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by Hilary L. Maybaum

Why do mass strandings of marine mammals happen? Researchers have been trying to answer this question scientifically for decades. Various hypotheses have been put forth, from parasitic diseases to anomalies in Earth’s magnetic field. Now, manmade sonar can be added to that list.

Sonar—an acronym for SOund Navigation And Ranging—is the technique of using sound propagation to navigate, communicate, or detect underwater objects. Originally deployed on a massive scale during World War I for enemy submarine detection, sonar has since become a mainstream navigation and fish-finding aid for military, commercial, and recreational vessels alike. It relies on variations in underwater sound speed, determined mainly by temperature, pressure, and salinity (saltiness). There are two kinds of sonar systems. Passive sonar simply “listens,” without transmitting any sound. Active sonar—such as that used in fish-finding—emits sound pulses, sometimes called pings, for detection of objects. The pulses can vary in frequency, loudness, and duration.

I actually did my Master’s Thesis on the effects of an active 3.3 kHz sonar systems on humpback whales in Hawaii. A group of sonar engineers in Massachusetts had a National Geographic grant to test a prototype system on its detection of humpback whales in Stellwagon Bank. They were getting lousy results, mostly because of the shallowness and sound opacity in the bank’s waters. I invited them to come test their system in Hawaii, where the waters were clear and deep. They did, and I investigated the behavioral effects of their system on the whales. It turned out there were some observable effects, and we all concluded it was not a plausible system to use for this purpose.

But I digress. This post is about beaked whales.

Public domain image of a Cuvier's beaked whale from the National Oceanic and Atmospheric Administration

In 2005, a group of scientists studied a mass stranding of beaked whales off the Canary Islands and found lesions apparently induced by mid-frequency sonar sounds. Previously, other scientists had suggested a link between mass stranding of beaked whales and mid-frequency sonar, but were unable to establish a causal relationship. This particular stranding event of 14 whales occurred about four hours after the onset of an international naval sonar exercise conducted between the coast of Fuerteventura Island and 40 km offshore.

Postmortem examinations of the whales found no external trauma or bruising, with the exception of one postmortem shark bite. No pre-existing health issues were indicated. The researchers did, however, find severe internal hemorrhaging, swelling, and congestion in the head and neck areas, including the jaws, ears, and brain. They also found evidence of nitrogen supersaturation (“the bends”) in the blood vessels and tissues of vital organs. Beaked whales are known for their deep diving capabilities, and don’t normally get the bends unless their dive behavior is dramatically altered (see this press release from Peter Tyack’s group at the Woods Hole Oceanographic Institution).

The Natural Resources Defense Council (NRDC) sued the Navy over the use of mid-frequency sonar after this and similar findings were published. I’m interested in finding out where that lawsuit stands.

Update: The lawsuit was settled this time last year, in favor of the Navy. A precedent had already been set by the Supreme Court. According to a 12/29/08 NY Times article:

…in a ruling on a council lawsuit challenging the Navy’s sonar training exercises off Southern California, the Supreme Court ruled that military training trumped protecting whales.

Chief Justice Roberts wrote that forcing the Navy to deploy an inadequately trained antisubmarine fleet would jeopardize the safety of the fleet. He also wrote that it was unclear how many marine mammals the Navy’s sonar exercises might harm.

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Many thanks to Dr. Joseph Mobley for sending me the original article on the mass stranding event from Vet Pathol 42:446-457 (2005).

Thanks also to Mary Beckman (@sciwriter) for the update on the Navy’s lawsuit.