Echolocation is utilised by many animals for communication, orientation, navigation and foraging (http://www.clickdetector.co.uk). In essence, the animal emits a click in a forward-facing beam, and gauges information about the environment from the returning echo. Directionality refers to the area ensonified by the beam, and is discussed commonly in terms of beam width; a click with a narrow beam ensonifies a smaller area than one with a wide beam, in the same way that a torch with a narrow beam of light illuminates less of a dark room than a torch with a wide beam.
Harbour porpoises (Phocoena phocoena) emit directional Narrow Band High Frequency (NBHF) clicks with mean –3 dB beam widths of ca. 13° in the horizontal plane and ca. 11° in the vertical plane (Koblitz et al., 2012). Beam width in this sense, expressed as -3 dB in degrees, means degrees away from the acoustic axis in the horizontal or vertical plane when the Sound Pressure Level (SPL) is reduced by 3 dB.
The narrowband nature of harbour porpoise clicks means the area covered by a single click is limited. In order to increase the range at which a click can be heard or a target detected, harbour porpoises make lateral and vertical movements with their heads whilst emitting biosonar signals (Kastelein et al., 2008; Clausen et al., 2010). Head movements are much quicker than body movements, and most likely require less energy (Akamatsu et al., 2010).
IMPACTS OF DIRECTIONALITY AND HEAD MOVEMENTS ON DETECTION
An echolocation click emitted straight onto a target is referred to as ‘on-axis’, whilst a click received at an angle is ‘off-axis’. Attenuation and distortion of the signal causes the off-axis click characteristics, including source level and frequency, to vary from on axis clicks (Herzing & Santos, 2004; Madsen et al., 2004; Atem et al., 2009; Clausen et al., 2010), which has implications for detection on a Passive Acoustic Monitoring (PAM) system (www.passiveacousticmonitoringsystem.co.uk).
Hanson et al. (2008) (http://bit.ly/1rDL3mQ) reported a reduction in porpoise click source levels of 27 dB, 37 dB and 44 dB with off axis angles of 45˚, 90˚ and 135˚ respectively, although the off axis hydrophones did detect all clicks. In terms of wild populations, Hanson et al. (2008) calculated that if two communicating porpoises are facing each other at an angle of 135˚, the range of detection for echolocation clicks with a source level of 160 dB re 1 µPapp will be reduced to ca. 60 m, which is in comparison to ca. 670 m estimated for on-axis communication. Similarly, Clausen et al. (2010) (http://bit.ly/1ohHH6q) calculated a 500 m reduction in detection distance between on and off-axis communication. This theory can be applied equally to Passive Acoustic Monitoring (PAM) systems such as T-PODS (www.t-pod.co.uk) and C-PODS (www.c-podclickdetector.co.uk). It should be mentioned that the thresholds of hydrophones are generally higher than that of porpoise hearing, which will decrease detection distances further than for porpoise to porpoise communication (Hanson et al., 2008).
Further complications arise if a harbour porpoise is moving its head and causing direction of the echolocation beam to vary constantly. A hydrophone placed directly in front of the animal will receive only part of the click train straight on, as the beam will alternate between on and off-axis in terms of the hydrophone.