東海大学海洋研究所報告第 36 号（2015）1 〜7 頁

原著論文（Original Article）

Bull Inst. Oceanic Res. & Develop. Tokai Univ.（2015）, 36, 1 ‑ 7設置型音響ブイによる御蔵島周辺海域に棲息するミナミハンドウ
イルカ（ Tursiops aduncus ）の夜間の分布について
─研報─

森阪 匡通 1,＊），酒井 麻衣 1,2 ），小木 万布 3 ）Detection of the Nighttime Distribution of Indo-Paciﬁc Bottlenose Dolphins (Tursiops aduncus) around Mikura Island with
Stationed Acoustic Buoys
Tadamichi Morisaka 1,＊）, Mai Sakai 1,2 ）and Kazunobu Kogi 3 ）Abstract
Knowledge for the distribution pattern of an animal species is fundamental for our understanding
and conservation of the animal. Since researchers cannot conduct visual surveys during night, the nighttime distribution pattern of the dolphins which can hunt prey in a dark condition is diﬃcult to study.
Here we describe the detection of the nighttime distribution of Indo-Paciﬁc bottlenose dolphins around
Mikura Island with custom-made stationed acoustic buoys.

Knowledge for the distribution pattern of anwith the use of boats, planes, and the other forms

animal species is fundamental for our understand-of transportation (e.g., Kogi et al. 2004; Karczmar-ing and conservation of the animal (e.g., Mota-Var-ski et al. 2005; Shirakihara et al. 2007). However,gas and Rojas-Soto 2012). Numerous studies havethese types of visual surveys cannot be conducted

revealed the finely detailed distribution patterns

in dark conditions, such as nighttime. Odontocetesof single species of odontocetes, or toothed whales,

search for and hunt prey during the night with1 ）東海大学創造科学技術研究機構（東海大学海洋研究所気付） 〒 424‑8610 静岡県静岡市清水区折戸 3‑20‑1
Tokai University Institute of Innovative Science and Technology, c/o Institute of Oceanic Research and Development,
Tokai University, 3‑20‑1, Orido, Shimizu-ku, Shizuoka-shi, Shizuoka 424‑8610, Japan2 ）日本学術振興会 〒 102‑0083 東京都千代田区麹町 5‑3‑1Japan Society for the Promotion of Science, 5‑3‑1, Kojimachi, Chiyoda-ku, Tokyo 102‑0083, Japan3 ）御蔵島観光協会 〒 100‑1301 東京都御蔵島村
Mikura Island Tourist Information Center, Mikurajima-mura, Tokyo 100‑1301, Japan＊）Correspnding author : chaka@tokai-u.jp（ 2015 年 2 月 6 日受付／ 2015 年 2 月 19 日受理）

Received, 2/6/2015, Accepted, 2/19/2015

2森阪 匡通

酒井 麻衣

their excellent echolocation abilities. In addition,
several odontocetes, including spinner dolphins,

小木 万布

Materials and Methods

are known to rest in shallower water near islands

The study site was the shallow water aroundaround noon (e.g. , Norris et al. 1994). This sug-

Mikura Island, Tokyo, Japan ( Fig. 1 ) , and thegests that by using visual surveys researchers

study subjects were Indo-Pacific bottlenose dol-have only studied the resting range of these ani-phins. Three types of custom-made stationed

mals and have not examined their feeding and/oracoustic buoys were used. Table. 1 lists the equip-

social ranges. Thus, studies of the nighttime distri-ment used with each stationed acoustic buoy. The

bution of odontocetes are important for an under-

overall frequency responses of these systems

standing of their entire home range.

ranged from about 1 to 16 kHz. A recorder and an

Acoustic monitoring is one of the methods for

ampliﬁer were housed in a plastic food container

the distribution study of odontocetes in dark condi-

anchored onto Styrofoam, which made the systemtions when visual surveys do not work well. Since

water resistant. A hydrophone that went throughodontocetes almost continuously produce echoloca-

the Styrofoam was located about 3 m below thetion sounds as a substitution for visual ability(Akamatsu et al. 2005), researchers rarely misswater surface. The system was deployed withthe location of odontocetes when they are withinlocation ( around 200 m of the shore and 20 m

about 60 m of rope and an anchor (Fig. 2) at theseveral hundred meters, depending on the condi-deep) where the dolphins are usually sighted intions. By using acoustic monitoring methods, one

the daytime. We deployed the systems before sun-can detect more eﬃciently single individuals thanset and retrieved them after sunrise. Sunset oc-by using visual method (Akamatsu et al. 2008).
The Indo-Pacific bottlenose dolphin (Tursiops

curred at 18 : 43, and sunrise occurred aroundaduncus) is a small odontocete (adult size: 2.7 m inlength, 230 kg in weight) found in shallow coastal
waters from around South Africa, through the Indian Ocean, to southeast Asia and Australia (Jefferson et al. 2008). Around a small oceanic island,Mikura Island (Japan), almost all of the Indo-Pacific bottlenose dolphins have been identified bytheir natural markings with underwater videoidentiﬁcation research since 1994. About 160 individuals have been identiﬁed around Mikura Island(Kogi et al. 2004). Throughout the year, these indi-

were ﬁrst converted into spectrograms with Avi-viduals inhabit the area around Mikura Island, andcy (see Morisaka 2009). Burst-pulse sounds are al-they are typically found within around 200 m ofso pulses, but the inter-pulse intervals are very

the shore and less than 20 m deep (Morisaka, per-short (<10 ms; Lammers et al. 2004). The number

sonal observation). This, however, has been ob-of whistles was counted for a period of 10 min, andserved only in the daytime, and the nighttime dis-the relative amount of clicks (many/few/none )tributions of the population has not yet been

was determined. Due to the high ambient noise

reported. In addition, the nighttime distribution

level, it was diﬃcult to count the number of clicks.
None indicated that no clicks were detected during

studies of this species at other sites have not been
performed. Here, we describe the detection of the4 : 41‑42 for the study period. The recorded sounds
soft-SASLab Pro version 4.0 software (Avisoft Bioacoustics, Glienicke, Germany) with a frequency
resolution of 93 Hz and a time resolution of 5.3 ms
with a Hamming window. Tonal sounds (whistles),echolocation clicks, and burst pulses were checkedvisually and acoustically with the Avisoft-SASLabPro software. A whistle is a tonal sound with anarrow band lasting from milliseconds to a few
seconds with frequency modulations, while clicks
are short pulse sounds with a broadband frequen-nighttime distribution of dolphins around Mikura

the 10-min bout either visually or acoustically. Few
meant a few weak clicks detected mainly by theIsland with custom-made stationed acoustic buoys.

hearing of the sound, but it was diﬃcult to create

Nighttime distribution of wild dolphins

3Fig. 1 The location of Mikura Island and the sound recording sites around the Island.

Table. 1 Equipments for four stationed acoustic buoy
Buoy
numberHydrophone & Ampliﬁer

RecorderStorage (memory capacity)

Recording type

Samplingfrequency

B1

OKI Whalephone

Roland Edirol R-1

Compact Flash (2GB)

mp3

44.1 kHz

B2

OKI Whalephone II

Sony Hi-MD MZ-RH10

Hi-MD (1GB)ATRAC3plus

32 kHzB3

OKI Whalephone

Roland Edirol R-1

Compact Flash (2GB)

mp3

44.1 kHz

B4

OKI custom-madeSony DAT TCD-D8

DAT (1.3GB)

LP mode32 kHzFig. 2 A picture of a stationed acoustic buoy(B2) deployed with a rope and anchor.

4

森阪 匡通

酒井 麻衣3a)

小木 万布3b)Fig. 3 Spectrogram of the sounds recorded by the stationed acoustic buoy at East Shoji-ne. a ) An example of the recording of a few clicks during the 10-min bout. Only the faint low-frequency
components of several clicks were heard from 3.0 s to 3.3 s. b) An example of many clicks during the 10-min bout. Many clicks were heard and visually detected. Whistles (Frequency modulation tonal sounds) are observed from 1.7 s to 4.2 s in the spectrogram.

a spectrogram due to the ambient noise (Fig. 3a).
Many signiﬁed many clicks detected visually andResults

acoustically (Fig. 3b). The rating of Many was usedeven if there was only one brief period of manyon July 14clicks during the 10-min period. Burst-pulse

sites where the buoys were deployed. Table. 2

sounds with rapid pulse repetitions following in-shows the time of buoy deployment, the recording

creasing echolocation clicks were also checked, be-durations, the buoys that were used, and the loca-

cause such burst-pulse sounds have been suggest-tions and depths of the recording sites. We missed

ed to have a relation with feeding (e.g., Akamatsu

the July 14

et al. 2010).

buoys due to human error. We could not deploy

Four stationed acoustic buoys were deployedthththand 16 , 2005. Figure 1 shows therecordings from the B1, B2, and B3

buoys in the southern part of the island because

Table. 2 Day, time, recording duration, location and depth of the recordings using stationed acoustic
buoys
RecordingDay

Time deployed

Recording
Duration

Buoy #

2005/7/14

17:00

3:49:05

B4

17:17

─

B1

17:31

─

B2

17:51

─

B3

17:34

5:43:27B1

17:45

16:40:00

B2

18:07

5:45:43

B3

18:21

3:53:21

B4

2005/7/16

N. L.
E. L.
33'53''
139'37''33'51''
139'37''33'51''
139'36''33'53''
139'34''33'54''
139'34''33'53''
139'34''33'53''
139'34''33'53''
139'34''

Location name

Depth (m)Ebi-Iso

16.8

East Moto-ne

20

Kawaguchi-Taki

14.4

Borosawa-Taki

15.3

East Shoji-ne

20.6

Kita-goshi

22South Shira-taki

22

Mitsu-ana

15.3

Nighttime distribution of wild dolphinsth5of rough seas on July 16 . The total recordingsite. Several burst-pulse sounds with rapid pulse

time (11 h, 51 min, and 36 s) included 2,246 whistles

repetitions following increasing echolocation clicks

and clicks with about 65 ten-min bouts. Figure.4

were heard from all four sites, except for Mitsu-shows the recorded whistles and clicks at eachana.

4a)4b)thFig. 4 The results of the sounds recorded at the ﬁve diﬀerent locations. a) Recording on July 14 atthEbi-Iso. b) Recordings from July 16 at East Shoji-ne, Kita-goshi, South Shira-taki, and Mitsuana. The points and lines indicate the number of whistles per 10-min bout, and the bars indicate the relative number of clicks per 10-min bout. A full bar length means Many, a half barmeans Few, and no bar means None.

6森阪 匡通

酒井 麻衣

Discussion

小木 万布cies. Examinations of the stomach contents of Indo-Pacific bottlenose dolphins around Zanzibar

Indo-Pacific bottlenose dolphins around thesupport the possibility that these dolphins feed at

Mikura Islands are usually found within 200 m ofshallower depths (Amir et al. 2005). The stomach

the coastline of the island and in shallow watercontents from two stranded dolphins around Mi-within a depth of 20 m during the day. Althoughkura Island contained mostly epipelagic fish andcephalopods, such as ﬂying ﬁsh (Cypselurus agoo)

we recorded for only 2 nights, the data clearly
showed that the dolphins (at least several individuals) were found in a similar habitat (near theand Japanese flying squid ( Todarodes pacificus )(Kakuda et al. 2002). These data support our data.

coastline and in shallower water) during the night.

We have rarely observed feeding behavior aroundEspecially at Kita-goshi, the site of the longest re-

the Island during the daytime surveys conducted

cording, 43 of the 60 ten-minute bouts recorded

from 2000 to 2014. This observation implies thatbetween sunset and sunrise had some sounds, sug-nighttime feeding but not daytime feeding might

gesting that the dolphins stayed there about 70%

be the main form of feeding for them.

of the time during the nighttime. Since our sta-

Despite the few sampling days, the present

tioned acoustic buoys only covered about half ofstudy is the ﬁrst report on the nighttime distribu-

the circumference of the Island, the percentage(70 %) was sufficiently high. Thus, we conclude

tion and feeding activity of Indo-Paciﬁc bottlenosethat at night several individuals of the population

buoy systems is needed to show the detailed

were found in a habitat similar to their daytimenighttime movement patterns and feeding places

one. In addition, these ﬁndings suggested that sev-of these dolphins. Such data will elucidate the en-

eral individuals of the population very much de-tire picture of the nighttime ecology of Indo-Pacif-

pend on the water around Mikura Island. As eco-

ic bottlenose dolphins, which is important for the

logical state of this region is critical for theconservation of the population.

dolphins. Further research with stationed acoustic

dolphin, their population would be greatly loweredby deterioration of their habitat environment.

Acknowledgments: This research was funded

Burst-pulse sounds with rapid pulse repeti-

by a grant from the Inui Tasuke Scientiﬁc Foun-tions following increasing echolocation clicks weredation to TM and JSPS KAKENHI Grant Number

heard from all four recording locations, except for23220006 to TM. We are grateful to Masahiko Fu-

Mitsu-ana. This suggested that dolphins aroundrusawa, Yoshinori Hirose, and the local people ofMikura Island feed on prey marine animals (ﬁsh,Mikura Island.

squid, etc.) during the night near the island and inshallow water where dolphins are found during

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和文要旨
動物の分布パターンを知ることは，その動物そのものを知り，保全を考えるうえで最も基礎的な情報となる． しかしながらイルカのように暗闇でも採餌できる種の夜間の分布パターンを知ることは，研究者が目視調査を行うことができないために難しい． 本研究において，自作の設置型音響ブイを用いて，御蔵島周辺海域に棲息するミナミハンドウイルカの夜間の分布を調べたので報告する．