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Beaked whales fleeing near surface at risk for bends?

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Extended Near-surface Dives May Be Cause of Beaked Whale BendsWalter, M.X. Simmer, Peter L. Tyack. Repetitive shallow dives pose decompression risk in deep-diving beaked whales. Marine Mammal Science, Volume 23 Issue 4 Page 888-925, October 2007.  ScienceNow summary of paper: [WEBSITE]It is apparent that beaked whales are especially sensitive to mid-frequency sonar, but the reasons have remained elusive. This study explores a new idea about how the whales’ dive patterns may be disrupted enough to cause decompression sickness (DCS, ie “the bends”). Some have suspected that exposure to sonar may cause the whales to surface more quickly than usual, but since the whales’ lungs are collapsed in dives deeper than 70m (thus preventing nitrogen from entering the bloodstream and infiltrating into tissues where it could cause damage), it is not readily apparent how faster surfacing would cause DCS. Instead, this study looks at the “recovery” period observed in beaked whales, during which they make a series of near-surface dives before embarking on a new deep foraging dive. The researchers modeled the possible physiological effects of having this recovery period extended longer than usual. The team incorporated known physiological data into a model that charts how the bubble size might increase in the circulatory system, brain, muscles, and fat tissues when a whale dives repeatedly to between 30 and 80 meters for as long as 3 hours. The team’s model predicts that if the whales’ lungs do not collapse during a long series of shallow dives, the increased pressure can cause nitrogen bubbles to diffuse into tissues, increasing the risk of bubble formation on ascent. Such behavior may result if the whales perceive sonar transmissions as a predator: repeated dives travelling horizontally to escape the percieved predator could put the animals at risk. When fleeing an orca (their primary predator), such avoidance would like be short duration, and not dangerous; however, if the sonar transmissions continued to be audible for long periods of time, the whales might continue the avoidance dives to the point where they begin to sustain injury. This could also explain the relatively unusual appearance of beaked whales close enought to shore to end up beaching: they might be chased far from their normal habitat by continuing sonar sounds (ed. note: it also begs the question of whether many other animals are similarly injured, but happened to be fleeing in other directions). The team concludes that limiting the duration of sonar testing may prevent the animals from diving in these harmful patterns.

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