Minke whale (Balaenoptera acutorostrata) boings detected at theStation ALOHA Cabled Observatory

Julie N. Oswalda,b) and Whitlow W. L. AuHawaii Institute of Marine Biology, University of Hawaii, P.O. Box 1106, Kailua 96374, Hawaii

Fred DuennebierDepartment of Geology and Geophysics, School of Ocean and Earth Science and Technology, University ofHawaii, Room 701, 1680 East-West Road, Honolulu 96822, Hawaii

(Received 23 July 2010; revised 14 March 2011; accepted 15 March 2011)

Minke whales (Balaenoptera acutorostrata) in the tropical North Pacific are elusive and difficult to

detect visually. The recent association of a unique sound called the “boing” to North Pacific minke

whales has made it possible to use passive acoustics to investigate the occurrence of this species in

Hawaiian waters. One year of recordings (17 February 2007–18 February 2008) made at the Station

ALOHA Cabled Observatory were examined to investigate the characteristics of boings and temporal

patterns in their occurrence at this site, located 100 km north of Oahu. Characteristics of boings

exhibited low variability. Pulse repetition rate and duration measurements matched those for “central”

or “Hawaii” boing types. Boings were detected from October until May, with a peak in March.

Although no boings were detected from June to September, the absence of boings does not necessar-

ily indicate the absence of minke whales. Significant diel variation in boing rate was not observed.

The absence of a diel pattern in boing production suggests that day- or night-time acoustic surveys

are equally acceptable methods for studying minke whale occurrence. Future research should include

efforts to determine what other sounds are produced by minke whales in this area, and which age/sex

classes produce boings. VC 2011 Acoustical Society of America. [DOI: 10.1121/1.3575555]

PACS number(s): 43.80.Ka, 43.30.Sf [JFF] Pages: 3353–3360

I. INTRODUCTION

North Pacific minke whales (Balaenoptera acutoros-trata) are among the smallest of the baleen whales. They are

typically encountered individually or in small groups of two

or three (although large aggregations occasionally form in

high latitudes). Their blows are inconspicuous and they sur-

face for only short periods of time (Perrin and Brownell,

2002). Because of these characteristics, minke whales are

notoriously difficult to detect using visual methods and the

species has been considered rare in Hawaiian waters (Hor-

wood, 1990). A cabled, ocean bottom observatory located at

Station ALOHA (A Long-term Oligotrophic Habitat Assess-

ment) provided an opportunity to study the occurrence of

minke whales at a deep ocean research site located 100 km

north of Oahu, Hawaii.

Because minke whales are so difficult to observe visu-

ally, other methods must be used to study their distribution,

behavior, and ecology. Most species of baleen whales pro-

duce relatively low frequency sounds that propagate well

underwater. As a result, passive acoustic methods have been

used extensively to study many baleen whales, such as blue,

humpback, fin, right, and bowhead whales (e.g., Clark, 1982;

McDonald et al., 1995; Norris et al., 1999; Stafford et al.,2005; Delarue et al., 2009). Passive acoustic studies have

revealed much about the biology of baleen whales, including

distribution patterns, migration routes, population structure,

and seasonal and diel behavioral patterns (e.g., Clapham and

Matilla, 1990; Stafford et al., 1999; Burtenshaw et al., 2004;

Stafford et al., 2005; Oleson et al., 2007a).

Minke whales have been reported to produce a variety

of sounds throughout their range. Low frequency down-

sweeps, higher frequency clicks, whistles, grunts, and pulse

trains have been recorded in the presence of minke whales in

the St. Lawrence Estuary (Edds-Walton, 2000) and the Ca-

ribbean (Winn and Perkins, 1976; Mellinger et al., 2000) as

well as in the presence of Antarctic minke whales (Balae-noptera bonaerensis) in the Ross Sea (Schevill and Watkins,

1972; Leatherwood et al., 1981). Dwarf minke whales on the

Great Barrier Reef, Australia produce a distinctive sound

that has been dubbed the “star wars” vocalization due to its

unusual, synthetic-sounding characteristics (Gedamke et al.,2001).

In contrast to Atlantic and Antarctic minke whales,

almost nothing was known about the sounds produced by

North Pacific minke whales until quite recently. Based on si-

multaneous visual observations and acoustic localization,

Rankin and Barlow (2005) were able to attribute the mysteri-

ous “boing” sound to this species. Boings are relatively ster-

eotyped calls that consist of a brief pulse followed by a long

frequency and amplitude modulated call with a pulsed struc-

ture and a peak energy of approximately 1.4 kHz (Thompson

and Friedl, 1982; Rankin and Barlow, 2005; Figs. 1 and 2).

The structure of boings exhibits geographic variation, and

two different boing types have been reported in the

a)Current address: Oceanwide Science Institute, P.O. Box 61692, Honolulu

96839, Hawaii.b)Author to whom correspondence should be addressed. Electronic mail:

oswald.jn@gmail.com

J. Acoust. Soc. Am. 129 (5), May 2011 VC 2011 Acoustical Society of America 33530001-4966/2011/129(5)/3353/8/$30.00

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literature. The “eastern” (or “San Diego”) boing has a pulse

repetition rate of 91–93 pulses/s, a mean duration of 3.6 s,

and has been detected east of 138�W. The “central” (or

“Hawaii”) boing has a pulse repetition rate of 114–118

pulses/s, a mean duration of 2.6 s, and has been detected

west of 135�W (Wentz, 1964; Rankin and Barlow, 2005).

The boing was first described from recordings made by U.S.

navy submarines off San Diego, California and Kaneohe,

Hawaii (Wenz, 1964) and is a common sound in the North

Pacific Ocean. They have been reported to occur seasonally

in Hawaiian waters (Thompson and Friedl, 1982).

Attributing the source of the boing to North Pacific

minke whales has finally given researchers a way to study

the occurrence, behavior, and ecology of this elusive species.

Using acoustic recordings made at the Station ALOHA

Cabled Observatory (ACO), we examined the characteristics

of minke whale boings in the waters north of Oahu, as well

as seasonal and diel patterns in their occurrence.

II. MATERIALS AND METHODS

Recordings were made using a seafloor-mounted hydro-

phone located at the ACO, 100 km north of Oahu, Hawaii

(22�450N, 158�000W, Fig. 3). A retired electro-optical

telecommunications cable (HAW-4 SL-280 electro-optical

cable) provided power and broadband Ethernet communica-

tions capability to the ACO, allowing real-time, continuous

acoustic monitoring. The factory calibrated hydrophone

[OAS (Optimum Applied System, Poughkeepsie, NY)

Model E-2PD] used at the ACO was originally installed for

FIG. 1. Spectrogram of a minke

whale boing (1024 point FFT, Hann

window). “Precursor” component is

indicated by an arrow.

FIG. 2. (Color online) Waveform of

a boing. The pulsed structure of the

amplitude modulated component

can be seen in the expanded wave-

form (inset).

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seismic work and had a flat frequency response from 10 Hz

to 1.3 kHz. Oscillations in the frequency response above

1.5 kHz were caused by the sound wavelength approaching

the dimensions of the sensor. These oscillations had a maxi-

mum amplitude of 65 dB. The hydrophone sampled up to

96 kHz with 24-bit resolution, resolving signals ranging

from 0.01 Hz to 40 kHz. It was located at a depth of 4.7 km

and floated approximately 10 m off the seafloor (Duennebier

et al., 2008). Data from the ACO were transmitted to the

AT&T Makaha Cable Station on Oahu and then over a T-1

datalink to the University of Hawaii, where they were

archived and analyzed. The original data were downsampled

to 24 kHz for storage as archived data. Archived data were

stored at a lower sampling rate in order to conserve storage

disk space at the University.

The ACO hydrophone was operational from 17 February

2007 until 22 October 2008. Recordings were continuously

made during this 20 month period, with small breaks due to

equipment problems. We examined 1 yr of these recordings

(17 February 2007–18 February 2008) using a data template

detector created with XBAT (Extensible Bioacoustic Tool)

software. XBAT’s data template detector is a spectrogram cor-

relation detector. It looks at the time cross-correlation

sequence between an example sound [in this case, both a

high signal-to-noise ratio (SNR) boing and a medium SNR

boing were used as example sounds] and the sound file being

analyzed. Events are detected when the correlation exceeds a

user-defined threshold. This detector was used to automati-

cally detect and log minke whale boings. The detector was

ground-truthed using 8 h of data recorded on 5th March

2007. An experienced acoustician manually identified boings

based on listening to the recording and examining a spectro-

gram, and ranked each boing as one of the five quality cate-

gories. The five quality categories ranged from 1 (audible,

but barely recognizable as a boing on the spectrogram) to 5

(very loud and clear boing). Low quality category one and

two boings had SNRs of approximately 6–12 dB above am-

bient noise in the 1–5 kHz frequency range. The results of

the manual detections were then compared to results of

detections made using XBAT on the same section of data.

A total of 783 boings were manually identified in the 8-

h recording that was used for ground-truthing the detector.

The automated detector identified 100% of category 5 boings

(n¼ 49), 99% of category 4 boings (n¼ 78), 91% of category

3 boings (n¼ 150), 59% of category 2 boings (n¼ 259), and

22% of category 1 boings (n¼ 247). Only 5% of detections

made by the XBAT detector were false detections.

Measurements were taken from a subset of boings ran-

domly selected from the year of data. In order to avoid sam-

pling multiple boings from a single individual and therefore

maintain independence of data, one boing was randomly

selected from each 24 h period during which boings were

detected and only boings that were at least 6 h apart were

measured. Measurements were made using OSPREY, an auto-

mated measurement software package (Mellinger and Brad-

bury, 2007). Boings were divided into two components. The

“precursor” component is an initial pulse that precedes the

longer, “amplitude modulated” (AM) component (Fig. 1).

The AM component has a pulsed structure, as seen in the

waveform in Fig. 2. Several measurements were taken from

each component, including minimum and maximum fre-

quency, peak frequency (the frequency at which peak inten-

sity occurs), and duration. In addition, several measurements

were taken only from the AM component: SNR of the first

0.5 s, SNR of the last 0.5 s, and pulse repetition rate. Pulse

repetition rate was measured using a custom written add-on

to OSPREY (S. Martin, SPAWAR).

To examine seasonal patterns in boing detection, the

mean number of boings detected per hour was plotted for

each month. Mean number of boings per hour (hourly boing

rate) was examined instead of absolute number of boings

detected in a day because, due to technical issues, some days

contained fewer than 24 h of recordings.

For analyses of diel patterns in boing occurrence, only

days during which calls were detected and days with less than

half an hour missing from the recordings were included. Each

day was divided into three light regimes defined by the alti-

tude of the sun (obtained from the United States Naval Ob-

servatory Astronomical Applications Department web site):

(1) Light: The hours when the altitude of the sun was greater

than 0� above the horizon, from approximately 06:55 to

18:15.

(2) Dusk: The hours when the altitude of the sun was

between 0� and –12� below the horizon, from approxi-

mately 06:00 to 06:55 and 18:15 to 19:09.

(3) Dark: The hours when the altitude of the sun was less

than –12� below the horizon, from approximately 19:09

to 06:00.

Mean adjusted boing rate (MABR) was then calculated

for each light regime in a given day. Mean adjusted boing

FIG. 3. Map showing the island of Oahu and the location of the Station

ALOHA Cabled Observatory (represented by the white star).

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rate was based on the mean hourly boing rate for each 24 h

day (BR24) and the mean hourly boing rate during each light

regime (BRlight, BRdusk, and BRdark) for that day. For

example:

MABRlight¼ BRlight � BR24:

A negative value for MABR means that the mean hourly

boing rate during that part of the day was less than the mean

hourly boing rate for that 24 h period and a positive value

means that the hourly boing rate during that part of the day

was greater than the mean hourly boing rate for that 24 h pe-

riod. This approach was taken to remove bias caused by the

large variation in the total number of calls detected each day

throughout the year (Stafford et al., 2005; Wiggins et al.,2005; Oleson et al., 2007a). Because MABR data were not

normally distributed, Kruskal–Wallis tests were used to test

the null hypothesis that the difference in boing rates among

light regimes was equal to zero.

III. RESULTS

Measurements were taken from a total of 111 boings and

are presented in Table I. Frequency measurements for both the

precursor and the AM components of boings exhibited a very

low variability. The fundamental frequencies of boings ranged

from 1 to 1.8 kHz, but harmonics extending to approximately

9 kHz were often observed. Duration exhibited the highest var-

iability, especially for the AM component, ranging from 1.4 to

4.2 s. Overall duration (precursor duration plus AM duration)

had a mean value of 2.5 s (SD¼ 0.4 s) and ranged from 1.6 to

4.5 s. SNR decreased an average of 5.3 6 2.2 dB from the

beginning to the end of the AM component.

Boings were commonly heard during the winter and

spring, with a total of 15 552 boings detected between

February 2007 and February 2008. Figure 4 shows the mean

number of boings detected per hour for each month. Boings

were detected from 22 October to 21 May and not at all dur-

ing the months of June to September. The occurrence of

boings increased quite suddenly in November, peaked in

March, and dropped off quickly after that. High variability

in hourly boing rate was observed from day to day, as shown

by the example in Fig. 5. Mean boing rate ranged from 0 to

62 boings/h during the month of January 2008 and large dif-

ferences were seen from 1 day to the next.

Figure 6 shows mean hourly boing rate for each hour of

the day, averaged over the year (n¼ 154 days). Hourly boing

rate was highest in the early morning and the middle of the

day, with a dip in the mid-morning and late evening. Mean

adjusted boing rates reflected this pattern, with higher values

during light hours (mean¼ 0.10, SD¼ 6.9, n¼ 154) than

during dark (mean¼ 0.02, SD¼ 6.4, n¼ 154) or dusk

(mean¼ –0.19, SD¼ 5.2, n¼ 154) hours. However, these

differences in MABR were not significant (Kruskal–Wallis

test, n¼ 154, df¼ 2, H¼ 20.8, p< 0.0001).

IV. DISCUSSION

A. Ground-truthing

Boings are relatively stereotyped calls that are well

suited for detection using techniques such as spectrogram

correlation. XBAT’s spectrogram correlation detector identi-

fied a very high percentage of high quality boings in the 8 h

of recordings used for ground-truthing. Most of the boings

that were missed by the detector were low quality, category

one and two boings. These boings had poor SNR and only a

portion of the boing was visible on the spectrogram.

Thompson and Friedl (1982) recorded boings using two

bottom-mounted hydrophones located north of Oahu and

used time-of-arrival differences to estimate that boings could

TABLE I. Mean (with standard deviation in parentheses), minimum and

maximum values for measurements taken from the precursor and amplitude

modulated (AM) components of boings.

Variable Precursor

component

AM

component

Peak frequency (kHz) Mean 1.4 (0.09) 1.4 (0.02)

Minimum 1.2 1.4

Maximum 1.6 1.5

Minimum frequency (kHz) Mean 1.2 (0.07) 1.3 (0.04)

Minimum 1.0 1.2

Maximum 1.3 1.4

Maximum frequency (kHz) Mean 1.6 (0.08) 1.6 (0.07)

Minimum 1.4 1.4

Maximum 1.8 1.9

Duration (s) Mean 0.28 (0.05) 2.2 (0.4)

Minimum 0.15 1.4

Maximum 1.6 4.2

Pulse repetition rate (pulses/s) Mean n/a 116.3 (0.8)

Minimum n/a 111.7

Maximum n/a 117.9

SNR of first 0.5 s (dB) Mean n/a 16.7 (2.1)

Minimum n/a 11.6

Maximum n/a 22.6

SNR of last 0.5 s (dB) Mean n/a 11.4 (2.3)

Minimum n/a 7.8

Maximum n/a 18.2

FIG. 4. Mean and standard deviation of hourly boing rate by month from 17

February 2007 to 18 February 2008. Number of days during which record-

ings were made for each month shown as solid line and triangles. Number

of days during which boings were detected for each month shown as dashed

line and squares.

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be detected at a distance of at least 10.5 km. Based on this

work and the fact that there was little ambient noise masking

boings in the ground-truth data, it is likely that our category

one and two boings were produced by animals that were a

significant distance from the hydrophone. As the goal of this

study was to examine the occurrence patterns of boings, we

felt that it was more important to consistently detect high-to-

medium quality boings produced relatively close to the

hydrophone with relatively few false detections, rather than

to increase the incidence of false detections in order to detect

every boing within a larger area. This approach provides a

good basis for relative comparisons among months, days,

and time periods within days. For other questions such

as estimating absolute numbers of animals or calling rates,

it would be more important to detect every boing. To

accomplish this, a lower threshold could be set for boing

detection, but this would lead to a greater number of false

positive detections.

The false positive (false alarm) rate for this study was

low. Most of the false positives were triggered by “moans”

produced by humpback whales, which are most likely part of

their songs. These sounds are low frequency, broadband, and

have relatively long durations, similar to boings. The

ground-truth data were taken at the height of the humpback

whale wintering season in Hawaii when noise from hump-

back whales was loud and relatively constant throughout the

8 h that were analyzed. It is expected that the false positive

rate would be even lower than 5% during days when there

FIG. 5. Mean and standard devia-

tion of hourly boing rate by day for

the month of January 2008.

FIG. 6. Mean and standard devia-

tion of number of boings per hour of

the day, averaged from 17 February

2007 to 18 February 2008 and

including only days during which

boings were detected and with less

than half an hour missing from the

recordings (n¼ 154 days). The x-

axis starts at midnight and ends at

23:00. Bar above histogram indi-

cates light regime: black¼ night;

gray¼ dusk; and white¼ day.

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was less humpback whale song, especially near the begin-

ning and end of the humpback whale season when there are

fewer whales in the area.

B. Characteristics of boings

Measurements taken from the precursor and AM com-

ponents of boings illustrate the stereotyped nature of these

sounds. Variability of the frequency measurements was on

the order of tens of hertz. It is not likely that this low vari-

ability was a result of measuring multiple boings from a sin-

gle individual, as only boings that were at least 6 h apart

were measured. While it is possible that one whale stayed in

the area for 6 or more hours, measured boings were an aver-

age of 72 h apart, increasing the likelihood that boings were

measured from different individuals.

Pulse repetition rate (PRR) and duration measurements

of the AM component matched those reported for “Central”

or “Hawaii” boings. Boings recorded at the ACO had a PRR

ranging from 112 to 118 pulses/s and a mean overall dura-

tion of 2.5 s. Rankin and Barlow (2005) reported a PRR of

114–118 pulses/s and mean duration of 2.6 s for their central

boings, and Thompson and Friedl (1982) reported a mean

pulse repetition rate of 115 pulses/s for boings recorded at a

bottom-mounted hydrophone located north of Oahu. These

similar measurements also illustrate the stereotyped and sta-

ble nature of boings.

Duration of the AM component was the characteristic of

boings that exhibited the highest variability, ranging from a

minimum of 1.4 s to a maximum of 4.2 s (Table I). Some of

this variability may have been due to the fact that the SNR

of boings generally decreases from the start of the AM com-

ponent to the end of the AM component (see Table I).

Because boings appear to “fade out,” those produced at

greater distances may appear to be shorter than those pro-

duced at closer range to the hydrophone. Another source of

variability among boings is the presence of the precursor

section. As the aim of this study was to examine characteris-

tics of both the precursor and the AM components of boings,

only boings that contained both components were measured.

However, not every boing contains a precursor component.

The short duration of this component relative to the AM

component as well as the fact that is it not always present

suggests that it is less important to the function of boings

than the AM component.

C. Seasonal and daily variability

The seasonal trend in boings detected at the ACO was

similar to that found by Thompson and Friedl (1982).

Thompson and Friedl (1982) conducted a long-term study of

sounds produced by several species of whales recorded using

two bottom-mounted hydrophones located 11.6 km apart and

at a depth of approximately 800 m off Kahuku Point, Oahu

(McDonald and Fox, 1999). They reported that boings were

heard from November to April and were most abundant in

February (Thompson and Friedl, 1982). Similarly, boings

were heard at the ACO from October to May, with a peak in

March and a steady drop-off after that. There was no second-

ary peak observed by Thompson and Friedl (1982) or in the

ACO recordings. The fact that boings were heard for an

extended period of time with no secondary peak suggests that

Hawaiian waters are an end-point destination for minke

whales, rather than a transitional location. If minke whales

were simply passing through Hawaiian waters on their way to

some other destination, one would expect to see two peaks in

their occurrence—one as minkes passed through the area on

their way to their destination, and another as they returned.

It is unlikely that minke whales are traveling to Hawai-

ian waters primarily to feed, as the North Pacific subtropical

gyre is generally considered to be an oligotrophic environ-

ment (Karl and Lukas, 1996; Sakamoto et al., 2004). Hump-

back whales migrate to Hawaiian waters to breed during the

winter and spring (Baker and Herman, 1981) and it is possi-

ble that minke whales are behaving similarly, as the season-

ality in boing detections closely matches that of humpback

whale presence in Hawaiian waters. In addition, it is thought

that most baleen whales feed in productive, high-latitude

waters during the summer months and spend winters at low-

latitude breeding grounds (Lockyer and Brown, 1981;

Bowen and Siniff, 1999). Although no boings were heard

from June to September, it is important to note that the ab-

sence of boings does not necessarily indicate the absence of

minke whales. Our recordings were made using a single, bot-

tom-mounted hydrophone and it is possible that minke

whales were out of range of our hydrophone, but still in Ha-

waiian waters, during the summer months. In addition,

minke whales may be present in Hawaiian waters year-round

and only produce boings at certain times of the year, similar

to blue whales, which exhibit seasonal separation in the pro-

duction of their B calls and D calls in the Southern Califor-

nia Bight (Oleson et al., 2007a). A variety of sounds have

been attributed to minke whales in other locations, including

low frequency downsweeps, higher frequency clicks, whis-

tles, grunts, and others (Winn and Perkins, 1976; Edds-

Walton, 2000; Mellinger et al., 2000). Based on this knowl-

edge, it is plausible that minke whales in the tropical north

Pacific may be producing other sounds in addition to boings.

Currently, nothing is known about other call types that may

be produced by north Pacific minke whales. Efforts should

be made to make recordings in the presence of minke whales

for extended periods and to attach acoustic or other remote-

locating tags to minke whales in order to determine what

other sounds they may be producing. When the vocal behav-

ior of north Pacific minke whales is more completely known,

it will be possible to gain a much deeper understanding of

their true seasonal distribution patterns in the Pacific Ocean.

In addition to seasonal variability, there was also high

daily variability in the occurrence of boings. For example,

Fig. 5 shows that the mean hourly boing rate on 12th Janu-

ary, 2008 was 62 boings/h, while the very next day, mean

hourly boing rate decreased to zero boings per hour. This

high variability is likely related to the number of minke

whales in the area each day. Thompson and Friedl (1982)

reported that when one sound source (minke whale) was

present, it produced a boing approximately once for every 6

min, but if two or more whales were present, the boing pro-

duction increased dramatically to an average of once for ev-

ery 30 s. So, boing rate not only increases because of an

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increase in the number of animals producing boings, but also

because each animal is producing boings more frequently.

Thompson and Friedl’s (1982) observation that boing

rates increase when two or more minke whales are present in

an area suggests that boings may function to maintain spac-

ing among animals. This function has been suggested for

humpback whale song (Frankel et al., 1995) and for the “star

wars vocalization” produced by dwarf minke whales off the

coast of eastern Australia (Gedamke et al., 2003). The struc-

tural similarities between the boing and the star wars sounds,

and the close evolutionary relationship between northern

hemisphere minke whales and dwarf minke whales

(Gedamke et al., 2001) suggest that the star wars sound and

the boing may be used in similar ways. Future studies should

include recordings made using multiple hydrophones, acous-

tic tags, and/or focal follows using passive acoustics (e.g.,

towed arrays) to further investigate the relationship between

the number of, and spacing among, minke whales and boing

rate. This information is not only important for determining

the function of boings, but also when considering methods

for estimating abundance based on calling rates (see Mar-

ques et al., 2009).

D. Diel variation

There was no significant diel variation in mean adjusted

boing rates at the ACO. This suggests that towed array sur-

veys that take place during daylight hours are an appropriate

method for studying minke whales. Surveys conducted dur-

ing daylight hours will reasonably sample vocal behavior

and provide opportunities for visual observations, biopsy

sample collection, and tag applications. These additional

data will provide information on how frequently individual

minke whales produce boings, what behaviors boing produc-

tion is related to, and which age/sex classes are producing

boings. Knowing which age/sex classes produce boings is

particularly important when using acoustics to determine

distribution and abundance. For example, if boings are pro-

duced only by males, as has been shown for certain sounds

produced by other species (e.g., humpback whales, Tyack,

1981; fin whales, Croll et al., 2002; blue whales, Oleson etal., 2007b), then using boings as a proxy for animal occur-

rence would be possible for only a portion of the population

(i.e., males), and nothing could be said about the distribution

and abundance of females or other individuals that do not

produce boings.

E. Future research

The amount of information that can be gleaned from a

single, stationary hydrophone is limited. It is difficult to

localize animals with accuracy using a single hydrophone

and therefore we do not know how many animals were pres-

ent or the distance at which boings were detected. Plans are

in place to install two hydrophones at the ACO in 2011 and

therefore it will likely be possible to obtain this information

from future recordings. This will not only allow us to deter-

mine how many vocalizing minke whales are in the ACO

area at any given time, but it will also allow us to monitor

the movements of whales within the area and to take a closer

look at the relationship between the number of whales pres-

ent, their proximity to each other and boing rate.

In addition to providing information regarding the distri-

bution and abundance of minke whales in Hawaiian waters,

the study of boings can also provide insight into why minke

whales are present and producing boings in these waters dur-

ing the winter and spring. There are near-monthly cruises to

Station ALOHA as part of the Hawaii Ocean time-series

(HOT) program to measure a variety of physical, chemical,

and biological properties of the water column and provide a

comprehensive description of the ocean at this site (Karl and

Lukas, 1996). An examination of the relationships between

variables such as primary production, plankton community

structure, and temperature and the presence of minke whales

would help illuminate the ways in which minke whales may

be utilizing Hawaiian waters.

ACKNOWLEDGMENTS

The authors would like to extend their thanks to Jim

Jolly for all of his help with accessing ACO data files. We

are grateful to Sofie Van Parijs, Denise Risch, and Kate Staf-

ford for their tireless help with XBAT. Thank you also to

Charlotte Fabjanczyk, Shannon Coates, and Michael Oswald

for their assistance with data analysis and to Steve Martin

for providing the code for measuring pulse repetition rate.

We thank Tom Norris, Shannon Rankin, and Michael

Oswald for their insights and helpful suggestions on drafts of

this manuscript. Paul Johnson kindly produced the map in

Fig. 3. The ACO is funded by the National Science Founda-

tion. J.N.O. was supported by the Hunt Fellowship from the

Acoustical Society of America. This is HIMB contribution

No. 1346. This is SOEST contribution No. 8115.

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