Richard Bogren, Lutz, C. Greg | 1/4/2011 1:14:56 AM
News Release Distributed 04/06/10
By Greg Lutz, LSU AgCenter aquaculture specialist
Springtime losses in fish ponds are common across the Southeast, and especially in Louisiana. These losses can be the result of oxygen problems, common diseases or a combination of causes. Many problems that become apparent in the spring actually begin in the fall, when hot weather can reduce oxygen levels in ponds and make fish susceptible to diseases. Overcrowding, over-feeding or over-fertilizing almost always contributes to these problems.
Cold temperatures during winter force fish into a state of slow motion in which they eat little and their immune systems respond slowly. If fish go into the winter in poor condition, they usually are still under stress when springtime arrives.
As water temperatures begin to increase in the spring, disease-causing organisms naturally present in ponds can more easily attack fish already in a weakened state. Stress caused by abrupt temperature fluctuations, such as many parts of Louisiana experienced in the past several weeks, often aggravates fish health problems by further suppressing immune responses.
All these factors increase the chances of springtime disease problems in Louisiana fish ponds. Bacteria capable of attacking fish are commonly present in almost any pond, and once 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 all can be affected.
Oxygen problems also are to blame for many fish kills this time of year. Nutrients tend to accumulate in a pond over time, and most of them go into solution in the water column where they are 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 hours, and this is usually a major source of oxygen in fish ponds. As pond water warms and the amount of sunlight increases, however, 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, oxygen levels remain fairly stable. Winter algae sometimes die off abruptly, however, causing low oxygen levels that may kill some fish outright or further weaken their immune systems.
Another springtime problem in Louisiana ponds involves “turnover” and “stratification.”
Turnover often occurs in ponds following severe spring weather fronts.
Stratification causes layering of pond water into warmer, oxygen-producing upper zones and cooler, oxygen-consuming bottom waters where toxic compounds such as hydrogen sulfide can accumulate because of natural decomposition processes. During particularly cold winters, layering effects can break down in some Louisiana ponds as surface temperatures become more similar to those found at greater depths. 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 rainwater or heavy winds on the pond surface breaks down layering patterns.
There is no guaranteed approach that will eliminate springtime fish losses to disease, turnovers or oxygen problems, but avoiding overcrowding and high levels of fertility throughout the year will help minimize the chances of a fish kill in the spring.
If pond owners don’t keep their fish thinned out through a regular management program, Mother Nature will eventually take the opportunity to do it for them.