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Biology

The problem with data sets: Cuvier’s beaked whales vs. Navy acoustic testing

Curvier's beaked whale (Ziphius cavirostris). Photo courtesy of the Smithsonian Institute

Curvier’s beaked whale (Ziphius cavirostris). Photo courtesy of the Smithsonian Institute.

Paper: Schorr GS, Falcone EA, Moretti DJ, Andrews RD (2014) First Long-Term Behavioral Records from Cuvier’s Beaked Whales (Ziphius cavirostris) Reveal Record-Breaking Dives. PLoS ONE 9:e92633. doi: 10.1371/journal.pone.0092633

Congratulations on the longest and deepest dive EVER! Please, ignore the regular acoustic testing….

The elusive Curvier’s beaked whale (Ziphius cavirostris) is officially the deepest diver in the sea mammal community, annihilating both the sperm whale and southern elephant seal for the illustrious title. Until now, its diving abilities have been underestimated owing to the paucity of direct observations and sufficient study periods. Ziphius cavirostris (hereafter Ziphius) is not only a species of remarkable divers, but also is thought to be acutely affected by Mid-Frequency Active (MFA) sonar exposure during military exercises. Schorr et al. use the largest data set ever collected on this mysterious cetacean to examine both incredible behavioral patterns and the possibility that they may be able to adapt to a certain amount of acoustic disturbance.

New records for Cuvier’s beaked whale
At the start of the journal article, Schorr et al. stress that caution should be exercised when drawing conclusions about behavior using short-term data sets. Ziphius is the most widely distributed beaked whale species in the world, but the only existing published data is from short-term logging tags with average deployment times of 12 hours.

Distribution of Ziphius is the most extensive of all beaked whales. It can now also take the titles of deepest diving and longest dive lengths. Photo courtesy of the Smithsonian Institute (http://vertebrates.si.edu).

Distribution of Ziphius is the most extensive of all beaked whales. It can now also take the titles of deepest diving and longest dive lengths. Photo courtesy of the Smithsonian Institute (http://vertebrates.si.edu).

In the San Nicholas Basin of Southern California, 8 individuals were tagged for periods up to three months and Schorr et al. collected 3732 hours of data that revealed record-breaking details – dives went to 2992m depth and lasted 137.5 minutes. This longer-term behavioral data showed variation between shallow and deep dives, with very short (< 2 minutes) surface bouts revealing exceptionally short gas exchange intervals. There is also a bias for staying closer to the surface at night and an average of 7 food foraging dives per day.

Plot A shows a sample dive trace from one of the tagged individuals. You can clearly see the short surface durations separated by periods of both deep and shallow dives. Plot B shows the length of dives in conjunction with depth of dives.

Plot A shows a sample dive trace from one of the tagged individuals. You can clearly see the short surface durations separated by periods of both deep and shallow dives. Plot B shows the length of dives in conjunction with depth of dives.

When the average 7 foraging dives per day are compared to the short-term published studies, it appears the Southern California Ziphius (long-term study) are foraging less frequently. At this, some jump to the conclusion that it is due to the presence of Navy acoustic testing in the study region. Schorr et al., again, caution that it is more complex than that. It could also be a factor of sample size or greater success in foraging. Ultimately, the variation means the behavioral range for Ziphius is broader than previously thought and a longer-term behavioral record from a given individual is the most appropriate context to evaluate behaviors.

Sonar and Long-term data sets

The causal relationship between stranded Ziphius cavirostris and Mid-Frequency Active (MFA) sonar is poorly understood because the beaked whales are so difficult to study and comprehensive records of sonar use during tag deployments are frequently unavailable. What is known is that from 2009 to 2014, the Navy is authorized to use up to 3408 hours of MFA sonar annually. This breaks down to about 6.8 hours/day of acoustic disturbance, not including other anthropogenic noise. Schorr et al. recognize that given the acoustic sensitivity of beaked whales and other odontocetes (toothed whales), it is hard to imagine that sonar does not displace whales occasionally.

But, just as daily weather does not always reflect broader climate change (ie: one snowy week does not negate global warming), 12-hour behavior surveys may not reflect long-term sonar influences.

The expanded data set sheds some light on previous thinking that Ziphius’ reaction to sonar testing is to increase dive depths and durations. When the long-term data is analyzed, however, it suggests that the MFA exposure is unlikely to be the primary factor in the long average dive durations, though it likely influences some dives. Whatever displacement is caused by sonar in the San Nicholas Basin, it appears temporary. Despite the high levels of acoustic disturbance, the whales return. Either the whales have adapted to the noise, or the region is extremely important habitat, causing the local advantages of being in the basin to outweigh the costs the acoustics impose.
Regardless of the reasons for whales returning or the newly expanded dive capabilities, everyone can agree that more long-term data is essential for understanding the true behavior of Ziphius around regular acoustic disturbances.

Sarah Fuller
With academic backgrounds in oceanography, geology, and environmental education, Sarah has traveled to far reaches of the planet to learn everything she can about our natural world. While exploring, she dabbles in photography and is rarely found without a book. She has no plans to stop any time soon.

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