Schools of herring exposed to progressive hypoxia show a peak in velocity d
uring severe hypoxia, at 15-34% oxygen saturation, followed by a decrease i
n swimming speed until school disruption occurred. The observed increase in
swimming speed during severe hypoxia reveals a graded response, since the
lower the fish's swimming speed prior to severe hypoxia (U95-50, the speed
at oxygen saturations between 95 and 50%), the greater the relative increas
e in swimming speed. The oxygen saturations at which both peak velocity and
school disruption occurred were lower for fish with lowest U95-50, suggest
ing that the fish with the slowest speed U95-50, reach their critical P-O2
(at which there is respiratory distress) last, i.e. at lower oxygen saturat
ion. At a functional level, it is suggested that herring encountering hypox
ia increase their speed in order to find more favourable conditions, and th
e magnitude of this increase is modulated by their respiratory distress. It
is also hypothesised that the observed increase in speed may be related to
an increase in the rate of position shifting within the school. Since the
oxygen saturation at which the response to hypoxia occurs and the magnitude
of the response are related to the fish's preferred speed prior to severe
hypoxia, it is suggested that such a preferred speed should be measured in
experiments testing the effect of hypoxia on fish behaviour. (C) 2000 The F
isheries Society of the British Isles.