Diel echolocation activity of harbour porpoises (Phocoena phocoena) around North Sea offshore gas installations

Abstract

Todd, V. L. G., Pearse, W. D., Tregenza, N. C., Lepper, P. A., and Todd, I. B. 2009. Diel echolocation activity of harbour porpoises (Phocoena phocoena) around North Sea offshore gas installations. – ICES Journal of Marine Science, 66: 734–745. Echolocation clicks of harbour porpoises (Phocoena phocoena) were detected with T-PODs, autonomous, passive, acoustic-monitoring devices, deployed from an offshore-exploration-drilling-rig and gas-production-platform complex in the Dogger Bank region of the North Sea from 2005 to 2006. Echolocation-click trains were categorized into four phases of the diel cycle: morning, day, evening, and night. Porpoises were present near (<200 m) the platform, and there was a pronounced diel pattern in echolocation activity; the number of porpoise encounters (visits) was greater by night than by day. The number of click trains with a minimum inter-click interval of <10 ms also increased at night. This was confirmed by a comparison of the ratios of feeding buzzes to search-phase clicks (feeding buzz ratios) and an analysis of the changes in pulse-repetition frequencies within each train. A reasonable interpretation of this pattern was that porpoises were feeding below or around the platform at night. The evidence for changes in activity during the morning and evening was less clear, so these may be transitional phases. The pattern of porpoise-echolocation behaviour around this platform is related most probably to the diel activity of their prey. If porpoises cluster regularly around such installations within 500-m shipping exclusion zones, they may be omitted from population surveys. We conclude that offshore installations may play an important role as nocturnal porpoise-feeding stations in an overfished environment, but that further replicated and controlled studies are required. These findings should be taken into consideration during offshore-installation-decommissioning decisions in the North Sea.

Using Static Acoustic Monitoring to Describe Echolocation Behaviour of Heaviside’s Dolphins (Cephalorhynchus heavisidii) in Namibia

“…To reduce the likelihood of this occurring, we further filtered our data by excluding time periods in which bottlenose dolphin-like clicks (50 to 70 kHz) were recorded. For data calculated per minute, any whole minutes of data in which bottlenose dolphin-like clicks were recorded, as well as the minute before and after that time, were (Carlström, 2005;Todd et al, 2009). Feeding-buzz ratio (FBR) has previously been used as an indicator of potential feeding behaviour for odontocetes (Todd et al, 2009).…”

“…For data calculated per minute, any whole minutes of data in which bottlenose dolphin-like clicks were recorded, as well as the minute before and after that time, were (Carlström, 2005;Todd et al, 2009). Feeding-buzz ratio (FBR) has previously been used as an indicator of potential feeding behaviour for odontocetes (Todd et al, 2009). The terms feeding buzz or click burst have been used to describe the very fast click trains (i.e., with small ICI values) produced as cetaceans or bats approach and capture targets such as prey items (e.g., Goodson et al, 1988Goodson et al, , 1994Miller et al, 1995;Johnson et al, 2004Johnson et al, , 2006Johnson et al, , 2008Akamatsu et al, 2005;Madsen et al, 2005;Melcón et al, 2007;Verfuβ et al, 2009).…”

“…For this study, we therefore chose to use ICI values below 10 ms as a possible indicator of feeding behaviour or target-locking. FBR values were calculated by dividing the number of click trains in each individual hour with a mean ICI of < 10 ms by the number with a mean ICI of ≥ 10 ms, creating a ratio of fast, possibly feedingassociated click trains to all other trains (Todd et al, 2009). A high ratio would thus suggest more time spent producing buzz trains and therefore possibly engaging in feeding behaviour.…”

“…The T-POD is a SAM tool which has been used extensively to monitor odontocetes (Leeney & Tregenza, 2006;Leeney et al, 2007;Philpott et al, 2007;Verfuß et al, 2007;Kyhn et al, 2008;Todd et al, 2009), including Hector's dolphins of New Zealand (Rayment et al, 2009a(Rayment et al, , 2009b, a species closely related to Heaviside's dolphins. The T-POD is a self-contained, submersible hydrophone and digital processor which recognizes and logs the echolocation clicks made by odontocetes.…”

“…The T-POD is a self-contained, submersible hydrophone and digital processor which recognizes and logs the echolocation clicks made by odontocetes. Todd et al (2009) provide a description of the T-POD hardware and of the processes of data collection and classification, while numerous studies have reviewed settings and T-POD functionality (e.g., Thomsen et al, 2005;Philpott et al, 2007;Kyhn et al, 2008;Simon et al, 2010). The onboard data logging nature of the T-POD makes it particularly suited to the study of odontocetes, which use narrow-band high frequency (NBHF) echolocation clicks, such as the Phocoenids (for which the instrument was initially designed; Goodson & Sturtivant, 1996) and members of the Cephalorhynchus genus, as the sampling rate required for real-time recording (> 250 kHz) makes long-term recordings unfeasible.…”