How Environmental Stressors Impact Fish

How Environmental Stressors Impact Fish

Stress Physiological stress and physical injury are the primary contributing factors to 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 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.

What is Stress?

    Stress is any condition that causes physical or mental discomfort that results in the release of stress-related hormones or results in specific physiological responses.
    Stress can be physical, psychological, or environmental.
    Stress can either be short and sudden, or long and chronic.
    Mild, short-term stress has few serious health effects, but long-term stress or severe, short-term stress contributes to many of the illnesses and deaths in aquarium fish.

Causes of Stress

    Elevated Ammonia
    Elevated Nitrate
    Improper PH level
    Fluctuation in Temperature
    Improper Salinity
    Low Oxygen level
    Medication and Water treatments
    Harassment from Other Fish

Water quality

    Do not exceed the 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.

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.


    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)


    Quarantine all new fish and containers, nets, and equipment to minimize transmission for mortality. 
    Send sam- sion of parasites and disease pies to a diagnostic laboratory from one population 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.


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.


Dr. M. K. Yadav
Assistant Professor
Sage University Bhopal (MP)

Enquire Now