Does Boat Noise Affect Dugong (Dugong dugon) Vocalization?

Abstract

In recent decades, human activities in coastal areas have increased worldwide. Activities such as coastal develop­ ment, vessel traffic, and excessive fishing have altered the marine environment in ways including destruction of sea­ grass beds, mangroves and estuaries, and water pollution from dredging and chemicals (Hinrichsen 1998; Orth et al. 2006). These alterations reduce the quantity and qual­ ity of favorable habitats for marine animals. Coastal ma­ rine mammals are exposed to direct and indirect threats like vessel collisions, entanglement in fishing nets, and disturbance and displacement of habitat. Potential human impacts on them have been given much attention in the last few decades (Reeves and Reijnders 2002). For exam­ ple, the disturbance caused by boat traffic displaces killer whales (Orcinus orca) away from feeding habitats, result in reducing feeding opportunities or increasing travel costs (Williams et al. 2006). These cumulative impacts on animals possibly affect their survival, and productivity of individuals, and may have detrimental effects on popu­ lations (Lusseau and Bejder 2007). In many species, however, the mechanisms linking short-term behavioral responses to long-term population effects are still under­ explored. Understanding whether and how animals could be influenced by human disturbances is essential for con­ servation efforts and management plans of target species and their environments. It supports designing of effective measures for mitigating the impacts of human disturbances. Anthropogenic noises generated by human activities, especially vessel traffic, have increased over the last few decades (Hildebrand 2005, 2009). Anthropogenic noise impacts on marine mammals have been a great concern, because noise may cause changes in animal behavior, dis­ tribution, and habitat selection (NRC 2005; Nowacek et al. 2007). Marine mammals rely on sound for communication, prey detection, and predator avoidance. Noises from natural and anthropogenic sources can interfere with an animal’s hearing and use of sound signals (Hildebrand 2005). Some marine mammal species alter their vocaliza­ tions in the presence of noise to overcome acoustic inter­ ference (e.g., Nowacek et al. 2007). For example, beluga whales (Delphinapterus leucas) responded to increases in ambient noise from boat traffic by increasing their call amplitude (Scheifele et al. 2005), repetition, and fre­ quency (Lesage et al. 1999). Killer whales (Orcinus orca) increased the duration of their stereotyped call as the number of whale-watching boats in their habitat increased (Foote et al. 2004). The dugong, Dugong dugon, is an herbivorous mam­ mal that lives in tropical and subtropical shallow coastal waters from East Africa to Vanuatu, between about 26° north and south of the equator (Marsh et al. 2002). They are in decline or extinct in at least one third of its range. Dugong deaths caused by boat collisions and entangle­ ments in fishing gears have been reported (Marsh et al. 2011). They are vulnerable to anthropogenic influences because they live in shallow coastal waters where various human activities take place. In fact, dugongs show sev­ eral behavioral changes in response to human activities, such as boat traffic. For example, they stop feeding and/or move toward deeper water in response to vessels approaching (Preen 1992; Hodgson and Marsh 2007). Dugongs also vocalize like other mammals and are thought to use acoustic signals for communication (Ichikawa et al. 2011). Various types of boats pass through shallow coastal waters where dugongs live, and it is likely that their sound frequencies overlap with dugong call fre­ quencies and interrupt their communication. Dugongs tend to vocalize at higher frequencies in response to high ambient noise levels (Ando-Mizobata et al. 2011). Manatees (Trichechus manatus), also members of the