Composite Training Unit Exercises and Joint Task ... - Govsupport.us

COMPTUEX/JTFEX EA/OEA Final Chapter 4but was a factor in the calculation of population densities. It was assumed that marine mammals wereexposed to the maximum receive levels calculated for the horizontal distance to the source. No attemptwas made to predict animal behavior in response to sound in the water or their location relative to thepoint where the source originates.Estimated marine mammal effects in which each acoustic source produced a total energy flux at or abovethe defined Level A and Level B harassment thresholds depend on a combination of the input data valuesfor each of the four parameters described above. Calculations based on the harassment thresholds wereperformed for each combination of operations, acoustic source, depth of water and season with resultssummarized by species, sonar system and operations. The goal was an unbiased prediction of the numberof exposures that are expected over the duration of one year’s training given these diverse and variablefactors.4.3.1.7.12 Other Effects ConsideredAcoustically Mediated Bubble GrowthOne suggested cause of injury to marine mammals is rectified diffusion (Crum and Mao, 1996), theprocess of increasing the size of a bubble by exposing it to a sound field. This process is facilitated if theenvironment in which the ensonified bubbles exist is supersaturated with gas. Repetitive diving bymarine mammals can cause the blood and some tissues to accumulate gas to a greater degree than issupported by the surrounding environmental pressure (Ridgway and Howard, 1979). Deeper and longerdives of some marine mammals (for example, beaked whales) are theoretically predicted to induce greatersupersaturation (Houser et al., 2001). Conversely, studies have shown that marine mammal lung structure(both pinnipeds and cetaceans) facilitates collapse of the lungs at depths below approximately 50 m thuspreventing nitrogen absorption (Kooyman et al., 1970). Collapse of the lungs would force air in to thenon-air exchanging areas of the lungs (in to the bronchioles away from the aveloli) thus significantlydecreasing nitrogen diffusion in to the body. Deep diving pinnipeds such as the northern elephant andWeddell seals typically exhale before long deep dives, further reducing air volume in the lungs (Kooymanet al., 1970). If rectified diffusion were possible in marine mammals exposed to high-level sound,conditions of tissue supersaturation could theoretically speed the rate and increase the size of bubblegrowth. Subsequent effects due to tissue trauma and emboli would presumably mirror those observed inhumans suffering from decompression sickness.It is unlikely that the short duration of sonar pings would be long enough to drive bubble growth to anysubstantial size, if such a phenomenon occurs. However, an alternative but related hypothesis has alsobeen suggested: stable bubbles could be destabilized by high-level sound exposures such that bubblegrowth then occurs through static diffusion of gas out of the tissues. In such a scenario the marinemammal would need to be in a gas-supersaturated state for a long enough period of time for bubbles tobecome of a problematic size. Yet another hypothesis has speculated that rapid ascent to the surfacefollowing exposure to a startling sound might produce tissue gas saturation sufficient for the evolution ofnitrogen bubbles (Jepson et al., 2003). In this scenario, the rate of ascent would need to be sufficientlyrapid to compromise behavioral or physiological protections against nitrogen bubble formation.Collectively, these hypotheses can be referred to as “hypotheses of acoustically mediated bubble growth.”Although theoretical predictions suggest the possibility for acoustically mediated bubble growth, there isconsiderable disagreement among scientists as to its likelihood (Piantadosi and Thalmann, 2004; Evansand Miller, 2003). To date, ELs predicted to cause in vivo bubble formation within diving cetaceans havenot been evaluated (NOAA, 2002b). Further, although it has been argued that traumas from recentbeaked whale strandings are consistent with gas emboli and bubble-induced tissue separations (Jepson et4-43 February 2007