The Northern Mariana Islands are part of the Commonwealth and, as a strategic location for the US Department of Defense, frequently experience exercises of antisubmarine warfare (ASW) that use mid-frequency activity sonar (MFAS). Initially, the study by Anne Simonis and colleagues (2020) was conducted to document the seasonal acoustic presence of beaked whales around the Northern Mariana Islands of Saipan and Tinian in the North Pacific Ocean from 2010 to 2014. After MFAS had been discovered in the acoustic data during the same time-frame as two Cuvier’s beaked whales stranded on Saipan in 2011, the researchers also focused on detection of MFAS in their data, available reports on sonar activities and corresponding stranding events between 2006 and 2019.
Through the acoustic data, three different beaked whale species were identified in the area. MFAS signals could be detected during a total of 35 days with multiple sonar packets at different frequencies, suggesting several sources. The range at the recoding locations included levels that elicited moderate to strong avoidance responses in beaked whales, as previous experiments had shown. Beside the detected correlation in 2011, records showed that out of the eight total stranding events between 2006 and 2019, 50% occurred within 6 days after a presence of multinational naval ASW training operations. Statistical analysis showed that this co-occurrence was highly significant.
However, there was no full disclosure of ASW training activities, thus limiting robust assessment of correlation. At the same time, the actual stranding number was rather low with a total of 10 to 11 beaked whales between 2006 and 2019, decreasing trustworthiness of the statistical analysis. It also needs to be added that the study solely showed a possible correlation, not a causal relationship between MFAS and beaked whale strandings.
Nevertheless, the paper did indicate the Northern Mariana Islands as an important habitat for beaked whales. The significant association of naval activities and 50% of beaked whale stranding events is “adding the Mariana Archipelago to a global list of locations, […] where sonar-associated beaked whale strandings have been documented”. Especially the pristine acoustic environment with very infrequent sonar activity might be the reason for such strong avoidance responses and inability to get habituated to the noise.
Further research needs to be conducted into the behavioural response, acoustic data and actual MFAS events to improve statistical power and reliability of conclusions.
Simonis, A.E., Brownell, R.L., Thayre, B.L. et al. (2020). Co-occurrence of beaked whale strandings and naval sonar in the Mariana Islands, Western Pacific. 2020. 287(1921): p. 20200070. https://doi.org/10.1098/rspb.2020.0070
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