The deaths of over 100 melon-headed whales, which stranded on the shores of a lagoon in northwest Madagascar in 2008, were likely primarily triggered by a form of sonar being deployed by an ExxonMobil survey vessel, according to a scientific review panel.
This is the first known marine mammal mass stranding event to be closely associated with what are known as high-frequency mapping sonar systems; but it is merely the latest in a long line of incidents in which industrial noise in the ocean has been implicated in deaths and injuries to marine mammals, and specifically cetaceans.
One area of particular focus is the use of active sonar by the United States and other navies. In 2004, the Scientific Committee of the International Whaling Commission concluded that “there is now compelling evidence implicating military sonar has a direct impact on beaked whales.” In 2001, the U.S. Navy acknowledged that its active sonar played a role in the stranding deaths of 14 beaked whales, two minke whales, and a dolphin in the Bahamas in 2000. Necropsies of the beaked whales revealed that the animals had suffered acoustic trauma resulting in hemorrhaging around the brain, in the inner ears, and in the acoustic fats located in the head that are involved in sound transmission.
Then there is the use of air guns to conduct seismic surveys for oil and gas deposits in the sea bed. Because of concerns that the intense sounds from these air guns can either cause physical damage to cetaceans or cause them to flee, the U.S. Marine Mammal Protection Act requires visual observers to examine the area for marine mammals for a period of at least 30 minutes. Assuming the coast is clear, the survey must ramp up slowly by firing first one seismic gun and then others over a period of between 20 and 40 minutes. However, if a whale or other marine mammal appears within an exclusion zone of 500 meters from the center of the seismic array, the operation must shut down, and visual examination must resume for 30 minutes.
The presence of seismic survey vessels was initially considered a likely reason for the 2008 Madagascar incident, not least because melon-headed whales are an open-ocean species that had never before (and have never since) been recorded in the island’s shallow tidal Loza Lagoon estuarine system; it seemed reasonable to infer that something had startled them into entering the area. Experts with the International Fund for Animal Welfare and the Wildlife Conservation Society attempted to rescue the whales, but with minimal success, largely because of the large distance (over 69 km/43 miles) from the point where they were becoming stranded to the open ocean.
Following the incident, the International Whaling Commission facilitated a review of the evidence and an Independent Scientific Review Panel (ISRP) of five experts was invited to examine that evidence. The ISRP has just published its findings, and concluded that air gun blasts from seismic survey vessels in the area were not to blame, as they in fact took place several days after the incident.
However, a vessel using a different type of surveying technique — a high-power 12 kHz, multi-beam echosounder system (MBES) — was, says the report, “moving in a directed manner down the shelf-break the day before the event, to an area approx. 65 km offshore from the first known stranding location. The ISRP deemed this MBES use to be the most plausible and likely behavioral trigger for the animals initially entering the lagoon system.”
Most attention has focused on the likely impacts of mid-frequency and low-frequency sound sources, because of their greater propagation through water; it is possible, the ISRP noted, that high-frequency sounds like the MBES also affect some cetaceans more than previously realized but that, under normal circumstances, those cetaceans swim away. It was to the considerable misfortune of those melon-headed whales five years ago that they were in the wrong place at the wrong time, and that when they turned tail and fled, they swam into an inhospitable area from which there was no escape.
“Implications go well beyond the hydrocarbon industry, as these sonar systems are widely used aboard military and research vessels for generating more precise bathymetry (underwater mapping),” Howard Rosenbaum, director of the Ocean Giants program for the Wildlife Conservation Society, said in a statement. “We now hope that these results will be used by industry, regulatory authorities, and others to minimize risks and to better protect marine life, especially marine mammal species that are particularly sensitive to increasing ocean noise from human activities.”