A short note on feeding behaviour of sei whales
observed in JARPNII
MIDORI ISHII1, HIROTO MURASE2, YOSHIAKI FUKUDA3, KOUICHI SAWADA3, TOYOKI SASAKURA4,5,
TSUTOMU TAMURA6, TAKEHARU BANDO6, KOJI MATSUOKA6, AKIRA SHINOHARA1, SAYAKA NAKATSUKA2,NOBUHIRO KATSUMATA2, KAZUSHI MIYASHITA7 AND YOKO MITANI7
1

Graduate School of Environmental Science, Hokkaido University, 20-5 Benten-cho, Hakodate, Hokkaido
040-0051, Japan
2
National Research Institute of Far Seas Fisheries, Fisheries Research Agency (FRA), 2-12-4 Fukuura,
Kanazawa-Ku, Yokohama, Kanagawa, 236-8648, Japan
3
National Research Institute of Fisheries Engineering, Fisheries Research Agency (FRA), Hasaki7620-7,
Kamisu, Ibaraki, 314-0408, Japan
4
Tokyo University of Marine Science and Technology, 4-5-7, Konan, Minato-ku, Tokyo, 108-8477, Japan
5
Fusion Incorporation, 1-1-1-806 Daiba Minato-ku, Tokyo 135-0091, Japan
6
The Institute of Cetacean Research, 4-5, Toyomi-chyo, Chuo-ku, Tokyo, 104-0055, Japan
7
Field Science Center for Northern Biosphere, Hokkaido University, 20-5 Benten-cho, Hakodate,
Hokkaido 040-0051, Japan
ABSTRACTDiving behaviour of sei whales and vertical distribution of their prey were recorded simultaneously in 2013 JARPNII to
study their feeding behaviour at micro scale. This was the first attempt of this kind of observation targeting on thisspecies. Small acoustic time depth transmitters (pingers) were attached to two sei whales and their behaviours wererecorded for 10.2 and 32.0 hours, respectively. Vertical distributions and densities (volume backscattering strength, SV)of their prey were recorded by an echosounder following swimming path of the individuals. The diving behaviour deeper
than 10 m was classified into two shapes (U-shape, V-shape). It was assumed that U-shape was related to feeding
behaviour, especially lunge feeding, while V-shape was related to prey searching dive. Sei whales showed diel patterns inmean diving depth (day: 19 ± 14 m and 16 ± 10 m, night: 12 ± 5 m and 10 ± 5 m). Dense scattering layers (presumably
zooplankton) were observed around 40 m during the daytime, and they migrated closer to the surface in the evening.
Diving depth of the whales followed the changes in the scattering layers (i.e. diving depth was same as depth of
scattering layers). U-shape diving was associated with higher SV values than V-shape diving in daytime. The number of
U-shape diving increased around dusk. The results suggested that sei whales frequently lunged to prey around dusk.
First-Passage Time (FPT) of sei whales as an indicator of feeding was calculated using the cruise track as a proxy ofhorizontal movement of tagged individuals. Large FPT can be considered as feeding behaviour. FPT was increasedaround dusk and it was corresponding to increase in U-shape diving. Combining these results, it could be said sei whales
actively fed on prey around dusk. Swimming depth of the whales was shallower than 10 m after sunset while deep
scattering layers (presumably myctophids) were migrated from below 60 m to around 30 m. The results might indicatethat they did not feed on prey in deep scattering layers at night. However, it could not preclude a possibility that sei
whales fed on prey near surface at night because data on behaviour and prey distribution near surface could not recordedby the acoustic devices (pinger and echosounder) used in this study. The results of this study revealed that sei whales
changed their diving behaviour in response to availability of their prey.

INTRODUCTION
Observations of feeding behaviours of baleen whales are widespread in recent years in parallel with
development of biologging devices. Feeding behaviours of blue (Friedlaender et al., 2015; Goldbogen etal., 2013; Goldbogen et al., 2015), fin (Friedlaender et al., 2015; Goldbogen et al., 2006), Antarctic
minke (Friedlaender et al., 2014) and humpback (Friedlaender et al., 2009; Goldbogen et al., 2008)
whales were reported in scientific literatures. However, feeding behaviour of sei whales has not reported
to date.
It is preferable to record both diving profile of whales and vertical distribution of their prey
simultaneously to clarify their feeding behaviours. A small acoustic time depth transmitter (pinger) is asuitable biologging device for this purpose because it allows us to follow swimming path of an individual
based on the acoustic signal. Vertical distribution of their prey along the path can be recorded anechosounder. Such attempts were made targeting on humpback and fin whales in the Gulf of Alaska(Witteveen et al., 2014; Witteveen et al., 2008) and similar approach was taken in this study. Pingers
were deployed to two individuals of sei whales in the western North Pacific in August 2013 as a part of
JARPNII to investigate their feeding behaviour. Echosounder data were recorded simultaneous while1following their swim paths. Net samplings were also conducted around the area to confirm species
composition of acoustic backscatterings recorded by the echosounder. This paper is a short note on the
result of the observations.
There are three spatiotemporal scale to link feeding ecology of cetaceans with distribution
pattern of them and the definitions are follows; at the macro scale, cetaceans migrate seasonally between
feeding and breeding grounds; at the meso scale, cetaceans move over days and weeks in search ofpreferred local abundance of food; and at the micro scale, whales dive and search for food within
localised areas (IWC, 2003). This study corresponds to the micro scale.
MATERIALS AND METHODS
Acoustic time depth transmitters (FPXG-1040-60P500T30, Fusion Inc., Tokyo, Japan) used in this study
send pulses (62.5 kHz) per second and the depth data were encoded in them. The weight of the transmitterwas 6 g in air. Maximum duration of the batteries and detection range were 48 hours and 800 m,
respectively. Transmitted signals were received by a transducer with four channels (bow, port, starboard
and bottom sides) and processed by a receiver (FRX-4001, Fusion Inc., Tokyo, Japan) onboard a research
vessel (RV), Shunyo-maru (887 gross tons). The transducer was hull-mounted at a depth of 4.3 m below
the sea surface. The transmitters were tethered to small titanium spearheads (3.8 cm in length, 1.45 cm inmaximum head width and 4 g in air) by polyethylene (Dyneema) fishing line with linear strength of 55 kg.
The spearheads were loosely attached to titanium shafts (12.15 cm in length) with an expectation that thetransmitters and spearheads were detached from the shafts once they hit whales. The titanium shafts wereattached to carbon bolt arrows. The spearheads and shafts were made by Koreisangyo (Yokohama, Japan).
The tags were shot by compound crossbows from the bow decks of the survey vessels. The length of thevessels is about 70 m and the height of the bow deck from sea surface is about 8 m. Based on visualobservations, it was considered that there was no obvious reaction by the individuals to the deployment
and in subsequent observations. The RV followed the tagged individuals based on relative swimming
direction processed by received signals.
A calibrated quantitative echosounder, Simrad EK60 (Simrad, Horton, Norway), with a 120 kHz
transducer was used to record volume backscattering strength, SV (dB), as index of prey density. The
transducer was hull-mounted at a depth of 4.3 m below the sea surface. Trawl and plankton net sampling
were conducted at 5 and 4 stations respectively around the area where the behaviours were observed. The
sampling was conducted at the predetermined stations rather than targeting specific acoustic backscatters.
Acoustic backscatter was processed by using a software, Echoview 4.9 (Myriax, Hobart, Australia).
Depth data were imported into programs IgorPro version 6.12 (WaveMatrics, Oregon, US), andEthographer package version 2.01 (Sakamoto et al., 2009) for Igor Pro were used to detect diving
behaviours. Starting and end points of diving behaviour were set as 3 m. Individuals were considered asin diving when they stayed more than 5 seconds in the water column deeper than 5 m. If there was a gap
more than 15 minutes without depth data between successive signals, the data were excluded from theanalysis. Mann-Whitney U-test was used to test whether maximum depth and duration of each whale
showed diel patterns. Considering sunrise and sunset at local time, from 5:00 to 19:00 was conventionallydefined as daytime while the rest as nighttime.
Time Allocation at Depth (TAD) index was calculated following Fedak et al. (2001). This index
varies 0 to 1 and represents difference between dive shapes. Diving behaviour deeper than 10 m wasclassified into two shapes based on TAD: U-shape (TAD ≧ 0.7) and V-shape (TAD < 0.7). Previous
studies suggested that U-shape dives of baleen whales indicate feeding dive, especially lunge feeding,while V-shape dives indicate travelling or prey searching dive (Croll et al., 2001; Goldbogen et al., 2006;
Martin et al., 1998). It is considered that the dive shapes reflected bottom time portion of dive time. SV
values at maximum dive depth in each dive were extracted from echosounder data. SV values were
integrated in every 5 min of the cruise tracks, and in each 1 m depth bin. The horizontal resolution (i.e. 5min) was set as the mean diving interval of two individuals was 5 min. Mann-Whitney U-test was used totest whether SV values were different between U and V shapes. Correlation between TAD and SV were
tested using simple linear regression.
Two behavioural states (travelling and area restricted search [ARS]) of sei whales in the
Atlantic were categorized using switching state-space model based on satellite tracking data (Prieto et al.,2014). ARS, which is assumed to be related to feeding behaviour (Bailey et al., 2009), can also be2

detected using First-Passage Time (FPT, in seconds [s]) (Fauchald and Tveraa, 2003). This is defined as
the time taken to cross a circle of a given radius, which is centred around an animal’s position (Bailey etal., 2009).Because the tracking system used in this study could not detect accurate horizontal location of
tracking individuals, the cruise track was considered as a proxy of horizontal movement of taggedindividuals for calculation of FPT as we followed them within 800 m range based on acoustic signals
from the transmitters. Positions of the RV were recorded by using GPS every second and GPS data wasresampled for 2 minutes interval to calculate FPT. FPT is defined as the time required for a whale to crossa circle with a given radius and can be considered as reasonable measure for the search effort along thepath. The radius of the circle which can be considered as scale of ARS was estimated by using
Ethographer. Large FPT in the circle implies ARS.
Near surface temperature was also recorded continuously along the paths of the RV. Mean
temperature along the paths was calculated to investigate oceanographic conditions encountered by thetagged individuals.
RESULTS AND DISCUSSION
Diving behaviour of one individual of sei whale (S-1) was observed for 10 hr 11 min from 10:16 to 20:27
on 13 August 2013, while the other individual (S-2) was observed for 31 hr 59 from 10:37 on 14 August
to 18:36 on15 August 2013. Because of technical problem of data recording, one (21 min) and two (1 hr
11 min and 1 hr 10 min) long gaps without signal were existed for S-1 and S-2, respectively. The numberof dives for S-1 and S-2 was 119 and 387 respectively while mean dive depths and the standard
deviations (SD) throughout the observation periods were 17.9±12.6 m with maximum depth of 57 m and14.2±9.6 m with maximum depth of 48 m, respectively. Mean dive durations for S-1 and S-2 were 3.2
min and 3.0 min, respectively. The result of U-test (U-test) suggested that dive durations in daytime werelonger than nighttime for both individuals (p < 0.05) (Table 1). They tended to dive deeper in daytime
than in nighttime although the difference was not statistically significant. Diving depth of both individuals
coincided with the changes in dense scattering layers observed in shallow water (Fig. 1).
Two prominent dense scattering layers were observed while tracking whales. The shallow one
was observed around 40 m in the daytime but it migrated to near the surface in the afternoon. In thenighttime, scattering layers migrated from water depth deeper than 60 m to around 30 m. Maximum SV
observed in the daytime and nighttime was around -80 dB and -72 dB, respectively. Species identification
based on echosounder data could not be conducted as multifrequency data were not available. However,results of trawl sampling indicated that Japanese anchovy (Engraulis japonicus) was distributed aroundthe target area near the surface in daytime while both anchovy and myctophids were distributed in thewater column. It could be assumed that deep scattering layers observed in the nighttime mainly consistedof myctophids. The results of plankton net sampling showed that copepods, mainly Neocalanus spp.,
were dominant in wet weight in the samples apart from gelatinous planktons. It could be assumed thatshallow scattering layer observed in the daytime was mainly consisted of copepods.
U-shape diving was associated with higher SV than V-shape diving for both individuals (U-test :p < 0.05) (Table 2). The result of linear regression suggested that TAD were positively correlated with
SV for both individuals (p < 0.05). The radius of the circle of S-1 and S-2 which could be considered as
scales of ARS were estimated as 2.7 km and 1.4 km respectively. FPT were getting larger from 15:00 to
20:00 and the decreased subsequently as indicated in the path of S-2 (Fig. 2,3). Large FPT was related tohigh SV and U-shape diving (Fig. 3). Means of near surface temperatures and the SDs encounter by S-1
and S-2 were 15.1 ± 0.08 °C and 15.0 ± 0.10 °C, respectively. Near surface temperature encountered by
sei whales were stable along the paths of horizontal movement. The results of spatial distribution study ofsei whales using a generalized model (GAM) suggested high density of this species was expected around
15 °C (Murase et al., 2016: SC/F16/JR7) while GAM for modelling prey consumption of this species was
not indicated such a peak along SST gradient (Tamura et al., 2016: SC/F16/JR16). Combining theseresults, it can be hypothesized that spatial distribution of sei whales at meso scale are largely determinedby oceanographic conditions such as SST instead of prey availability. Sei whales then search for theirprey within the optimal oceanographic conditions at micro scale.
Both individuals dived to shallow scattering layer continuously in the daytime though the diving
depths varied widely. However, they dived sequentially once the density became relatively high and the
diving depths were concentrated to the dense scattering layers especially in the afternoon. After sunset,

3

swimming depth became shallower around 10 m. Neocalanus spp. is one of the important prey species ofsei whale in the western North Pacific (Konishi et al., 2009). It was reported that Neocalanus spp. wasmainly distributed at shallower than 40 m and have weak diel vertical migration (Kirby et al., 2007). It
was also reported that Neocalanus spp. was distributed around 30 m, and migrate around 10 m at night(Seki and Shimizu, 1998). It was observed that Neocalanus spp. formed dispersed small-dense patches atdawn, while it formed large patches from dusk to midnight (Tsuda et al., 1993). These observations
reported in the past corresponded to diel change of shallow dense scattering layers observed in this study.
It can be said that sei whales change their diving depth in response to change in distribution depth of theirprey to maximize their feeding efficiency.
The results of this study revealed that sei whales swam near surface and rarely dived to the depthof deep scattering layers distributed around 30 m at night where myctophids were distributed. The
behaviour corresponded to the fact that myctophids were not recognized as main diet of sei whales inrecent years (Konishi et al., 2009). However, it can not preclude a possibility the sei whales fed nearsurface in night as data on behaviour and prey distribution near surface could not recorded by the acoustic
devices (pinger and echosounder) used in the study. Small FPT values in nighttime might not be impliedthat sei whales do not feed in night because they are known as skimming feeder as well and they might
not change their swimming path frequently while they are skimming. Generally, baleen whales are
considered as visual predators (Friedlaender et al., 2009; Hazen et al., 2009). However, it was
hypothesized that sei whales were feeding near surface in night based on the observations in thesouthwestern Gulf of Maine that rates of their calling were reduced in night when a copepod, Calanus
finmarchicus, moved to near surface due to the diel migration (Baumgartner and Fratantoni, 2008). A
Digital acoustic recording tag (DTAG) can be an alternative biologging device to detect feeding
behaviour near surface an as analysis method was developed for this purpose recently (Owen et al., 2015).
Recording of prey distribution using echosounder near surface is challenging because of existence of nearsurface dead zone. Net sampling and/or continuous plankton recorder might be alternative options forechosounder.
ACKNOWLEDGEMENT
The authors express their thanks to the crews and researchers who participated in the surveys to collectthese valuable data. Dr. Shingo Minamikawa and Mr. Shigetoshi Nishiwaki provide us useful commentsof development of deployment system of pinger. We thank Dr. Kenji Minami and Dr. Hokuto Shirakawa
for their support on analysis, Mr. Genki Sahashi and Mr. Keizo Ito for their advice. We also thank to theirsupport by Fisheries Agency of Japan, Fisheries Research Agency and Institute of Cetacean Research.
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Table 1. Means and standard deviations (SD) of maximum diving depth (m) and diving durations (seconds, s) by
day and night for two individuals of tagged sei whales (S-1 and S-2). Maximum recorded values are also shown.
Results of Man-Whitney U test to test difference between day and night are also shown.

ID
S-1

Max. depth (m)Duration (s)S-2Max. depth (m)Duration (s)

Day

Night

Mann-Whitney U test

mean±SD

19 ± 1412.2 ± 5.4

p = 0.083

Max.

57

26

mean±SD

207 ± 141134.4 ± 126.4

Max.

736

545

mean±SD

16 ± 109.5 ± 5.1

Max.

48

31

mean±SD

201 ± 159

120 ± 95.4

Max.

657

460

p < 0.05

p < 0.05

p < 0.05

Table 2. Means of volume backscattering strength (SV, dB) value of each diving types (V- and U-shapes) for
two individuals of tagged sei whales (S-1 and S-2).

SV mean (dB)

Mann-Whitney

ID

V-shapeU-shapeU test

S-1

-86.0

-80.0

p < 0.05

S-2

-80.6

-78.8

p < 0.05

6

Fig.1. Maximum diving depth of each diving (filled circle) of sei whales (upper panel: S-1, lower panel: S-2) is overlaid on the echogram. Strong volume backscatteringstrength (SV) is assumed to be related to high densities of prey. Grey shadow indicates gaps without depth data and black bar along the timeline means nighttime (19:00 to5:00).

7

Fig. 2. Cruise tracks of survey vessel as a proxy of swimming path of tagged sei whales (upper panel: S-1, lower
panel: S-2). First-Passage Time (FPT) along the track was also shown. White circle indicates the starting point of
tracking. Black and white arrows indicate time of sunset and sunrise respectively.

8

Fig.3. Number of U-shape (black bar) and V-shape (white bar) and first passage time (FPT) along the tracking paths of tagged sei whales (upper panel: S-1, lower panel: S-2).
Grey shadow indicates gaps without depth data.

9