An assessment of the audibility of sound from human transport by breeding Weddell seals (Leptonychotes weddellii)

Anthropogenic noise generated through travel in the Antarctic has the potential to affect the region’s wildlife. Weddell seals (Leptonychotes weddellii) in particular can be exposed to anthropogenic noise because they live under, and breed on, the fast ice on which humans travel. To investigate the potential effects of anthropogenic noise on Weddell seals we developed sound profiles for pedestrian travel, over-snow vehicles, aircraft and watercraft operating at various distances and altitudes from breeding seals. The received 1/3-octave noise levels were then related to an assumed detection threshold for the Weddell seal. We found that most noise levels generated by the pedestrian, quad (4-wheeled, all-terrain vehicle) and Hagglunds (tracked, all-terrain vehicle) were commonly categorised in the inaudible and barely audible range of detection (both in-air and underwater), while noise levels generated by the helicopter, Twin Otter aircraft and Zodiac boat were categorised more commonly in the barely audible and clearly audible range. Experimental underwater recordings of vocal behaviour of Weddell seals exposed to continuous low-amplitude over-snow vehicle noise (i.e. Hagglund operation) were also made. Weddell seals underwater did not alter individual call types in response to low-level Hagglunds noise, but they did decrease their calling rate. Introduction Human activity in the Antarctic has been steadily increasing since the continent was discovered in 1820 (Kimball 1999). Early human activities included harvesting of wildlife (primarily seals and whales), exploratory expeditions and scientific research. In recent decades, activity has been largely limited to science and tourism. Sounds of varying frequencies and intensities are associated with most human activities in the region and many activities may affect the wildlife (see, for example, Richardson et al. 1995; National Research Council 2003). Sound is important to marine mammals for foraging and social facilitation, suggesting that alterations of the acoustic medium are potentially adverse for the wildlife. Despite this, very little research has been conducted to establish whether Antarctic wildlife is affected by anthropogenic noise. Studies have investigated the effect of helicopter operations on the behavioural response of king penguins (Aptenodytes patagonica) (Cooper et al. 1994), emperor penguins (Aptenodytes forsteri) (Giese and Riddle 1999), Adélie penguins (Pygoscelis adeliae) (Culik et al. 1990; Wilson et al. 1991) and southern elephant seals (Mirounga leonina) (Burton and van den Hoff 2002). However, these studies have not differentiated between the acoustic and visual components of the stimuli to which animals were exposed, so it is difficult to draw conclusions about the relative importance of acoustic effects. Studies on marine mammals elsewhere suggest that anthropogenic noise can cause: (1) changes in behaviour, such as the cessation of feeding and mating, increased alertness, vigilance and agnostic behaviour or increased avoidance and escape behaviour, as suggested by the reactions of harbor seals (Phoca vitulina) (Myrberg 1990) and ringed seals (Phoca hispida) (Born et al. 1999), (2) changes in vocal behaviour, such as the cessation of calls, or changes in call duration, repetition rate, frequency (kHz) and loudness, as evident from responses of beluga whales (Delphinapterus leucas) (Lesage et al. 1999), bottlenose dolphins (Tursiops truncatus) (Scarpaci et al. 2000), Pacific humpbacked dolphins (Sousa chinensis) (van Parijs and Corkeron 2001) and killer whales (Orcinus orca) (Foote et al. 2004), (3) changes in movement patterns such that animals temporarily or permanently leave an area, as illustrated from studies of harbour seals (Henry and Hammill 2001) and killer whales in Canada (Morton and Symonds 2002), (4) masking of important sounds, affecting communication, navigation, and predator–prey interactions, as reported for killer whales in Canada (Morton and Symonds 2002), (5) temporary or permanent hearing loss, or (6) physical injury or death (Richardson et al. 1995; National Research Council 2003). Various measures and conventions designed to control human travel in the vicinity of Antarctic wildlife exist under the Antarctic Treaty System (Kimball 1999). In addition to these, the International Association of Antarctic Tour Operators (IAATO) has developed guidelines for vessel and aircraft operations in the vicinity of wildlife (IAATO 2004). Many of the Antarctic Treaty Nations with research bases in the region, including the Australian Antarctic Division (AAD), have also developed specific operational requirements for vehicles; however, most of these are not based on scientific studies and have not been tested to determine whether they are actually sufficient to minimise or eliminate noise impacts to wildlife. The Weddell seal (Leptonychotes weddellii) is the only Antarctic marine mammal that lives under, and breeds on, the same fast ice that people utilise for travel. As a consequence, seals near research bases or tourist operations are often exposed to anthropogenic noise. The vocal behaviour of Weddell seals is sophisticated, compared with that of other Antarctic phocids, and they may therefore be especially vulnerable to acoustic interference (Ray and deCamp 1969; Evans et al. 2004). Quantifying the effects of noise on the behaviour (and potentially the physical state) of Weddell seals requires knowledge of the auditory threshold of the Weddell seal, the factors affecting the audibility of noises, the sound levels produced by various forms of transport (i.e. their sound profile) and how seals might respond to anthropogenic noise. The aims of this study were to (1) determine audibility by Weddell seals of a number of commonly used Antarctic vehicles and (2) determine, from an experiment of vocal response, whether continuous vehicle noise affected the vocal behaviour of Weddell seals underwater. The spectra of in-air and underwater noises were compared to the assumed detection thresholds for Weddell seals. This enabled us to determine the frequency (kHz) at which there was the greatest amplitude of the noise above the detection threshold. In turn, this process permitted us to estimate detection ranges (in quiet surroundings) of noises independent of their frequency. Materials and methods Study sites, experimental stimuli and experimental design