Johnny Morgan, Lutz, C. Greg
News Release Distributed 04/10/15
BATON ROUGE, La. – Many Louisiana ponds experience partial fish die-offs during the spring due to a combination of disease and low oxygen stress, according to LSU AgCenter aquaculture specialist Greg Lutz.
“Overcrowding, over-feeding or over-fertilizing almost always compound these problems,” Lutz said.
Low temperatures during winter force fish into a state of slow motion in which they eat very little, and their immune systems respond very slowly.
“When temperatures begin to rise in the spring, disease-causing organisms, already naturally present in a pond, can get the upper hand on fish that are in a weakened state,” Lutz said.
Stress caused by abrupt temperature fluctuations, such as many parts of the state experienced in the past several months, often aggravates fish health problems by further suppressing immune responses, he said.
Nutrients in the water are taken up by the algae bloom, which are single-celled plants suspended in the water. Like all green plants, algae produce oxygen during the daylight hours, and this is usually a major source of oxygen in fish ponds.
As pond water warms during the springtime and the amount of sunlight increases, algal species that predominated in the bloom during the winter die back and other species more suited to summer conditions replace them.
“When the winter bloom dies off abruptly, insufficient oxygen levels may persist for several days,” Lutz said. “This may kill some fish directly or cause sufficient stress to weaken their immune systems.”
Other springtime problems caused by algal blooms, especially in deeper ponds, involve “turnovers.”
“This occurs when cool rain water or heavy wind on the pond surface breaks down layering patterns,” Lutz said.
Turnovers can cause oxygen levels to drop to a point where fish begin to suffocate.
Mechanical aeration is required to raise oxygen levels once a depletion has begun. Pumps can be used to pull water from a depth of 2 to 3 feet and spray it back into the pond.
“Water should not be drawn from greater depths because this will only intensify the turnover effect,” he said. “Emergency aeration will be most effective in smaller ponds. But the success of any aeration practice will depend on the severity of the oxygen depletion.”
There is no guaranteed approach that will eliminate springtime fish losses to disease or oxygen problems, but avoiding excessive fish densities and high levels of fertility throughout the year will help minimize the chances of a fish kill in the spring.