In contrast to the ideal conditions that normally prevail in the home aquarium, wild
fishes are sometimes faced with situations where little oxygen is present in the water.
In places such as swamps and pools, the absence of wind and water currents prevents
the mixing of the oxygen-rich upper layer with the rest of the water column. 1 In
other places, an overabundance of aquatic animals or decomposing matter depletes
oxygen because of increased consumption. These conditions are not uncommon, and
therefore it is not surprising to learn that fishes have evolved special behaviours to
cope with low oxygen levels. Good aquarists should have no cause to ever witness
these behaviours, but biologists, being the curious lot that they are, have sought or
created hypoxic (low oxygen) conditions and catalogued the reactions of fishes to
them. Here is what they found.