Synthetic communication signals influence wild harbour porpoise (Phocoena phocoena) behaviour
We used our novel and programmable Porpoise Alarm (PAL, patd.) to synthesize lifelike, electronic harbour porpoise communication signals based on those described for captive animals. In the Little Belt, Denmark, we employed PAL (source level 158 ^ 1 dB pp re 1mPa@1 m; centroid frequency 133 ^ 8.5 kHz) to synthesize three aggressive click train types termed A, F3 and M1 to naive, free-living harbour porpoises. Via theodolite tracking (372 h of total visual effort spread over 10 expeditions) we found that, depending on signal type, porpoises either avoid or become attracted to PAL: Signal types A and F3 are slight deterrents, porpoises increasing minimum range (þ23 to 32 m, respectively), whereas M1 attracts porpoises, reducing range (by 229 m). As determined via archival acoustic detectors (AADs), both signals F3 and M1 led the animals to significantly intensify their click rate (by þ10% and 68%, respectively) while signal A led to a significant reduction (259%). We propose that equipping fishing gear with PAL emitting signal F3 could potentially reduce porpoise by-catch by increasing (1) awareness through enhanced echolocation and (2) distance to the nets. Detection probability and radius of PAL/AAD tandems could be improved by emitting signal M1 to focus porpoise echolocation signals on the AAD. The signal may also be useful in luring animals away from hazards, which may be helpful for conservation measures prior to the onset of harmful acoustic activities such as pile-driving, seismic exploration or ammunition clearance.
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