Stress
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:
increased fish density and poor water quality (i.e., low dissolved oxygen, undesirable temperature or pH, increased levels of carbon dioxide, ammonia, nitrite, hydrogen sulfide, organic matter in the water);
injury during handling (i.e., capture, sorting, shipping);
inadequate nutrition; and
poor sanitation.
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.
Defense against infection: Mucus
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 fishs body through damaged areas in the skin.
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 fishs 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 fishs 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
Use capture methods that minimize physical injury and stress.
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 available, 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.
Conclusions
Stress compromises the fishs 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 fishs 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.
Source: Southern Regional Agricultural Center and the Texas Aquaculture Extension Service - February 1992
