A study published this week in the British journal Biology Letters has raised a bit of a ruckus, with headlines suggesting that it confirms that sonar can deafen dolphins (even Nature and New Scientist ran such heads). The paper is a brief report (3pp) on a study in which a captive dolphin was exposed to recordings of actual mid-frequency active sonar signals (5.6kHz) at gradually increasing volumes until a consistent temporary threshold shift (TTS) was induced. Surprisingly, considering the long-running controversies over these sonars, this is the first study to use actual sonar sounds in this way, rather than generic stand-in sounds. The researchers found that they had to ramp up the sonar sounds to 203dB SPL rms (214dB SEL) to “consistently” induce a TTS of 6db; such exposures would occur only within about 40m of sonar vessels, though in some situations (multiple ships, clear sound propagation conditions), could occur at greater distances. This shift faded rather rapidly, to 4dB at 10 minutes, and back to baseline hearing in 20 to 40 minutes. This is not “deafness,” but rather what one of the researchers termed a “rock concert effect” whereby the animal would need sounds to be 6db louder than normal in order to be heard (and, louder sounds will appear fainter than normal), for a period after the intense exposure. The ability of sounds over 200dB to create TTS was no surprise; more notable was that a current assumption that SEL (cumulative sound exposure) can be a consistent measuring stick to assess risk of TTS was undermined by this study–the short-duration sonar signals required a higher SEL to trigger TTS than longer duration sound do. (I’ll spare lay readers a detailed analysis of the metrics questions here, but suffice to say, this questioning of the “equal energy model” will cause some consternation in the sound exposure modeling community!) While the media ran with headlines suggesting that this study will provide new fuel for anti-sonar advocates, the US Navy’s first response was that “This would mean that the Navy may have vastly overestimated impacts of mid-frequency active sonar on marine mammals in its environmental planning documents.” (The Navy has been using a level of about 195dB SEL, which occurs at roughly 150m from sonar vessels, as the TTS-prevention threshold.)
Now, before dashing to any other rash conclusions, we should note that the TTS results only apply to the Navy’s estimates of Level A Harassment (injury or death), which are consistently very low or even zero; the far more worrisome Level B Harassment numbers (behavioral changes) are unrelated to any hearing loss impacts, and can occur out to 50km or more from sonar vessels. Current Navy EISs predict that marine mammals will hear and react in some way to sonar signals millions of times over the coming five years in formal Navy training ranges along coastlines throughout the US; how many may respond to the hundreds of US and allied Navies’ routine use of mid-frequency active sonar throughout the world is unknown.
Mooney, Nachtigall, Vlachos. Sonar-induced temporary hearing loss in dolphins. Biology Letters, doi: 10.1098/rsbl.2009.0099