Case Report: Oceanic migratory behavior of chum salmon in the summer Bering Sea using a pop-up satellite archival tag

doi:10.21203/rs.3.rs-7337965/v1

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Case Report

Case Report: Oceanic migratory behavior of chum salmon in the summer Bering Sea using a pop-up satellite archival tag

https://doi.org/10.21203/rs.3.rs-7337965/v1

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Background

The Bering Sea is a key foraging habitat for Pacific salmon, but their migration ecology remains poorly understood due to the region’s remoteness. Chum salmon (Oncorhynchus keta) is one of the most abundant Pacific salmon species, and spends much of its life in the open ocean. To obtain preliminary information on its migration ecology, we deployed a pop-up satellite tag (PSAT) on a chum salmon (fork length: 530 mm; ocean age: 3) captured and released in the central region (56.0°N, 174.9°W).

Results

The PSAT surfaced 75 days later in the North Pacific Ocean, near the waters off Amchitka Island in the Aleutians (50.8°N, 179.3°E), transmitting depth, water temperature, light, and acceleration-based activity data. From 65 days post-release, the PSAT recorded sea surface temperatures above 20°C, implying predation by an endothermic fish. The estimated track indicated a southward movement within the Bering Sea, suggesting it continued foraging migration. The fish predominantly occupied surface waters (above 10 m) but occasionally dived below the thermocline (deeper than 40 m). No consistent diel vertical migration was observed; however, increased activity was detected during twilight and in dives occurring during the daytime and twilight periods, potentially linked to foraging behavior.

Conclusion

This pilot case study provides the first documentation of chum salmon migrating in the summer Bering Sea over a two-month period and new insights into their open-ocean behavior, such as crepuscular activity. However, further study with additional individuals is required to explore the variation and consistency of these patterns.

Pacific salmonid

open ocean

diving behavior

activity time series

crepuscular activity

Pacific salmon (Oncorhynchus spp.) are iconic and commercially valuable fishes throughout the northern Pacific Rim [1]. Most species are anadromous and typically spend 1–4 years in the ocean [1]. The Bering Sea functions as a crucial foraging habitat, particularly in summer, when various populations of Pacific salmon from Asia (Russia, Japan, and Korea) and North America (the United States and Canada) migrate into the region to feed and become widely distributed. Notably, for some stocks, both abundance and adult size depend on the feeding environment in the summer Bering Sea, raising concerns about the impact of ongoing global warming and increasingly frequent ocean heatwaves on their growth and survival [2]. However, knowledge of how salmon behave within and depart from this region is fragmented, and their oceanic migration ecology in this area remains poorly understood due to limited accessibility.

Chum salmon (O. keta) are one of the most abundant Pacific salmon species in the Bering Sea. They exhibit a brief freshwater life history, as the fry migrate to sea soon after emergence [3]. Consequently, they spend most of their lives in the ocean, generally returning to their natal rivers at ocean age (OA) of 3, the number of years spent overwintering at sea [1, 3]. Prior to return, chum salmon migrate widely across the North Pacific Ocean [1, 3]. However, oceanic distributions and migration routes vary among populations depending on geographic origin [1]. In the summer Bering Sea, Asian chum salmon are mainly distributed in the western and central region, whereas North American fish are distributed primarily in the east [4]. Distribution also differs even among Asian stocks: Russian fish dominate the western region, especially those from eastern Kamchatka, whereas Japanese fish increase towards the central area. Genetic stock identification has further suggested that Russian chum salmon migrate to the western North Pacific in winter, while Japanese and North American fish move into the eastern North Pacific, such as the Gulf of Alaska [5]. These stocks are thought to overwinter in the North Pacific Ocean and migrate seasonally between the Bering Sea and the North Pacific until initiating their spawning migration in spring or summer [4, 5]. Although the migration patterns of chum salmon stocks have been hypothesized, their detailed timing and routes throughout the life history (i.e., foraging and spawning migrations) remain unclear.

Animal-borne electronic tags have provided direct insights into fine-scale behavior and migration routes of fishes in the open ocean, such as the trans-oceanic migration of tuna [6] and the long-distance spawning migration of eels [7]. For chum salmon, only a few studies have employed animal-borne data loggers to investigate the swimming behavior (e.g., swimming depth, ambient temperature, and swim speed) of homing individuals returning from the Bering Sea to the coastal area [8, 9], or acoustic tags to describe short-term horizontal and vertical movements of homing or foraging salmon in this region [10]. With advances in miniaturization, pop-up satellite archival tags (PSATs) have been increasingly applied to track ocean-migrating salmonids (e.g., Chinook salmon O. tshawytscha: [11, 12]; Dolly varden Salvelinus malma: [13]; Steelhead O. mykiss: [14]), and provide direct information on their migration routes [15], vertical movements [11, 13, 14], and predation [11]. More recently, PSATs equipped with acceleration sensors have enabled the estimation of activity levels in fish [16]. In this study, we deployed a PSAT on a chum salmon in the central Bering Sea during summer to explore its behavioral patterns associated with foraging or spawning migration.

PSAT and Tagging

This study was conducted as part of a Japanese high-seas salmonid research cruise of the R/V Hokko maru from July 17 to August 27, 2024 [17]. Live, healthy chum salmon caught using hook-and-line gear were placed in an onboard recovery tank. A relatively large fish, caught at the northeast monitoring station in the Aleutian Basin (56.0°N, 174.9°W) on the evening of July 30 (UTC + 12H), was selected for tagging. The fish measured 530 mm in fork length (approximately 2 kg, estimated from length) and was identified as OA3 based on scale analysis. The salmon was equipped with a PSAT (MiniPAT: Wildlife Computers, Redmond, WA, USA; 118 mm in length, 38 mm in diameter, 61 g in air, approximately 3% of the tagged fish body mass) under anesthesia using FA100 (eugenol, 107 mg·ml\(\:{}^{-1}\); Bussan Animal Health Co., Ltd., Osaka, Japan) at 0.1 mg·l\(\:{}^{-1}\). Before tagging, the PSAT was connected to a 3D-printed attachment with Zylon braid thread. The base was secured with stainless steel wire at two points on the lateral side beneath the dorsal fin. The wires, threaded through the fish’s body, were passed through a backing plate on the opposite side and secured by twisting the ends. After an hour of recovery from anesthesia, the fish was released on July 30 at 20:10 (UTC + 12H).

The PSAT records depth, ambient temperature, acceleration, and light measurements at 3, 5, 0.1, and 15 s intervals, respectively. The records were summarized internally prior to transmission, and the resulting dataset—including time-series data downsampled to 10-min intervals and daily sea surface temperature (SST)—was then sent via the Argos satellite system. Due to the high sampling frequency, acceleration data were converted into an activity time series (ATS), defined as the number of high-activity events occurring within each 10-min interval. High-activity events were characterized by dynamic body acceleration (DBA) values exceeding an threshold determined by the distribution for each eight-hour time blocks (i.e., 00:00–08:00, 08:00–16:00, and 16:00–24:00). The threshold was derived from the minimum value, located on the negative side of the DBA distribution centered around the mean and projected toward the positive side [16]. The release mechanism of the PSAT was triggered either after a programmed deployment period or when constant pressure was recorded for three consecutive days. The programmed release was scheduled for 75 days after tagging.

Geolocation

Daily locations were estimated using a hidden Markov model developed by the tag manufacturer (WC-GPE3), a method widely used for marine fish tracking [18]. WC-GPE3 applies a gridded model based on the methods of a previous study [19], using sunrise/sunset times, SST, and maximum depth, and a random walk movement model to estimate likelihood profiles across a state space of 0.25 grids. Periods of presumed predation were excluded from the estimation. The maximum daily migration speed for calculating the migration path was set to 0.6–1.2 m·s\(\:{}^{-1}\), based on empirical values from a propeller data logger (0.5–0.9 m·s\(\:{}^{-1}\), [9]) and the values used in previous studies on steelhead and Chinook salmon (100 km·day\(\:{}^{-1}\) \(\:\approx\:\) 1.16 m·s\(\:{}^{-1}\), [14]). Estimation scores provided by WC-GPE3 were compared across models to assess fit. Total distance traveled was calculated by summing great-circle distances between daily positions using the distGeo function in the “geosphere” package in R (v. 4.1; [20]).

Time-series data analysis

Depth, temperature, and ATS data at 10-minute intervals were processed using Igor Pro V8.1 with the “Ethographer” add-on [21], including labeling and extraction. Statistical analyses were conducted in R. To examine diel patterns, solar altitude was calculated using the sunAngle function in the “oce” package in R. Coordinates for the calculations were set as the midpoint between the release and pop-up locations. Daytime and nighttime were divided by solar altitude at − 6°, and twilight as − 6–6°.

Pacific salmon are known to exhibit repeated V-shaped vertical movement patterns, referred to as “diving bouts” [9]. In this study, the V-shaped vertical movements were defined as diving behavior, and diving bouts were identified as periods when the fish was deeper than 10 m, including the preceding and following time points (i.e., descent and ascent phases), following the criteria of the previous study [9]. The proportion of time spent diving was compared among diel periods using Pearson’s chi-squared test. When significant, post hoc pairwise proportion tests with Bonferroni-adjusted p-values were applied.

To examine the effects of diel period and diving behavior on activity, a generalized linear model (GLM) was applied, treating both factors as categorical variables. As the activity data comprised positive integers and high-activity events were rare, a negative binomial distribution was assumed for the residuals (glm.nb function in “MASS” package). Pairwise comparisons among levels of the fixed effects (e.g., diel period: daytime vs. nighttime) were conducted using the “emmeans” package, with p-values adjusted by the Bonferroni method. To further investigate the relationship between activity and solar altitude, a generalized additive model (GAM) was applied (gam function in “mgcv” package), incorporating thin plate regression splines for solar altitude and stratified smooth terms for dive behavior (i.e., s(solar altitude, by = dive)). GLMs and GAMs were compared using Akaike’s information criteria (AIC) and Bayesian information criteria (BIC) to identify a parsimonious model.

Tracking profiles

The PSAT surfaced as scheduled 75 days after the tagged salmon was released in the North Pacific near the waters off Amchitka Island, Aleutians (50.8°N, 179.3°E), approximately 700 km southwest of the release site (Fig. 1). Over the following seven days, the PSAT transmitted data, and 50.4% of the time-series data on depth, water temperature, and ATS were successfully retrieved (Fig. 2). From 65 days post-release, the PSAT recorded SST exceeding 20°C (Fig. S1), suggesting predation by an endothermic fish. Therefore, the migration path was estimated excluding the suspected predation period. The model scores increased with the assumed swim speed, and the configuration with a maximum daily migration speed of 1.2 m·s\(\:{}^{-1}\) yielded the highest score (Table S1). The estimated migration path showed a southward movement from the release site but remained within the Bering Sea (Fig. 1). The cumulative distance of the likely migration paths was 588 km during 65 days (Fig. 1B), corresponding to an average speed of approximately 8.6 km·day\(\:{}^{-1}\).

Depth, temperature, and activity profiles

The tagged chum salmon spent most of its time in the surface layer (< 10 m), and predominantly experienced the surface temperature of 9–10°C (Fig. 2A; Fig. 3). The ATS, representing the number of high-activity events, ranged 3–125 and was also occasionally elevated (Fig. 2B). The fish occasionally dived below 10 m during certain periods (Fig. 2A), and some dives extended beneath the thermocline (20–40 m). The frequency of deep dives (> 30 m) decreased around August 19, when the water mass structure changed, and the thermocline deepened from approximately 20 m to 40 m (Fig. 2A; Fig. 3). After August 20, the salmon rarely experienced water temperature below 7°C (Fig. 3).

Clear diel patterns in the diving bouts and ATS were not consistently observed (Fig. 4A, C); however, around twilight (solar altitude: −6°–6°), the tagged salmon tended to exhibit diving behavior (Fig. 4B) and the ATS increased (Fig. 4C). Additionally, ATS was elevated during diving, particularly during daytime and twilight (Fig. 4D). Indeed, the time proportion accounted for diving significantly differed between diel periods (chi-squared test: \(\:p<0.01\)), and the GLM of ATS, including diel period and diving behavior, showed the lowest AIC and BIC (Table S2).

To further explore diel patterns in ATS and their link to diving behavior, GAM was used to examine the relationship among the variables. The GAM, including solar altitude, diving behavior, and their interaction (i.e., solar altitude stratified by diving behavior) as fixed effects, had the lowest AIC and BIC (Table S3). Model predictions supported (1) increased in ATS around twilight (solar altitude: −6–6°) during both diving and non-diving periods, with higher values during diving, and (2) elevated ATS during daytime diving (Fig. 5, Fig. S2).

The Bering Sea has long been recognized as a crucial foraging area for Pacific salmonids [2, 4, 22, 23], yet their migration ecology in this area remains elusive due to its remote location. A pioneering study in the 1990s tracked the Pacific salmon in the Bering Sea using acoustic telemetry [10], but the tracking lasted a few days, and longer tracking is needed to explore their migration. The advent of PSATs has enabled the monitoring of the migratory behavior of marine fishes, spanning periods from months to years [7, 24]. In contrast to the earlier short-term study, this study marks the first successful deployment of a PSAT on a chum salmon migrating offshore in the Bering Sea, documenting its swimming behavior—including migration path and acceleration-based activity—over a two-month period, albeit for a single individual. The origin of the tagged fish was unclear, but it was likely of Asian origin, particularly Russian or Japanese stocks, as Asian fish dominate in the central region (> 80%) [5], with 76.3% from Russia and 18.6% from Japan in summer 2024 based on genetic stock identification (K. Honda, unpublished data).

Predation

The PSAT recorded high SST exceeding 20°C 65 days after the tagged salmon was released, indicating its predation by endothermic animals, particularly fish (Fig. S1). Salmon shark (Lamna ditropis) is a major predator of Pacific salmon in the Bering Sea, and the temperature around 20°C coincides with their stomach temperature [11]; therefore, the tagged salmon was likely predated upon by salmon shark. In addition to sharks, Pacific salmonids are targeted as prey by a variety of marine predators, including marine mammals and ectothermic fish [11, 25], and are known to respond to their attacks [25]. Deep, steep dives have been suggested as a predator-avoidance strategy in Pacific salmonids [25], and some of the short deep dives (> 50m) observed in the tagged salmon may therefore reflect evasive responses to predators.

Migration speed and the state of tagged fish

The tagged salmon was estimated to have traveled 588 km over 65 days, 9.0 km·day\(\:{}^{-1}\). Pacific salmon generally travel at speeds of 30–60 km·day\(\:{}^{-1}\) when returning to coastal waters [9, 23], which is faster than the tagged salmon in this study. Little is known about the factors affecting the migration speed of Pacific salmon, but as a possible reason, the mature condition is considered a potential factor. A previous study using acoustic telemetry to track presumably immature and maturing individuals of four Pacific salmon species, including chum salmon, over 1–3 days reported the differences in their migration speed and path: mature salmon migrated faster and followed straighter routes than immature salmon [10]. The maturing salmon in the study moved at speeds of 40–74.5 km·day\(\:{}^{-1}\) (n = 2), while the presumably immature salmon moved at speeds of 27.9–53.5 km·day\(\:{}^{-1}\) (n = 4) [10]. Additionally, Russian and Japanese chum salmon return to their natal river in August–November, September–December, respectively [3], so it is difficult to expect that the tagged fish could reach those areas by November or December. Given its relatively slow migration speed, the tagged salmon was likely still engaged in foraging migration. If so, the migration pattern would be consistent with the migration hypothesis for Asian chum salmon based on fishing surveys [5], which suggests these fish migrate out of the Bering Sea and into the northern Pacific and the Gulf of Alaska between September and November.

Vertical distribution and movement

The tagged salmon primarily remained in the surface layer (< 10 m) and occasionally dived. No consistent diel patterns were observed in its vertical distribution or diving behavior, generally aligned with previous studies on Pacific salmonids [12, 14, 15]. Meanwhile, their diel vertical migration has been observed under some circumstances [12, 26], and multiple factors, such as season, hydrographic conditions, prey availability, and predator avoidance, may influence their vertical movement patterns [12, 14]. Indeed, also in this study, the number of deep dives of the tagged salmon decreased after August 19, possibly due to changes in the vertical stratification of the water column (Fig. 3). Although no diel patterns in vertical distribution were observed, the time proportion occupied by diving was highest during twilight (Fig. 4B), suggesting increased activity during this period. Consistently, ATS increased during the period, supporting the interpretation of elevated activity during twilight (Fig. 4C, D; Fig. 5C). Twilight is typically a period of high activity in both lake and oceanic environments, when zooplankton migrate from deep to shallower layers, their predators aggregate to feed on them, and many animals exhibit heightened foraging activity [27], and since foraging is recognized as a key driver of diving behavior in marine pelagic fishes [14, 28], the increased dive time proportion and elevated ATS during twilight likely reflect the foraging activity of the tagged salmon.

The activity data further provides insights into diel differences in diving. During daytime and twilight, ATS increased during dives compared to when the fish remained at the surface layer, whereas, at night, the difference in ATS between diving and surfacing diminished (Fig. 4D; Fig. 5C). The cause of this difference in diving’s effect on ATS remain unclear, but it may relate to the foraging behavior of chum salmon. In the summer Bering Sea, chum salmon forage continuously throughout the day, indicated by stomach content analysis [29, 30], but the prey items vary depending on the time of day [22]. During the daytime, their diet consists largely of juvenile and larval fish, while at night, it is dominated by planktonic prey, such as gelatinous zooplankton (medusae, ctenophores, and salps), copepods, euphausiids, and hyperiid amphipods [22]. Since chum salmon are visual predators, they likely forage actively for nektonic prey during daytime and twilight dives, which may explain the increased ATS during twilight.

This study marks the first successful deployment of a PSAT on a chum salmon in the summer Bering Sea, likely during foraging migration, offering new insights into their oceanic migratory behavior, despite the limited sample size. While vertical movement patterns similar to those reported in previous studies were observed, the integration of acceleration-based activity data provided new evidence suggesting a crepuscular foraging pattern. The activity data also indicated elevated behavioral vigor during daytime and twilight dives, potentially linked to prey availability. The Bering Sea serves as a crucial foraging habitat for Pacific salmon, but recent ongoing global climate changes have raised concerns about the negative effects on their growth and further survival; however, knowledge about their spatio-temporal patterns in this area is still limited. Further electronic tagging efforts involving multiple fish over multiple years would reveal the consistency and variability of oceanic migratory behavior in chum salmon, ultimately contributing to the mechanistic understanding of climatic effects on the Pacific salmon species.

AIC

Akaike’s information criteria

ATS

activity time series

BIC

Bayesian information criteria

DBA

dynamic body acceleration

GAM

generalized additive model

ocean age

PSAT

pop-up satellite archival tag

Ethics approval and consent to participate

All experimental procedures regarding tagging were conducted following the guidelines of the Animal Ethics Committee of the University of Tokyo, and the protocols of the study were approved by the same committee (P24-5).

Consent for publication

Not applicable.

Availability of data and materials

The datasets used and/or analyzed during the current study are available from the corresponding author upon reasonable request.

Competing financial interests

The authors have no conflict of interest, financial or otherwise, in relation to this study.

Funding

This study was financially supported by the Research and Assessment Program for Fisheries Resources of the Fisheries Agency of Japan, the Japan Society for the Promotion of Science (JSPS) [Grant numbers 16H06541, 19H04927, 21H05296, 23KJ1983 and 25K02080], and Japan Science and Technology Agency (JST) Core Research for Evolutional Science and Technology (CREST) [Grant number JPMJCR23P2 to ST].

Author contributions statement

Conceptualization: T.K.A., T.K.; Methodology: T.K.A.; Software: T.K.A.; Validation: T.K.A.; Formal Analysis T.K.A.; Investigation: T.K.A., K.H., S.O, T.S., Y.I.; Writing: T.K.A; Supervision: T.K., K.H.; Project administration: T.K., K.H.; Funding acquisition: K.H., T.K., T.K.A., Y.M., S.T.

Acknowledgments

We sincerely thank the crew of R/V Hokko maru for their support in specimen collection for this study. This research was supported by the Cooperative Program [Program number JURCAOSKAV24-70] of the Atmosphere and Ocean Research Institute, the University of Tokyo. Additionally, we thank Sabrina Garcia for her valuable assistance during the research voyage. During the preparation of this study, we used Grammarly and ChatGPT to correct grammar. After interacting with them, we meticulously reviewed and made the necessary adjustments to the content.

ORCID

Takaaki K. Abe: 0000-0002-3021-323X
Kentaro Honda: 0000-0003-1967-9263
Yuki Iino: 0000-0003-3425-3789
Tomoki Sato: Not applicable
Satoki Oba: Not applicable
Yuya Makiguchi: 0000-0001-8148-9394
Susumu Takahashi: 0000-0001-6257-4550
Takashi Kitagawa: Not applicable

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No competing interests reported.

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Case Report: Oceanic migratory behavior of chum salmon in the summer Bering Sea using a pop-up satellite archival tag

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