The phrase "stress-related illness" is perhaps something that we would normally associate with humans, but it is equally applicable to the majority of fish health problems. Indeed, the majority of disease outbreaks and unnatural fish deaths can be attributed to some form of stress, often caused as a result of and in conjunction with poor water quality.

What is Stress?

"Stress" can be difficult to envisage and often it is assumed to be a purely mental process. However, the physical effects of stress are very real, and it is these effects that are responsible for any ensuing health problems. Although there are numerous definitions for stress, it can broadly be described as.

"The sum of all the physiological responses by which an animal
tries to maintain or re-establish a normal metabolism
in the face of a physical or chemical force".

Basically, it refers to alterations in the physiological workings of the fish in response to a stressful situation, such as being chased by a predator or being exposed to poor water quality. The body adapts to help the fish escape or deal with these situations, the so-called "fight or flight" response, and then must try to maintain and re-establish its normal state. In the wild, this response is beneficial as it increases a fish's ability to overcome adverse situations. Once out of danger, the fish can then recover relatively quickly. In captivity, the fish may not be able to escape and the response will therefore be prolonged. Eventually what started as a beneficial (or "adaptive") response, soon becomes a detrimental one ("maladaptive"), as the fish cannot maintain its heightened physiological state. This is why captive fish are more at risk from stress than wild ones. .

What Causes Stress?

Any stress causing event or situation is termed a "stressor", and there are numerous examples. In captivity, these could include physical stressors (e.g. netting, handling, transport, water changes), chemical ones (e.g. high ammonia or nitrite, low oxygen, incorrect pH), and biological ones (e.g. aggression and territorial disputes). Given the range of stressors, a small aggression and territorial disputes). Given the range of stressors, a small amount of stress is usually unavoidable in captive fish. In most cases this is not a problem, provided the stressor is short lived and the fish is given chance to recover. Stress begins to cause problems when the stressor is either prolonged, or occurs in conjunction with other stressors (as they tend to have an additive effect). Prolonged (or "chronic" stress) as opposed to "acute" stress does not allow the fish chance to recover and the detrimental effects become long-term, inevitably resulting in health problems. .

The Stress Response

When a fish is exposed to a stressor, a set of physiological responses are put into place that occur in response to the release of two sets of hormones. The following diagram illustrates a generalised stress response, but it is important to be aware that different species of fish respond in slightly different ways:

Upon the fish's sensory systems interpreting a stressful situation, signals are sent to the hypothalamus which is a major part of the brain. Initially, the hypothalamus initiates a nervous response that causes the chromaffin tissue in the kidney to release hormones called "catecholamines".

These include adrenalin and noradrenalin, and are released within a minute or so (depending on temperature). These hormones result in the mobilisation of energy-producing glucose into the blood and an increase in heart rate to pump it to the muscles involved in the "fight or flight" response. In addition, the gills are dilated to let in more oxygen and the numbers of oxygen-carrying red blood cells are increased. This serves to provide the extra oxygen needed to release this energy.

At the same time as this nervous response, the hypothalamus releases "corticotrophin releasing factor" which stimulates the pituitary gland (found in the brain) to produce "adrenocorticotrophic hormone" (ACTH). ACTH is carried via the circulatory system to the interrenal tissue in the kidney, where it stimulates the production and release of corticosteroid hormones (e.g. cortisol and cortisone). These hormones perpetuate the mobilisation of energy reserves and the dilation of the gills in an attempt to maintain this heightened state. Whilst it takes longer for corticosteroid hormones to be released, their effects can last much longer than those of adrenalin.

Detrimental Effects of Stress

Whilst in the short term these physiological changes help the fish to escape or fight a dangerous situation, in the long run they can cause specific problems. The most broad ranging effect of stress is that it redirects energy away from processes such as growth, reproduction and the immune system, in order to fuel the increased level of activity.

In addition to this, the increased oxygen demands of the response means that the fish is less able to cope with poor environmental conditions. Clearly, this will have an effect on the vitality and overall performance of the fish.

In addition to this, there are also more specific consequences. For example, the dilation of the gills allows an influx of water and a loss of body salts in freshwater fish (the reverse happening in marines). Whilst the fish may be able to cope with this in the short term by using up extra energy, in the long run it will be unable to maintain a correct concentration of salts in its body fluids and will either become ill or ultimately die. In addition to this, the corticosteroid hormones released during the response have a direct effect on the immune system, suppressing its effectiveness at fighting off infection.

The net result of long-term stress is therefore to predispose fish to infection and ill health.

Reducing Stress

By trying to minimise stress it is possible to maintain a much healthier collection of fish. Clearly, the most obvious way of doing this is to ensure that the environment is suitable for the fish. This includes not just water quality, but also the physical environment. Areas of refuge where the fish can hide are important for many species, as are open swimming areas. Keeping fish in the correct social grouping is also important, e.g. large numbers for shoaling species and avoid mixing in aggressive fish with placid ones. Where stressful events are unavoidable, such as during water changes, it is important to perform the task quickly and to allow the fish time to recover afterwards. It is especially prudent to avoid performing multiple stressful tasks all at once (e.g. performing a water change straight after introducing a new fish). Additionally, when fish have been stressed there are steps that can be taken to prevent a disease problem occurring. Try leaving the lights off for a while and not feeding them for a day afterwards, and if possible increase oxygen levels with an air pump. Adding a stress-relieving product, such as AquaSafe. to the tank will also help to protect the fish.

By being aware of the effects of stress and what can cause it. it is possible to prepare for it and help the fish to overcome it. This will go a long way towards maintaining a healthy pond or aquarium.

Last updated January 2005