Aleutian Islands beaked whale echolocation signals

Abstract

Beaked whales are an elusive group of marine mammals. They are infrequently encountered as they are pelagic, deep diving foragers with short surface intervals between long dives (Tyack et al. 2006). In recent years, research has shown that beaked whales produce frequency modulated (FM) upsweep echolocation signals (Zimmer et al. 2005, Johnson et al. 2006, Gillespie et al. 2009, McDonald et al. 2009, Baumann-Pickering et al. 2010), which appear to be species specific in their spectral and temporal characteristics. Their typical echolocation behavior during foraging consists of FM pulses with very regular interpulse intervals (IPIs) while searching for prey, and discrete click series with short IPIs when closing in on a potential prey target, called a buzz (Johnson et al. 2004, Madsen et al. 2005). Around the Aleutian Islands, only three species of beaked whales are currently known to occur. Sightings or strandings of Baird’s beaked whales (Berardius beardii), Cuvier’s beaked whales (Ziphius cavirostris), and Stejneger’s beaked whales (Mesoplodon stejnegeri) have been reported (Jefferson et al. 2008). Stejneger’s beaked whales are found in cold-temperate and subpolar waters, and they are endemic to the northern North Pacific. Based on stranding records because live sightings are rare, the central Aleutian Islands are considered to be the center of this species’ range (Loughlin et al. 1982, Walker and Hansen 1999). Stejneger’s beaked whale are mostly encountered in small groups of two or three animals and occasionally in larger groups up to 15 animals (Loughlin and Perez 1985). These groups make coordinated dives with a series of five to six short dives, followed by a longer dive of 10–15min (Loughlin et al. 1982). The species is most often found in water depths ranging from 730 to 1,560 m. Their habitat is the steep slope of the continental shelf, which drops off into the Aleutian Basin to the north and into the Aleutian Trench to the south, but they are not likely to occur in the shallow northern or eastern Bering Sea (Mead 1989). Investigative effort to study the behavior of beaked whales has increased due to several mass strandings, which have been linked to military sonar exercises (Cox et al. 2006). Numerous mass strandings of Stejneger’s beaked whales in the Aleutian Islands have been documented since 1975 (Walker and Hansen 1999), yet to date, no correlation between these strandings and human activities has been established. Passive acoustic monitoring provides a cost-effective technique to investigate the behavioral ecology of these animals over long periods and in remote locations, particularly for species whose echolocation signals are distinguishable (McDonald et al. 2009, Baumann-Pickering et al. 2010).