Echolocation activity of harbour porpoises (phocoena phocoena) around an offshore gas-production platform-drilling-rig complex.
conference contributionposted on 26.03.2012, 13:39 authored by V.L.G. Todd, I.B. Todd, Paul LepperPaul Lepper, N.C. Tregenza
Harbour porpoises (Phocoena p. phocoena L.) are vocal animals and their activity can be monitored effectively using underwater, autonomous, passive-acoustic cetacean-click detectors called T-PODs [e.g. 1, 2, 3]. The characteristics of porpoise-echolocation clicks have been described in great depth over the last forty years [4-10]; clicks can be emitted singularly or in groups known as “trains”. There is a linear correlation between porpoise-echolocation pulse intervals and target range [11, 12] with a peak in repetition rate as the animal nears the target, analogous to the “terminal buzzes” repeatedly observed in echolocating bats . Determination of a successful prey-capture event in wild echolocating bats has been achieved effectively [e.g. 14] but for wild porpoises, underwater filming of prey-capture attempts is extremely tedious. Moreover, in the wild, without visual confirmation, any correlation between porpoise buzz activity and feeding success cannot be assumed a priori without experimental evidence, because a high buzz rate may simply be associated with increased foraging effort for the same amount of prey. Nonetheless, it is conceivable that by using acoustics alone, a proxy of feeding activity could be surmised by examining the relative incidence of increasing click rates, emitted during range-locking echolocation behaviour, and the associated decreasing interval between clicks, known as “inter-click-intervals (ICI)” [see 2]. A link between feeding and decreasing ICI has been established for foraging Blainville's beaked whales, Mesoplodon densirostris  and harbour porpoises .
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