Inflammation:
Inflammation is a natural immune response by the cells to a foreign protein, such as bacterium, virus, parasite, fungus, or toxin. Inflammation is characterized by swelling, redness, and loss of function.
It is a protective response, an attempt by the body to wall off and destroy the invader. Any stress causes hormonal changes which decrease the effectiveness of the inflammatory response.
Temperature stress, particularly cold temperatures, can completely halt the activity of the immune system, eliminating this defense against invading disease organisms.
Excessively high temperatures are also extremely detrimental to the fish’s ability to withstand infections. High water temperature may favor rapid population growth of some pathogens.
High temperature also reduces the ability of the water to hold oxygen and increases the metabolic rate and resulting oxygen demand of the fish.
Antibodies:
Unlike inflammation and other nonspecific forms of protection, antibodies are compounds formed by the body to fight specific foreign proteins or organisms. The first exposure results in the formation of antibodies by the fish which will help protect it from future infection by the same organism.
Exposure to sublethal concentrations of pathogens is important for fish to develop a competent immune system. Animals raised in a sterile environment will have little protection from disease.
Young animals may not have as effective an immune response as older animals and therefore, may be more susceptible to pathogens in the environment. Stress impairs the production and release of antibodies.
Temperature stress, particularly rapid changes in temperature, severely limits the fish’s ability to release antibodies, giving the invader time to reproduce and overwhelm the fish. Prolonged stress reduces the effectiveness of the immune system, increasing the opportunities for disease-causing organisms.
Disease prevention:
Numerous books and articles have been written on the diagnosis and treatment of specific fish diseases; however, prevention through good management practices is the best control measure to minimize disease problems and fish kills.
Good management involves maintaining good water quality, preventing injury and stress during handling providing good nutrition, and using sanitation procedures, The following are management practices that help prevent stress and the resulting fish kills.
Water quality:
- Do not exceed carrying capacity of fish in ponds and tanks.
- Monitor water quality parameters.
- Maintain dissolved oxygen levels above 5 mg/L. Sub-optimum levels of dissolved oxygen, while not immediately lethal, may stress fish, resulting in delayed mortality.
- Prevent the accumulation of organic debris, nitrogenous wastes (ammonia and nitrite), carbon dioxide, and hydrogen sulfide.
- Maintain appropriate pH, alkalinity, and temperature for the species.
Handling and transporting:
- When possible, use knitted mesh nets rather than knotted nets to reduce injury and scale loss.
- Speed and gentleness when handling fish are of utmost importance.
- Minimize the number of times the fish are lifted from the water, and work as quickly as possible when transferring fish.
- Harvest, handle, and transport fish at times when fish are least susceptible to stress and infection.
- Transport and holding tanks should be large enough to allow complete freedom of movement of fish and have no sharp corners or edges that might injure the fish.
- Maintain optimum water conditions while capturing, hauling, and handling fish.
- A high level of dissolved oxygen is crucial for rapid recovery of the fish from the struggle of capture and handling.
- Salt (0.3 to 1.0 percent) maybe used in the transport water to minimize osmotic stress and bacterial infection of freshwater fish.
- Ice may be added to the water during hauling to prevent an increase in water temperature which reduces the ability of the water to hold oxygen and increases the metabolic rate and resulting oxygen demand of the fish.
Nutrition:
- Feed a high quality diet that meets the nutritional requirements of the species.
- Use proper feeding rate (either over-feeding or starving the fish should be avoided).
- Store feed in a cool dry place to prevent deterioration. If avail- able, a freezer is ideal for storing fish feed.
Sanitation:
- Quarantine all new fish and observe for mortality. Send samples to a diagnostic laboratory to be examined for parasites and evaluated for viral and bacterial disease.
- Prevent disease-carrying fish from living in hatchery water supply (e.g., reservoir ponds, springs, streams).
- Remove all dead fish from a production system as soon as they are observed.
- Dispose of dead fish properly to prevent spread of disease.
- Use good sanitation practices resulting in clean equipment, ponds, and tanks. Disinfect containers, nets, and equipment to minimize transmission of parasites and disease from one population to another
Authors:
R.W. Rottmann, R. Francis-Floyd , and R. Durborow
