Beaked Whales Perform Extreme Dives to Hunt Deepwater Prey


October 19, 2006

A study of ten beaked whales of two poorly understood species shows
their foraging dives are deeper and longer than those reported for any
other air-breathing species.  This extreme deep-diving behavior is
of particular interest since beaked whales stranded during naval sonar
exercises have been reported to have symptoms of decompression
sickness. One goal of the study was to explore whether the extreme
diving behavior of beaked whales puts them at a special risk from naval
sonar exercises.

Scientists from the Woods Hole Oceanographic
Institution (WHOI) teamed with colleagues from the University of La
Laguna in Spain, the University of Aarhus in Denmark, Bluwest and the
NATO Undersea Research Centre in Italy. The team studied
Cuvier’s beaked whales (Ziphius cavirostris) and Blainville’s beaked
whales (Mesoplodon densirostris) in Italian and Spanish waters using a non-invasive digital archival
tag or D-tag developed at WHOI by one of the authors, engineer Dr. Mark
Johnson.  Their findings are reported in the current online issue
of the Journal of Experimental Biology.

The D-tag, about the size of a sandal,  has a variety of sensors to
record sounds and movements, and is attached to the animals with four
small suction cups using a handheld pole. It is programmed to release
from the animal within a day and is recovered with help from a
VHF radio beacon in the tag. The 3-6 Gbytes of audio and sensor data are then off-loaded to a computer for anaylsis.

Dr. Peter Tyack, a senior scientist in the WHOI Biology Department and
lead author of the study, says they found some similarities with the
much better studied sperm whales and elephant seals, but also some
major differences. “These two beaked whale species make long, very deep
dives to find food, and then make shallow dives and rest near the
surface. By contrast, sperm whales and elephant seals can make a series
of deep dives without the need for prolonged intervals between deep
dives. We think that beaked whales return to the surface after deep
dives with an oxygen debt and need to recover before their next deep
dive.”

Tyack said the team’s analysis suggests that the normal deep diving
behavior
of beaked whales does not pose a decompression risk. “Rather, it
appears that their greatest risk of decompression sickness would stem
from an
atypical behavioral response involving repeated dives at depths between
30 and 80 meters (roughly100 to 250 feet),” Tyack said. “The reason for
this is that
once the lungs have collapsed under pressure, gas does not diffuse from
the lungs into the blood. Lung collapse is thought to occur shallower
than 100 meters (330 feet), so deeper parts of the dive do not increase
the risk of
decompression problems. However, if beaked whales responded to sonars
with repeated dives to near 50 meters (165 feet), this could pose a
risk.”

The Cuvier’s beaked whales were tagged in June 2003 and 2004 in the
Ligurian Sea off Italy, while the Blainville’s beaked whales were
tagged in October 2003 and 2004 off the island of El Hierro in the
Canary Islands. Both field sites were in deep water, between 700 and
2,000 meters (2,300 to 6,500 feet) with steep bottom topography. Tags
were attached to seven Cuvier’s beaked whales and three Blainville’s
beaked whales, and they remained attached to the whales for an average
of 8 hours and 12 hours, respectively.

“Although this study was limited to ten animals, it provides the
first detailed information available about the diving, acoustic, and
movement behavior of two species of beaked whales,” Tyack said.
“Shallow dives seem to be performed between deep dives, and both
species dive very deep to hunt for prey. They seem to
spend equal time ascending and descending in shallow dives, but take
longer to ascend from deep dives.”

The slow ascent from deep dives is a
major mystery. “Why don’t they stay longer at depth to feed, and then
come up more rapidly?” Tyack said.  “Avoidance of decompression problems by slow
ascent, as in scuba divers, cannot account for this behavior if the lungs
of these breathhold diving marine mammals are collapsed at depths
greater than 100 meters (330 feet).”

Very little is known about these two species of beaked whales since
they spend little time on the surface and it is difficult to tag
them.  The much better studied sperm whale can dive for more than
one hour to depths greater than 1,200 meters (roughly 4,000 feet), but
typically dives for 45 minutes to depths of 600-1,000 meters (1,968 to 3,280 feet). Elephant
seals, another well known deep diver, can spend up to two hours in
depths over 1,500 meters (nearly 5,000 feet), but typically dive for
only 25-30 minutes to depths of about 500 meters (1,640 feet).  Marine mammals
seem to have adapted to the effects of diving deep and optimizing
their oxygen supplies.

The Cuvier’s beaked whales dove to maximum depths of nearly 1,900 meters
(about 6,230 feet) with a maximum duration of 85 minutes, while the
Blainville’s beaked whales dove to a maximum depth of 1,250 meters (4,100 feet) and
57 minutes in duration. The dives near 1,900 meters constitute the
deepest confirmed dives reported from any air-breathing animal. While
people often focus on the maximum dives of breathhold diving animals,
breathhold divers are not at a track meet and it is the average of the
deep foraging dives that is more important. Regular echolocation clicks
and buzzes and echoes of what appears to be prey were recorded on the
tags, suggesting the whales were hunting for food on the deep dives.
The average foraging dive for Cuvier’s beaked whale went to a depth of
1,070 meters (about 3,500 feet) with a duration of 58 minutes, while the
Blainville’s beaked whales dove to an average depth of 835 meters (2,740 feet) and
46.5 minutes in duration.  These represent the deepest and longest
average dives reported for any breathhold-diving animal.

These two beaked whale species have been reported to mass
strand during naval sonar exercises in the area.  It is unclear
how these beaked whale species respond to the sonar sounds and whether
their responses cause physiological changes that increase the risk that  they will strand
and die.  This study suggests the paradoxical result that even
though beaked whales are extreme divers, their normal diving behavior
does not seem to put them at greater physiological risk for sonar
exposure. Rather it suggests that physiological risk would stem from a
specific behavioral response to the sonars.

“No matter what the precise cause of the strandings is, we need to
develop effective mitigation strategies to reduce the accidental
exposure of beaked whales to bay sonar,” Tyack said. “The information
in this study provides critical data to design efficient acoustic and
visual detection methods for these at-risk species of marine mammals.”

Funding for the tag development was provided by a Cecil H. and
Ida M. Green Technology Innovation Award at WHOI and the U.S. Office of Naval Research.
Funding for field work was provided by the Strategic Environmental
Research and Development Program  (SERDP), the National Ocean
Partnership Program, the Packard Foundation, the Canary Islands
Government, and the Spanish Ministry of Defense. Fieldwork support was
provided by BluWest, NATO Undersea Research Center, and the Government
of El Hierro.