The US government is proposing to waive environmental laws so as to enable the Navy to deploy a controversial sonar system for submarine detection. The sonar has been the subject of intense debate because of its potentially adverse effects on marine mammals.

The Surveillance Towed Array Sensor System, with a Low Frequency Active upgrade, listens for reflections bouncing off submarines in response to bursts of sound emitted by the system. The Navy says the system is needed because modern submarines are too quiet to be detected by passive systems. The sonar has so far been fitted on only one ship, but the Navy has funding for a second and hopes to build two more to obtain global coverage.

Tests on the system were classified information until about six years ago, when environmentalists learned about it. They threatened the Navy with lawsuits based on concerns that it might cause hearing loss or disorientation in marine mammals. The Navy responded by agreeing to assess the system's environmental effects. It also supported additional research on the sonar's effects on the animals.

The research found that the system might significantly affect the mammals by, among other things, altering their migration patterns. Two recent whale strandings have also been linked to sonar use, although the Navy says these events involved factors, such as proximity to land, that would not apply to their plans for deploying the system.

Based on the assessment and related research, the National Marine Fisheries Service (NMFS), part of the National Oceanic and Atmospheric Administration, has just released a draft of a new rule that would give the Navy a five-year exemption from the Marine Mammal Protection Act. The act prohibits the harassment or killing of marine mammals by US entities in or outside US waters.

The rule outlines mitigation measures that the Navy and the NMFS say will ensure that the system has little effect on marine mammal populations. They include a promise not to use the system within 20 kilometres of the coast, to shut it down whenever mammals are detected within one kilometre, and to stay out of regions where mammal aggregations are common.

Environmentalists and some academics say that, although there are insufficient data to be certain about the system's effects on marine life, the mitigation measures probably do not go far enough. “I simply do not believe that the Navy has addressed the most fundamental problems,” says Joel Reynolds, director of marine mammal protection at the Natural Resources Defense Council, an environmental group.

Jonathan Gordon, who studies marine mammal acoustics at the University of St Andrews in Scotland, expresses satisfaction that the effects of the military technology have been assessed at all. He points out that France and Britain are developing comparable systems without any public scrutiny. But he remains concerned that the US proposal is based on inadequate information.

Ken Hollingshead, a fisheries biologist at the NMFS, says that the Navy recognizes the need for better information, and will continue to seek it. But Roger Gentry, coordinator of the NMFS acoustics team, says he believes there is enough information available to say with reasonable certainty that the system will cause no significant harm.

Public comment is being invited on the proposed rule, and a final decision will be announced later in the year.