Stress: Its Role in Fish
by Ruth Francis-Floyd
Courtesy of the Institute of Food and Agricultural Sciences
Stress is a condition in which an animal is unable to
maintain a normal physiologic state because of various factors adversely
affecting its well-being.
Stress is caused by placing a fish in a situation
which is beyond its normal level of tolerance. Specific examples of
things which can cause stress (stressors) are listed below:
Poor water quality - low dissolved oxygen,
Pollution - intentional pollution: chemical
treatments - accidental pollution: insect spray, spills
Diet composition - type of protein, amino acids
Nitrogenous and other metabolic wastes -
accumulation of ammonia or nitrite
Population density - crowding
Other species of fish - aggression,
territoriality, lateral swimming space requirements
Microorganisms - pathogenic and non-pathogenic
Microorganisms - internal and external parasites
Alarm reaction (fight or flight
Increases in blood sugar are caused by a
secretion of hormones from the adrenal gland. Stored sugars, such as
glycogen in the liver are metabolized. This creates an energy
reserve which prepares the animal for an emergency action.
Osmoregulation is disrupted because of changes in
mineral metabolism. Under these circumstances, a freshwater fish
tends to absorb excess water from the environment (over-hydrate). A
saltwater fish will tend to lose too much water to the environment
(dehydrate). This disruption requires that extra energy be used to
Respiration increases, blood pressure increases,
and reserve red blood cells are released into the circulation.
The inflammatory response is suppressed by
hormones released from the adrenal gland.
Stress triggers a chain of events which result in an "alarm
reaction" (fight or flight response) by the fish which then
triggers a series of hormonal changes. As the fish tries to adjust to
the insult it uses up energy reserves but during this time it is able to
resist or compensate for the insult. If the insult is not removed, its
energy reserves become depleted and the fish becomes
"exhausted." At this phase its ability to resist disease
organisms, with which it is in constant contact, is severely compromised
and the fish may become sick or die.
An animal is able to adapt to stress for a finite
period of time. During this period the animal may look and act normal,
but is depleting energy reserves because of the extra requirements place
The animal's reserves have been depleted and
adaptation fails because the stress was too severe or lasted too long.
What is disease?
Disease is an abnormal condition characterized by
a gradual degeneration of a fish's ability to maintain normal
physiologic functions. The fish is not "in balance" with
itself or its environment.
All fish do not get sick and die each time a
disease outbreak occurs. There are many factors which affect how an
individual responds to a potential pathogen. The pathogen (bacteria,
parasite, or virus) must be capable of causing disease. The host (fish)
must be in a susceptible state, and certain environmental conditions
must be present for a disease outbreak to occur (see
Defense against disease
Protective barriers against
Mucus (slime coat) is a physical barrier
which inhibits entry of disease organisms from the environment into
the fish. It is also a chemical barrier because it contains enzymes
(lysozymes) and antibodies (immunoglobulins) which can kill invading
organisms. Mucus also lubricates the fish which aids movement
through the water, and it is also important for osmoregulation.
Scales and skin function as a physical barrier
which protects the fish against injury. When these are damaged, a
window is opened for bacteria and other organisms to start an
Disease rarely results from simple contact between the host (fish) and
potential pathogen. Mitigating circumstances, such as poor water
quality, excessive crowding, or similar stressor, are usually present
before fish become sick. Identification and correction of these problems
is essential for successful control of disease outbreaks.
Inflammation (non-specific cellular response) is
a cellular response to an invading protein. An invading protein can
be a bacteria, a virus, a parasite, a fungus, or a toxin.
Inflammation is characterized by pain, swelling, redness, heat, and
loss of function. It is a protective response and is an attempt by
the body to wall off and destroy the invader.
Antibodies (specific cellular response) are
molecules specifically formed to fight invading proteins or
organisms. The first time the fish is exposed to an invader,
antibodies are formed which will protect the fish from future
infection by the same organism. Exposure to sublethal concentrations
of pathogens is extremely important for a fish to develop a
competent immune system. An animal raised in a sterile environment
will have little protection from disease. Young animals do not have
an immune response which works as efficiently as the immune response
in older animals and therefore, may be more susceptible to disease.
Effect of stress on
Any stress causes chemical changes in mucus
which decrease its effectiveness as a chemical barrier against
invading organisms. Stress upsets the normal electrolyte (sodium,
potassium, and chloride) balance which results in excessive uptake
of water by fresh water fish and dehydration in salt water fish. The
need for effective osmoregulatory support from mucus components is
Handling stress physically removes mucus from the
fish. This results in decreased chemical protection, decreased
osmoregulatory function (at a time when it is most needed), decreased
lubrication thereby causing the fish to use more energy to swim (at a
time when its energy reserves are already being used up
metabolically), and disruption of the physical barrier against
Chemical stress (i.e. disease treatment) often
damages mucus resulting in loss of protective chemical barrier, loss
of osmoregulatory function, loss of lubrication, and damage to the
physical barrier created by mucus.
Scales and skin
Scales and skin are most commonly damaged by
Handling Stress. Any break in the skin, or removed scale, creates an
opening for invasion by pathogenic organisms.
Trauma caused by fighting (Reproductive Stress or
Behavioral Stress) could result in breaks in the skin or scale loss.
Parasite infestations can result in damage to
gills, skin, fins, and loss of scales which could create breaks in
the skin for bacteria to enter. Many times, fish which are heavily
parasitized actually die from bacterial infections; but the parasite
problem, associated physical damage, and stress response create a
situation which allow the bacteria in the water to invade the fish,
causing a lethal disease.
Any stress causes hormonal changes which
decrease the effectiveness of the inflammatory response.
Temperature stress, particularly cold temperatures,
can completely halt the activity of "killer cells" of the
immune system, thus, eliminating an important first defense against
invading organisms. Excessively hot temperatures are also very
detrimental to fish, although the precise impact of sudden increases
in temperature on the immune system is not known.
Temperature stress, particularly a sharp
decrease in temperature, severely impairs the fishes ability to
quickly release antibodies against an invading organism. The time
lapse required to mount an antibody response gives the invader time to
reproduce and build up its numbers, therefore giving it an advantage
which may allow it to overwhelm the fish.
Prolonged stress severely limits the effectiveness
of the immune system, thereby increasing the opportunities for an
invader to cause disease.
Prevention of stress
The key to prevention of stress is GOOD
MANAGEMENT. This means maintaining good water quality, good nutrition,
Good water quality involves preventing accumulation
of organic debris and nitrogenous wastes, maintaining appropriate pH and
temperature for the species, and maintaining dissolved oxygen levels of
at least 5 parts per million. Poor water quality is a common and
important STRESSOR of cultured fish and precedes many disease outbreaks.
Feed a high quality diet that meets the nutritional
requirements of the fish. Each species is unique and the nutritional
requirements of different species will vary. Supplementing diets with
fresh vegetables and live food is a good way to provide a balanced diet
for fish which have poorly understood nutritional requirements. Fish in
ponds have an advantage over fish raised indoors, because of the variety
of natural foods available.
Proper sanitation implies routine removal of debris
from fish tanks and disinfection of containers, nets, and other
equipment between groups of fish. Organic debris which accumulates on
the bottom of tanks or vats is an excellent medium for reproduction of
fungal, bacterial, and protozoal agents. Prompt removal of this material
from the environment will help decrease the number of agents the fish is
exposed to. Disinfection of containers and equipment between groups of
fish helps minimize transmission of disease from one population to
Prevention of disease
Fish farm management should be designed to minimize
stress on fish in order to decrease the occurrence of disease outbreaks.
When disease outbreaks occur the underlying cause of mortality should be
identified, as well as underlying stress factors which may be
compromising the natural survival mechanisms of the fish. Correction of
stressors (i.e. poor water quality, excessive crowding, etc.) should
precede or accompany disease treatments.
Stress compromises the
fish's natural defenses so that it cannot effectively protect itself
from invading pathogens. A disease treatment is an artificial way of
slowing down the invading pathogen so that the fish has time to defend
itself with an immune response. Any stress which adversely effects the
ability of the fish to protect itself will result in an ongoing disease
problem; as soon as the treatment wears off, the pathogen can build up
its numbers and attack again. Rarely would a treatment result in total
annihilation of an invading organism. Disease control is dependent upon
the ability of the fish to overcome infection as well as the efficacy of
the chemical or antibiotic used.
The keys to minimize disease outbreaks on your fish farm are
maintenance of good water quality, proper nutrition and sanitation.
Prevention of disease outbreaks is more rewarding and cost-effective
than treatment of dying fish. Disease treatments should never be applied
in a haphazard fashion. When needed, chemical or antibiotic treatment
should be targeted at a specific problem. Any management deficiencies in
water quality management, nutritional management, or sanitation should
be corrected. Fish which do not respond to a correctly administered
treatment should be reevaluated by a fish health professional.