These losses can be the result of oxygen problems, common diseases or a combination of these factors, said LSU AgCenter aquaculture specialist C. Greg Lutz.
“Many problems that become apparent in the spring actually had their beginnings back in the fall when temperatures were high and oxygen levels were less than optimum,” Lutz said.
Although fish may show no sign of disease during cold weather, when temperatures begin to increase in the spring, disease-causing organisms that were already present can get the upper hand on fish that went into the winter in a weakened state.
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. “These factors make fish particularly susceptible to environmental stress during the springtime, especially low dissolved oxygen levels.”
Many Louisiana ponds experience partial fish die-offs during the spring due to a combination of disease and stress from low oxygen.
“Oxygen problems usually kill larger fish first,” Lutz said, adding, “Overcrowding, over-feeding or over-fertilizing almost always contributes to these problems.”
As nutrients accumulate in a pond, most go into solution in the water column where they are used by microscopic plants called algae. Algae produce oxygen during the daylight as a by-product of photosynthesis, and this is usually a major source of oxygen in fish ponds.
However, algae blooms can be unstable, especially during springtime conditions, Lutz said.
“As pond water warms and the amount of sunlight increases, algal species that predominated during the winter die back, and other species more suited to summer conditions multiply and replace them,” he said. When this process proceeds gradually, conditions remain fairly stable.
Unfortunately, the cold-water algae sometimes die off abruptly, causing insufficient oxygen levels for several days.
Low oxygen levels may kill some fish directly or cause sufficient stress to weaken their immune systems, Lutz said. “In these cases, bacterial infections usually occur within the next several days to two weeks.”
Bacteria capable of attacking fish are commonly present in any pond, and if day-to-day stressors weaken the fishes’ resistance, bacterial infections can often be seen in the form of sores, bruises and discoloration on the skin and fins.
Bream, bass and catfish can all be affected, he said. “During the winter months, cold-water fungus can also attack fish, especially on the skin and gills where bacterial problems have already begun.”
There is no guaranteed approach that will eliminate springtime fish losses to disease or oxygen problems, but keeping the fish population thinned out and avoiding excessive levels of fertility throughout the year will help minimize the chances of a fish kill in the spring, Lutz said.
For additional tips on pond management, check out the LSU AgCenter’s list of topics at www.LSUAgCenter.com/aquaculture.