Differences in the response of a striped dolphin (Stenella coeruleoalba) and a harbour porpoise (Phocoena phocoena) to an acoustic alarm.

Small cetacean bycatch in gillnet fisheries may be reduced by deterring odontocetes from nets acoustically. However, different odontocete species may respond differently to acoustic signals from alarms. Therefore, in this study a striped dolphin and a harbour porpoise were subjected simultaneously to sounds produced by the XP-10 experimental acoustic alarm. The alarm produced 0.3s tonal signals randomly selected from a set of 16 with fundamental frequencies between 9 and 15kHz, with a constant pulse interval of 4.0s (duty cycle 8%) and a Source Level range of 133-163dB re 1muPa (rms). The effect of the alarm was judged by comparing the animals' respiration rate and position relative to the alarm during test periods with those during baseline periods. As in a previous study on two porpoises with the same alarm, the porpoise in the present study reacted strongly to the alarm by swimming away from it and increasing his respiration rate. The striped dolphin, however, showed no reaction to the active alarm. Based on harbour porpoise audiograms and the specific audiogram of the striped dolphin in the present study, and the low background noise levels during the experiment, both animals must have heard the alarm signals clearly. This study indicates that cetacean species are not equally sensitive to human-made noise disturbance. Therefore, source levels of acoustic alarms should be adapted to the species they are supposed to deter. In addition, alarms should be tested on each odontocete species for which they are intended to reduce bycatch.