How man-made interference might cause gas bubble emboli in deep diving whales

Abstract

Recent cetacean mass strandings in close temporal and spatial association with sonar activity has raised the concern that anthropogenic sound may harm breath-hold diving marine mammals. Necropsy results of the stranded whales have shown evidence of bubbles in the tissues, similar to those in human divers suffering from decompression sickness (DCS). It has been proposed that changes in behavior or physiological responses during diving could increase tissue and blood N2 levels, thereby increasing DCS risk. Dive data recorded from sperm, killer, long-finned pilot, Blainville's beaked and Cuvier's beaked whales before and during exposure to low- (1–2 kHz) and mid- (2–7 kHz) frequency active sonar were used to estimate the changes in blood and tissue N2 tension (PN2). Our objectives were to determine if differences in (1) dive behavior or (2) physiological responses to sonar are plausible risk factors for bubble formation. The theoretical estimates indicate that all species may experience high N2 levels. However, unexpectedly, deep diving generally result in higher end-dive PN2 as compared with shallow diving. In this focused review we focus on three possible explanations: (1) We revisit an old hypothesis that CO2, because of its much higher diffusivity, forms bubble precursors that continue to grow in N2 supersaturated tissues. Such a mechanism would be less dependent on the alveolar collapse depth but affected by elevated levels of CO2 following a burst of activity during sonar exposure. (2) During deep dives, a greater duration of time might be spent at depths where gas exchange continues as compared with shallow dives. The resulting elevated levels of N2 in deep diving whales might also make them more susceptible to anthropogenic disturbances. (3) Extended duration of dives even at depths beyond where the alveoli collapse could result in slow continuous accumulation of N2 in the adipose tissues that eventually becomes a liability.

Nitrogen solubility in odontocete blubber and mandibular fats in relation to lipid composition

“…Thus, fats have been considered 'slow' loading tissues because of assumed low perfusion and/or generally modest vascularization relative to 'essential' tissues (e.g. Zimmer and Tyack, 2007;Fahlman et al, 2009Fahlman et al, , 2014. However, recent studies have demonstrated that blood flow in diving marine animals may be more variable during different dive phases and conditions, such as elevated exercise and foraging/feeding (Zapol et al, 1979;Butler, 1982;Noren et al, 2012).…”

“…In tissues, these bubbles can produce mechanical and biochemical problems, which can be fatal (reviewed in Vann et al, 2011). The occurrence of decompression sickness in air-breathing divers depends on the animal's diving regime (Fahlman et al, 2014). During a dive, tissues can become saturated or supersaturated with inert gases, such as nitrogen (N 2 ), and if ambient pressure decreases faster than the tissues can release the gas, decompression sickness can result (Boyle, 1670;Vann et al, 2011;Hooker et al, 2012).…”

“…Zimmer and Tyack, 2007;Kvadsheim et al, 2012;Fahlman et al, 2014). The first four compartments are all assumed to have N 2 solubility values equivalent to the value derived for blood: ∼0.014 ml N 2 ml Not all odontocetes contain WEs in their blubber, but every species examined to date contains WEs in its acoustic fats Litchfield and Greenberg, 1974;Litchfield et al, 1975;Koopman et al, 2006;Koopman, 2007), which are specialized cranial adipose depots involved in echolocation and hearing (Norris, 1968).…”

Human Disturbances Might Cause Dangerous Gas Bubbles to Form in Deep-Diving Whales

“…For this reason, no additional gas is taken up and the gas that was taken up in the intermediate region is now circulating between the blood and tissues (figure reproduced from Ref. [5]). may be at higher risk of forming bubbles.…”

“…For every 10 m of depth, the pressure increases by 1 ATA, and by measuring the pressure these instruments were able to record the depth to which the whales were diving while foraging, or hunting for food. We used the pressure data and the Gas Solubility to estimate the amount of gas the whales had dissolved in the blood and tissue, and these estimates gave us an idea of how likely the whales were to experience gas bubbles [4,5]. We compared these estimated values of dissolved gas based upon the whales' diving behavior before, during, and after exposure to sonar.…”