Diving habits of beaked whales (Family Ziphiidae) are of management interest for two primary reasons. Firstly, animals which spent long periods of time beneath the water’s surface are likely to be missed during shipboard or aerial surveys, and without taking diving patterns into account, abundance estimates may be negatively biased. Based on observational studies, beaked whales are known to dive for very long periods, but collecting detailed surfacing data is problematic. Secondly, beaked whales are known to mass-strand in response to high-intensity sonar. What makes them susceptible to such impacts remains unclear, but received sound pressure levels are thought to be lower than levels that would cause direct physical harm. Theoretically, indirect physical harm could be caused by behavioral reactions in several ways, if whales: 1) surface excessively fast (causing gas bubble formation); 2) stay at the water’s surface for too long (if tissues are supersaturated with nitrogen); 3) dive prematurely (if whales spend extended periods at the surface to eliminate nitrogen); or 4) stay at depth for too long, forcing an overly rapid ascent. Given the paucity of information on normal diving behavior of beaked whales, it is not known which of these possibilities is most likely. We studied diving behavior of Cuvier’s and Blainville’s beaked whales in Hawaiian waters from 2002-2005, using suction-cup attached time-depth recorder/VHF tags. Beaked whales were encountered on 30 occasions in 922 hours of effort (17 sightings of Cuvier’s, 11 sightings of Blainville’s, and 2 sightings of unidentified beaked whales). Six whales were tagged, two Cuvier’s and four Blainville’s, and 41 hours of dive data were collected. Several aspects of diving were similar between the species: 1) both dove for 48-68 minutes to depths greater than 800 m (maximum 1,408m for Blainville’s, 1,450m for Cuvier’s) with one long dive occurring on average every two hours; 2) ascent rates for long/deep dives were substantially slower than descent rates, while for shorter dives there were no consistent differences; and 3) both spent prolonged periods of time (66 – 155 minutes) in the upper 50 m of the water column. We suggest that the frequent extremely long dives push the animals’ physiological limits, resulting in behavioral mechanisms (slow ascent rates and prolonged periods of time at the surface to purge excess dissolved nitrogen) to compensate. Indirect physical harm from abnormally rapid ascents or premature dives seem plausible as mechanisms for beaked whale mass-strandings in relation to high-intensity sonar.
Citation:
Baird, R.W., D.L. Webster, D.J. McSweeney, A.D. Ligon and G.S. Schorr. 2005. Diving behavior and ecology of Cuvier’s (Ziphius cavirostris) and Blainville’s beaked whales (Mesoplodon densirostris) in Hawai‘i. 2005. Report prepared under Order No. AB133F-04-RQ-0928 to Cascadia Research Collective, Olympia, WA from the Southwest Fisheries Science Center, NMFS, La Jolla, CA.
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