Purdue News

Most farm-pond fish kills happen naturally, expert says

WEST LAFAYETTE, Ind. -- Every summer, fish die in farm ponds.

According to Purdue University aquaculture expert LaDon Swann, most of these fish kills happen because the pond water lacks oxygen -- not because of chemical contamination.

"Most are due to either an algae die-off or a physical process called thermal destratification or pond turnover," said Swann, an aquaculture specialist with Purdue's Cooperative Extension Service. "Both problems result in decreases in dissolved oxygen levels."

Swann said that when oxygen levels are too low, fish are asphyxiated or die from diseases brought on by the lack of oxygen.

Microscopic green algae plants are essential oxygen-producers in farm ponds. Healthy ponds normally have a faint green color, with water clear enough to see about 2.5 to 3 feet deep. "If nutrients are over-abundant in the water, algae blooms grow, giving the water a green, pea-soup color," Swann said. "Water clarity decreases, limiting visibility to only about six inches."

This is the first warning sign that a fish kill may happen. It's also a critical time in the pond's life cycle, because if the oxygen-providing blooms don't get adequate sunlight, they may die.

Swann said temperature also is a key life-supporting component of farm ponds. It determines the levels of dissolved gases such as oxygen, carbon dioxide and nitrogen in the water. The warmer the water, the less soluble the gas.

While there is no way to completely prevent fish kills, Swann said there are a few precautions farm pond owners can take to reduce the risk:

• Follow recommended fish-stocking densities. Farm ponds generally can support about 300 pounds of fish per acre.
  
• Eliminate unnecessary nutrients by reducing fish feed. This will lower the risk of excessive algae populations, which can lead to fish kills. Various fish feeds contain nutrients that promote algae growth. "Even though it's enjoyable to watch fish eat," Swann said, "feeding does increase the risk of excessive algae buildup."
  
• Control aquatic plants.

To learn more about this and other aquaculture subjects, such as fish farming, visit this site on the World Wide Web: http://www.ansc.purdue.edu/aquanic.
CONTACT: Swann, (765) 494-6264, e-mail: lswann@ansc.purdue.edu

Compiled by Chris Sigurdson, (765) 494-8415; E-mail, sig@ecn.purdue.edu
Purdue News Service: (765) 494-2096; e-mail, purduenews@purdue.edu

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