Lawrence Crum (lac@apl.washington.edu),
Michael Bailey, Steven Kargl, and Thomas Matula
Applied Physics Laboratory
University of Washington
1013 NE 40th Street
Seattle, WA. 98105
http://cimu.apl.washington.edu/
Popular version of paper 2pAB8
Presented Tuesday, November 16, 2004
148th ASA Meeting, San Diego, CA
Both mid-range and low frequency active sonar systems have been implicated in the mass stranding of several species of cetaceans, principally beaked whales [1-4]. Indeed, the US Navy has agreed that their own sonar systems have probably contributed to these strandings. Since these sonar systems are critical to the successful operation of naval maneuvers and for the safety of their own ships, considerable attention has been directed toward the biophysical conditions whereby active sonar systems could result in such strandings.
In an earlier study performed several years ago [5], we modeled the effects of low-frequency-active sonars on bubble growth by a process called "rectified diffusion" in which an acoustic pulse from a sonar system could cause small microbubbles, present in a marine mammal, to grow to macroscopic sizes and result in deleterious bioeffects. In this study, we concluded that this bubble growth was unlikely because relatively high levels of gas supersaturation were required. Supersaturation results when higher-than-normal levels of gas is dissolved in a liquid-such as carbonated beverages. At that time, no data was available on levels of supersaturation in marine mammals.
Recently, however, Houser and his colleagues [6] were able to measure these supersaturation levels in trained dolphins. They determined that diving whales could reach supersaturation levels on the order of 300%--a relatively large value. Although we found these high supersaturation levels of great interest, we did not redo the calculations to determine if bubble growth was now to be expected.
The concept of sonar-induced bubble growth was raised again when Jepson and his colleagues [1] reported that they were able to observe gas bubbles present in the tissues of stranded whales. They suggested that these whales suffer decompression sickness (the "Bends") induced in some way by these sonars.
We have investigated this suggestion with some preliminary experiments that support the "decompression sickness" hypothesis. In particular, we produced supersaturation levels of approximately 300% in ex vivo bovine liver tissue and then exposed this supersaturated tissue to low frequency acoustic pulses similar to those generated by Navy sonar systems. We found that many bubbles were produced almost immediately after insonification by the sonar.
We propose two mechanisms whereby Navy sonars could result in decompression sickness in marine mammals such as beaked whales:
1. Whales that have dived to their normal diving depths, and have become supersaturated--beaked whales often dive to depths of 1000 meters and remain there for several minutes--are frightened by a Navy sonar and rise immediately to the surface. In this way, they do not slowly decompress as they would normally and thus develop bubbles in their tissues.
2. Whales that have dived and returned to the surface, and have high levels of supersaturation, are exposed to Navy sonars, which trigger bubble formation by rectified diffusion or microbubble destabilization. These bubbles then grow to macroscopic sizes as a result of the high level of tissue gas supersaturation.
References:
[1] P.D. Jepson, et al., "Gas-bubble lesions in stranded cetaceans",
Nature, 425, 575-576 (2003).
[2] "Mass stranding in the Bahamas," MMPA Bulletin, Issue 18, p. 3 (2000); "Update on mass stranding in the Bahamas," MMPA Bulletin, Issue 19/20, p. 3 (2000); http://www.nmfs.noaa.gov/prot_res/-PR2/MMPA_Bulletin/mmpabulletin.html
[3] "Joint Interim Report, Bahamas Marine Mammal Stranding, Event of 15-16 March 2000", U. S. Department of Commerce; Donald L. Evans, Secretary; Secretary of the Navy, Gordon R. England, December 2001.
[4] "Summary of the report on the atypical mass stranding of Beaked Whales in the Canary Islands in September, 2002 during naval exercises", by Vidal Martel.
[5] L.A. Crum and Y. Mao, "Acoustically enhanced bubble growth at low
frequencies and its implications for human diver and marine mammal safety,"
J. Acoust. Soc. Am., 99, 2898-2907 (1996).
[6] D.S. Houser, R. Howard, and S. Ridgway, "Can Diving-induced Tissue
Nitrogen Supersaturation Increase the Chance of Acoustically Driven Bubble Growth
in Marine Mammals?", J. Theor. Biol, 213, 183-195 (2001).