After encountering an area of low oxygen content, fish often start to swim rapidly and
to zig-zag all over the place. This is probably an adaptive response to escape the
danger zone. However, if hypoxic conditions persist, most fish will greatly curtail
their general activity. The advantage of inactivity in the face of hypoxia is simple: less
muscular work means less need for oxygen. As part of his doctoral research at Laval
University in Quebec City, Fred Whoriskey observed sticklebacks breeding in tide
pools along the St-Lawrence Estuary, and he reported that during periods when
dissolved oxygen levels dropped, the fish became very sluggish, even failing to show
normal fright responses when an object flew over the pool (the object could be a bird
or, if nature did not oblige, a frisbee thrown by the resourceful researcher). In the
same vein but this time in a lab at McGill University in Montreal, Don Kramer and
his students observed that guppies swim less, eat less, and court less when oxygen
levels are lower than normal.5 Inactivity can even reach extremes, as in the case of
the South American cichlid Biotodoma cupido (the greenstreaked eartheater) which
appears to fall “asleep” when oxygen is scarce