Although the same fish were used in trials 2 and 3, each trial re-
sulted in different behavioural responses to air gun noise. In trial 2
the fish swam to the bottom of the cage where they remained for
the duration of noise exposure and fast swimming was observed in
relation to noise exposure. In trial 3 the fish did not display a pref-
erence for any portion of the cage and fast swimming was occa-
sionally observed but was not related to noise exposure. There
are several possible explanations for this difference in behaviour.
For example, the fish may have become habituated to the noise
from trial 2 and did not associate the noise with danger. Also, in
trial 3 a different air gun noise regime was used, with air gun noise
levels beginning 5 dB lower that in trial 2 which may have resulted
in the fish becoming habituated to the noise before higher noise
levels were reached. Alternatively, the damage that resulted to
the ears of the fish in trial 2 and reported in
McCauley et al.
(2003)
compromised the fishes hearing ability. Finally, it is possi-
ble that the longer acclimation time to the cage (i.e. 70 days) by
the time trial 3 had begun, had an effect on fish behaviour.