Volume 31

Issue 4

Article 14

Impact of Multiple Water Temperature Structures on the Behavioral Patternof Chum Salmon (Oncorhynchus keta) on the Coast of the Shiretoko
Peninsula
Xinyi Li
Graduate School of Env. Science, Hokkaido University, Hakodate, Hokkaido, Japan, xinyi.li.p1@elms.hokudai.ac.jp

Hiroshi Yamaguchi
Graduate School of Env. Science, Hokkaido University, Hakodate, Hokkaido, Japan

Hokuto Shirakawa
Demersal Fish Resources Division, Fisheries Stock Assessment Center, Fisheries Resources Institute, National
Research and Development Agency, Japan Fisheries Research and Education Agency, Japan

Kenji Minami
Field Science Center for Northern Biosphere, Hokkaido University, Hakodate, Japan

Nobuhiko Sato
Field Science Center for Northern Biosphere, Hokkaido University, Hakodate, JapanSee next page for additional authors

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Recommended Citation
Li, Xinyi; Yamaguchi, Hiroshi; Shirakawa, Hokuto; Minami, Kenji; Sato, Nobuhiko; Zhu, Yanhui; Miyakoshi, Yasuyuki;
Miyashita, Kazushi; and Miyashita, Kazushi (2023) "Impact of Multiple Water Temperature Structures on the
Behavioral Pattern of Chum Salmon (Oncorhynchus keta) on the Coast of the Shiretoko Peninsula," Journal of Marine
Science and Technology: Vol. 31: Iss. 4, Article 14.
DOI: 10.51400/2709-6998.2721
Available at: https://jmstt.ntou.edu.tw/journal/vol31/iss4/14
This Research Article is brought to you for free and open access by Journal of Marine Science and Technology. It has been
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Impact of Multiple Water Temperature Structures on the Behavioral Pattern ofChum Salmon (Oncorhynchus keta) on the Coast of the Shiretoko Peninsula
Authors
Xinyi Li, Hiroshi Yamaguchi, Hokuto Shirakawa, Kenji Minami, Nobuhiko Sato, Yanhui Zhu, Yasuyuki
Miyakoshi, Kazushi Miyashita, and Kazushi Miyashita

This research article is available in Journal of Marine Science and Technology: https://jmstt.ntou.edu.tw/journal/
vol31/iss4/14

RESEARCH ARTICLE

Impact of Multiple Water Temperature Structures on
the Behavioral Pattern of Chum Salmon(Oncorhynchus keta) on the Coast of theShiretoko Peninsula
Xinyi Li a,*, Hiroshi Yamaguchi a, Hokuto Shirakawa b, Kenji Minami c, Nobuhiko Sato c,
Yanhui Zhu c, Yasuyuki Miyakoshi d,e, Kazushi Miyashita caGraduate School of Env. Science, Hokkaido University, Hakodate, Hokkaido, Japan
Demersal Fish Resources Division, Fisheries Stock Assessment Center, Fisheries Resources Institute, National Research andDevelopment Agency, Japan Fisheries Research and Education Agency, Japanc
Field Science Center for Northern Biosphere, Hokkaido University, Hakodate, Japand
Salmon and Freshwater Fisheries Research Institute, Hokkaido Research Organization, 3-373 Kitakashiwagi, Eniwa, Hokkaido,061-1433, Japane
Kitami Salmon Enhancement Program Association 1-5-17 Shin-machi, Abashiri, Hokkaido, 093-0046, Japanb

Abstract
During the migration of chum salmon (Oncorhynchus keta) towards the Shiretoko Peninsula in Hokkaido, Japan, theconvergence of coastal currents results in the formation of intricate water temperature structures. However, the impact of
this phenomenon on the behavioral tendencies of chum salmon remains largely unexplored. In 2012 and 2013, we
conducted a study to document the vertical water temperature proﬁles and uninterrupted swimming patterns of homingchum salmon in the coastal waters surrounding the Shiretoko Peninsula. The results indicated that chum salmon
exhibited a preference for avoiding thermoclines characterized by rapidly decreasing water temperatures. They werepredominantly observed swimming either in the surface or deep seawater layers (210e230 m), away from the thermocline. This ﬁnding demonstrates that homing chum salmon adapt their behavioral patterns in response to the verticaltemperature distribution in the water. The ﬁndings presented in this study contribute to our understanding of theswimming behavior of chum salmon, providing valuable insights that can be utilized to develop effective management
strategies aimed at enhancing sustainability.
Key words: Chum salmon, Behavioral pattern, Water temperature, Data logger

1. IntroductionC

hum salmon (Oncorhynchus keta) is one ofseven Paciﬁc salmon species, with a global
annual catch of 32,300 t [1]. It is an ecologically [2,3]and economically [4] important species in the North
Paciﬁc. As one of Japan's most important ﬁshery resources, chum salmon return to their natal river innortheastern Japan from September to November ofReceived 28 June 2023; revised 3 November 2023; accepted 7 November 2023.
Available online 15 December 2023
* Corresponding author.
E-mail address: xinyi.li.p1@elms.hokudai.ac.jp (X. Li).

https://doi.org/10.51400/2709-6998.2721
2709-6998/© 2023 National Taiwan Ocean University.

annually [4]. Returned chum salmon are primarilycaught using salmon trap nets (96 %) [5]. The Shiretoko Peninsula in Hokkaido, Japan, has the highest
chum salmon yield area in Japan, and salmon trap
net ﬁsheries are highly developed [6]. However, in
recent decades, there has been a signiﬁcant decline
in the catch of chum salmon in Japan, from 70 % of
the total catch in the North Paciﬁc in 1991 to 40 % atpresent [7]. To maintain the sustainable development

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JOURNAL OF MARINE SCIENCE AND TECHNOLOGY 2023;31:515e528

of chum salmon ﬁshery resources, there is an urgent
need to investigate the behavioral pattern of homingchum salmon along the coast of the Shiretoko
Peninsula and the factors that may cause changes in
the regional catch.
In nature, climate change [8,9], density dependence [10], and artiﬁcial release [11,12] are crucial
factors affecting ﬁsh population dynamics. Beamish[13] and Eriyama et al. [14] suggested that artiﬁcial
release and hatchery technologies are the main
catalysts for chum salmon return. However, between 1990 and 2022, the number of releases ofchum salmon in Hokkaido, Japan, was generally
consistent [15] but catch recurrence changed andgradually decreased. Beamish et al. [16] and Hareet al. [17] investigated the combined effects of density dependence, climate change, and artiﬁcial releases on chum salmon population dynamics andsuggested that the changes in catches could belargely explained by climate change. However, there
have been cases where different patterns of catch
variation have occurred in different areas of thesame sea. According to statistical data from 2012, thecatch of the Utoro Fishing Association on the westside of the Shiretoko Peninsula was 80 % that of theprevious year, while that of the Abashiri Fishing
Association in the same marine waters was 10 %higher than the previous year [18], the discrepancy
suggests that factors beyond climatic conditions
may also be inﬂuencing the catch volume.
Salmon trap nets are usually set up in shallow
waters, waiting for salmon to enter [19,20]. Thismethod utilizes the behavioral ecology of the target
species for net ﬁshing [21]. In other words, changes
in the behavioral patterns of chum salmon could
directly lead to changes in the salmon trap net
catch. Chum salmon is a typical cold water ﬁsh,surviving at optimum water temperatures ranging
from 13 to 15  C [22,23]. As a temperature-changing
animal, the temperature of seawater determines thetemperature of salmon and affects them at all stagesof life [24]. High water temperatures alter thegrowth rate and migration period of chinook(Oncorhynchus tshawytscha) [25,26] and pink salmon(Oncorhynchus gorbuscha) [26]. Sockeye (Oncorhynchus nerka) [27] and chinook salmon [26] use
cold water blocks in the ocean for behavioral thermoregulation under high water temperatures.
Adult chum salmon are believed to adapt their
behavioral strategies according to the water temperature structure [28] to achieve a balance between
utilizing directional cues about their natal rivers
and temperature regulation [29-31]. In other words,
water temperature affects the behavioral patterns ofchum salmon.

The Shiretoko Peninsula bordering the Sea of
Okhotsk, is a major producer of chum salmon in
Japan. The Soya Warm Current (SWC), Eastern
Sakhalin Current Water (ESCW), and Coastal Oyashio Water (COW) converge [32-34] along theShiretoko Peninsula, and the variable sea conditions
have been well-established [35]. The SWC exhibits
its strongest interaction with ESCW from
September to October [36,37], during which the
SWC leads to the formation of surface cooling zones
and subsurface dome structures, signiﬁcantly
impacting the coastal water temperature structure[38,39]. This period coincides with the return periodof chum salmon; at that time, the homing chumsalmon will be in a multilayered water temperature
structure. The complex structure of multiple watertemperatures may alter the behavioral patterns ofchum salmon. To verify this, we attached micro dataloggers to homing chum salmon in the coastal waters around the Shiretoko Peninsula to record their
behavioral patterns in 2012 and 2013. The verticalwater temperature structure and salinity distributions in the same water were extrapolated using a
conductivity-temperature-depth (CTD) proﬁler. We
concurrently collected water temperature andsalinity data from both the eastern and western
coasts. The primary aim of this study was to explore
the effects of vertical changes in water temperatureon the behavioral patterns of chum salmon bycomparing swimming behavior proﬁles under
different water temperature structures.

2. Materials and methods
2.1. Study area
This study was conducted in the Shiretoko
Peninsula, Hokkaido, Japan (43 60 e44 60 N, 144 30 e
145 80 E) (Fig. 1). The experiments were conducted
between September 2012 and September 2013.
Considering that the SWC extends from the Sea ofJapan to the northeastern part of Hokkaido, themarine environment on the western side of thepeninsula, which is directly affected by the SWC,
differs from that on the eastern side of the peninsula, which is indirectly affected. In this study, thecoast of the Shiretoko Peninsula was bounded byCape Shiretoko and divided into western andeastern sides (Fig. 1).
2.2. Environmental data collection
On September 22e23, 2012, and September 18e19,
2013, we measured vertical water temperature andsalinity using a CTD proﬁler (19plus V2 SeaCAT;

JOURNAL OF MARINE SCIENCE AND TECHNOLOGY 2023;31:515e528

517

Fig. 1. Schematic diagram of the ocean currents around Hokkaido and geographic location of surveys (upper left corner). The larger map shows thestudy area of this study around the Shiretoko Peninsula (43 60 e44 60 N, 144 30 e145 80 E), Hokkaido, Japan. The broken line extending from Cape
Shiretoko divides the Shiretoko Peninsula into ‘West Part’ and ‘East Part.’ The solid and hollow circles indicate the release points in 2013 and 2012,
respectively. The black triangles are the four measured points of the CTD proﬁler.

Sea-Bird Scientiﬁc, Bellevue, WA, USA) aboard the‘Ushio-maru’ (179t, a practice ship attached to
Hokkaido University, Faculty of Fisheries); themeasurement points are shown in Fig. 1. The vertical swimming depth of adult chum salmon
generally does not exceed 200 m [40-42], and the
maximum drop depth of the CTD was set to 500 m
in this study.
2.3. Logger experiments on salmon
Adult chum salmon homing from the Bering Sea
toward their natal river in Japan ﬁrst reach theShiretoko Peninsula [43,44]. The release experiments
were conducted on September 3, 2012, andSeptember 6, 2013, and the release point was set to
the west side of Shiretoko Peninsula, as shown in
Fig. 1. The chum salmon used for the release
experiment were caught in the Utoro Sea (Fig. 1);
injured and bled individuals were excluded. Allremaining individuals were placed in 2-phenoxyethanol diluted to 0.04 % with seawater for anesthesia for approximately 3 min 2-phenoxy ethanol
appears to have no adverse effects on the homing
behavior of chum salmon [45]. After anesthesia cameinto effect, we attached a micro data logger to the518

JOURNAL OF MARINE SCIENCE AND TECHNOLOGY 2023;31:515e528

ﬁsh just below the anterior segment of the dorsal ﬁn.
A nylon strap was passed through the ﬁsh via a
perforated tube, and the micro data logger was sutured to the left side of the ﬁsh. Finally, the fork
length, weight, sex, and maturity of the ﬁsh wererecorded. The entire process of anesthesia and datalogger attachment took approximately 5 min. The
marked individuals were allowed to recover in aseawater tank for 10e30 min and were released
sequentially until the effect of anesthesia passed.
Eight and twenty individuals were released in 2012
and 2013, respectively. Swimming depth andambient temperature were monitored simultaneously using the micro data logger (Lat 1400; Lotek,
Newmarket, ON, Canada). Each data logger was
35 mm in length and 11 mm in diameter, weighed
5.1 g in air and 1.5 g in seawater, had a resolution of
0.25 m, an observed water temperature range of 5
to 35  C, and the maximum range of the depthchannel was 2000 m. The data loggers were set torecord empirical data at intervals of 1 min. Similar totagging, externally attached data loggers may reduce
the swimming performance of salmonids [46,47].
However, a tag load of less than 2 % (i.e., tag mass/
body mass; [48]) does not signiﬁcantly affect swimming behavior [49]. The minimum weight of thereleased individuals in this study was 2.9 kg, and the
tag load was far less than 2 %. Therefore, the resultsof this experiment are credible. Furthermore, The
CTD proﬁler recorded vertical water temperatureand salinity simultaneously. Released individuals
were recovered with the assistance of ﬁshing associations and anglers. All animal handling, anesthesia, and surgical procedures followed the
University of NebraskaeLincoln Institutional Animal Care and Use Committee-approved protocol no.
1691.
2.4. Statistical analyses
The Shiretoko Peninsula was divided into east andwest sides by Cape Shiretoko (Fig. 1), and thereleased individuals were labeled according to theirrecapture locations. For example, ‘West’ and ‘East’
mean individuals were recaptured on the west andeast sides, respectively. Because chum salmon show
reduced activity and passive sinking for 6e8 h after
being marked [50], data from 8 h after the release
experiment were excluded. In addition, marked individuals were mainly recovered by salmon trap
nets, which were usually collected once a day between 4:00 and 6:00 am, meaning that marked individuals were in the salmon trap nets for a
maximum of 24 h. Therefore, data collected within
24 h before net collection were excluded. If the day
before the net collection was a ﬁshing-off day, theFig. 2. Vertical distribution of water temperature and salinity at the release points. The ordinate represents water depth; water salinity is indicated on
the upper abscissa, and water temperature on the lower abscissa. Water temperature is represented as a solid line and salinity as a broken line. (a)Vertical distribution of water temperature and salinity at the release points on September 3, 2012. (b) Vertical distribution of water temperature andsalinity at the release points on September 6, 2013.

JOURNAL OF MARINE SCIENCE AND TECHNOLOGY 2023;31:515e528

exclusion time became 24 h to accommodate theﬁshing-off day time. After processing the data as
described above, individuals with insufﬁcient
recording times were excluded from the analysis.
According to the deﬁnition provided by Tanaka [51],chum salmon dropping below 10 m and rising again
to 10 m at shallow depths was deﬁned as one vertical movement. We recorded the number of verticalmovements, frequency of vertical movements perunit time (times/h), and maximum and minimum
swimming depths (m). Statistical analyses wereconducted using ‘Ethographer,’ a suite of functions
designed for visualizing and analyzing bio-logging
data within IGOR Pro ver9.0 (WaveMetrics Inc.,
Portland, OR, USA). Various statistical metrics forthe time series data can be obtained via the graphical user interface, including event ID, start time,end time, average swimming depth, minimum
swimming depth, maximum swimming depth,
duration, standard deviation, and sum of diving
events. In addition, if a thermocline is detected, thenumber of individuals passing through the thermocline will be calculated using R Statistical Software (v4.1.2; [52]). Assuming a thermocline depth is
X meters, the number of points where the swimming depth-time series graph intersects with y ¼ X
can represent the number of times individuals passthrough the thermocline. A thermocline is a transition layer between warmer mixed water on theocean surface and colder deep water [53], and thetemperature changes rapidly with depth [54]. This
study used the deﬁnition suggested by Gray [55],
who deﬁned a thermocline as a water layer with atemperature change greater than 0.5  C within adepth interval of 1 m.

519

prominent differences was the presence of a wellmixed water mass in 2013, whereas the watertemperature dropped rapidly at a depth of 50 m in2012.

3. Results
3.1. Vertical changes in water temperature at therelease point
Fig. 2 shows the water temperature and salinity
obtained using the CTD at the release points in2012 and 2013. At the release point in 2012, thewater temperature from the surface to a 50-m
depth slowly decreased from 19 to 17  C, and below
50 m, the water temperature sharply decreased
from 17 to 10.5  C at a depth of 80 m (Fig. 2a). The
salinities at the surface and bottom were 33.63 and33.72 PSU, respectively. At the release point in2013, the sea surface temperature was approximately 19.2  C, decreasing phasically to 13.5  C at a
depth of 100 m. The sea surface salinity was 33.55
PSU, with a phased increase to 33.83 PSU from thesea surface to the bottom (Fig. 2b). One of the mostFig. 3. Vertical distribution of water temperature and salinity at four
CTD proﬁler observation points on September 22e23, 2012, andSeptember 18e19, 2013. The ordinate represents water depth, watersalinity is indicated on the upper abscissa, and water temperature isindicated on the lower abscissa. Water temperature is represented as asolid line and salinity as a broken line. (a) Observations at point 1 in the‘East Part’ in 2012. (b) Observations at point 2 in the ‘East Part’ in
2012. (c) Observations at point 3 in the ‘West Part’ in 2012. (d) Observations at point 4 in the ‘West Part’ in 2012. (e) Observations atpoint 1 in the ‘East Part’ in 2013. (f) Observations at point 2 in the ‘East
Part’ in 2013. (g) Observations at point 3 in the ‘West Part’ in 2013. (h)Observations at point 4 in the ‘West Part’ in 2013.

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JOURNAL OF MARINE SCIENCE AND TECHNOLOGY 2023;31:515e528

Fig. 4. Release and retrieval points in 2012 and 2013. Solid and hollow circles represent the release points in 2013 and 2012, respectively. Solid andhollow rectangles represent the retrieval points in 2013 and 2012, respectively. The solid line illustrates the anticipated swimming route of individuals
retrieved on the west side of the Shiretoko Peninsula, whereas the dashed line illustrates the anticipated swimming route of individuals retrieved on
the east side.

3.2. Vertical changes in the water temperature in
the coastal waters around Shiretoko Peninsula
On the east side of the peninsula on September
22e23, 2012, the water temperature slowly
decreased from 19 to 6  C from the sea surface to theseaﬂoor. At observation point 1, the salinity was 33.4
PSU at the sea surface, rose to 33.7 PSU at a depth of
40 m, and then dropped back to 33.4 PSU. At
observation point 2, the variation in salinity was
minimal at approximately 33.4 PSU at the sea surface and 33.6 PSU at the bottom (Fig. 3ab). The water

Table 1. Data of retrieved individual chum salmon
Logging Depth (m)Temperature ( C)
Diving events (10 m)
50- m depth passage
duration Median Maximum Median Minimum Maximum Number Frequency Mean dive
Frequency (n/h)
(h)
(n)
(n/h)
duration (minutes)
West’12 76
East’12 60
West’13 614

41.3
14.6
44.8

362.4
144.3
258.7

12.3
16.6
15.6

0.2
4.1
1

20.4
19.4
20.6

35
100
1099

0.46
1.67
1.79

59.7
20.4
25.4

0.17
0.65
0.63

From left to right: logger recording time (h), median swimming and maximum diving depth (m), median experienced water temperature( C), maximum and minimum ambient temperature ( C), number of diving events (times), frequency of diving (times/h), mean diving
duration (minutes), and frequency of 50-m depth passage (times/h).

JOURNAL OF MARINE SCIENCE AND TECHNOLOGY 2023;31:515e528

temperature and salinity distribution on the westernside signiﬁcantly differed from those on the easternside. On the west side of the peninsula, the watertemperature was unchanged at 20  C from the water
surface to a depth of 50 m. In parallel, seawatersalinity remained at 33.7 PSU. There was a strong
decrease in salinity and temperature at a water
depth of 50 m; the water temperature dropped toless than 8  C, and the salinity dropped sharply toapproximately 33.4 PSU (Fig. 3cd).
On the east side of the peninsula on September
18e19 2013, the water temperature was approximately 18  C from the surface to a 40-m depth,
decreasing slowly to 12  C from 40 m to the seaﬂoor
at observation point 1. The salinity at the sea surface
was recorded at 33.4 PSU, with a gradual increase to33.75 PSU at the seaﬂoor (Fig. 3e). At observation
point 2, the water temperature varied fromapproximately 18  C at the surface to a 60-m depth,
gently decreasing to 12  C between 60 m and theseaﬂoor. Sea surface salinity measured 33.55 PSU,
with a gradual rise to 33.73 PSU at the seaﬂoor(Fig. 3f). On the west side of the peninsula, the sea
surface temperature was approximately 19  C,
gradually decreasing to 12  C on the seaﬂoor. The
sea surface salinity was 33.2 PSU, increasing to 33.8
PSU on the seaﬂoor (Fig. 3gh). According to the
CTD data, the structure of seawater temperature521

and salinity on the west side of the peninsula in 2012
considerably differed from that on the east side in2012 and on both sides in 2013. From at least
September 3 to 23, 2012, a thermocline in watertemperature at a 50-m depth formed two stratiﬁed
water masses, with water temperature suddenly
dropping to approximately 10  C.
3.3. Retrieved individuals
Four of the eight individuals released in 2012 wererecaptured. After removing the data loggers 8 h
after the release experiment and 24 h before
recapture, two individuals were excluded because of
insufﬁcient recording times. The remaining two individuals were marked as West’12 (ID3211) andEast’12 (ID2081) according to their retrieved locationand year of release (Fig. 4). Twelve of the twenty
individuals released in 2013 were recovered, andﬁve individuals with insufﬁcient recording times
were excluded. The remaining seven individuals(ID2221, ID2206, ID2609, ID2610, ID3385, ID3377,
and ID2561) were all recaptured on the west side of
the Shiretoko Peninsula and were marked asWest’13 for analysis (Fig. 4). The total analysis timewas 76 h for West’12 (76 h for each individual), 60 h
for East’12 (60 h for each individual), and 614 h forWest’13 (87.7 h for each individual) (Table 1).

Fig. 5. Swimming depth and ambient temperature proﬁles of homing chum salmon retrieved on the west side of the Shiretoko Peninsula in 2012. The
vertical axis on the left represents the swimming depth (m), while the right vertical axis represents the ambient temperature ( C).

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West’12 more frequently swam on the sea surface
and at a 210e230-m depth (Fig. 5), with a bimodal
swimming depth distribution. The swimming depth
for West’12 was negatively correlated with ambient
temperature. Most of the time, the ambient temperature decreased as the depth increased; however, there were shallow swimming depths with low
water temperatures (Fig. 6). West’12 had a bimodal
swimming depth distribution. The median swimming depth for West’12 was 41.3 m, and the
maximum swimming depth was 362.4 m (Table 1).
The ambient temperature for West’12 also showed abimodal distribution, being mostly 1, 2, and 16  C(Fig. 6). The median, minimum, and maximum
ambient temperatures for West’12 were 12.3, 0.2,
and 20.4  C, respectively. The hourly diving frequency was 0.46/h, and the mean dive duration was
59.7 min. In addition, West’12 had a passage frequency of 0.17/h at a depth of 50 m, where a thermocline appeared on the west side of the Shiretoko
Peninsula in 2012 (Table 1).
East’12 more frequently swam in shallow water
below 100 m (Fig. 7), with a median swimming
depth of 14.6 m and a maximum swimming depth ofFig. 6. Relationship between the ambient temperature and swimming depth of West’12. The vertical axis represents the ambient temperature ( C),
while the right vertical axis represents swimming depth (m). The upper and right histograms show the frequency distributions of swimming depth andambient temperature for West’12, respectively.

JOURNAL OF MARINE SCIENCE AND TECHNOLOGY 2023;31:515e528

523

Fig. 7. Swimming depth and ambient temperature proﬁles of homing chum salmon retrieved on the east side of the Shiretoko Peninsula in 2012. The
vertical axis on the left represents the swimming depth (m), while the right vertical axis represents the ambient temperature ( C).

144.3 m (Table 1). The swimming depth for East’12
was negatively correlated with ambient temperature, and ambient temperature decreased withincreasing swimming depth (Fig. 8). The ambient
temperature for East’12 was concentrated from 12 to
18  C (Fig. 8), with median, minimum, andmaximum ambient temperatures of 16.6, 4.1, and19.4  C, respectively. The diving frequency was 1.67/
h, which was 3.7-fold higher than that for West’12.
The mean dive duration was 20.4 min, signiﬁcantly
shorter than that for West’12. The 50-m depth passage frequency was 0.65/h, 3.8-fold that of West’12(Table 1).
Seven recaptured West’13 individuals showedsimilar swimming patterns; ID3385 is a typicalexample (Fig. 9). Both West’13 and East’12 spent
most of their time swimming in the shallow layer
below 100 m. The swimming depth for West’13 wasnegatively correlated with ambient temperature,and the ambient temperature decreased with anincrease in swimming depth (Fig. 10). West’13
exhibited a median dive depth of 44.8 m and amaximum depth of 258.7 m (Table 1).
The ambient temperature for West’13 wasconcentrated from 11 to 19  C (Fig. 10), with median,minimum, and maximum ambient temperatures of
15.6, 1, and 20.6  C, respectively. The diving frequency was 1.79/h, 3.9-fold that of West’12. The
average single diving dwell time was 25.4 min(Table 1), signiﬁcantly shorter than that of West’12.

The 50-m depth passage frequency was 0.63/h,
approximately 3.7-fold that of West’12.
Since swimming depth and ambient temperaturearenotnormally
distributed,
separate
KruskaleWallis tests were conducted for swimming
depth and ambient temperature in West’12, East’12,
and West’13 using the R. The test results reveal
signiﬁcant differences among West’12, East’12, andWest’13 in terms of swimming depth (KruskaleWallis chi-squared ¼ 2503, df ¼ 2, p-value<2.2e-16). Likewise, signiﬁcant differences were also
observed in ambient temperature (KruskaleWallis
chi-squared ¼ 1316.2, df ¼ 2, p-value <0.01). Theseﬁndings suggest notable variations in swimming
patterns among West’12, East’12, and West’13.

4. Discussion
CTD data revealed that (Figs. 2 and 3), only in
2012, a thermocline was formed at a 50-m water
depth on the west side of the Shiretoko Peninsula,where the water temperature was approximately
18  C and salinity was approximately 33.7 PSU fromthe surface to a 50-m depth. At a depth of 50 m, thewater temperature rapidly decreased to approximately 8  C, and seawater salinity dropped to 33.3
PSU. Because the salinity of ESCW is 32.0e33.0 PSU
and that of the SWC is more than 33.6 PSU [37], thethermocline in 2012 is thought to have been formedby the conﬂuence of the SWC and ESCW.

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Fig. 8. Relationship between the ambient temperature and swimming depth of East’12. The vertical axis represents ambient temperature ( C), while
the right vertical axis represents swimming depth (m). The upper and right histograms show the frequency distribution of swimming depth andambient temperature for East’12, respectively.

Based on the results of the release experiments,
the swimming patterns of homing chum salmon
could be divided into two main patterns. The ﬁrst
pattern involves swimming at a shallow depth of
100 m and making frequent vertical movements
similar to East’12 and West’13. This behavioral
pattern is similar to that observed in homing chumsalmon [51] in September 1994 [41] and December
1997 on the coast of Sanriku, Japan, and can beconsidered the general behavioral pattern of homing chum salmon. The second pattern, exhibitedby West’12, involves swimming in shallow seawateror at depths of 210e230 m with low diving activity.
The vertical movement frequency of East’12 andWest’13 was 3.71- and 3.98-fold that of West’12.
It can be considered that these individuals inhibited body temperature increases through frequentvertical movement. The reason West’12 could
stay in shallow water for a long time when theambient temperature was considerably lower
than that of East’12 and West’13 is probably theappearance of low-temperature water blocks on
the surface of the swimming area of West’12 (Fig. 6)[51,56].

JOURNAL OF MARINE SCIENCE AND TECHNOLOGY 2023;31:515e528525

Fig. 9. Typical swimming depth and ambient temperature proﬁles of homing chum salmon number 3385 retrieved on the west side of the Shiretoko
Peninsula in 2013. The vertical axis on the left represents the swimming depth (m), while the right vertical axis represents the ambient temperature ( C).

We calculated the passage frequency of thereleased individuals based on a depth of 50 m where
the thermocline appeared. West’13 and East’12
passed through the thermocline at frequencies of
0.65 and 0.63/h, respectively, 3.8- and 3.7-fold that ofWest’12, respectively. Chum salmon is an ectothermwith a narrow temperature tolerance [23], and its
body temperature changes in response to changes
in water temperature. A dramatic drop in watertemperature (e.g., thermocline) as a stressor cancause many physiological, behavioral, and healtheffects in ﬁsh [57], including cold shock, which, in
extreme cases, can even lead to ﬁsh death [58]. Fishoften show avoidance tendencies when theyencounter a thermocline; this tendency has also
been observed in both Atlantic cod (Gadus morhua)[59] and Atlantic blueﬁn tuna (Thunnus thynnus) [60].
The low passage rate and distinct swimming patternof West’12 at a depth of 50 m appeared to beinﬂuenced by the thermocline. In addition, among
the individuals used for analysis in the present
study, two individuals were recovered from the west
and east sides of the Shiretoko Peninsula in 2012;
however, at the same time, all seven individuals in2013 were recovered from the west side of theShiretoko Peninsula. We speculate that some of thehoming chum salmon in 2012 changed their homing
paths to avoid the thermocline that appeared on thewestern side of the Shiretoko Peninsula.

Although classiﬁed as having similar behavioral
patterns, East’12 and West’13 dove to depths >50 m
more often than the homing chum salmon on the
coast of Sanriku in 1994 [41] and 1997 [51]. This
could be due to the greater thermal difference inseawater as the sea surface warms [61]. The deepdiving behavior of East’12 and West’13 minimized
metabolic energy costs [51] while searching for theirnatal rivers [30,31] or regulated body temperature
before spawning [62]. In fact, the surface temperature of the coastal waters of the Shiretoko Peninsula
during this experiment was approximately 20  C,
while that along the Sanriku coast was 10.5e12.5  C
in 1994 and 12  C in 1997, much lower than thatduring the present study.
Salmon trap nets passively trap ﬁsh, and catchesdepend on how often the target ﬁsh appear aroundthe nets. Salmon trap nets in the Shiretoko Peninsula typically extend from the surface to a depth ofapproximately 30 m. In other words, the catch increases when the frequency of chum salmon
swimming in waters shallower than 30 m increases.
As mentioned above, chum salmon prefer shallow
water when the surface temperature decreases andthe thermal difference between surface and deep
water becomes smaller [63]. Under these circumstances, the frequency of chum salmon occurringaround the salmon trap net increases, which is inagreement with Yatsu [64] and Kubo [65], who

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JOURNAL OF MARINE SCIENCE AND TECHNOLOGY 2023;31:515e528

Fig. 10. Relationship between the ambient temperature and swimming depth of West’13. The vertical axis represents ambient temperature ( C), while
the right vertical axis represents swimming depth (m). The upper and right histograms show the frequency distribution of swimming depth andambient temperature for West’13, respectively.

argued that catch increases when low sea surface
temperatures occur. Comparing the two behavioral
patterns of homing chum salmon observed in thisstudy, it is easily seen that East’12 and West’13 spent
most of their time moving at depths shallower
than 100 m. Conversely, West’12, which preferred tobe active at the surface and in deeper waters from
210 to 230 m, will less frequently be around thenets, as shown in Figs. 6, 8 and 10. This suggeststhat the behavioral patterns and surface occurrencesof chum salmon during their homing migration
have been altered due to the inﬂuence of the thermocline. The formation of a thermocline due to the

conﬂuence of currents is likely one of the reasons forregional variations in catch in the coastal areas of
the Shiretoko Peninsula.
In conclusion, our study clearly demonstrated thathoming chum salmon adjust their behavioral patterns according to the vertical water temperature
structure. Multiple water temperature structures
alter the behavioral patterns of chum salmon along
the coast of the Shiretoko Peninsula. Moreover, thisstudy provides evidence of homing chum salmon
avoiding thermoclines. Chum salmon tended tomove in the surface layer of seawater or deeper
waters away from the thermocline, which decreased

JOURNAL OF MARINE SCIENCE AND TECHNOLOGY 2023;31:515e528

the frequency of their appearance around thesalmon trap nets, which may be one of the reasonsfor the decrease in catch. One limitation of thisstudy is that the data logger recorded the continuous swimming behavior of East’12 from the time it
was released on the west side of the Shiretoko
Peninsula until it was retrieved on the east side, but
due to the absence of location tracking experiments,
we cannot determine when East12 entered theeastern waters of the Shiretoko Peninsula. Anotherlimitation is that due to a limited number of released
individuals and a short data collection period, we
couldn't establish the behavioral patterns of chumsalmon with varying caudal fork length, weight, sex,
and maturities. It's currently unclear whether
salmon with these differences exhibit distinct behaviors when encountering a thermocline, a question we will explore in future research.

Funding
Funding acquisition for this research was provided by Kazushi Miyashita, one of the authors.

Conﬂict of interest
The authors declare there are no competinginterests.

Acknowledgments
The authors would like to express their gratitude
to the individuals who have supported and assisted
in the development of this paper. Speciﬁcally, we
would like to acknowledge the staff of the Abashiri
Fishery Cooperative, Utoro Fishery Cooperative,
and Shari Fishery Cooperative for their valuable
support of our study. We also extend our recognition to the ‘Ushio-maru’ research vessel afﬁliatedwith Hokkaido University, Faculty of Fisheries, fortheir assistance in conducting logger experiments
and collecting environmental data. Additionally, we
would like to express our appreciation to the ﬁshermen and anglers who recaptured marked salmon,
as their participation was crucial to the success of
our research. Lastly, we would like to thank Editage(www.editage.com) for their professional assistancein editing the English language of the manuscript.

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