The Cuvier's beaked whale, part of a mass stranding that occurred in 1996 in the Kyparissiakos Gulf, Greece.
Credit: Alexandros Frantzis / Splashdowndirect / Shutterstock
Naval sonar has been associated with mass whale stranding for almost two decades, but the precise mechanisms of how it affects whales have misled scientists. Now researchers have explained important details of how this disruptive signal triggers behavior in some whales that end in death.
Previously, necropsies of multiple-slope beak whales found nitrogen bubbles in their body tissues, a characteristic of decompression sickness, or "the curves." This dangerous condition also affects divers when they rise very quickly from the deep waters; It can cause pain, paralysis and even death.
Whales are adapted for deep-sea diving, and whale sharks are the record holders of the longest and deepest dives. But the new research explains how sonar at certain frequencies disorientates and terrifies some beaked whales, such that the experience negates an important adaptation to deep diving: a slower heart rate. Extreme fear accelerates a whale's heart rate, which can lead to decompression sickness; the intense pain of this condition incapacitates the whales, so they spread on the beaches and end up dying, scientists reported in a new study. [Whale Photos: Giants of the Deep]
Cuvier's beak mass whales (Ziphius cavirostris) were almost unknown before 1960, but this has changed with the introduction of active mid-range sonar (MFAS) in offshore naval drills. This type of sonar, developed in the 1950s for the detection of submarines, operates in a range of 4.5 to 5.5 kHz, according to the study. After this sonar appeared, mass-beached events soon shot at billfish, with 121 of those stranding occurring between 1960 and 2004, the researchers wrote.
Scientists first observed a connection between massive strandings of Cuvier's beaked whales and naval exercises using sonar in the late 1980s, led study author Yara Bernaldo de Quirós, a researcher at the Institute of Animal Health and Food Security at the University of Las Palmas de Gran Canaria. in Spain, Live Science said in an e-mail.
This bond strengthened after similar events in Greece in 1996 and in the Bahamas in 2000, Quíros added. And in September 2002, when 14 whale whales stranded in the Canary Islands during a NATO naval exercise, veterinary pathologists discovered injuries to animals that were "compatible with a decompression sickness," Quiros said.
Fight or run away
In 2017, biologists studying the beaked whales gathered for a workshop to review the findings about the stranding of the past few decades, noting the mass stranding connected to nearby naval exercises using sonar.
Between 2002 and 2014, six mass chains occurred in Greece, the Canary Islands and Almeria, in southeastern Spain, but the dead whales did not appear to be malnourished or sick. However, they showed "abundant gas bubbles" in all veins, multiple organ blood clots and "variable gravity" microscopic hemorrhages in body tissues.
Stranded whales may have experienced a "fight or flight response" that nullified a major adaptation of the dive: a decrease in heart rate, which reduces oxygen consumption and prevents nitrogen buildup. The result was bleeding and "massive formation of blisters in their tissues," explained Quiros.
These symptoms of decompression sickness probably afflicted whales after they were hit by sonic blasts, according to the study.
"The temporal and spatial association with naval exercises using sonar is very clear," Quíros said in the email. In addition, behavioral studies have shown that whales that have never encountered ringing (or have been exposed to it only occasionally) typically exhibit a stronger response than animals living near military outposts, she added.
In 2004, Spain banned sonar in the waters of the Canary Islands, a critical mass point. No mass strand has occurred since the ban was enacted, "proving the effectiveness of this mitigation," said de Quíros.
Based on their findings, the study's authors recommended more widespread bans on military drills using sonar across the Mediterranean Sea, where atypical mass strikes of beaked whales still occur. More research will determine the long-term impact of mass beaching on beaked whale populations, the authors wrote in the study.
The results were published online today (30 January) in the journal Proceedings of the Royal Society B.
Originally published in Living Science.