Page 24 - April 2008

COMING TO TERMS WITH THE EFFECTS OF OCEAN NOISE ON MARINE ANIMALS
Mardi C. Hastings
Applied Research Laboratory, Pennsylvania State University State College, Pennsylvania 16804
From the sound that threatened “to
deafen whales” with a low frequen-
cy hum throughout the world’s
oceans (Anderson, 1991) to present day
military exercises using active sonar,
controversy about the effects of sound
on the marine environment has contin-
ued for over two decades. Marine ani-
mals use sound for communication,
navigation, detection of predators and
prey, and identification of their habitats.
But the ocean is filled with many inter-
fering sounds, some naturally occurring
such as earthquakes and volcanic eruptions, and others resulting from human activity. The largest contributor to anthropogenic (human-caused) sound in the ocean is com- mercial shipping, which accounts for over 90% of interna- tional commerce. But other contributors, such as active sonar and seismic air guns, have very high source levels even though they affect smaller defined areas. Active sonar is used not only by the world’s navies to detect and track potentially hostile underwater intruders, but also by scien- tific researchers to study the ocean environment and the animals that live there. Likewise sound pulses created by arrays of seismic air guns are used for geophysical research to understand structures and processes beneath the seafloor as well as by oil and gas companies to locate and quantify reserves of hydrocarbon fuels. The challenge is to balance these activities so they do not impact the health and safety of creatures, large and small, that live in the sea.
The public is acutely aware of potentially harmful inter- actions between marine animals and anthropogenic sound (Simmonds et al., 2003; Wartzok et al., 2003/04; Jasny et al., 2005). As this article goes to press, the U.S. Navy finds itself in a 30-day period during which it can file an appeal to the U.S. Supreme Court for exemption from environmental laws1 that protect whales and other marine mammals so that it can fully conduct sonar training exercises off the coast of California. President Bush had exempted the Navy from applicable environmental laws on the basis of national secu- rity so that sonar training activities could continue without restrictions. But on February 29th of this year the U.S. Court of Appeals for the 9th Circuit upheld a lower court ruling that requires the Navy to limit sonar training off the California coast to minimize harm to marine life (The Washington Post, 2008). Now for the first time, it appears that the debate over effects of sound on marine mammals could be headed to the highest court in the land.
How did it begin?
Concern about potential adverse effects of anthro- pogenic sound on marine life accelerated in the early 1980’s
“The challenge is to balance our sound producing activities so they do
not impact the health and safety of creatures, large and small, that live in the sea.”
when endangered gray whales off the coast of California and bowhead whales in the Beaufort Sea displayed avoidance responses when exposed to playbacks of noise from drillships and dredges (Richardson et al., 1990). But beginning in 1990, considerable public awareness about the effects of anthropogenic sound on marine animals materialized when plans for the Heard Island Feasibility Test (HIFT) coincided with advanced devel- opment of the U.S. Navy’s Surveillance Towed Array Sensor System Low
Frequency Active (SURTASS LFA) Sonar. Because sound at low frequencies can travel further underwater than sound produced at higher frequencies, SURTASS LFA was designed to emit signals at frequencies between 100 and 500 Hz using an array of 8 transducers, each with a source level of 215 dB re 1 μPa at 1 m (DoN, 2001), which could propagate from tens to hundreds of nautical miles (nm), effectively covering an ocean basin, to provide longer detection ranges for small, relatively quiet diesel-electric submarines and thus more
2
the National Science Foundation (NSF), the Department of Energy (DOE), and the National Oceanic and Atmospheric Administration (NOAA) were sponsoring researchers led by Walter Munk at the Scripps Institution of Oceanography to plan and conduct the HIFT in attempt to demonstrate a method to monitor climate changes on a global scale. Taking advantage of known, long distance sound wave paths often referred to as the “deep sound channel,” the Heard Island experiment was designed to show that measuring sound speed in the world’s oceans could be used to monitor global warming (Cohen, 1991). The experiment consisted of trans- mitting acoustic signals at a frequency of 57 Hz (sometimes described by the press as a “low frequency hum”) from Heard Island in the southern Indian Ocean and then measuring their time of arrival at various points around the world. The speed of sound in water increases with increasing tempera- ture, so small changes in temperature of the ocean basins could be determined by measuring the amount of time it took for these sounds to travel from one point to another (Baggeroer and Munk, 1992; Munk et al., 1994). This exper- iment created an outcry from several environmental groups, whose members feared that the HIFT sound transmissions would interfere with low frequency communications among large baleen whales (mysticetes)3 or even physically harm these animals. The experiment began in January 1991, but with scientists onboard ships to monitor marine mammal activities in the vicinity of the sound source. No adverse reac- tions were observed (Bowles et al., 1994).
time for defensive action.
At the same time, the Office of Naval Research (ONR),
22 Acoustics Today, April 2008