Are fish really cold-blooded animals, or is it more complicated than that?
Like reptiles and amphibians, fish are cold-blooded poikilothermous vertebrates —meaning they get their body temperature from the surrounding water.
Therefore, as body temperature is directly linked to water temperature, and changes in body temperature have an effect on how the body works, these can be stressful or deadly.
At higher water temperatures water holds less dissolved oxygen, so when water warms it affects fish respiration and they have to move their gills more rapidly to extract the oxygen they need.
Temperature also affects metabolism and metabolic processes occur quicker in warmer water. This also adds to the amount of oxygen fish require.
Some fish, such as tuna and certain types of shark, are more warm-blooded and able to warm their red muscle tissues to 26-32°C/79-90°F when the ambient water temperature is between 6-30/43-86°F.
This item was first published in the October 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.