There is perhaps not a single topic that is more controversial than decompression sickness in marine mammals. Historically it was believed that marine mammals are not prone to decompression sickness, after all they have been adapted to deep diving and life at sea for eons. However it was discovered that they are not only prone to decompression sickness, but are also even more vulnerable to it because of nitrogen’s solubility in fats.
Everybody who has been scuba diving knows all about decompression sickness, how dangerous it is and what needs to be done to avoid it. Due to decompression sickness bubbles form in a diver’s body and migrate to various places where they produce effects ranging from some moderate pain to instant death.
In regard to marine mammals it is thought that decompression sickness can affect them in two ways. First, they are getting spooked by sonar or seismic exploration airguns and ascend faster than they are supposed to, thus getting decompression sickness. Second, it was proposed that direct exposure to intense sound such as sonar produces embolism on the spot. The video below shows what happens with nitrogen saturated experimental vessel when it gets exposed to sonar-like sound. Very dramatic presentation, it starts around 5 minutes 40 seconds:
Additionally, several concerns have been expressed that not only marine mammals are vulnerable to decompression sickness and embolism, but also that they are highly vulnerable to it. It happens for 2 reasons (Koopman & Westgate, 2009):
1. Nitrogen (which marine mammals accumulate a lot) is highly soluble in oil. In fact overweight human divers are more at risk for decompression sickness than others with lower BMI. Marine mammals have a lot of blubber and fats all over their body. So not only they have a lot of nitrogen but also a lot of fat to absorb it.
2. Deep water divers like kogias and beaked whales have special waxes in their body the purpose of which are not fully understood. It turned out that these waxes have even higher affinity for nitrogen and even greater potential for embolism (Koopman & Westgate, 2009).
Naturally, the Navy and Big Oil are aware of all that and try to distance themselves from causing embolism in marine mammals as much as they can. In recently published study funded by the Navy it was of course argued that “actually observed behavioral responses of cetaceans to sonar in our study, do not imply any significantly increased risk of DCS” (DCS=decompression sickness).
So it is clear that in order to argue decompression sickness in stranded cetaceans the bubbles not only have to be found but also their content has to be sampled as well, in order to find nitrogen. There is a problem however. Soon after death, the body starts to decompose. The process of decomposition creates gasses and of course bubbles. These have to be distinguished from decompression related gasses and bubbles. So the sampling needs to be done asap and special precautions have to be taken not to contaminate the air sample content from the bubble. We all know how difficult it is to perform such analysis and also know that if analysis can be stalled by various factors and parties, both intentionally and unintentionally, the probability of finding decompression related sickness and evidence reduces greatly.
Recently, a post appeared in MARMAM about decompression sickness and gas sampling. The post is a direct response (in our opinion) to Peru’s mass die off and ORCA’s insistence on decompression sickness as reason for strandings. WHOI has a long history of being funded by the Navy and in our opinion it embodies a classic case of conflict of interests as described in our previous blog post. That is why it is a bit unsettling to see the invitation to send emboli samples to WHOI for analysis. Given how controversial and charged the discussion about strandings, sonar, Big Oil and decompression sickness is, now it is more important than ever to have independent researchers sampling stranded marine mammals and giving their own perspective on causes and findings. Perhaps the better way would be to establish some sort of standardized protocol, but also to make sure that independent (aka not funded by the Navy and Big Oil) researchers have free and unrestricted access to sampling and are given freedom to arrive at their own conclusion. So if you want to send your sample to WHOI make sure you have a sample for yourself as well, because the conflict of interests is a reality and has been hovering over issues of anthropogenic noise, strandings and marine mammals for decades.
Koopman, H., & Westgate, A. (2009), Solubility of nitrogen gas in odontocete blubber: Are deep divers more vulnerable to nitrogen absorption, Paper presented at the 18th Biennial Conference on the Biology of Marine Mammals, 12-16 October, 2009, Quebec, Canada.
EDIT: The Navy is currently accepting public comments in regard to their proposed sonar tests ranges off East Coast and Pacific/Hawaii. The public can comment up until July 10 on both. Please consider commenting if you can, as the Navy is planning to not only increase ranges during upcoming years but also to increase their sonar tests. Please comment here for Hawaii/Pacific and here for East Coast range.