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.

To be clear I'm not looking for "chemical x is released and that leads to the fish dying, I'm looking for why these chemicals are released at all. So to give an example if my question had of been "why does adrenaline shock kill" I'd be looking for "adrenaline release is usually a good thing, it's released in response to a fright and prepares the body for a fight or flight response. However this heightend preparedness can cause additional strain on the body which can cause death in some cases"

  • $\begingroup$ As others have said fish can die from an overload of hormones or muscle exaustionn. Mammals are likely to get hypothermia from the dramatic temperature drop killing them. $\endgroup$
    – Anonymous
    Nov 6 '17 at 9:10
  • $\begingroup$ How fast can a lake or a river change from 20-30 degrees? 2 days. Inevitable thermal shock is not something fish dealt with a single time in the last 300 million years. Every chemical reaction in the fishes body suddenly goes at 1/2 speed if you cool it from 30 to 20. some fish can go easily from 15 to 25, they can dive for cover one hour and then bask at the top where they can grow faster. $\endgroup$ Nov 6 '17 at 20:27
  • $\begingroup$ The question that you're posing isn't "why does temperature changes kill Fish", but why these physiological responses exist in the first place. The above answers clearly explained what the effects are of temperature shock, and what biological responses can occur that can harm the fish. I suggest you research and ask people the rephrased question "Why do these processes occur in fish?" Maybe check this article about metabolic rate and oxygen consumption to start your research: fishbio.com/field-notes/the-fish-report/… $\endgroup$
    – Aman
    Dec 26 '17 at 23:31
  • $\begingroup$ You might have an easier time if you look at the reverse question, sudden temperature changes are pretty hard on living things, even something as simple as membrane permeability can be drastically changed by temperature. We don't think about it becasue we are descended from terrestrial organisms, and air can and does change temprature quickly unlike water, so our ancestors had to develop defenses against these sudden swings. most fish never had to bother. $\endgroup$
    – John
    Dec 27 '17 at 3:48

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
  • $\begingroup$ 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 $\endgroup$ Jul 28 '14 at 11:42
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    $\begingroup$ 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) $\endgroup$ Jul 28 '14 at 11:43

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