They must also keep swimming to keep from sinking, since most have largely or completely lost the swim bladder, the gas-filled sac that helps most other fish remain buoyant.

Lastly, on Wednesday the Daily Mail ran with the scare headline "Swimming too often in chlorinated water 'could increase risk of developing bladder cancer', claim scientists".

Fish generally respire through gills. most bony fish es have a swim bladder a gas-filled organ used to adjust swimming depth.