A universal reaction to low oxygen is increased ventilation of the gills. In the same
way that people start to breathe faster in order to bring more air to their lungs, fishes
increase the rate of water flow through their gills. A convenient measure is the rate at
which gill covers open and close under different levels of dissolved oxygen. Thus
three-spined sticklebacks have been reported to increase their rate of gill cover
movements from 95 to 165 per minute after the oxygen content of their water was
experimentally cut down to half of its normal level. One could also hypothesize that
ram-ventilating sharks (fishes that bring oxygenated water to their gills by swimming
with their mouths open) would increase their swimming speed and would open their
mouths wider when oxygen is scarce. This is indeed what has been observed.