maculatus), banded kokopu (G.fasciatus), common
smelt (Retropinna retropinna), torrentfish (Cheimarrichthys
fosteri), common bullies {Gobiomorphus
cotidianus), shortfinned and longfinned elvers
{Anguilla australis and A. dieffenbachii), and shrimp
{Paratya curvirostris). Juvenile rainbow trout
(Oncorhynchus mykiss) were also tested for
comparison.
The numbers and sizes of animals collected are
shown in Table 1. Once in the laboratory, the test
animals were kept at least 5 days before being tested.
During this acclimation period, they were held in
tanks containing 50 litres of aged, aerated Hamilton
city tap water at 15°C (±0.5°C) which was replaced
regularly. To reduce the risk of disease, 5 litres of
sea water was added to each tank. Test animals were
exposed to a 12:12 dark/light regime during
acclimation and testing. Fish were fed daily with live
food (Daphnia spp.) during acclimation, but were
not fed for 12 h before or during experimentation.
Shrimp were provided with aquatic macrophytes for
food and shelter during acclimation.
The experimental apparatus consisted of a 40 litre
plastic tank containing 30 litres of aged, aerated
Hamilton city tap water. Diffusing rods made of spun
fibreglass were positioned along both sides of the
tank and fed with a continuous supply of nitrogen
and air. A small submersible pump was used to
ensure that water was mixed within the tank and to
maintain water flow over an oxygen probe connected
to a YSI Model 55 oxygen meter that constantly
recorded oxygen levels in the water. A transparent
plastic lid was placed on top of the tank to allow
observation, prevent fish escaping, and limit gas
transfer between the headspace of the tank and the