3.1. General behavior
In general, P. hypophthalmus showed high levels of swimming activity
andwere rarely observed stationary in the water column. This species is
known to be sensitive to disturbance, with vigorous swimming and
escape behavior after even the slightest stimulus. Surfacing and airgulping
were observed infrequently. Young fish (up to 150 g) had a
tendency to surface synchronously in large groups. During air-breathing,
the fish would swim towards the surface at an angle of approximately
45° and, after surfacing where the breathing cycle seemed to be rapidly
completed, turn quickly to submerge. Air bubbleswere occasionally seen
to be expelled through the gill slits during the descent from the surface.
When confined in a respirometer without access to air, the fish were
generally agitated for the first hour exhibiting pronounced gill
ventilation. Following this initial period, the fish continued to show
regular bouts of spontaneous activity interspersedwith quiescence, even
after many hours in the respirometer. The fish continued to be sensitive
to disturbance, for instance, when the flush pump had to be turned off
during measurement of Pcrit. It was therefore important that human
disturbancewas eliminated duringmeasurement and that the laboratory
was completely undisturbed during this time. The fish seemed to be less
sensitive and calmer in a respirometer where access to air was allowed.