levels can result in low oxygen availability to tissues,
which may cause physiological and behavioural
modifications in an effort to compensate. Typically,
these compensatory mechanisms include: leaving the
water completely to respire in air; gulping air from
above the water surface; surfacing to skim the top
few millimetres of water that have an appreciably
higher oxygen content; reduced activity as energy is
shifted to increase water volumes passed across the
gills; or increased activity for escape. These mechanisms
may be adequate for short-term exposure to
low oxygen levels, but as the severity and duration
increase, the costs in terms of energy expenditure and
vulnerability to predation also increase (Birtwell
1989).
There are many studies on the dissolved oxygen
requirements of salmonids (salmon, trout, and char),
which are considered to be the most sensitive group
of freshwater fish, especially at the time of hatching
(e.g., Alabaster & Lloyd 1984). In comparison, little
is known about the tolerances of New Zealand's
native fish species to low levels of dissolved oxygen,
and the purpose of this study was to examine them
in native fish species commonly occurring in lowland
streams. Juvenile rainbow trout were included
in the study for comparison.