Southern Regional Aquaculture Center
R.W. Rottmann, R. Francis-Floyd1, and R. Durborow2
Physiological stress and physical
injury are the primary contributing
factors of fish disease and mortality
in aquaculture. Stress is
defined as physical or chemical factors
that cause bodily reactions
that may contribute to disease and
death. Many potential fish disease
pathogens are continually present
in the water, soil, air, or fish. In nature
fish are often resistant to these
pathogens, and they are able to
seek the best living conditions
available. Food fish reared under
commercial aquaculture conditions
are confined to the production
unit and are weakened by
stress conditions including:
These conditions can result in decreased resistance by the fish, resulting in the spread of disease and parasite infestation. Stress and injury initially trigger an alarm reaction (fight or flight response), which results in a series of changes within the fish. A blood sugar increase occurs in response to hormone secretion from the adrenal gland as liver glycogen is metabolized. This produces a burst of energy which prepares the animal for an emergency situation. In addition, the inflammatory response, a defense used by fish against invading disease organisms, is suppressed by hormones released from the adrenal gland. Water balance in the fish (osmoregulation) is disrupted due to changes in the metabolism of minerals. Under these circumstances, freshwater fish absorb excessive amounts of water from the environment (over-hydrate); saltwater fish lose water to the environment (dehydrate), This disruption increases energy requirements for osmoregulation. Respiration increases, blood pressure increases, and reserve red blood cells are released into the blood stream.
Fish are able to adapt to stress for a period of time; they may look and act normal. However, energy reserves are eventually depleted and hormone imbalance occurs, suppressing their immune system and increasing their susceptibility to infectious diseases.
Mucus (slime layer) is the first physical barrier that inhibits entry of disease organisms from the environment into the fish. It is also a chemical barrier, containing enzymes and antibodies which can kill invading disease organisms. Mucus also lubricates the fish, aiding their movement through water, and is important for osmoregulation.
Injury as a result of handling (i.e., capture, transport, etc.) and certain chemicals in the water (i.e., poor water quality, disease treatments) remove or damage the mucous layer, reducing its effectiveness as a barrier against infection at a time when it is needed most. This damage decreases the chemical protection of the slime layer and also results in excessive uptake of water by freshwater fish and dehydration by saltwater fish. Decreased lubrication causes the fish to expend more energy to swim at a time when its energy reserves are already depleted.
Scales and skin
Scales and skin function as a physical barrier which protects the fish. These are injured most commonly by handling, rough surfaces of tanks or cages, and by fighting caused by overcrowding or reproductive behavior. Parasite infestations can also result in damage to gills, skin, fins, and loss of scales. Damage to scales and skin of the fish can increase the susceptibility to infection. It also causes excessive uptake of water by freshwater fish or loss of water from marine species (osmotic stress). Fish which are heavily parasitized may die from bacterial infections which gained initial entrance to the fish's body through damaged areas in the skin.
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.
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.
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.
Handling and transporting
Stress compromises the fish's natural defenses against invading pathogens. When disease outbreaks occur, the underlying stress factors, as well as the disease organism, should be identified. Correcting stress factors should precede or accompany chemical disease treatments. A disease treatment is only an artificial way of slowing down an infection so that the fish's immune system has time to respond. Any stress which adversely affects the fish will result in an ongoing disease problem. Prevention of disease outbreaks is more cost-effective than treating dying fish.
The work reported in this publication was supported in part by the Southern Regional Aquaculture Center through Grant No. 89-38500-4516 from the United States Department of Agriculture.
1 Institute of Food and Agricultural Services, University of Florida
2 Kentucky State University