Marine Mammals Still Imperiled After Sonar Ruling (Science 11 Jan 08)

Benjamin Lester*

LOS ANGELES, CALIFORNIA–Marine mammals won some protection last week from the U.S. Navy’s submarine-chasing sonar technology. A federal judge imposed significant restrictions on use of the technology, known as mid-frequency active (MFA) sonar, in training exercises taking place off the southern California coast through January 2009. Environmental groups that brought the suit hailed the ruling. But researchers say it still leaves the most vulnerable species with little added protection.

MFA sonar detects ultraquiet submarines by bouncing powerful sound waves off their hulls. For more than 10 years, mass strandings of beaked whales and other marine mammals have been linked to its use, although the mechanism is not clear. Ruling on 3 January that the sonar posed a risk to species in the waters off southern California, Judge Florence-Marie Cooper of the U.S. District Court for the Central District of California ordered the Navy not to use it within 22 kilometers of the coast. She also told the Navy to use shipboard observers, aircraft, and hydrophones to monitor for marine mammals before and during the exercises and to turn off an MFA system if a marine mammal was detected within a 2000-meter “safe zone.”

The Navy had sought less stringent restrictions. But Cooper called one such proposal–a safe zone of 180 meters–“grossly inadequate to protect marine mammals from debilitating levels of sonar exposure.” At the same time, Cooper said that granting the 45-kilometer exclusion zone sought by plaintiffs, led by the Natural Resources Defense Council (NRDC), a New York City-based nonprofit, would “prevent the Navy from training to detect submarines in the very bathymetry [deep submarine canyons] in which submarines are most likely to hide.”

Those canyons are also a popular foraging spot for beaked whales, the group most susceptible to MFA sonar. Robin Baird, a marine mammalogist with the Cascadia Research Collective in Olympia, Washington, who gave expert testimony in the case, says the zone will protect porpoises and migrating whales, but it does little for beaked whales. Likewise, a 2000-meter safe zone around sonar sources won’t necessarily protect beaked whales, who Baird says are “more or less invisible” even at 300 meters.

Baird says that sea-floor hydrophone arrays, such as the Navy operates off San Clemente Island, can detect the whales’ clicks and warn vessels of their presence. In other areas of the exercise zone, however, the downward directionality of the whales’ clicks, coupled with the high speed of Navy vessels, makes ship-mounted hydrophones “fairly ineffective,” according to Baird.

Necroscopies of stranded individuals point to decompression sickness–bubbles in the blood formed by rapid changes in pressure–as the cause of death. However, says Baird, “there’s still a lot of uncertainty about what could lead to those symptoms.” One possible reason that MFA sonar is particularly problematic for beaked whales is that its pings mimic the calls of killer whales, their primary predators. In a recent study, Peter Tyack of the Woods Hole Oceanographic Institution in Massachusetts speculates that the false calls might prompt the whales to leave the area rapidly by making a series of short, shallow dives that promote bubble growth.

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