Charles Lutz, Merrill, Thomas A. | 4/13/2007 2:39:13 AM
Springtime losses in fish ponds are common throughout the Southeast – especially in Louisiana, according to LSU AgCenter expert Dr. Greg Lutz, who says these losses can be the result of oxygen problems, common diseases or a combination of causes.
"Many problems that become apparent in the spring had their beginnings back in the fall," Lutz explained. "Cold temperatures during winter force fish into a state of slow motion in which they eat very little, and their immune systems respond very slowly, but when temperatures begin to rise in the spring, disease-causing organisms naturally present in a pond can get the upper hand on fish that are in a weakened state."
Lutz also said stress caused by abrupt temperature fluctuations, such as the ones many parts of the state have experienced in the past several months, often aggravate fish health problems by further suppressing immune responses.
"All these factors make fish particularly susceptible to other forms of environmental stress during the springtime, especially low-dissolved oxygen levels," he said.
Many Louisiana ponds experience partial fish die-offs during the spring because of a combination of disease and stress from low oxygen levels, the LSU AgCenter expert explained.
"Overcrowding, over-feeding or over-fertilizing almost always contribute to these problems," he said, adding, "As nutrients accumulate in a pond, most go into solution in the water column where they are used by microscopic plants and animals. Except for situations where excessive weeds are present, most nutrients in the water are normally taken up by the ‘algae bloom’ – the collection of microscopic single-celled plants suspended in the water that give it a greenish tint."
These algae produce oxygen during the daylight as a byproduct of photosynthesis, and that usually is a major source of oxygen in fish ponds.
"But 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."
When this process proceeds gradually, Lutz said conditions remain fairly stable.
"Unfortunately, the winter algae sometimes die off abruptly, causing insufficient oxygen levels for several days," he explained, adding, "This may kill some fish directly or cause sufficient stress to weaken their immune systems. In these cases, bacterial infections usually occur within the next several days to two weeks."
Other springtime problems in fish ponds involve stratification and "turnovers," according to the expert, who said stratification causes layering of the pond water into warm, oxygen-producing upper zones and cooler, oxygen-consuming bottom waters.
"During the winter months, stratification can break down in some ponds as surface temperatures become more similar to those found at greater depths, and this situation can put stress on fish," Lutz said. "Occasionally fish kills will occur if the oxygen-poor bottom waters are mixed rapidly with the rest of the pond."
This type of mixing, referred to as a turnover, occurs when cool rain water or heavy wind on the pond surface breaks down layering patterns, which is why turnovers often are observed in ponds following severe spring weather fronts, Lutz explained.
"Bacteria capable of attacking fish are commonly present in almost any pond, and once day-to-day stressors weaken the fishes’ resistance, bacterial infections often can be seen in the form of sores, bruises and discoloration on the skin and fins," Lutz said, adding, "Bream, bass and catfish all can be affected."
Unfortunately, the LSU AgCenter expert said there is no guaranteed approach that will eliminate springtime fish losses to disease or oxygen problems, but he offered some suggestions.
"Avoiding excessive fish population densities and high levels of fertility throughout the year will help minimize the chances of a fish kill in the spring," he said, adding, "If pond owners don’t thin out their fish through a regular management program, Mother Nature will eventually take the opportunity to do it for them."
For tips on all aspects of pond management, visit the Aquaculture section listed under Crops and Livestock on the LSU AgCenter’s Web site at www.lsuagcenter.com or contact an agent in your parish’s LSU AgCenter Extension office.