Acoustic characteristics of underwater tail slaps used by Norwegian and Icelandic killer whales (Orcinus orca) to debilitate herring (Clupea harengus)
1 Institute of Biology, University of Southern Denmark-Odense,
Denmark
2 Sea Watch Foundation Cymru, New Quay, Wales, UK
3 Department of Zoophysiology, Institute of Biological Sciences, University
of Aarhus, Denmark
* Author for correspondence (e-mail: mjsimon{at}bi.ku.dk)
Accepted 29 March 2005
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Summary |
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Key words: killer whale, Orcinus orca, foraging, prey debilitation, tail slap, cavitation, acoustics
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Introduction |
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Prey debilitation prior to capture has also been noted in invertebrate
predators, such as snapping shrimp, Alpheidae spp.
(MacGinitie and MacGinitie,
1949), which debilitate their prey by producing abrupt pressure
changes when rapidly closing the snapper claw
(Versluis et al., 2000
). This
action generates cavitation: the ambient pressure falls to such low levels
that dissolved gas in the water is released. When the pressure returns to
ambient, the gas bubbles collapse and a distinct, very loud, sound pulse is
produced. Cavitation is a well-known phenomenon associated with rotating
propellers and other vibrating man-made structures
(Medwin and Clay, 1998
).
However, it is also produced by biological systems as in snapping shrimp
(mentioned above), and during photosynthesis and xylemic action inside plants
(Nardini et al., 2001
).
Many toothed whales; Odontoceti, produce loud sounds with source levels as
high as 236 dB RMS re. 1 µPa at 1 mmeasured from sperm whales
(Møhl et al., 2003).
Prey debilitation using intense sounds has been discussed as a possible
hunting strategy among toothed whales
(Bel'kovich and Yablokov, 1963
;
Norris and Møhl, 1983
).
Playing loud, short duration sound signals to cephalopod and fish species to
test the acoustic prey debilitation hypothesis showed no debilitation effects
(Zagaeski, 1987
;
Mackay and Pegg, 1988
).
However, long duration sounds had a debilitating and, in some cases, lethal
effect on guppies, Lebistes reticulatus
(Zagaeski, 1987
). Underwater
tail slaps produce several transient sounds of a long total duration, which
potentially could have a debilitating effect on the fish. It has been
suggested that the sound produced by underwater tail slaps of bottlenose
dolphins and killer whales is caused by cavitation around the tail moving
through the water (Smolker and Richards,
1988
; Similä and Ugarte,
1993
). Intense sound pressure associated with cavitation could
potentially be the cause of prey debilitation. The fish may also be
debilitated by the physical impact of the tail, or by turbulence, large
movements of water particles and pressure changes that could affect the
lateral line system of fish (Smolker and
Richards, 1988
; Similä
and Ugarte, 1993
; Domenici et
al., 2000a
; Coombs and Braun,
2003
). While the damaging effects of physical impact on herring
are obvious, the effects of pressure changes, turbulence and water particle
movements have, to our knowledge, not been described in detail. Likewise, the
intensity of the sounds produced by the underwater tail slaps of killer whales
has not previously been estimated. To understand the sound production
mechanism and the possible effect of the sounds on the herring, it is
important to know the sound intensity and frequency content of signals from
the tail slaps.
The aim of the present study was to record underwater tail slaps of Norwegian killer whales using broadband recording equipment and a hydrophone array and to analyse the acoustic characteristics of these sounds. To investigate whether cavitation is caused by underwater tail slaps, we compare these analyses with cavitation sounds from documented sources. In addition, we present evidence that killer whales in Icelandic waters also use underwater tail slaps when foraging on herring.
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Materials and methods |
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Sound recordings of Norwegian killer whales
Recordings of Norwegian killer whales were made from a 30 ft cabin cruiser
during OctoberDecember 2001 in Vestfjord and adjacent fjords in
northern Norway. Here killer whales gather in late fall and winter to feed on
the overwintering schools of Norwegian spring-spawning herring (Simlä et
al., 1996). The depth at the recording sites varied from 50 to 500 m. Birds
taking fish among whales that dived repeatedly in one area, and fish or fish
parts on the surface, identified foraging activity. When a group of foraging
killer whales had been located, the boat was placed approximately 30 m upwind
and the engine turned off so that the boat could drift across the feeding
spot. This procedure gave minimal disturbance of the herring and whales.
The recording system consisted of an array of four omnidirectional Reson TC 4034 hydrophones (frequency response within 3 dB from 0.1 kHz to 300 kHz, Reson, Slangerup, Denmark). Three peripheral hydrophones were placed symmetrically at a distance of 0.5 m from a central hydrophone (Fig. 1). The hydrophones were fitted on PVC tubes minimising reflections from the array. The array was mounted on a pole and positioned so that the depth of the centre hydrophone was 1.5 m below the water surface.
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The analogue recordings were played 8 or 16 times slower when digitalised on a computer with a sampling rate of 48 kHz (effective sampling rate: 384 kHz or 768 kHz), using CoolEdit Pro (Syntrillium Software, Phoenix, AZ, USA) and a sound card with built-in antialiasing filter.
Cross correlation programs were written in MatLab (The MathWorks, Inc.
Cambridge, MA, USA) and were used to calculate the difference in arrival times
of sounds at the four hydrophones. The time-of-arrival differences were used
to calculate the distance to the sound source and the apparent source level
(ASL, defined as the sound level 1 m from a sound source oriented in an
unknown direction; Møhl et al.,
2000) of the tail slap (Au and
Herzing, 2003
). Underwater tail slaps that were recorded on all
four channels and located at a distance of <15 m from the array were chosen
for further analysis.
Sound recordings of Icelandic killer whales
Underwater sound recordings of foraging Icelandic killer whales were made
from a 36 ft gaff-rigged sloop from June to August 2002 around Vestmannaeyjar,
Iceland. Just as in Norway, foraging activity was identified from birds taking
fish among whales, which dove repeatedly in one area, and fish or fish parts
on the surface. Recording sessions of 10 min duration were obtained using a
custom-built hydrophone (Woods Hole Oceanographic Institute, with a ±4
dB response up to 20 kHz) connected to a DAT recorder (Sony, TCD-D8, sampling
frequency: 48 kHz).
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We measured the energy content of the same pulses used to measure the ASL by applying a 100 µs time window. By assuming spherical spreading and compensating for the frequency-dependent sound absorption, the energy content was back-calculated to 1 m distance from the tail, rendering the energy content in dB re. 1 µPa2s at 1 m.
ASL estimates and other signal analyses were made using CoolEdit Pro (Syntrillium Software, Phoenix, AZ, USA), BatSound Pro (Petterson Elektronik, Uppsala, Sweden), MatLab (The MathWorks, Inc. Cambridge, MA, USA) and SigPro (S. B. Pedersen, Centre for Sound Communication, Denmark).
Comparing sounds of Icelandic and Norwegian killer whales
The limitation in the recording bandwidth of the Icelandic recordings
prevented direct comparison of the source levels and bandwidth with the
Norwegian recordings. Therefore, the signals recorded in Norway (bandwidth:
150 kHz) were low-pass filtered at 20 kHz to obtain the same bandwidth as the
sounds recorded in Iceland. The differences between two parameters
(E97BW and f0), measured in pulses from
multi-pulsed sounds (potential tail slaps) from Icelandic and Norwegian killer
whales, were tested with non-parametric, two-way ANOVA
(Barnard et al., 2001).
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Results |
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Feeding was divided into two phases; first a herding phase, then a feeding
phase (Similä and Ugarte,
1993). During the herding phase, the killer whales were observed
swimming alone or in groups of up to nine individuals close together under and
around the herring school, with their white ventral side along the edge of the
school. Calves often swam in synchrony with an adult (N=36).
Sometimes a whale swam directly through the school without splitting it into
smaller groups. In the beginning of the herding phase, the water was very
clear (visibility >20 m). The visibility deteriorated as the whales emitted
air bubbles (see video 1 in supplementary material), air was dragged down from
the surface by the whales' bodies and flukes, and ascending air bubbles were
released by the herring schools (see video 2 in supplementary material).
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Ninety-two underwater tail slaps were recorded close enough to the camera to assess whether or not fish had been debilitated. In our observations, adult killer whales had greater success at debilitating herring with underwater tail slaps (76.5% tail slaps debilitated one or more herring, N=51) than did calves (36.6% tail slaps debilitated one or more herring, N=41). Due to limited resolution of the video recordings, it was difficult to obtain an exact count of the number of fish debilitated by each underwater tail slap, but this number varied from zero to more than seven. Only herring in the immediate vicinity of the whale tail were debilitated. The approximate depth of 126 underwater tail slaps was determined. Of these, 51% were made at depths of 05 m, and 24% occurred at depths of 510 m. The remaining 25% were made at depths >10 m, but we could not determine the exact depth because the surface was not visible. However, we estimate that the whales did not use tail slaps below depths of about 20 m.
Most of the video recordings started after the killer whales had surrounded the herring or ended before the feeding ceased due to the boat drifting away from the whales or to changes in weather and light conditions. Only one event was filmed from the beginning to the end. This event involved approximately 15 killer whales and a herring school of 1218 m in diameter. It took 3.5 min for the killer whales to drive the fish from 2030 m depth to the surface. The herding phase lasted for 7 min and 45 s. During the following 1 min and 15 s there were three tail slaps marking the beginning of the feeding phase. The whales did not begin to consume fish until after 1 min. Then there was a 30 min period of active feeding with a series of tail slaps at gaps of 15 s to 1.0 min. The whales then swam close to the school and consumed the debilitated fish. During the 30 min feeding phase the whales made 146 underwater tail slaps. The last tail slap was made 37 min and 45 s after the beginning of the feeding session. Shortly after, the whales left the herring school, which remained close to the surface for another 2 min. There was no noticeable change in the size of the fish school suggesting that the killer whales ate a relatively small number of the fish.
Sounds of Norwegian killer whales
A total of six underwater tail slaps were recorded from the Norwegian
killer whales with the hydrophone array. Four of the tail slaps were recorded
by all four hydrophones and were within 15 m of the array, and these were used
for further analysis. Three of the analysed tail slaps were recorded at a tape
speed of 30 ips (recording bandwidth: 150 kHz), with the fourth tail slap was
recorded at a tape speed of 60 ips (recording bandwidth: 300 kHz). This tail
slap was filtered through a 150 kHz low pass filter to make it comparable to
the tail slaps recorded at 30 ips before further analysis in SigPro (S. B.
Pedersen, Centre for Sound Communication, Denmark). The sound signals produced
by underwater tail slaps consisted of multiple bursts of pulses and had a
total average duration of 318 ms (S.D.=99, N=4)
(Fig. 2). The number of bursts
of pulses within each tail slap varied with a mean of 16 (S.D.=5.3;
Table 1). Each burst of pulses
could contain up to 10 single pulses.
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The frequency bandwidth and the centre frequency were measured for the single pulses of highest peak amplitude (Fig. 2, arrow, and Fig. 4A) selected from all four tail slaps (N=13). The 97% energy bandwidth (E97BW) and the 3 dB bandwidths were 130.5 kHz (S.D.=17.5) and 36.8 kHz (S.D.=22.5), respectively. The centre frequency (f0) was 46.1 kHz (S.D.=22.3). Fig. 4B shows the frequency spectrum of one such pulse. These intense pulses are extremely broadband and contain frequency components up to the limit of the recording system, 150 kHz or 300 kHz depending on the tape speed of the recordings.
Pulses containing the highest received sound pressure level in each tail slap (Fig. 2, arrow) were chosen for calculating apparent source levels. The underwater tail slaps containing these pulses occurred 1115 m from the centre hydrophone of the array and gave a mean apparent source level of 186 dB (pp) re. 1 µPa at 1 m (S.D.=5.4 dB, N=4; Table 1). The mean energy content of the tail slap pulses was 169 dB re. 1 µPa2s at 1 m (S.D.=3.3 dB, N=4; Table 1).
Sounds of Icelandic killer whales
Sound recordings of foraging Icelandic killer whales revealed signals
consisting of multiple bursts of pulses with an average total duration of 226
ms (S.D.=54, N=10), and an average of eight bursts of
pulses (S.D.=3.6, N=10) per tail slap
(Fig. 5 and
Table 1). Each burst of pulses
could contain up to seven single pulses. The pulses were broadband with a 97%
energy frequency bandwidth (E97BW) of 17.6 kHz
(S.D.=2.4, bandwidth of the recording system was 20 kHz) and a
centre frequency (f0) of 7.8 kHz (S.D.=4.4)
(Table 1). These sounds were
produced in 100% of the recording sessions (N=22 10 min recordings of
foraging killer whales). Herring was the only observed prey species. During
the recordings we picked up nine debilitated fish from the surface by hand.
These fish wriggled when touched and swam off when released.
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Comparison between sounds of Icelandic and Norwegian killer whales
Table 1 shows the parameters
measured from multiple bursts of pulses recorded in Iceland and filtered
sounds of underwater tail slaps recorded in Norway for comparison. There was
no significant difference between either the 97% energy bandwidths
(E97BW) or the centre frequencies (f0)
of signals recorded from Icelandic and Norwegian killer whales (non-parametric
two-way ANOVA, E97BW: H=2.60, d.f.=1,
P=0.11. f0: H=1.24, d.f.=1,
P=0.27; Barnard et al.,
2001).
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Discussion |
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An acoustic signal generated by cavitation consists of two pulses. There is
a low frequency pulse, the precursor, created as air bubbles are formed due to
the drastic reduction in pressure from ambient. The precursor is followed by a
very broadband transient, the main-pulse, caused by the collapse of the air
bubble when the pressure returns to ambient
(Young, 1989). We did not
observe any clear precursors in our recordings. However, precursors have low
intensity (Versluis et al.,
2000
) and they may have been masked by background noise in the
pulse burst. The similarities between the acoustic characteristics of
cavitation sounds and the sounds produced by underwater tail slaps strongly
suggest that underwater tail slaps cause cavitation.
The incipient cavitation number (relationship between temperature, density,
velocity, vapour pressure and pressure) defines the onset of cavitation. Under
controlled laboratory conditions, the incipient cavitation number is reached
when the pressure decreases under the saturated vapour pressure
(Brennen, 1995). High
concentrations of bubbles and particles, as well as turbulence in the water,
increase the incipient cavitation number, which means that cavitation occurs
before the pressure reduces to the saturated vapour pressure
(Brennen, 1995
). Furthermore,
cavitation is more likely to occur at a shallow depth with lower ambient
pressure because the pressure gradient needed to produce cavitation is lower
(Medwin and Clay, 1998
;
Ross, 1976
). The video
recordings revealed that the water in which the killer whales were feeding on
herring had high concentrations of air bubbles and particles, as well as
turbulence created by the movement of the fish and the whales. Such an
environment facilitates cavitation. In addition, the video analyses showed
that a substantial number of tail slaps were recorded within the upper 5 m of
the water column where cavitation is more likely to occur
(Medwin and Clay, 1998
). If
the incipient cavitation number (Brennen,
1995
) could be determined for the waters where killer whales were
performing tail slaps, calculations might confirm that cavitation is a
plausible sound producing mechanism for tail accelerations measured during
underwater tail slaps (maximum 48 ms2 as measured by
Domenici et al., 2000a
). Our
study shows that several characteristics of some of the tail slap sound
components resemble cavitation-generated signals.
While cavitation could explain some of the pulses observed in the sound
produced during tail slaps, it may not explain all of them. Some pulses are
likely to be sound resulting from physical contact between the whale tail and
the herring, as discerned from the video sequences showing physically damaged
fish and as previously suggested by Similä and Ugarte
(1993) and Domenici et al.
(2000a
). It seems unlikely
that contact sounds could produce the very high frequencies in pulses observed
in the tail slaps. Recordings of physical impact sounds on herring by an
artificial killer whale tail fluke might confirm this.
Debilitation of herring
A photograph published by Sigurjónsson et al.
(1988) shows a school of
herring that was herded to the water surface by a group of Icelandic killer
whales. This photograph suggests that killer whales in Icelandic waters use
hunting techniques similar to those used by killer whales in Norwegian waters
(Similä and Ugarte,
1993
). The acoustic similarities between the signals produced by
Icelandic killer whales and those of underwater tail slaps recorded from
Norwegian killer whales (Fig. 5
and Table 1) indicate that
Icelandic killer whales use underwater tail slaps. In addition, the
observations of debilitated herring on the surface above foraging Icelandic
killer whales resemble the debilitated herring observed in the video
recordings of Norwegian killer whales. We conclude that Icelandic killer
whales most likely use underwater tail slaps to debilitate prey, as do
Norwegian killer whales.
Herring have a sensitive hearing system with a direct connection between
the ear, the lateral line system (acoustico-lateralis) and the swim bladder
(Coombs and Braun, 2003). This
is probably the reason for their high sensitivity to sound, both in terms of
sound pressure and particle displacement
(Enger, 1967
). Only a few
herring are debilitated after each tail slap, and only in the immediate
vicinity of the killer whale tail fluke. The sound pressure levels measured in
this study were probably not intense enough to cause debilitation of fish. The
fact that only fish in the immediate vicinity of the tail were debilitated
further suggests that the sound pressure alone is not intense enough to
debilitate the herring. Apart from sound pressure, the herring in the
immediate vicinity of the tail would be exposed to a number of other factors,
which may seriously affect its sensory system and cause debilitation. These
factors include: steep pressure gradients, high levels of water acceleration
and particle movements and physical contact with the tail or other fish.
To our knowledge, most underwater tail slaps have been reported from
predators feeding on schooling herring, exemplified by sharks feeding on
clupeids, Norwegian and Icelandic killer whales feeding on Atlantic herring,
and Pacific bottlenose dolphins feeding on Perth herring, Nematolosa
vlaminghi (Muus et al.,
1988; Smolker and Richards,
1988
; Similä and Ugarte,
1993
; present study). In addition to direct contact between the
fluke and the fish, the acoustic pressure changes and particle movements in
close vicinity of underwater tail slaps could contribute to the effectiveness
of this hunting strategy for schooling prey with good hearing and sensitivity
to hydrodynamic flow, such as clupeid fish. Sensory overloading and loss of
buoyancy control may explain the occurrence of herring floating on their sides
on the surface and quiescent in the water following underwater tail slaps, and
why some of these debilitated fish regained their swimming abilities.
The analysis of the acoustic signals from killer whale underwater tail slaps revealed insights both into the sound production mechanism and the function of the tail slaps in prey capture. More studies on the acoustic biology of killer whales and herring are needed to reveal further insights into the intricate predatorprey interactions between these species. Such studies should include play back trials on tail slap sounds and hydrodynamic action upon herring schools, as well as more detailed acoustic and video recordings of tail slapping killer whales.
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Acknowledgments |
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Footnotes |
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