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Bioacoustics
The International Journal of Animal Sound and its Recording
ISSN: 0952-4622 (Print) 2165-0586 (Online) Journal homepage: http://www.tandfonline.com/loi/tbio20
Vocalizations of common minke whales
(Balaenoptera acutorostrata) in an eastern North
Pacific feeding ground
Katrina Nikolich & Jared R. Towers
To cite this article: Katrina Nikolich & Jared R. Towers (2018): Vocalizations of common minke
whales (Balaenoptera�acutorostrata) in an eastern North Pacific feeding ground, Bioacoustics, DOI:
10.1080/09524622.2018.1555716
To link to this article: https://doi.org/10.1080/09524622.2018.1555716
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Published online: 11 Dec 2018.
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Vocalizations of common minke whales (Balaenoptera
acutorostrata)inaneasternNorthPacific feeding ground
Katrina Nikolich
a
and Jared R. Towers
b,c
a
Biology Department, University of Victoria, Victoria, Canada;
b
Marine Education and Research Society, Port
McNeill, BC, Canada;
c
Bay Cetology, Alert Bay, BC, Canada
ABSTRACT
The minke whale (Balaenoptera acutorostrata) is a small species of
baleen whale with a cosmopolitan distribution. Despite extensive
study on the vocalizations of other balaenopterids, the acoustic
repertoire of minke whales is not well known. Individuals of the
North Pacific subspecies (B. acutorostrata scammoni) produce
unique vocalizations (‘boings’) during their putative breeding sea-
son from fall to spring. However, no vocalizations have been
previously reported for this subspecies in any eastern North
Pacific feeding ground. We present two call types recorded in
the presence of six minke whales, two of which were confirmed
as female, in Cormorant Channel, British Columbia, Canada, during
the summer of 2012. The calls consist of downsweeps and pulse
chains. These call types share some characteristics with calls
described elsewhere, although they are not identical to similar
call types observed for other populations. Calling rates for minke
whales in this study region are very low compared to those
reported for this subspecies on its putative breeding grounds, as
well as for other subspecies on their feeding grounds. We propose
predation risk, sexual segregation and acoustic masking as poten-
tial causes of the low calling rates observed for minke whales in
Cormorant Channel.
ARTICLE HISTORY
Received 19 July 2018
Accepted 14 November 2018
KEYWORDS
Common minke whale;
vocalizations; vocal
behaviour; eastern North
Pacific
Introduction
The common minke whale is a small baleen whale found in all oceans of the world. In
the northern hemisphere, two subspecies are recognized. Balaenoptera acutorostrata
acutorostrata is found in the North Atlantic Ocean and B. acutorostrata scammoni is
found in the North Pacific Ocean (Stewart and Leatherwood 1985). In the southern
hemisphere, a dwarf form of common minke whale exists as an unnamed subspecies
(Jefferson et al. 2015). Several types of vocalizations have been attributed to the three
subspecies of common minke whales.
In the southern hemisphere, the vocalizations of the dwarf minke whale are best known
from the waters around the Great Barrier Reef in Australia (Arnold 1997). In this region,
minke whales are known to make a complex, multi-component, amplitude and frequency
modulated call spanning 50 Hz–9.4 kHz that lasts up to 2.5 s (Gedamke et al. 2001). This
CONTACT Katrina Nikolich katrinan@uvic.ca
Supplemental detail for this article can be accessed here.
BIOACOUSTICS
https://doi.org/10.1080/09524622.2018.1555716
© 2018 Informa UK Limited, trading as Taylor & Francis Group
call is commonly referred to as the star wars vocalization and has characteristics consistent
with songs produced by other baleen whales advertising reproductive potential. Another
less common minke whale call recorded during at least 59% of encounters in this region
was a 0.2–0.3 s downsweep ranging in frequency from 250 to 50 Hz (Gedamke et al. 2001).
A similar downsweep vocalization has also been attributed to minke whales from the
St. Lawrence River estuary in the North Atlantic Ocean. In this region, 0.4 s downsweep
vocalizations typically ranging from 100 to 200 Hz to <90 Hz were detected during 51%
of minke whale encounters (Edds-Walton 2000). These calls are also similar in duration
and structure to some vocalizations of Antarctic minke whales (Balaenoptera bonaer-
ensis) (Schevill and Watkins 1972). Both species were found to make these calls in
summer feeding grounds. Other types of vocalizations made by minke whales in the
North Atlantic primarily include trains of low frequency pulses in the 50–400 Hz range
lasting from 10 to 60 s. These pulse trains have been recorded speeding up, slowing
down and with consistent inter-pulse intervals in the Caribbean in winter (Winn and
Perkins 1976; Mellinger et al. 2000) and offof Massachusetts in spring and fall (Risch
et al. 2013). It is also hypothesized that North Atlantic minke whales emit high
frequency clicks (Beamish and Mitchell 1973) and pings (Winn and Perkins 1976).
In the North Pacific, only one type of vocalization has so far been attributed to
minke whales. This call commonly referred to as a boing has only been recorded from
fall to spring, usually in low latitude oceanic regions offthe coasts of Mexico, Hawaii
and the Mariana Islands (Rankin and Barlow 2005; Norris et al. 2012). However, boings
with characteristics similar to those recorded in Hawaii have also been detected in the
Bering Sea (Delarue et al. 2012). These central Pacific boings have a mean pulse
repetition rate of 115 s
−1
and duration of approximately 2.6 s. In the eastern tropical
North Pacific, minke whale boings have been recorded with a lower pulse repetition
rate (92 s
−1
) and longer duration (3.6 s) (Rankin and Barlow 2005). Like the star wars
vocalizations, these boings are detected mainly in regions where, and at times when
minke whales are thought to breed (Thompson and Friedl 1982; Oswald et al. 2011).
From spring to fall, small numbers of North Pacific minke whales can be found
distributed in temperate waters of western Canada (Towers et al. 2013). In this region,
they are usually only documented over or near shallow banks where they can some-
times be observed feeding (Towers et al. In review). One such area offnorth-eastern
Vancouver Island is Cormorant Channel (Towers et al. In review). In order to deter-
mine frequency of use patterns, behaviour and the acoustic signals of minke whales in
this area, we made concurrent shore-based observations and acoustic recordings during
the summer of 2012. This report presents the acoustic results of this study including
details on two types of vocalizations previously undocumented for this subspecies.
Materials and methods
Data collection
A Wildlife Acoustics SM2M outfitted with an HTI-96-min transducer (sensitivity: −165 dB
re. 1 V/µPa with manufacturer reporting flat frequency response above 100 Hz) was set to
make continuous recordings at a sampling rate of 16 kHz (Bit rate: 16). It was anchored to
the seafloor at 50° 36.2 N, 126° 56.7 W in Cormorant Channel at 20 m depth on June 10th,
2K. NIKOLICH AND J. R. TOWERS
2012. This location was 208 m at a magnetic compass bearing of 40° northeast from an
observation point at the high tide line on the shore of Cormorant Island. Visual observa-
tions of minke whales were conducted from the observation point from June 11th to
August 15th (Figure 1). Observers used naked eyes and 15 × 80 Steiner binoculars with
built-in magnetic compass and reticle bar mounted on a levelled Velbon C-600 fluid-
panning tripod to conduct visual surveys for up to 13 h a day when Beaufort sea state levels
were ≤2. Data including Beaufortsea state,tide height and notes on visibility were recorded
every 30 min of survey effort. The time, compass bearing, and reticle distance from the
Malcolm Island shoreline were recorded each time a minke whale surfaced and also when
other marine mammals were observed (Towers et al. In review). The behaviour of cetaceans
was noted when apparent and effort was taken to visually identify individual minke whales
based on their unique natural markings portrayed in Towers (2011). Effort to biopsy these
known whales using the technique outlined in Barrett-Lennard et al. (1996)wasalso
undertaken in Cormorant Channel in the summers of 2013 and 2014.
Figure 1. Map of study area in Cormorant Channel, British Columbia. The X denotes the shore-based
visual observation site from which minke whale presence was confirmed. The light grey dot denotes
the location of the SM2M omnidirectional hydrophone and acoustic recorder. The grey lines portray
the arc of the area surveyed by observers and the shading between the lines includes all surveyed
waters 3 or more reticles from the Malcolm Island shore. The lighter tone of grey shading denotes the
area in which the 3 reticle limit fluctuated due to the semi-diurnal tidal activity. All acoustic data
recorded from 5 min before to 5 min after minke whales were observed within the shaded area were
analyzed. Map created by Christie McMillan, Marine Education and Research Society, Port McNeill, BC,
Canada; used with permission.
BIOACOUSTICS 3
Data analysis
Prior to manual analysis, a detection algorithm was designed and applied to the entire
acoustic dataset to check for the presence of minke whale boing vocalizations (Rankin
and Barlow 2005). The detector used was a programmable Band-Limited Energy detector
in Raven Pro 64 v.1.5 (Cornell Lab of Ornithology). Detector parameters were as follows:
Bandwidth 1000–1600 Hz; duration 1.4–5.0 s; minimum separation 6.0 s; minimum
occupancy 50%; SNR threshold 10.0 dB re: 1 µPa; block size 15 s; percentile 20%.
Detailed acoustic analysis was conducted for the time periods when minke whales were
observed within 998 m of the moored hydrophone. These time periods were defined as the
interval of time including 5 min before a minke whale was first seen surfacing within 3 or
more reticles (≤1176 m) of the observation point, to 5 min after the whale was seen within
that radius (Figure 1). Sightings were considered part of the same time interval if sightings
of minke whales within the defined radius were no more than 10 min apart, to account for
possible missed surfacings in between. This method of subsampling the acoustic data was
chosen to maximize the probability of observing calls heard by minke whales with high
signal-to-noise ratio (SNR), and minimize the possibility of attributing the calls of other
animals to minke whales due to lack of visual confirmation.
Odontocetes were sometimes present at these time intervals; however, the only
cetacean regularly observed in the area that may produce similar calls to a minke
whale was the humpback whale (Megaptera novaeangliae). Calls closely resembling
known humpback whale vocalizations were discarded from further analysis. In addi-
tion, candidate minke whale calls that were recorded within two hours of a humpback
whale sighting in the study area were discarded, as humpback whale calls from outside
visual range may have been heard. In the event that multiple call types were heard
within one analysis period, the SNR of calls was to be compared to determine which call
type was more likely to come from close range; however, this overlap did not occur in
our analysis. Other vocalization sources in the study area include harbour seals (Phoca
vitulina) and several species of soniferous fish. Harbour seal vocal repertoires from this
region have been documented (Nikolich et al. 2016; Matthews et al. 2017) and were
observed in several analysis periods. Any candidate minke whale calls that resembled
portions of harbour seal roars were removed from further analysis. Candidate call types
were compared to known vocalizations of fish in the area, and any calls matching fish
vocal characteristics were discarded.
Recordings were analysed manually using Raven Pro 64 v.1.5 (Cornell Lab of
Ornithology) to identify biological sounds both visually using spectrograms (Hann
window, 2560-point fast Fourier transform [resolution = 160 ms × 6.25 Hz], 50%
overlap) and aurally by listening to the recordings. Signals that matched previous
descriptions of minke whale calls from other locations were manually selected from
the spectrograms. Signals that appeared mammalian in origin, but could not be
classified to another marine mammal, were also selected. All selected calls were then
placed into categories based on similar spectral contour and acoustic properties.
Measurements of duration (seconds), upper and lower frequency (Hz), and peak
frequency (Hz) were taken for all calls categorized in this way. Mean and standard
deviation was calculated for each measurement, for each call type.
4K. NIKOLICH AND J. R. TOWERS
For pulse chain call types, spectrograms were examined using a 400-point fast Fourier
transform (resolution = 25 ms × 40 Hz; all other spectrogram parameters remained the
same as above). This increased resolution in thetime domain, which allowed more accurate
measurement of inter-pulse intervals. Pulse chains which had a sufficient SNR to be
measured (i.e. their beginning, end, high and low frequencies were clearly visible in
spectrographic analysis) were divided roughly into thirds of their durations, and inter-
pulse interval within the chain was measured once per third to determine whether pulse
chains showed increasing, decreasing, or constant pulse rates. Duration, peak frequency,
high frequency, and low frequency were compared among pulse chain types using analysis
of variance (ANOVA) when parametric assumptions were met, or Kruskal-Wallis rank-
sum tests when parametric assumptions were not met.
Each time interval that was analysed was qualitatively assessed for anthropogenic
noise level. Anthropogenic noise was considered to be noise attributable to boat motors,
fishing gear, or aircraft. Each time interval was given an average noise rating between 0
(no anthropogenic noise for most of the interval) to 4 (very loud anthropogenic noise
for most of the interval). All intervals were found to have some anthropogenic noise;
thus, noise level ratings ranged from 1 to 4. To determine whether noise masking had
an effect on manual selection of potential calls, a Kruskal-Wallis rank sum test was used
to detect statistical differences between the number of calls selected at each noise level,
and whether there were statistical differences in average noise level during intervals in
which calls were selected versus intervals in which no calls were selected.
DNA was extracted from each biopsy sample using standard phenol:chloroform
procedures (Sambrook and Russell 2001; Wang et al. 2009). Samples were then sexed
using the method described in Gilson et al. (1998).
Results
Minke whales were observed at three or more reticles from the Malcolm Island
shore for a total of 721 min during 77 surfacing bouts in June; 896 min during
104 surfacing bouts in July; and 210 min during 24 surfacing bouts in August. The
total number of surfacings recorded was 890, 157 of which occurred within 5 min of
when one or more minke whales were observed surfacing within the 3 reticle arc
(Figure 1;Table 1). Surface feeding behaviour was observed within 3 or more
reticles of the observation point on 6 occasions (Table 1). Observer experience,
weather, and surfacing behaviour allowed for the identities of 6 unique minke
whales (M001, M002, M003, M004, M006, M022; see Towers 2011)toberecognized
Table 1. Summary of minke whale sightings (by number of surfacings observed) that occurred
close to the hydrophone (i.e. 3 or more reticles from the Malcolm Island shore), and sightings that
occurred within this range within 5 min of the vocal activity described in this study. ‘≥2 Animals’
column refers to sightings that occurred within 5 min of when two or more different minke
whales were observed or suspected >3 reticles.
Total Alone ≥2 Animals Feeding ID confirmed
≥3 Reticles 890 733 157 6 132
Vocal activity 54 54 0 1 9
BIOACOUSTICS 5
on 132 occasions (Table 1). DNA extracted from biopsy samples of two of these
individuals (M001 and M006) confirmed that they are both female.
The detection algorithm for boings yielded no positive results. By manual analysis,
1,491 min of acoustic data were analysed from June, 1,936 min were analysed from July,
and 450 min were analysed from August. Initial manual examination of the data yielded
only 4 downsweep calls which were similar to documented minke whale vocalizations
in other regions. We then expanded our search to include biological signals that may
not closely match known minke whale calls, but could also not be classified to other
species. In this way we annotated a total of 115 calls heard over 8 of the 43 days (19% of
days) that minke whales were visually observed within 3 or more reticles of the
observation point (calling rate was therefore 1.79 calls*hour
−1
or 0.030 calls*min
−1
).
These calls were placed into two categories based on spectral characteristics: Pulse
Chain (PC; N= 111) and Downsweep (DS; N = 4). Two other interesting vocalization
types were recorded, each on only one occasion: the Broadband Pulse (N= 2) and the
Tonal Waver (N= 22) (Supplemental Materials 1). Due to the fact that these calls were
only observed once each in the presence of minke whales, we do not include them as
candidate call types here. Examples of acoustic recordings of each call type, including
the two calls heard only on one occasion each, have been provided as supplemental
materials (Supplemental Materials 2–5).
Pulse chains were observed on June 17
th
, June 21
st
, June 29
th
, July 1
st
and July 28
th
,at
times when minke whales were the only marine mammal seen in the area (Figure 2).
Those recorded on June 21
st
were made when female minke whale M006 was observed
Figure 2. Spectrogram showing the pulse chain candidate call type (300-point fast Fourier transform,
Hann window with 50% overlap) including three call subtypes: pulse chains which (a) increase in
pulse rate, (b) decrease in pulse rate and (c) remain constant in pulse rate over their duration. Calls
were recorded in Cormorant Channel, British Columbia, Canada, during times when minke whales
were the only baleen whale visually observed.
6K. NIKOLICH AND J. R. TOWERS
near the hydrophone during nine consecutive surfacings (Table 1). Most pulse chains
(95/111) were observed on the morning of July 28th, between 8:00 and 11:00 local time.
Of the total number of pulse chains observed, 70 (63%) were deemed suitable for
further analysis. Of these 70 pulse chains, inter-pulse interval increased over the
duration of 39 calls (decreased in pulse rate), and decreased over the duration of 7
calls (increased in pulse rate). The remaining 24 pulse chains were constant in pulse
rate, meaning the inter-pulse interval did not increase or decrease more than 10 ms
over the duration of the call. There were no significant differences in low frequency
(overall mean = 327 ± 173 Hz), high frequency (overall mean = 1205 ± 459 Hz), or peak
frequency (overall mean = 676 ± 246 Hz) among the different pulse chain types
(ANOVA and Kruskal-Wallis tests, all p> 0.05). However, pulse chains that decreased
in pulse rate were significantly longer in duration than pulse chains which had constant
pulse rates (ANOVA, F
2,67
= 3.26, p= 0.045; Tukey post-hoc pairwise tests). Pulse
chains which increased in pulse rate did not have a significantly different duration from
either of the other pulse chain types (Table 2).
Downsweeps (Figure 3) were heard on June 29
th
and July 5
th
. A downsweep recorded
on June 29
th
occurred 4 min 23 sec before a minke was observed lunge feeding at the
surface (Table 1). Downsweeps had an average high (starting) frequency of 142 ± 69 Hz
and a low (ending) frequency of 38 ± 11 Hz. The average duration of the downsweeps
was 648 ± 152 ms (Table 2).
Vocalizations could not be significantly correlated with any specific behaviours
recorded by observers, due to the low number of calls recorded and infrequency with
which specific activities other than surfacing to breathe were observed. Anthropogenic
noise levels were not statistically different between time intervals with calls found and
time intervals with no calls found (Wilcoxon rank sum test, W= 5180, p= 0.649).
Similarly, the number of calls selected was not statistically different among noise levels
(Kruskal-Wallis rank sum test, Χ
2
= 0.639, p= 0.887).
Discussion
We present two call types that were recorded in the presence of North Pacific minke
whales in Cormorant Channel, neither of which have been previously documented for
this subspecies. However, because calls could not be localized in space, the source of the
calls could not be positively concluded. Neither of the two calls described here
resembled the boing, which is the only call type previously described for North
Pacific minke whales (Rankin and Barlow 2005). However, both call types are similar
Table 2. Summary of acoustic parameters describing the two call types attributed to minke whales
in Cormorant Channel, British Columbia, including call subtypes for pulse chains. All statistics are
reported as mean ± standard deviation. Subscripts denote significant difference between call
subtypes for the indicated parameter.
Call type Call subtype
Sample
size
Low frequency
(Hz)
High frequency
(Hz)
Duration
(ms)
Peak frequency
(Hz)
Pulse chain Decreasing pulse rate 39 323 ± 146 1142 ± 233 885 ± 275
a
673 ± 127
Constant pulse rate 24 334 ± 214 1249 ± 610 704 ± 281
b
915 ± 508
Increasing pulse rate 7 331 ± 180 1401 ± 760 831 ± 224
ab
1070 ± 691
Downsweep –4 38 ± 11 142 ± 69 648 ± 152 105 ± 36
BIOACOUSTICS 7
to those found in other subspecies (Winn and Perkins 1976; Edds-Walton 2000;
Mellinger et al. 2000; Gedamke et al. 2001; Risch et al. 2013). An additional two call
types, heard on only one occasion each, are undescribed for minke whales elsewhere.
Because of the small sample size of these two call types and our inability to localize
sound sources, we can only infer that these signals were produced by minke whales
until further investigations can confirm that this species is the true source of these
signals (Supplemental Materials 1).
The downsweep vocalizations observed in Cormorant Channel are similar in frequency
range and frequency transition to those described for common minke whales in the
St. Lawrence estuary, Canada and the Great Barrier Reef, Australia by Edds-Walton
(2000) and Gedamke et al. (2001), respectively. However, while St. Lawrence estuary and
Great Barrier Reef minke whales produced downsweeps that were 0.2–0.4 s in duration, the
mean duration of the downsweeps observed in this study was 0.7 s (Table 2). One down-
sweep was heard within 5 min of a surface feeding event (Table 1). Edds-Walton (2000)
reported few observations of feeding behaviour, but proposed that downsweeps may be
used to maintain contact between individuals while feeding on patchy prey. However,
Gedamke et al. (2001) only recorded downsweeps during the suspected calving and
breeding season, when minke whales were not observed or expected to be feeding.
The pulse chains observed in Cormorant Channel were broadband extending from
~300 to 1400 Hz whereas the pulse trains described in the Atlantic by Mellinger et al.
(2000) and Risch et al. (2013) were lower in frequency and narrowband, only extending
from ~100 to 400 Hz. However, pulse chains have similar pulse durations and inter-
pulse intervals to the ‘zip-like’calls described by Winn and Perkins (1976) in the
Atlantic, which are short pulse bursts extending up to 14,000 Hz with clicks approxi-
mately 50 ms apart. In this study, we further classify pulse chains into three categories
based on the way that pulse trains have been described elsewhere (Mellinger et al. 2000;
Risch et al. 2013,2014). In the North Atlantic, these pulse trains were produced in
Figure 3. Spectrogram showing the downsweep candidate call type (2500-point fast Fourier trans-
form, Hann window with 50% overlap). Calls were recorded in Cormorant Channel, British Columbia,
Canada, during four occasions when minke whales were visually observed.
8K. NIKOLICH AND J. R. TOWERS
a stereotyped series of call types resembling song and were associated more with
travelling or socializing than feeding (Mellinger et al. 2000; Risch et al. 2014). This
call type may be correlated with the age or sex of the caller (Risch et al. 2014) and has
been proposed to serve a reproductive function (Risch et al. 2013) or to maintain group
spacing (Risch et al. 2014). The pulse chains that we observed were produced by single
animals and none were recorded when more than one minke whale was observed
within 3 reticles of shore, suggesting that they may have been made to communicate
with other whales throughout or beyond the study area. One of the individuals
documented emitting a pulse chain is a known female. However, considering that
summer is thought to be a time when these whales are focused primarily on feeding
(Towers et al. 2013;In review), pulse chains were not likely made in relation to
reproductive behaviour.
Very few calls were identified using manual analysis even though minke whales were
observed during 224 of 565 h of effort during the study period (presence per unit effort:
44% –see Towers et al. In review). The calling rate in Cormorant Channel (2.79
calls*hour
−1
or 0.030 calls*min
−1
) was much lower than that observed for individual
minke whales in a migration corridor in the North Atlantic (average 48.6 calls*hour
−1
;
Risch et al. 2014), and 2 orders of magnitude less than peak rates reported in another
summer feeding ground in the St. Lawrence estuary in eastern Canada (up to 3.6
calls*min
−1
; Edds-Walton 2000). Minke whales in Cormorant Channel are also much
less vocal in this summer feeding ground than they are in their putative breeding
grounds in the tropics (Oswald et al. 2011; Martin et al. 2013). The relative vocal silence
in this area may be explained by several factors discussed below.
Mammal-eating Bigg’s killer whales are relatively common and widespread in coastal
waters of BC (Ford et al. 2013), but during the course of this study the presence per unit
effort of Bigg’s killer whales was only 2% (Towers et al. In review). However, even in
areas where killer whale density is low, this species is known to inhibit the vocal activity
of their prey (Rankin et al. 2012). We therefore suggest that minke whales may remain
relatively quiet in Cormorant Channel due to predation risk, especially considering
killer whales have been observed chasing minke whales in Cormorant Channel and
attacking and killing them in other nearby waterways (Ford et al. 2005).
The relatively low calling rates for minke whales in Cormorant Channel may also be
in part due to sexual segregation. Male and female minke whales are known to have
little overlap in some feeding grounds (Laidre et al. 2009) and to date, all eight minke
whales stranded in coastal waters of BC and Washington for which sex was recorded
were female, whereas the only two minke whales killed in offshore waters of BC for
which sex were recorded were male (Ford 2014). Additionally, two of the most
commonly observed individuals during this study were confirmed to be female through
analysis of DNA acquired in biopsy samples and no boings were recorded. Only male
minke whales are suspected to produce boings (Martin et al. 2013), similar to the vocal
behaviour of males of other baleen whale species (e.g. humpback, fin, blue) advertising
reproductive potential (Tyack 1981; Croll et al. 2002; Oleson et al. 2007). If so, it is
speculative but possible that the relative quietude of female minke whales may help
enable them to exploit prey in near-shore habitat despite the increased risk of predation
by killer whales, whereas males may forage further offshore.
BIOACOUSTICS 9
Cormorant Channel experiences high levels of vessel trafficduringthesummer
from small recreational and commercial vessels all of which produce underwater
noise in the frequency band occupied by previously described minke whale calls
(Edds-Walton 2000; Mellinger et al. 2000;Rischetal.2014). Therefore, it is likely
that acoustic masking is taking place. Other sources of masking noise include
surface disturbance, flow noise and bottom movement produced by strong current,
and the low-frequency vocalizations of other marine animals such as harbour seals,
fish, and invertebrates (Hanggi and Schusterman 1994;Širovićand Demer 2009;
Richardson et al. 2013;Staatermanetal.2013).Whilewedidnotseeaquantifiable
effect of ambient noise on the number of calls observed at any given time, there
were no recordings analysed during which the ambient soundscape was completely
free of vessel noise. While it is unlikely that acoustic masking prevented the
detection of all or most minke whale vocalizations, it is possible that a small
percentage of calls were not identifiable as such because the source level of the
calls was low, leading to attenuation at short distances, or because transmission of
the calls was hindered by masking noise. Other environmental factors such as
temperature or mineral content can also affect propagation; however, these condi-
tions were unlikely to fluctuate appreciably throughout the recording period.
Although source levels for minke whale pulse trains in the Atlantic (Risch et al.
2014) were higher than those reported for humpback whale pulse trains in Alaska
(Fournet et al. 2018), it is also possible that minke whales lower the source level or
rate of calling in areas with a high density of predators.
This report provides the first evidence that minke whales produce vocalizations while
on feeding grounds in the eastern North Pacific, albeit at a lower rate than has been
documented for other subspecies in other regions. We also introduce novel call types,
which have not been described for this species, suggesting that some vocalizations of
minke whales in the North Pacific may be distinct from calls produced by other
subspecies on feeding grounds. The vocal behaviour presented in this paper will provide
acousticians with reference material when analysing new and existing acoustic datasets
from the North Pacific. This is especially important because minke whales can often be
difficult to track visually (Norris et al. 2017), and therefore typically go undetected
during environmental impact assessments used to inform development projects. We
hope that our findings will provoke further study leading to a better understanding of
the acoustic ecology of minke whales in this region.
Acknowledgements
We thank Christie McMillan, Rebecca Piercey, Debra Hughes, Josephine Mrozewski, Leticiaá Legat,
Bart Willis, Angelica Rose, Nicole Borowczak, Ivan Ng, Joe Ng and Samuel Ng for their dedicated
field survey efforts to visually verify species and thus, enable ground-truthing of the acoustic data
presented in this paper. We also thank Timothy Frasier at Saint Mary’s University for confirming sex
of biopsied minke whales, Jackie Hildering and Andy Hanke for their underwater assistance
recovering the recording device, Barb Koot for preliminary acoustic analysis, and Francis Juanes
for reviewing a draft of this manuscript. This research was supported in part by the North Island
Marine Mammal Stewardship Association (NIMMSA) and Mountain Equipment Coop (MEC).
Biopsy samples of minke whales were collected under Canadian marine mammal research license
MML-01.
10 K. NIKOLICH AND J. R. TOWERS
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was supported by the Mountain Equipment Co-op [12–9145]; North Island Marine
Mammal Stewardship Association [2018–05].
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12 K. NIKOLICH AND J. R. TOWERS
