What Do We Know About Whale/Dolphin Mass Strandings and Naval Exercises?

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Humpback Whale

Hawaii’s Humpback Whales have not stranded due to sonar. Image courtesy of NOAA

There is much information being put forward around the recent Navy application to the National Oceanic and Atmospheric Administration (NOAA) for an incidental ‘take’ permit, which would allow for potential accidental harassment, injury or death of marine mammals during naval exercises.

However, not every cetacean mass stranding is related to navy testing or exercises. There are other reasons for strandings that must be considered. To begin with, some information on strandings and the permit under consideration.

What is a Cetacean Mass Stranding?

In order to intelligently examine the situation with military exercises at sea and whale and dolphin mass strandings, we must first have a clear understanding of what a mass stranding is.

In the US, the National Oceanic and Atmospheric Administration (NOAA)’s National Marine Fisheries Service (NMFS) defines a mass stranding as two or more animals, other than cow/calf pairs, found ashore either dead or alive, and which are spatially and temporally related – that is, in a similar location, or during a brief span of time.

Not All Whale and Dolphin Mass Strandings are the Result of Naval Exercises

Public perception of mass strandings of cetaceans is that they only happen in areas where sonar has been used. This is disproven by the fact that there are recorded mass strandings of whales and dolphins on Cape Cod and in New Zealand in the 1800s. In fact there are three international stranding ‘hotspots.’ One is in Australia, one in New Zealand and, Cape Cod in the US. Each of these sites experiences mass strandings on a regular basis.

In these places, experts think that a combination of local topography, extreme tides and the strong social bonds that are common in the species which most often strand, each play a role. Also, many of the species that mass strand are highly pelagic (living in the open ocean) and unlikely to be familiar with gently sloping beaches, nearshore currents and tidal changes.

At one time, researchers theorized that the animals were following fish. On Cape Cod, where many of the stranded animals that do not survive undergo complete necropsies (non-human autopsies), it is rare to find food in the digestive tract, suggesting that following food is not likely to be a primary cause for mass strandings.

Human-caused cetacean mass strandings have also been linked to oil and gas exploration, the most famous incident having occurred off of Madagascar causing the stranding of more than 100 melon-headed whales. An international independent review panel determined that Multi-beam Echosounder Systems (MBES) were “the most plausible and likely behavioral trigger for the animals initially entering the lagoon system” where the strandings occurred.

What We Know About How Sonar Affects Cetaceans

Melon-headed whales have stranded following sonar use Image courtesy of the U.S. Minerals Management Service

The known facts about whales, dolphins and sonar are much more limited than most people realize. Here’s what we do know:

  • Offshore species, beaked whales in particular, are most often impacted and Cuvier’s beaked whales more than other beaked whale species. Melon-headed whales have stranded in relation to both military and oil and natural gas exploration, also in significant numbers.
  • Pilot whales, pygmy sperm whales, striped dolphin and minke whales have also potentially been impacted. However, we can interpret this information in the light of the fact that mass strandings of pilot whales have been recorded long before sonar came into use, and that some of the animals found during these mass stranding events had health problems not related to sonar use, such as emaciation and heavy parasite loads.
  • Injuries linked to sonar exposure include auditory and brain injury, hemorrhage and multiple organ congestion. Specifically, there is often hemorrhage in the intracranial space, and around the ears and ‘acoustic fat’ found in the lower jaw of cetaceans which plays a role in sound collection. Researchers have also seen hemorrhage around abdominal organs, particularly the kidneys. Lung congestion is another common finding. In some situations, examiners have noted air emboli (pockets of air outside of the lungs).

Behavioral changes may be an important factor in human-caused mass stranding. In at least one case, pelagic whales that moved into shallow waters returned to their normal deepwater habitat once sonar operations ceased. In another, often cited, case – 15 harbor porpoises and one Dall’s porpoise stranded in Washington state while the USS Shoup was using mid-range sonar nearby. It was the first recorded mass stranding of harbor porpoise in that area. The animals, which were necropsied, did not, however, show signs of acoustic trauma.

Unfortunately, there were problems with bodies being partially decomposed by the time they were collected and storage issues prior to the work being done.

This leaves three possible explanations.

  • One, there was no acoustic trauma, in which case there may have been behavioral impacts which caused the animals to strand despite there being no injuries similar to those found in other species.
  • Two, there was some acoustic trauma but the condition of the carcasses precluded it being positively determined.
  • Three, this mass stranding was unrelated to the sonar use.

Further Study Necessary to Better Understand Sonar-Related Strandings

At present, researchers do not completely understand why these sonar-related mass strandings occur, as not all sonar use results in mass strandings. Possible reasons for this include aspects of the individual species’ physiology, habitat use and the type of coastline where the sonar is deployed.

Why are beaked whales the most sensitive to sonar? Explanations include the beaked whale’s unique anatomy and physiology, and their regular use of deep ocean trenches not far offshore.

One theory scientists are advancing is that sound can be trapped near the surface in these areas and, as the whales surface they are suddenly hit with the intensified sound. Researchers are looking at the anatomical and physiological differences between beaked whales and other cetaceans for possible clues.

Also under investigation is the level at which permanent hearing threshold shifts (PTS or permanent hearing loss) and temporary hearing threshold shifts (TTS or temporary hearing loss) occur. This level appears to be different for different species, based on their normal hearing range and the type of sonar used, and may help explain why some species where there is regular sonar use do not seem to be as seriously affected.

Researchers have not looked at behavioral impacts, including stress-related changes and the energetic costs of avoidance, in detail. As we can see by the situation in Washington state, there may be further ways in which cetaceans are affected by sonar. Until we have more data, however, these cases remain unsolved.

Naval sonar use has impacted some whales and dolphins. Image courtesy of the U.S. Navy

The NOAA Incidental Take Permit Situation

It is a fact that, under certain circumstances, sonar use can have a severe, sometimes fatal impact on some cetacean species. But the marine mammal research world is just beginning to understand many of the co-factors which are involved in scenarios creating the most severe impacts.

In September of 2013, a federal judge ruled that NOAA must reconsider the Navy’s take permits in light of new scientific data on the impact of sonar on whales and dolphins.

It is important to realize that the incidental take permit does not grant the Navy permission to intentionally kill whales and dolphins. This permit is given under adaptive management rules which allow NOAA to make changes to the permit based on findings during the previous year(s). The Navy is also required to attempt to prevent a ‘take,’ or inadvertent injury to marine mammals, and to report any incidents which involve marine mammals. If they report significant losses, NOAA is authorized to rescind the permit.

Managing Sonar Use and Other Anthropogenic Noise in Cetacean Habitat

There has been much emphasis on mass strandings occurring in the wake of sonar use. Given that we know so little about how and why these strandings happen, and that study of cetaceans is difficult at best, it would seem sensible to attempt to limit the use of sonar at sea as much as possible.

Strandings due to sonar use, however, may be only one easily-observed consequence of human-generated ocean noise. A 2003 National Research Council document, Ocean Noise and Marine Mammals, noted that anthropogenic noise from shipping has been increasing by 3 decibels per year.

The report likens this to some of our human workplaces, where hearing damage and stress-related illness are a concern – and we are not as dependent on our hearing as many marine animals are. A long, hard look at all forms of ocean noise and its impact on marine wildlife is necessary.

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