The acoustical conditions can confuse whales and may cause them to seek refuge by surfacing or actively swimming into the quieter zones that form directly in front of ships. Observations of whales surfacing in front of ships and the high incidence of strikes on North Atlantic right whale females and calves suggests that whales near the surface are unaware of ships approaching in their direct path. While the size and geometry of the shadows ahead of ships may vary with hull dimensions, propeller noise is more intense off the port and starboard sides than it is directly ahead an approaching vessel. Direct measurements reveal the confluence of acoustical shadowing, Lloyd’s Mirror Effect and spreading loss, which together make ship noise at the bow indiscernible from the ambient noise. However, near the surface, these lower frequencies are severely attenuated or canceled by a boundary interaction phenomenon known as the Lloyd’s Mirror Effect. Only very low frequency sounds with wavelengths larger than stern dimensions can diffract around the ship’s hull to the bow. Ship noise with wavelengths less than the ship stern dimensions are reflected back off the stern and reflect to the sides but do not propagate forward to the bow. The majority of ships that kill whales have propeller configurations above keel depth, and this shields the sounds produced by the propellers. It’s caused when the sound rays from a ship’s propellers are blocked from projecting forward by the ship’s hull. Perhaps the most confounding acoustical challenge to both whales and manatees is acoustical shadowing at the bow. Some individuals have been hit 50 different times by boats they cannot hear approaching. The incidence of collisions with small boats is so prevalent that individual manatees are routinely identified by their characteristic scars patterns from multiple encounters. Most survive collisions with smaller boats, while collisions with larger slow moving barges are fatal. Measurements of controlled ship passages through vertical hydrophone arrays demonstrate a confluence of propagation factors and near surface effects that obscure the sounds of approaching vessels which then pose serious detection challenges for marine mammals.įigure 2. The problem is a major concern for the remnant population of North Atlantic right whales and other marine mammals, such as the West Indian manatee. Whales, manatees and other animals cannot react to sounds that become indiscernible from ambient background noise nor can they react to sounds that never reach their ears. Here the acoustical laws of reflection and propagation significantly limit the ability of marine mammals to hear and locate the sounds of approaching vessels. A common denominator is that they all occur near the surface. Though more commonly identified and reported in busy coastal areas, collisions are not restricted to shipping lanes or shallow water environments. With increased worldwide commerce and international shipping, vessel collisions with marine mammals have become a global concern. Whales and other marine mammals are vulnerable to boat, barge and ship collisions. Parametric Projectors Protecting Marine Mammals from Vessel CollisionsĮdmund R.
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