Echolocation activity of harbour porpoises (phocoena phocoena) around an offshore gas-production platform-drilling-rig complex.

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 [13]. 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 [15] and harbour porpoises [16]