fish gulping at the surface. Here are other conditions that cause oxygen depletion:

1. Successive, hot, still, cloudy days reduce photosynthesis and oxygen production.
2. Overfeeding or excess fertilizing prompts nutrient decomposition.

How do you correct low oxygen?

1. Reduce fish volume below 1,000-pounds of fish per acre.
2. Have an emergency aeration plan. Back a boat on a trailer into the pond. Run the motor in a fixed position to circulate water and increase oxygen.
3. Add fresh well water, but aerate it well before it enters the pond.
4. Circulate water with a pump. It’s important to intake water from the surface and spray outflow over the surface. Don’t draft from the bottom.
5. Add bottom diffused aeration systems to prevent stratification.

Let’s make a plan to prevent these potential events in your pond.