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]

Do harbour porpoises target offshore installations as feeding stations?

Two sets of field trials were performed from offshore installations in the German Entenschnabel sector of the Dogger Bank, North Sea. Trial 1 was undertaken from the jackup drilling rig Noble Kolskaya and its support vessel Northern Seeker, at locations B4-5 and B11-4 over three discrete periods (October/November 2004 and December 2004/January 2005). The purpose of these trials was to perform measurements of acoustic noise levels generated by the rig during routine activities and to undertake preliminary passive acoustic monitoring (using T-PODs) of porpoises (Cetacea: Phocoena phocoena) around the rig. Trial 2 was a six-month study (August 2005-January 2006) using T-PODS around the A6-A gas-production platform when it was isolated, when the Noble Kolskaya was docked alongside, and after the rig’s departure. Sound levels generated by the Kolskaya were similar to previous measurements from metal-legged bottom-founded platforms, both in level (120 dB re 1μPa) and in the frequency range of dominant tonals (2-1400 Hz). Sound levels were highly variable over short periods, shifting 15-20 dB between quietest (holding) and loudest (drilling) operations. The rig was significantly quieter than its associated support vessels; rig high frequency sound levels dropped rapidly > 8 kHz. With the exception of rig-docking/rig-departure manoeuvres, porpoise activity appeared to be independent of platform/rig activity. The greatest porpoise activity was observed during the winter months; porpoise activity and feeding rates were significantly reduced in periods of heavy weather. Here we present data to suggest that harbour porpoises may be using installations in the Dogger Bank on a seasonal basis and that installations may enhance porpoise habitat by acting as artificial reefs and feeding stations, especially during the winter months. However, these are preliminary snapshot findings. To answer completely the question posed in the title of this paper, we would need to undertake baseline and controlled and replicated longer-term studies if the story of harbour porpoise usage of offshore installations is to be fully told