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As is advised with most fish you should avoid sudden changes in water temperature (for example by introducing new water that hasn't been left to come to room temperature). I have, however, never understood why that is the case, in particular given that most fish are cold blooded so there is no issue with needing to "ramp up" the metabolism when in cold water.

Say a particular species of fish can survive in water between 20°C and 30°C what causes a fish that was previously in 30°C water to die when suddenly plunged into 20°C water? I understand this effect is called "shock" but I'm looking for the biological reason for it.

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This due to a phenomenon called "cold shock". This induces a number of physiological changes in the fishs metabolism and also in its behaviour and can lead to death.

The first paper cites some reasons in table 1:

  • Brain and central nervous system response: Changes in neuronal activity
  • Catecholamine and corticosteroid response: Release of hormones due to the shock.
  • Haematological and metabolic response: Changes in red and white blood cells and lactic acid in the blood.
  • Protein expression and molecular responses: Changes in the expression of proteins of the heat shock family.

These changes (especially in brain and nerves and also the hormones) can deregulate the fishs metabolism so much that it can die. Shock is a critical condition. Obviously a lot of research is going on, but I recommend reading the two articles below for further details.


  1. Cold shock and fish
  2. Physiological stress responses in the warm-water fish matrinxã (Brycon amazonicus ) subjected to a sudden cold shock
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So, would it be correct to say in terms of points 1 and 3 that although the fish's overall metabolism isn't ramped up/down (being cold blooded) there are other processes that are ramped up/down to cope with the current temperature so if those "settings" are in the wrong place for the suddenly changed temperature then the fish will die (as it takes time to ramp up/down the processes). Basically I understand that the fish dies because "This process occurs which leads to the fishes death" I'm looking for why those processes even exist in the first place – Richard Tingle Jul 28 '14 at 11:42
e.g. A sudden fright can kill people because adrenalin is released which stimulates the heart rate etc to increase. Usually this would be beneficial giving a "fight or fright" response but in extremes can be lethal (in particular to a weakened system) – Richard Tingle Jul 28 '14 at 11:43

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