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Stress in fish is an immensely complex subject. There is a great deal of mystery surrounding the subject of stress in fish and for many their concept of stress is vague. Most of what we know about stress in fish comes from research targeted to commercial fisheries. Science has really just begun to scratch the surface of what appears to be an enormous field of study.

When people talk about stress, it is usually a dialogue about mental and emotional stress. However, stress has biochemical and physiological consequences. Stress causes an internal hormonal response that is remarkably similar in all vertebrate animals, including fish.

What then is stress, and is it easily defined? First, we must differentiate between a source of stress, which we will refer to as a stressor, and the body’s internal reaction to it. A simple definition of a stressor in fish could be a stimulus that requires a physiological response by the animal in an attempt to adapt to that stimulus.


Stress can be described as the physiological response to a stressor. In other words, stress is an internal physiological state that is caused by external conditions. Stress can also be described as an internal hormonal response of a living organism caused by environmental or other external factors that moves that organism out of its normal physiological resting state, or homeostasis (Selye, 1973). Stress can disturb the normal physiological equilibrium or homeostasis of fish by forcing a reallocation of energy within its system.

Stress is categorized as acute or chronic and severe or mild. The degree to which stress affects any particular fish is determined largely by the severity of the stress, its duration and the health of the fish.

Respiration in water requires a great deal more energy consumption than in air. Gas concentrations in water generally vary to a much greater degree than in air. Water suspends fish and since water is hundreds of times denser than air, movement in water requires more metabolic energy. Even though fish are well equipped for these environmental differences, they still consume large amounts of metabolic energy.

Stress affects fish in two ways: it produces effects that disrupt or threaten homeostatic equilibrium and it induces adaptive behavioral and physiological responses (Wendelaar Bonga, 1997). Along with the release of stress hormones and the subsequent physiological and chemical changes, come behavioral modifications. These changes are essentially adaptive and function to enhance the likelihood of survival when threatened by noxious or challenging situations.

In the natural environment, the stress response protects fish and insures their survival. One example of this is when a predator threatens fish. When the fish senses the threat, the primary stress response is to release catecholamine into the blood stream. This essentially creates an energy boost that helps the fish evade or escape predation. Intense chronic stress can cause these responses to lose their adaptive value and become dysfunctional.

Fish respond to chemicals and other stressors at intensity levels that are often far below those that can be detected by terrestrial animals (Wendelaar Bonga, 1997). Fish are more sensitive to stressors than many other vertebrates because their physiological homeostasis is intimately bound to and dependant upon the water in the surrounding environment. Disturbance of water and ion homeostasis during stress is due to the very intimate relationship between body fluids in the gills and the ambient water. The high bioavailability of chemicals in water is also a factor. Fish are exposed to pollutants through delicate respiratory surfaces of the gills. Marine fish also drink the saltwater.

Fish respond to stress on three levels. This integrated stress response includes: primary, secondary and tertiary levels. The primary response is the release of the stress hormones, corticosteriods and catecholamines, into the bloodstream. The secondary response is the effects of these hormones at the cellular level including the mobilization and reallocation of energy, osmotic disturbance and increases in cardiac output, oxygen uptake and transfer. The tertiary response extends beyond the cellular level to the entire animal. It inhibits immune response, reproduction, growth and the ability to tolerate additional stressors (Pickering, 1987. Maule, et al, 1989. Barton. et al, 1986. Mesa, 1994).

The internal hormonal response to stress in fish has many similarities to that of mammals. The most widely accepted indicator of stress is the blood plasma cortisol level. The two major actions of cortisol in fish are regulation of hydromineral or osmotic balance and energy metabolism. Some consider the role of corticosteriods to be protecting the body from overdoing with its own defense mechanisms (Munck et al, 1984).

Corticosteroid and catecholamine hormones are released in response to stressors in an attempt to adapt or avoid the cause of stress. Although both types of stress hormones are released into the bloodstream as part of the integrated stress response to acute and chronic stressors they do play different roles. Catecholamines or CA is the primary hormone released in the “Fight or Flight” response to acute stressors. Corticosteriods, predominately cortisol, are the primary hormone released in response to chronic stressors.

The catecholamine hormones epinephrine (adrenalin) and norepinephrine (noradrenalin) are associated with more immediate reactions to stress, and are released when situations requiring a fight or flight response occur. The release of catecholamine hormones into the bloodstream causes increases in cardiac output, blood sugar, respiration, oxygen uptake, and blood flow to the gills. This prepares fish to better cope with threats to territory and safety. These responses are usually short in duration and are considered acute stresses.

Corticosteriods, primarily cortisol, are associated with chronic stress as the animal attempts to adapt. Cortisol is released in response to all stressors, but its effects become greater the longer the stress reaction continues. In an aquarium, common factors that could lead to a chronic stress response would include such things as poor water quality, inappropriate temperatures, or toxins.

When stress is continuous or chronic and cannot be avoided, as is more likely in the confined environment of an aquarium, the hormonal and behavioral responses cease to be a tool for adaptation and survival. At this point, these responses become maladaptive.

This is the first installment in a series of articles detailing stress in fish. This series will cover why stress is important to understand, what causes stress, the behavioral indicators, how various stressors affect behaviors and how to control stress in fish. Part two in the series will continue with “Why You Should Care about Stress in Fish”.


  1. Barton, B.A. Schreck, C.B. & Sigismondi, L.A. Multiple Acute Disturbances Evoke Cumulative Physiological Stress Responses in Juvenile Chinook Salmon. Transactions of the American Fisheries Society, 115, 245-251, 1986.
  2. Maule, A.G. Tripp, R.A. Kaattari, S.L. & Schreck, C.B. Stress Alters Immune Function and Disease Resistance in Chinook Salmon. Journal of Endocrinology, 120, 135-142, 1989.
  3. Mesa, M.G. Effects of Multiple Acute Stressors on the Predator Avoidance Ability and Physiology of Juvenile Chinook Salmon. Transactions of the American Fisheries Society, 123, 786-793, 1994.
  4. Munck, A. Gurye, P.M. & Holbrook, N.J. Physiological Functions of Glucocorticoids in Stress and their Relationship to Pharmacological Actions. Endocrine Reviews, 5, 25-44, 1984.
  5. Pickering, A.D. Stress Responses and Disease Resistance in Farmed Fish. In Aqua Nor 87, Conference 3: Fish Diseases – a Threat to the International Fish Farming Industry. Pp. 35-49. Norske Fiskeoppdretteres Forening, Trondheim, 1987.
  6. Seyle, H. The Evolution of the Stress Concept. American Scientist, 61, 692-699, 1973.
  7. Wendelaar Bonga, S.E. The Stress Response in Fish, Physiological Reviews 77(3):591-625, July 1997.

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