J-STAGE Advance published date: October 8, 2021

ACOUSTIC MONITORING OF LARGE WHALES IN JAPANESE WATERSA SHORT REVIEW OF ACOUSTIC MONITORING OF
LARGE WHALES IN JAPANESE WATERS
Tomonari AKAMATSU
Ocean Policy Research Institute, the Sasakawa Peace Foundation,
1–15–16 Toranomon, Minato-ku, Tokyo 105–8524, Japan
Corresponding author: t-akamatsu@spf.or.jp

Abstract
Passive acoustic monitoring studies of large whales in the Japanese EEZ are reviewed inthis paper. Submarine cables to monitor seismic events have been installed on the PaciﬁcOcean side of Japan because of the risk of earthquake strikes. Low frequency ﬁn whale Balaenoptera physalus calls have been detected mostly in winter time. Many ﬁn whale calls wereobserved in the waters oﬀ Kushiro in the northern part of Japan. A scientiﬁc submarine cablein Sagami Bay, 100 km west of Tokyo, revealed frequent presence of sperm whales Physeter
macrocephalus year around. In the Ogasawara and Okinawa archipelagos, known as breeding
grounds of humpback whales Megaptera novaeangliae, songs were recorded using stationed
or boat-based recordings.
Key words:

passive acoustic monitoring, ﬁn whale, sperm whale, humpback whale, underwater sound.

Introduction
As is well known, baleen whales produce low frequency sounds during the breeding season. For
example, blue whales produce frequency modulated sounds as low as 15.3 Hz (Staﬀord et al., 2001),
which is similar to the frequency range of earthquake vibrations. In the early 90’s, underwater sounddata received by military submarine cables was made available for scientiﬁc purposes to monitor baleen whales passively. Earlier, Dr. Ohsumi suggested application of passive acoustic monitoring fora survey of baleen whales (Ohsumi, 1994), referring to SOSUS and SURTASS systems (Nishimuraand Conlon, 1994). Passive acoustic monitoring is the method of observing animals by receiving their
vocalizations. Especially in the ocean, passive acoustic monitoring is a powerful method to identify
presence, location, and movement of phonating animals in the water.
However, the passive acoustic method was not widely used in Japanese waters until recent years.
Most of the Japanese studies are not accessible from outside of Japan or not yet published. As thosestudies are not well known internationally, there is the mistaken impression of a “vacuum zone” ofpassive acoustic monitoring of large whales in the western Paciﬁc Ocean. This paper brieﬂy introducespassive acoustic monitoring studies of large whales in Japanese waters. In contrast, passive acoustic
monitoring of small odontocetes has been extensively conducted in Japanese and Asian waters. Manyof these scientiﬁc papers appeared in international journals, which can be found by searching “A-tag”
and “porpoise”:
https://scholar.google.com/scholar?hl=0%2C5&q=A-tag+porpoise.

Review
Japan is located at the edge of the Paciﬁc Rim and has suﬀered severe earthquake strikes many
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Fig. 1. Location of submarine cables in the Japanese EEZ and two breeding grounds of humpback whales(Okinawa and Ogasawara). Hatsushima cable is for scientiﬁc research purposes. The other four cables(Kushiro-Tokachi, DONET1, DONET2 and S-net) are deployed to observe seismic events nearby the trenchshafts. The S-net shown in dotted line has seismometers, which provides water acceleration associated withlow frequency sounds; no hydrophone data is available.

times. To monitor seismic events, submarine cables have been installed in the Japanese EEZ (exclusive economic zone), as shown in Fig. 1. For example, the Kushiro-Tokachi cable oﬀ Hokkaido wasdeployed in 1999 (Hirata et al., 2002). S-net is the newly installed 5,500 km submarine cable in operation since 2016, which covers the eastern side of Honshu Island from oﬀ Hokkaido, Aomori, Sanriku
and the Boso Peninsula (https://www.seaﬂoor.bosai.go.jp/S-net/). The installation of DONET1 (DenseOcean Floor Network System for Earthquakes and Tsunamis) observational equipment on 20 stationsat Kumano-Nada was completed in 2011 (https://www.jamstec.go.jp/donet/e/cable/). In addition,
DONET2 was deployed oﬀ Cape Muroto. DONET systems are currently operated by the National Research Institute for Earth Science and Disaster Resilience (https://www.seaﬂoor.bosai.go.jp/). Hatsushima cable in Sagami Bay has recorded a human audible range of underwater sounds since 1993
(Iwase et al., 2001). These cable systems have hydrophones or seismometers to record underwatersound pressure or water acceleration.
Passive acoustic monitoring of ﬁn whales Balaenoptera physalus
Low frequency calls of ﬁn whales were detected in winter time oﬀ Hokkaido by the Kushiro-Tokachi
cable (Matsuo et al., 2017). Four stations (DSO, OBS1, OBS2, OBS3) with hydrophones are locatedbetween 41°40.050′N–42°15.168′N, 144°20.454′E–145°3.372′E and 2,124 m–3,428 m in depth. Overa total of 2,981 recording days, 2,865,403 calls were identiﬁed by an automatic detector designed forthis study. Clear seasonal diﬀerences were observed, being high during October and February, and low
in the summertime. The number of calls received in a month showed a double peak structure in Octoberand January. Call detection did not show a diurnal pattern. Nishida et al. (2018) developed a web system to visualize detections of ﬁn whale calls. Yoshida (2017) independently analyzed Kushiro-Tokachi
cable data and reached a similar conclusion of the seasonal presence of ﬁn whales.
Ocean bottom seismometer data of S-net well supported the above results. Using 150 points ofS-net observatory data, Nakamura (2020) documented the frequent call detections during October to298

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ACOUSTIC MONITORING OF LARGE WHALES IN JAPANESE WATERSApril. In areas oﬀ Kushiro and Aomori, the number of detections were high during December to January. After January, the area of the highest reception shifted to the south-west, in waters oﬀ Aomori andSanriku. In addition, calls were detected on the shore side of the trench shaft, approximately 200 kmoﬀ Honshu Island. The seismometers in the southern part of S-net located oﬀ Boso Peninsula received
ﬁn whale calls in winter time, although the number of received calls was far smaller compared withthose observed oﬀ Kushiro.
According to the DONET1 data, no ﬁn whale calls were identiﬁed year round (Ikuo Matsuo, pers.
comm.). The location of DONET1 is approximately 400 km southwest of Boso Peninsula. This area
might not be a breeding ground of ﬁn whales. For DONET2, data analysis of baleen whale sounds has
not been conducted.
Passive acoustic monitoring of sperm whales Physeter macrocephalus
Hatsushima Island Cabled Observatory (Hatsushima cable) is located on the seaﬂoor at a depth of
1,175 m about 7 km southeast oﬀ Hatsushima Island in the western part of Sagami Bay, which is approximately 90 km southwest of Tokyo. Iwase (2012) retrieved audio sounds recorded from the soundtrackof videotape, which was received by the hydrophone of this observatory. Sperm whale clicks including
creaks (series of very rapid clicks, inter-click intervals less than about 0.2 second) were observed. Iwase(2012) reported that no seasonality of the detections of sperm whale clicks was observed during the1994–2012 period. The data suggested that sperm whales appeared around Hatsushima cable all year.
Amano et al. (2014) recorded sperm whale sounds by towed or suspended hydrophone oﬀ theKumano-Nada coast and Ogasawara waters. Statistical diﬀerences in the number of clicks andinter-click intervals were identiﬁed. They suggested diﬀerent vocal clans in the two study sites.
Passive acoustic monitoring of humpback whales Megaptera novaeangliae
Since Payne and McVay (1971) reported the songs of humpback whales, song structure of baleen
whales produced in the breeding season is one of the major subjects of underwater bioacoustics.
Maeda’s (2002) doctoral dissertation at Nagasaki University gives a systematic description of the songstructure of humpback whales in Okinawa waters. Using boat-based recordings during 1991–1998,
unit structure and unit sequence in a phrase were analyzed. Individual variations and year-by-yearchange of song structure were reported. In addition, Maeda (2002) compared song recordings between
Okinawa and Ogasawara waters during 1992–1995 and found no signiﬁcant diﬀerence in song structures of humpback whales. As Darling and Mori (1993) reported, the same humpback whales wereidentiﬁed in both waters, consistent with the comparisons of acoustic features.
In recent years, singing behavior and the location of humpback whales in Ogasawara waters wereexamined to assess the eﬀect of ship noise (Tsujii et al., 2018). They installed two ﬁxed underwater recorders separated by 3.0 km. Recorders were exchanged every two or three weeks before their batteries
ran out. Each recorder had a stereo hydrophone. This monitoring method enabled localization of singers by triangulation from the two locations, simultaneously with the recording of song structure. Fewerwhales sang nearby, within 500 m, of the shipping lane. Humpback whales reduced sound production
after a ship passed, when the minimum distance to the whale from the ship trajectory was 1,200 m.

Discussion
Passive acoustic monitoring is a powerful method to observe presence, location, and movementsof cetaceans. Even density can be estimated when phonating intervals, directionality, and source levelare available. In Japanese waters, several already-existing submarine cables are available. Most of thedata is available for research purposes judged to be conducive to the management of baleen whales in
the Japanese EEZ. To accelerate passive acoustic studies of large whales, several future directions canCetacean Popul. Stud. (CPOPS), Review
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be suggested.
1. Encourage open data policy of underwater sounds. Data classiﬁcation and masking will be neededto avoid conﬂict with national security in the low frequency range. For high frequency sounds ofsperm whales and other large odontocetes, this constraint will be relaxed.
2. For automatic detection and classiﬁcation of species or families, collections of species’ annotatedsounds are required to ensure acceptable performance of machine learning applications.
3. Improvement of density estimation models will serve for abundance estimation of oﬀshore baleen
whale populations. In the winter time when survey vessels are diﬃcult to operate, the area of breeding
grounds of large baleen whales will be an appropriate target for passive acoustic monitoring studies.

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Received: January 31, 2021
Accepted: May 24, 2021
Published online: October 8, 2021

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