The U.S. Navy has issued a National Defense Exemption under the Marine Mammal Protection Act to continue testing and operating its Surveillance Towed Array Sensor System Low Frequency Active Low Frequency Active towed-sonar array.

The move would extend the exemption for another two years or until final rules are issued by the National Marine Fisheries Service on mitigating possible harm to marine wildlife by the SURTASS LFA array.

Active sonar works by using sound waves for underwater object detection and mapping. The intensity of the sound used can be extremely loud, ranging up to 200 decibels. It is used by the Navy for detecting enemy submarines and other underwater objects as well as for commercial uses like oil drilling and fishing.

Many marine biologists believe that powerful active sonar can be harmful to wildlife, such as whales, panicking them and causing them to beach themselves en masse. This has led to lawsuits and a 2008 Supreme Court decision that sonar was vital to national security and outweighed environmental concerns, though the 9th Circuit ruled in 2016 that low-frequency sonar use must be limited to protect wildlife.

Whales and other marine mammals have sensitive ears to help communicate with each other over long distances and would be particularly susceptible to harm from sonar.

Whales beach themselves accidentally for a variety of reasons not fully understood, but many scientists say sonar is likely to be a factor in some cases. The Navy has denied in the past that sonar has more than a minimal effect with marine wildlife, and the National Marine Fisheries Service has concurred with that view. Rules concerning sonar use are still being determined.

The Surveillance Towed Array Sensor System Low Frequency Active is a sonar system under development by the Navy that is a more powerful version of earlier SURTASS arrays.

It is designed for detecting submarines at extreme ranges using both active and passive sonar and is towed by surface ships.

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