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stress in fish

Stress in Fish

What is Stress

Stress in fish should be taken seriously because it is real and it has many causes most of which may go unnoticed by aquarist especially when new to the hobby. When a fish encounters an unfavourable stimulus which are likely to adversely affect the fish it reacts in a to protect itself. Such unfavourable stimuli are referred to as stressors. In the short term where a mild stressors is involved the fish will adjust and cope well in most circumstances. If the stressor is more long term (chronic) or more severe (acute) then the fish is likely to cope less well and this may impact on its health or may even cause premature death.

Stress is very difficult to sum up because the processes involved are varied and complex. When a fish is placed under stress various changes take place, some almost immediately and others over hours or even days later depending on the severity of the stressor.

stress in fishWhat causes stress in fish?

There is a huge list of potential stressors where pond and aquarium fish are concerned.

  1. Poor water quality: this could mean the presence of nitrogenous waste such as ammonia, nitrite or excess nitrate, low oxygen levels, high organic content, high dissolved solids, excess suspended matter, the presence of chlorine or chloramines.
  2. The wrong water chemistry: even if the water is clean and fresh having the wrong water chemistry for a species will still cause stress. pH to high or to low, water which is too hard or too soft may act as a stressor particularly where wild caught fish are concerned.
  3. Wrong temperature: Tropical fish need tropical temperatures, most temperate fish have a wider range.
  4. Wrong tank mate:, some fish need the security of a shoal while others are confirmed loners, the presence of a tank bully which could actually be stressful to both perpetrator and victim. because being chased and attacked is obviously stressful especially in the long term but less obviously having an intruder on your territory which you cannot chase away could also cause some stress. Large boisterous fish kept with more timid species even if they appear peaceful will act as a stressor. Over crowding which isn’t the same as over stocking, some ‘loner’ species may feel over crowded by the presence of another fish.
  5. Nutrition: wrong type of diet, malnutrition,
  6. Wrong environment: if a shy timid species is kept in a very sparsely decorated tank with no hiding places it will be stressful for the fish, bright lighting, photo period.
  7. Sudden changes in the environment: It is important that this is fully understood by the aquarist. Moving a sick and stressed fish from a tank with very poor water quality to a tank with excellent water quality as quickly as possible may seem like a good idea, but it isn’t. The gills of a fish are semi-permeable, it is this which allows gas exchange to occur without which the fish would quickly die. But it also means that a fish’s internal body chemistry is in some ways linked to the water containing the fish and sudden changes could have the potential to make the fish go in to shock and possibly even die.
    It is the SUDDEN CHANGE which is harmful even when the change is from a poor environment to a good one. All changes must occur slowly to avoid stressing the fish. The only exception to this is when the environment is so poor that it is also a great threat to the life of the fish, for example when a fish has been put in an immature aquarium and as a result there is a very high level of either ammonia or nitrite present.
  8. Catching, handling and transport: All of these will cause great stress in the short term and should only be done when absolutely necessary.

What are the effects of stress in fishes?

There are two types of stress which may affect fish. Chronic stress and acute stress. Chronic stress occurs when the stressor is constant over a long period or constantly recurring. Acute stress occurs rapidly and usually over a short period of time. Chronic stress is usually less severe than acute stress but this isn’t always the case.
An example of chronic stress in the aquarium is when the nitrate level is allowed to rise unchecked until it reaches toxic levels. Most fish will adapt to high levels of nitrate but it is a stressor and it will eventually have some detrimental effect on the aquariums occupants.
An example of acute stress in the aquarium would be when a thermostat fails and the temperature rises beyond the fishes tolerance level. If left unchecked death will occur quite quickly.

Most experienced aquarists will quickly recognise a very stressed fish just through observation because some but not all of the effects of stress can be seen depending on the cause of the stress.
Fish will respond to stress by initially reacting to it on a physiological level. When acute stress strikes the fish will respond by enhancing its immune system, producing more mucus, increasing its breathing rate, working harder to maintain the right osmotic balance, releasing more glucose in to the blood for an energy boost for fighting or escaping and in most instances of acute stress which passes quite quickly these responses will help to protect the fish from any harm. But when the stress is chronic the opposite is likely to occur.
With chronic stress the immune system will be suppressed and the fish is more likely to fall victim to opportunistic infections caused by normally harmless background microbes.

What are the outward signs of stress in fishes?

Possible signs of stress, particularly chronic stress will include:

  • Poor growth.
  • Poor health, as the immune system fails different infections will take hold which will respond poorly to normally effective treatments. White spot, dropsy, pop-eye, fin rot and head – lateral line erosion disease are all classic signs of fish under long term stress.
  • Low fertility and lack of breeding.
  • Lethargy, clamped fins and possibly abnormal swimming behaviour.
  • Gasping at the surface.
  • Loss of condition.
  • Multiple deaths with no apparent cause.

Can stress in fishes be treated?

Prevention should be the number one approach to dealing with stress in fish. But when treating the condition the most important first step to treating a fish suffering from either chronic or acute stress is to remove the stressor, unless this is done nothing else will work. Once the stressor has been identified and dealt with the effects of the stress should be treated appropriately and with no stress the fish should respond to the treatment very well and that’s it. It really is so simple.

Good husbandry is the key to avoiding the cause of stress coupled with learning about a species before keeping it.

About Andy Rapson

I've been interested in fish for about fifty years and I have kept many different species in that time. I have also worked in the fish trade running my own fish shop and I'm a Fishbase collaborator. I'm now mainly interested in fish husbandry, fish health, native marines and fish photography.

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