New research adds to the bank of evidence that suggests military exercises involving sonar can be deadly for whales. In a study published in the Proceedings of the Royal Society B, researchers have linked several stranding events in the Mariana Archipelago to naval submarine operations involving mid-frequency active sonar (MFAS).
Beaked whales, a group containing 23 species of deep-diving cetaceans, are particularly susceptible to MFAS. The work focused on the Mariana Archipelago, where MFAS tests have been conducted by groups of nations almost every year since 2006.
As the authors wrote in their paper, "no beaked whale strandings were reported from the Mariana Archipelago between 1962 and 2006, but from 2007 to January 2019, eight beaked whale stranding events (10-11 individuals) have been reported on Guam and Saipan."
The team found that half of the eight beaked whale stranding events from June 2006 until January 2019 happened within six days of the MFAS exercises. Four events might not seem significant on its own but it adds to an extensive literature of cases. Since MFAS between 4.5 and 5.5 kHz were introduced in the 1960s, 12 beaked whale mass strandings have been linked to MFAS exercises and 27 other mass strandings have been witnessed near a naval base or ship.
Previous research suggests that 9 percent of all mass strandings are due to naval operations where MFAS are in use, but the authors consider this a conservative estimate as prior studies only focused on the strandings of multiple animals. In the Mariana Archipelago, most of the strandings were of a single individual (six out of eight) and two of them were associated with MFAS.
One possible reason for why the naval exercises might be so detrimental is the production of noises that are infrequent and unpredictable, scaring the animals. Previous research has shown that beaked whales purposely avoid unexpected MFAS both near and far. Even beaked whales living near naval bases (and supposedly more accustomed to human-made sound production) changed their habits when MFAS was switched on.
"In the waters surrounding the Mariana Archipelago, the infrequent sonar activity, in conjunction with quiet ambient noise levels, may increase the severity in the behavioural response of beaked whales to sonar compared to populations living with higher ambient noise levels or those which have become habituated to frequent MFAS activity," wrote the authors.
The team's recommendations are simple: “We strongly recommend more visual monitoring efforts, at sea and along coastlines, for stranded cetaceans before, during and after naval exercises."