Underwater click detectors are used to detect actively vocalising whales. The following information summarises recent studies on the use of Passive Acoustic Monitoring (PAM) to track beaked whales. For more information on PAM and how to detect echolocating whales with click detectors, please see www.pamoperator.co.uk, www.dolphindetectors.com, www.c-podclickdetector.com, and www.echolocationclickdetectors.co.uk.
Beaked whales are members of the Odontoceti suborder (toothed whales, dolphins and porpoises), which is slightly misleading, as only most have only a handful of teeth, only one species of beaked whale has a full set of teeth, and females of another species lack teeth all together. Beaked whales are found in every ocean, from the poles to the equator, typically in water deeper than 300 m; however, little is known about these elusive species because of their deep diving behaviour. Some, in fact, have been only identified to species level from stranded animals, and have never actually been observed alive at sea. All beaked whales, however, use echolocation (http://en.wikipedia.org/wiki/Animal_echolocation) for orientation, navigation, and foraging. The animals produce short pulses of sound and interpret returning echoes to gain a snapshot of their environment (Madsen et al. 2013).
BEAKED WHALE DIVING
Cuvier’s & Blainville’s beaked whales
Tyack et al. (2006) used sound and orientation recording tags to analyse dive profiles of ten beaked whales from two species: Cuvier’s (Ziphius cavirostris) and Blainville’s (Mesoplodon densirostris) (http://jeb.biologists.org). Each tag comprised a pressure sensor, three-axis accelerometers, and magnetometers to calculate and record whale orientation, and a hydrophone to record acoustic data.
All whales tagged for longer than 15 minutes undertook deep dives. Maximum depth recorded 1,888 m (Cuvier’s) and 1,251 m (Blainville’s). Duration of deep dives was correlated significantly to maximum depth in Blainville’s (i.e. duration increased with an increase in depth), whereas no correlation was observed in Cuvier’s. Descents were always faster than ascents, with each species descending at their own constant speed, regardless of final depth, and at a very steep angle, 60°–83°. In contrast, ascents had shallow angles, and speed changed during each ascent. Foraging and non-foraging dives were distinguished, with foraging dives defined as those with long sequences of echolocation clicks, combined with echoes from objects located by those clicks. Conversely, non–foraging dives were relatively silent with only a few isolated noises. The author’s results showed that foraging dives were far deeper (always in excess of 500 m) than non-foraging dives.
In a similar study in Hawai’i, Baird et al. (2006) recorded similar data to Tyack et al. (2006), with maximum dive depths of 1,450 m (Cuvier’s) and 1,408 m (Blainville’s) (http://swfsc.noaa.gov). They also found that Blainville’s beaked whales spent more time in the upper portion of the water column compared to Cuvier’s. Cuvier’s also performed dives close to maximum depth more frequently. Akin Tyack et al. (2006), the authors found that ascent rates for deep dives were consistently slower than the descent rates, and a series of shallower dives always followed a deep dive.
Northern bottlenose whales
Hooker and Baird (1999) attached time-depth recorder tags to northern bottlenose whales (Hyperoodon ampullatus) off the coast of Nova Scotia (http://www.ncbi.nlm.nih.gov). Authors found that northern bottlenose whales dive > 800 m regularly, with a maximum depth of 1,453 m. Many of dives were close to the sea floor. They also found that descent and ascent rates varied with dive depth during deep dives, and that longer dives had significantly faster descent than ascent rates. Authors also used an active fish finding sonar system to track non-tagged whales as they dived. They were able to document depth and time from the sonar system, which allowed them to calculate descent rates. Descent rates from sonar tracking were similar to those recorded from tagged whales undertaking deep dives. The sonar system had a limited range of 600 m, but none of the sonar traces appeared to level out, suggesting the whales were still diving when the traces were lost.
DIEL (24-HR) PATTERN OF BEAKED WHALE DIVES
Baird et al. (2008) investigated diel variation in diving behaviour of Blainville’s and Cuvier’s beaked whales using time-depth recorders. Similar patterns to the Tyack et al. (2006) study regarding depth, and ascent and descent rates were observed. For both Blainville’s and Cuvier’s beaked whales, deep foraging dives occurred as often during the day as at night. There were also no significant diel differences for ascent or descent rates, mean or maximum depths, or durations of dives for Blainville’s beaked whales. Similar trends were evident in Cuvier’s but unfortunately sample size prevented statistical analysis. Dives of Cuvier’s to mid-water depths (100–600 m), however, occurred 4.5 times more frequently during the day than at night, and they spent more time in shallow water at night than during the day. The increased time spent in shallow water at night is thought to be because at night there is reduced predation pressure from near-surface, visual predators, such as large sharks, so they do not need to spend as much time ‘hiding’ at mid-water depths. Being able to stay near the surface is advantageous, as it allows them to rest and recover between deep, foraging dives. Baird et al. (2006) also observed Blainville’s beaked whales spending greater amount of time near the surface during the night. There was not enough data to perform a comparison for Cuvier’s beaked whales.
BEAKED WHALES AND MILITARY SONAR
For information on beaked whales and military sonar, please see www.staticacousticmonitoringsystem.co.uk