The acoustics of vessel collisions with marine mammals

Whales and other marine mammals are vulnerable to boat, barge and ship collisions. Though more commonly identified and reported in busy coastal areas, collisions are not restricted to shipping lanes or shallow water environments. A common denominator is that they all occur near the surface. Here the acoustical laws of reflection and propagation can significantly limit the ability of marine mammals to hear and locate the sounds of approaching vessels. The problem is a major concern for the remnant North Atlantic right whale population, as 35% of known mortalities are attributable to ship strikes (Knowlton et al., 2001). With an estimate of only 350 surviving individuals, these whales are on the brink of extinction, and there is an urgency to understand and reduce the underlying causes of collisions. This paper addresses some of the acoustical causes for vessel collisions that apply to all whales and marine mammals at risk. Defining the physics of near surface acoustical propagation as it relates to vessel noise and marine mammal hearing is central to our understanding and to successful mitigation of the vessel strike problem. To address the problem one must consider the psychoacoustical hearing characteristics of marine mammals, in addition to acoustical characteristics of their habitats, vessel noise, and near surface propagation. Analysis of acoustical data reveals multiple factors that can affect the ability of whales and other marine mammals to detect and locate the sounds of approaching ships, including: (i) downward refraction in negative temperature gradients (ii) spreading loss from the stern to bow of large ships, (iii) Lloyd's mirror effect, an embodiment of Snell's Law which governs the transmission and reflection of sound from different boundaries, (iv) acoustical shadowing, a phenomenon that occurs when ship propellers are located above keel depth and sound rays are physically blocked at the stern, and (v) masking of approaching ship noise by ambient noise and sounds from distant ships. The confluence of these acoustical factors poses significant ecological challenges for marine mammals with respect to their ability to detect and locate the sounds of approaching ships. Whales, manatees and other animals can not react to sounds indiscernible from ambient noise nor can they react to sounds that never reach them. These conditions are demonstrated with data from controlled vessel passages recorded with vertical hydrophone arrays at the surface.