Physiological stress and physical injury are the primary
contributing factors of shad mortality. Stress is defined as physical or
chemical factors that cause bodily reactions that may contribute to disease and
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. In captivity they are subject to:
- 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;
- 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. The 3 day syndrome?
Shad's Defense Against Infection
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. Scale loss most commonly happens by handling, rough
surfaces of tanks or cages, and overcrowding. 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.
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, (the red
nose) and loss of function.
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.
- 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
High water temperature favors 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.
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.
Stress and Disease Prevention
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.
These practices will help prevent stress and the resulting fish kills.
- Do not exceed carrying capacity of fish in tanks.
A good number if your keeping shad in a storage tank is 1 shad to 3 gallons
of water. Shad over 8 inches 1 to 1 gal.
With a good boat bait tank with proper environment a 1-1 ratio will work for
short periods of time.
When in doubt - error on hauling fewer shad.
- 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 shad 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
- 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