Trans-Atlantic movements of Atlantic white-sided dolphins, Leucopleurus acutus

Abstract

Atlantic white-sided dolphins are wide-ranging and abundant cetaceans of the North Atlantic, yet their movements remain poorly understood. Using satellite telemetry, we tracked 23 dolphins tagged in the Faroe Islands to investigate their movement patterns, habitat use, and diving behavior. Our findings confirm a strong association with the shelf edge and identify the Irminger Sea and the Faroe-Shetland Channel as important regions. The observed movements align with oceanographic features that enhance productivity and prey availability, including strong mixing zones and the Irminger Gyre with deep mixed layers. Three dolphins independently undertook trans-Atlantic migrations to the Irminger Sea, where two remained for extended periods (26 and 63 days). Together with dive records and lower move persistence, this suggests that the Irminger Sea functions as an important autumn feeding ground. Dive data (n = 4) revealed a wide depth range (3–616 m) and diel diving patterns consistent with exploitation of vertically migrating mesopelagic prey. Identifying critical habitats and understanding trans-Atlantic connectivity are essential for effective conservation and management of this species, particularly given ongoing environmental changes in the North Atlantic. The post-release separation of tagged dolphins reflects their fluid social structure, suggesting a panmictic stock in the central and eastern North Atlantic. The study highlights the value of movement data for understanding habitat use, distribution, effect of ocean dynamics, and population structure in pelagic predators.

1 Introduction

The management and conservation of migratory marine top predators benefit from understanding their movements and how these relate to oceanographic features (Block et al., 2011; Hays et al., 2019). These features, influenced by both natural variability and anthropogenic environmental changes, complicate the understanding of species movements. The cumulative impact of environmental change is particularly pronounced in the Subpolar North Atlantic (SPNA), driven by both convective and advective processes (Hátún et al., 2017). This region, with the subpolar gyre (SPG) dominating the physical dynamics, maintains a productive ecosystem that sustains several marine predators, including Atlantic white-sided dolphins (Leucopleurus acutus) (Cipriano, 2018), hereafter referred to as white-sided dolphins. Over recent decades, environmental variability has altered SPNA ecosystems (Beaugrand, 2009; Hátún et al., 2009). This has involved declines in pre-bloom silicate concentrations (Hátún et al., 2017) and declines in prey such as copepods (Hátún et al., 2009) and krill (Edwards et al., 2021) throughout the SPG, potentially driving baleen whales (Víkingsson et al., 2015), and boreal fish species like mackerel and blue whiting from their spawning grounds west of the British Isles, toward the nutrient-richer waters of the Irminger Sea (Pacariz et al., 2016; Post et al., 2020). These ecosystem shifts indicate that the Irminger Sea may function as a multi-species feeding hotspot in the SPNA.

The white-sided dolphin inhabits cold-temperate to sub-polar waters (5°-16°C) along the continental shelf and slope, from the Gulf of St. Lawrence and the Labrador Sea in the west to the North Sea and the Norwegian Sea in the east (Cipriano, 2018). They primarily prey on mesopelagic and pelagic fish species, such as blue whiting (Micromesistius poutassou), Trisopterus sp., mackerel (Scomber scombrus), and myctophids (lanternfish) in northeast Atlantic waters (Couperus, 1997; Hernandez-Milian et al., 2016). Genetic studies suggest strong connectivity across the North Atlantic (Fernández et al., 2016; Gose et al., 2023). There is evidence of panmixia (random mating across the population), low individual relatedness (minimal genetic similarity between individuals), and a fission-fusion social structure (dynamic group formation and separation), indicating a lack of kin-associated bonds (Pugliares-Bonner et al., 2021; Gose et al., 2023).

White-sided dolphins are considered sensitive to distributional shifts driven by climate change (MacLeod, 2009; Skern-Mauritzen et al., 2022) and are subject to bycatch, and to hunts in Greenland and the Faroe Islands (Cipriano, 2018; NAMMCO, 2023). Yet, information on their offshore movements and habitat use remains limited. Satellite telemetry can help fill these gaps and improve our understanding of the species’ biology (Nowacek et al., 2016; Hays et al., 2019), which is critical for effective conservation and management (Braulik, 2019; NAMMCO, 2023). By providing information on distribution, residence time, and diving behavior, tracking top predators also helps identify biologically important areas, thus supporting ecosystem-based management (Hays et al., 2019; Skern-Mauritzen et al., 2022). Such data are especially important in the context of environmental change in the SPNA (Ramírez et al., 2017; Heide-Jørgensen et al., 2022). Here, we present movement data from healthy, free-ranging white-sided dolphins tagged in the Faroe Islands, revealing individual trans-Atlantic journeys and identifying key areas within their range. Our study provides new insights into the large-scale movements of this understudied marine predator and highlights the ecological significance of the Irminger Sea as a potential feeding hotspot.

2 Materials and methods

2.1 Study area

In the Subpolar North Atlantic, the SPG plays a key role in shaping physical oceanography, nutrient cycling, and biological productivity (Figure 1; Hátún et al., 2009). Although a limb of the SPG extends into the Iceland Basin, biological production is higher in the Irminger Sea (Gudfinnsson et al., 2008), mainly due to strong winter convection. Interannual variability in convection can be tracked by the winter mixed layer depths (MLD; Hátún et al., 2016), and is most pronounced in the Irminger Gyre, at the center of the southern Irminger Sea (Figure 1). During periods of intensified convection, MLD within this ‘mixing hole’ can exceed 1500 m (de Jong and de Steur, 2016; Sterl and de Jong, 2022). Such periods are associated with increased zooplankton abundances (Hátún et al., 2016), which serve as prey for krill and mesopelagic fish (Silva et al., 2014).

Figure 1

NOAA, 2022), with the 500 m isobath (GEBCO), representing the shelf edge, marked in white. Map generated with the R package ggOceanMaps (Vihtakari, 2024).

Myctophids dominate the mesopelagic biomass in the Irminger Sea, with the widely distributed glacier lantern fish (Benthosema glaciale) being particularly abundant (Dolgov, 2015) and feeding mainly on krill and Paraeuchaeta copepods (Sigurðsson et al., 2002; Dolgov, 2015). The Deep Scattering Layer varies in thickness, but its upper boundary remains stable at ~400–500 m throughout the year, with parts of the layer undergoing diel vertical migration (Magnússon, 1996). When the SPG weakens and sea surface temperatures rise, pelagic species extend into the slope waters of the northwestern Irminger Sea (MacKenzie et al., 2014; Post et al., 2020), while mesopelagic biomass continues to dominate the central Irminger Sea (Sigurðsson et al., 2002).

A ‘corridor’ between the northern SPG boundary and the Greenland-Scotland Ridge is influenced by Subpolar Mode Waters (McCartney and Talley, 1982). This corridor is characterized by strong vertical mixing and deep MLDs (Hátún et al., 2021), strong nutrient upwelling (Hátún et al., 2017), and high biological productivity (Pacariz et al., 2016). In contrast, the central Iceland Basin becomes stratified early in summer, leading to nutrient depletion near the surface and low zooplankton abundances (Pacariz et al., 2016). Deep overflows from the Faroese channels induce mixing in the overlying Subpolar Mode Waters, south of the Iceland-Faroe Ridge, south of the Iceland slope (de Marez et al., 2024), and on the Wyville-Thomson Ridge (Sherwin et al., 2008).

2.2 Capture, tagging, and release of dolphins

A total of 23 satellite transmitters were deployed on dolphins across six tagging events (two in 2009, one in 2022, and three in 2023; Table 1), with support from Faroese traditional drive hunting expertise. When a group was sighted close to shore, local boats guided the dolphins to the nearest authorized sandy beach with government approval. In ~40 cm of water, bystanders held the dolphins upright to prevent them from rolling over or beaching entirely (Bloch et al., 2003). Boats remained nearby to guide the group back to open water after tagging. Each operation—from herding to release—lasted 45–50 minutes.

We selected mature dolphins for tagging based on body size. The length of each selected dolphin was measured by standard procedures, and sex was initially determined by physical examination when possible (Bloch et al., 1993; Table 1). While two people held each dolphin upright, others attached the tag to its dorsal fin. Three holes were made in the dorsal fin using an 8 mm cork borer attached to a battery-powered drill. Through these holes, the tags were secured with Delrin or stainless-steel pins covered with silicone tubing, steel nuts, and a rubber backplate to protect the fin. Following Nielsen et al. (2018), our setup was designed for long-term attachment to collect data for the entire duration of the transmitters’ battery life. We used a new cork borer for each dolphin and saved the tissue samples for genetic analysis at the Globe Institute, University of Copenhagen. Sex was genetically determined from whole-genome sequencing data generated for another project, based on the sequencing depth ratio between sex chromosomes and (ChrX: ChrY ≈ 1:1 = male; markedly lower depth on ChrY = female) (Table 1).

On 30 August 2023, we deployed a stomach temperature pill (STP) that recorded the dolphin’s stomach temperature, where abrupt drops indicated feeding events (Heide-Jørgensen et al., 2014). The capsule-shaped sensor was inserted into the esophagus using an intubation tube. The dolphin was also fitted with a dorsally mounted SPLASH Mk10 satellite tag, secured with three 8 mm Delrin pins. The satellite tag received temperature and depth data from the STP and transmitted these along with other recorded data.

2.3 Transmitter settings and data processing

We used SPOT and SPLASH Mk10 fin-mounted satellite tags (Wildlife Computers, Redmond, WA; Table 1), which transmitted location and temperature data via the Argos system when the wet/dry sensor registered dry conditions. Tags were programmed to transmit primarily during periods of high satellite coverage (based on Argos Pass Prediction) to maximize the likelihood of high-quality location data. Variation in transmission schedules and data collection settings reflects the involvement of different researchers and the evolving understanding of tag performance across deployments (Supplementary Table S1). No duty cycling was applied, although transmission settings were relatively conservative to preserve battery life. Data were retrieved from the Wildlife Computers portal and processed in R 4.5.1 (R Core Team, 2025).

Individual raw Argos location data were combined into a single dataset, and positions recorded prior to tag deployment were removed. To filter and time-regularize the tracks while accounting for Argos-specific location uncertainty, we applied a random walk (RW) state-space model using the fit_ssm function from the R package aniMotum (v1.2-14; Jonsen et al., 2023). A correlated random walk (CRW) model was initially considered, but due to large data gaps (mean interval: 0.6–5.9 h across individuals; maximum: 46.1 h) it introduced unrealistic looping artefacts when interpolating through the data gaps. The maximum swim speed threshold was set to 3 m s⁻¹ (Mate et al., 1994; Sampson et al., 2012), and a 3-hour time step was applied to preserve finer-scale movement details and biological interpretability (Jonsen et al., 2023). Only tracks with >20 observed locations (n = 16) were included. The resulting time-regularized and filtered tracks were used in all subsequent analysis.

2.4 Movement analyses

To estimate total distance travelled, we summed distance between consecutive locations calculated using the Haversine formula. For the three dolphins making trans-Atlantic movements, we defined journey segments between fixed lines at the Faroe Shelf (63.66°N, 14.47°W to 59.94°N, 15.28°W) and past the Reykjanes Ridge (63.11°N, 25.46°W to 59.86°N, 30.90°W). Within these segments, point-to-point speeds were calculated as distance divided by elapsed time, with unrealistic speeds removed using the interquartile range method. Weighted average speeds were calculated by weighting each speed by its time interval. Journey duration was measured as time between the first and last points within the segment. For the two dolphins remaining in the Irminger Sea, residence time was calculated from the first entry (crossing the end line) until the end of their tracks.

2.4.1 Move persistence analysis

To contextualize the movement patterns, we applied a move persistence model to the regularized tracks derived from the RW state-space model with the fit_mpm function from the R package aniMotum (v1.2-14; Jonsen et al., 2023). A move persistence value (gamma, γ; range: 0 to 1) is assigned to each predicted position, where values near 1 indicate directed travel (transit) and values near 0 reflect tortuous movements characteristic of area-restricted search (ARS) that may be associated with foraging. We chose a move persistence model (model = mpm) with an unpooled random variance parameter to preserve within-individual behavioral variation and improve identification of potential ARS behavior along each track.

Temporal gaps in transmissions resulted in interpolated positions, which can bias move persistence estimates by propagating values across these intervals. Therefore, we interpreted move persistence values qualitatively, focusing on regional patterns of higher versus lower persistence rather than fine-scale variation at individual steps.

2.4.2 Habitat associations of horizontal movements

We assessed horizontal dolphin movements in relation to bathymetry (GEBCO 1-min grid; www.gebco.net) and an oceanographic variable. For each predicted dolphin location, we calculated the shortest geodesic distance to the 500 m isobath, which we used to represent the shelf edge (Pacariz et al., 2016). Dolphin distances were compared with distances from an equal number of randomly sampled locations within the study area, restricted to >100 m depth, using a Wilcoxon rank-sum test. This comparison was done for all dolphins combined and separately for the five dolphins that travelled west.

Winter mixed layer depth (MLD) was used as a proxy for biological productivity along the dolphin migration route. Winter MLD was generated by averaging the monthly CMEMS dataset [March 1993–2019; GLORYS12V1. E.U. Copernicus Marine Service Information (CMEMS)] for periods when the maximum MLD within 58°–65°N, 45°–35°W exceeded 1000 m, following Sterl and de Jong (2022). For the five westward-travelling dolphins, each predicted location was matched to the nearest MLD grid point and compared to background MLD values across the study area (56°–66°N, 43°–10°W). Both dolphin tracks and background datasets were matched with GEBCO bathymetry and filtered for >200 m depth. Differences between dolphin-associated and background MLD values were tested using a Wilcoxon rank-sum test.

Additional environmental and biotic data, such as mixing zones and prey distribution, were obtained from published literature.

2.5 Dive and behavioral data

In addition to location data, SPLASH Mk10 tags collected dive data. These were summarized over 6- or 24-hour periods depending on tag settings (Supplementary Table S2). Within each period, dives were grouped into predefined bins by depth, duration, time-at-depth, or time-at-temperature, with bin ranges varying by deployment. Four tags also recorded real-time behavior messages that summarized clusters of typically five consecutive dives, including depth, duration, and shape; these were linked to the predicted locations via timestamps. For each dive, we calculated the mean of the recorded minimum and maximum values for depth and duration, and used these values for further analysis.

To assess diel diving patterns, the day (24 hours) was divided into three light-based periods: daylight, night, and twilight. Each dive was assigned to one of the periods based on the solar time of the time and location it was recorded, calculated using the suncalc package (v0.5.1; Thieurmel and Elmarhraoui, 2022). To account for varying period durations, dive frequency was calculated as dives per hour for each period. We used a linear model (lm) to test for differences in dive rate between periods. For dive depth and duration, we used linear mixed-effect models (lmer) with individual dolphin ID as a random effect to account for non-independence of multiple observations from the same individual. Pairwise comparisons were done with Tukey’s HSD test. Model assumptions were assessed by examining residual plots and testing normality with Shapiro-Wilk tests.

The tag paired with the stomach temperature pill (STP) was configured to generate both behavioral and stomach temperature messages. Ingestion events were detected based on abrupt temperature drops (>0.6°C min⁻¹) or absolute stomach temperature thresholds (<34°C), with fine-scale sampling during events and coarse-scale sampling otherwise. The maximum duration of an ingestion event was set to 120 min. The data were explored in Excel.

3 Results

3.1 Tag performance

Satellite tag performance varied considerably across deployments, with 21 of 23 tags providing location data (Table 1). Tags #8 and #17 did not transmit, and tag #7 only recorded one location and was therefore discarded. The remaining 20 tags yielded a combined total of 521 tracking days from June to December, with an average track duration of 26.1 days (SD = 40.8, range: 2–175 days). Nine tags transmitted location data for more than ten days.

3.2 Horizontal movements

All dolphins left the Faroe Shelf within two days of tagging, moving to the shelf edge and deeper waters (Supplementary Figure S2). Dolphins tagged together separated within days, a pattern consistent across all six tagging events. Travelled distances ranged from 270−10,956 km (mean = 2,153 km) and extended 167−1,899 km (mean = 558 km) from the first position.

Movement patterns varied by tagging year. Of the 17 dolphins tagged in 2009 and 2023, 12 visited the Faroe-Shetland Channel (FSC), and nine remained there for most of their tracking period—particularly those with shorter transmissions. In contrast, none of the six dolphins tagged in 2022 entered the FSC.

Five dolphins (IDs 10 and 11 in 2022, and IDs 15, 20, and 22 in 2023) travelled westward at different times (Figure 2A). Two stopped transmitting near the Iceland-Faroe Ridge, while three crossed the Reykjanes Ridge to reach the Irminger Sea in October. Dolphins #10 and #22 remained in the Irminger Sea, respectively for 26 and 63 days, until transmission ceased. Dolphin #11 travelled more offshore, along the Reykjanes Ridge, with transmissions ceasing when it reached the Irminger Sea. These three trans-Atlantic crossings lasted 5 to 11 days (mean = 8.0 days, SD = 2.9 days), with weighted average swim speeds ranging from 3.6 to 4.8 km/h (mean = 4.3 km/h, SD = 0.7 km/h).

Figure 2

NOAA, 2022), with the 500 m isobath (GEBCO) as a white contour line. (B) Deepest depths of dives recorded from dolphin #22, with each bar representing one dive. Dives were transmitted in clusters of five (except for numbers 5 and 6, where the tag managed to transmit two clusters), collected within 24-hour periods and shown grouped by date. Each cluster shares a number that matches the map in panel (A). Each dive is color-coded by time of day, based on the solar time corresponding with the time and location of the dive.

3.2.1 Move persistence

The move persistence model revealed distinct regional patterns in movement behavior across the North Atlantic (Figure 3). Move persistence values were generally high during westward trans-Atlantic movements, indicating travel. In contrast, lower move persistence was identified north of the Faroe Shelf edge, in the FSC, the Wyville-Thomson Ridge, and the Irminger Sea, indicating ARS.

Figure 3

NOAA, 2022).

3.2.2 Habitat associations

Dolphin movements (n = 16) showed strong associations with bathymetric and oceanographic features. All dolphins occurred significantly closer to the shelf edge than expected by random distribution (Wilcoxon rank-sum test: p < 0.001), with an average of 64.8 km from the 500 m isobath (SD = 71.8 km, maximum: 404.7 km). The five dolphins that travelled westward maintained similar proximity to shelf edges (mean = 90.5 km, SD = 85.3 km) and showed significant association with deeper mixed layer depths. The average MLD along these five tracks was 490.1 m (SD = 209.7 m), compared to 366.4 m (SD = 159.8 m) across the ocean basin in the study area (Wilcoxon rank-sum test, p < 0.001; Figure 4B).

Figure 4

3.3 Dive and behavioral data

Two types of dive data were collected across the three deployment years: binned dive summaries (n = 8) and detailed behavioral dive profiles (n = 4) (Table 1).

Binned data revealed that 94.5% of the dives (weighted mean, SD = 9.1%, n = 3887) occurred shallower than 100 m. Similarly, dive durations were generally short, with 53.7% of the dives (weighted mean, SD = 19.7%, n = 5105) lasting under 60 seconds.

The behavioral dataset comprised a total of 70 dives, with uneven sampling: dolphin #22 recorded 55 dives while dolphins #13, #18, and #23 recorded five dives each (Table 2). The average dive depth ranged from 3 to 616 m (mean = 171 m, SD = 186 m) and average duration from 26 to 418 s (mean = 191 s, SD = 94 s). Most dives (41%) were shallow (0–40 m), while 19% reached between 400–616 m. The single deepest dive reached 616 m and lasted 6.8 minutes.

3.3.1 Diel dive patterns

Dive behavior varied significantly across diel periods (Figure 5). Dive rate differed significantly across daylight, twilight, and night periods (F(2, 23) = 24.9, p < 0.001, R² = 0.68; Figure 5B). Despite having the fewest recorded dives, twilight showed the highest dive rate (1.71 dives/hour, SE = 0.18), significantly exceeding both night (0.43 dives/hour, SE = 0.07; p < 0.0001) and daylight (0.40 dives/hour, SE = 0.07; p < 0.0001). No significant difference was observed between night and daylight dive rates (p = 0.97).

Figure 5

Dive characteristics also showed strong diel variation (Figures 5C, D). Dives were significantly deeper and longer during twilight and night compared to daylight (all p < 0.0001). Pairwise comparisons showed that daylight dives were ~279 m shallower and ~142 s shorter than twilight dives, and ~257 m shallower and ~114 s shorter than night dives (all p < 0.001). No significant differences were found between night and twilight for either depth (p = 0.94) or duration (p = 0.63).

3.3.2 Individual behavioral profiles

The 55 dives recorded by dolphin #22 throughout its transmission period offer further insights into the trans-Atlantic journey (Figure 2). Dive clusters were transmitted approximately every seven days, with dives recorded south of the Faroe Islands (nr 1), in the Iceland Basin (nr 2), and along East Greenland (nr 3-9). In the Irminger Sea (nr 7–9), 10 of 15 dives exceeded 400 m depth. These deep dives occurred mainly at night, whereas shallow dives were typically recorded during daylight (Figure 2B).

Dolphin #23 equipped with a stomach temperature pill (STP) provided limited feeding data. The first detected feeding event occurred 36 hours after tagging, at night, followed by two additional events—one within the same hour and another five hours later around 8:00. These three feeding events, with maximum depths of 17.5 m and 18.5 m, occurred on the Faroe Shelf before the STP left the stomach prematurely.

4 Discussion

Our study shows, for the first time, the capacity of Atlantic white-sided dolphins to undertake cross-basin migrations at consistent travel speeds over multiple days. The tracks of 20 tagged dolphins confirm that the species is usually encountered near the continental shelf and slope (Cipriano, 2018). Dolphin locations were significantly closer to the 500 m shelf edge than expected by chance. The five dolphins that went west were also associated with regions of deep mixed layer depths (MLD). Two areas of specific interest emerged: the Faroe-Shetland Channel (FSC) and the Irminger Sea.

The dolphins tagged during the same event dispersed shortly after tagging, likely reflecting their fluid social structure (Gose et al., 2023), rather than an effect of tagging. Eleven dolphins moved east after leaving the Faroe Shelf, with nine remaining in the FSC throughout most of their transmission periods. This aligns with models predicting year-round presence of the species in the area, with peak abundance in summer (Waggitt et al., 2019). The dolphins’ extended use of the FSC further underscores the ecological importance of the region, which hosts a diversity of cetaceans and seabirds (Waggitt et al., 2019; Virgili et al., 2024), along with commercially important prey such as blue whiting and mackerel (ICES, 2023, in-house data) and has been identified as an area of international importance (Skov et al., 2002). South of the FSC, near the Wyville-Thomson Ridge, dolphin #22 dove to depths of 200–300 m, possibly targeting blue whiting, myctophids, and pearl side (Maurolicus muelleri) (Couperus, 1997; Hernandez-Milian et al., 2016). The region is characterized by intense mixing driven by the equatorward flow of deep-water (Sherwin et al., 2008), and fisheries surveys confirm the high abundance of mesopelagic fish and other prey species (ICES, 2023, in-house data). Five dolphins travelled westward, all but one appearing to follow the productive southern slope of the Iceland-Faroe Ridge (de Marez et al., 2024). Two tags stopped transmitting, while the three others showed independent trans-Atlantic journeys—separated in time—to the Irminger Sea, a region frequented by top consumers like baleen whales, harbor porpoises (Phocoena phocoena), and migratory seabirds (Víkingsson et al., 2015; Nielsen et al., 2018; Davies et al., 2021). The average travel speed and short duration of these movements suggest a targeted migration rather than exploratory travel. The dolphins followed the productive corridor above the Iceland Basin (Hátún et al., 2017, Hátún et al., 2021), which is commonly used by migratory species such as tuna, mackerel, and seabirds (Bogdanova et al., 2011; MacKenzie et al., 2014; Pacariz et al., 2016). Their westward movements stayed within this corridor, where MLD is deeper than farther south (Figure 4), indicating its significance as migration corridor. Upon reaching the Irminger Sea in October, two dolphins remained southwest, near the Irminger Gyre (Figure 4), until transmissions ceased.

The move persistence analysis, though constrained by data gaps and interpolation, provides additional support for our interpretation of dolphin movements. High persistence during the trans-Atlantic crossings is consistent with directed travel at consistent speed. In contrast, areas of lower persistence—reflecting slower, more tortuous movements consistent with area-restricted search—were concentrated along the shelf edges, in the FSC, and at the Wyville-Thomson Ridge (Figure 3). Additional areas of lower persistence occurred along the east Greenland slope and in the Irminger Sea—particularly the Irminger Gyre. Taken together with the dolphins’ extended residency and the presence of boreal prey such as mackerel (Pacariz et al., 2016) and blue whiting (Post et al., 2020), as well as other marine mammals in the region (Víkingsson et al., 2015; Nielsen et al., 2018), these findings suggest that the Irminger Sea may serve as an important autumn feeding ground for white-sided dolphins.

While little is known about dolphin feeding habits in these waters, tagged harbor porpoises from West Greenland moved offshore into the Labrador Sea and the Irminger Sea in winter to target mesopelagic fish, likely the myctophid B. glaciale (Nielsen et al., 2019). Given that white-sided dolphins appear to target B. glaciale along the Reykjanes Ridge (Doksæter et al., 2008) and feed on myctophids in the northeastern Atlantic (Couperus, 1997; Hernandez-Milian et al., 2016), they likely exploit B. glaciale in the Irminger Sea. The pelagic fish species (i.e. mackerel and blue whiting) migrate eastward during autumn and winter to their main spawning grounds—opposite to the direction of the tagged dolphins—strengthens the idea that B. glaciale is a primary prey west of the Reykjanes Ridge (Doksæter et al., 2008).

The maximum (616 m) and mean (171 ± 186 m) dive depths recorded in our study exceed previous reports of the dive capacity of white-sided dolphins (Sampson et al., 2021; maximum: 191 m). The wide range in dive depths, along with significant differences in depth and duration across time periods, may reflect flexible foraging strategies—likely influenced by spatial and temporal variations in MLD and the Deep Scattering Layer across the North Atlantic, as well as individual differences. Significantly deeper nighttime dives, together with feeding events recorded at night by the STP, support the hypothesis that these dolphins exploit the diel vertical migration of mesopelagic prey, a known strategy among odontocetes (Baird et al., 2001; Nielsen et al., 2019).

In the Irminger Sea, dive depths recorded for dolphin #22 (Figure 2: nr 7–9) coincided with the upper boundary of the Deep Scattering Layer (400–500 m; Magnússon, 1996) and occurred at night or in the morning. This further supports the idea that white-sided dolphins forage on myctophids in this region, taking advantage of their diel vertical migration similarly to harbor porpoises (Nielsen et al., 2019).

The Irminger Sea emerges from our study as a previously unrecognized key area for white-sided dolphins and may function as a multi-species feeding hotspot, likely due to its unique oceanographic features, including the Irminger Gyre with deep MLDs, and associated high productivity (de Jong and de Steur, 2016; Hátún et al., 2016). The mixing hole, created in the central Irminger Sea during winters with intense ocean heat loss (Sterl and de Jong, 2022), resembles the processes observed in the northwestern Mediterranean Sea. In the latter, strong convection enhances nutrient upwelling, primary production, and fish recruitment (Mir-Arguimbau et al., 2022), attracting top predators such as striped dolphins (Stenella coeruleoalba) and fin whales (Balaenoptera physalus) (Virgili et al., 2024). Likewise, the Irminger Sea’s deep mixing processes appear to sustain a productive ecosystem that supports various marine species, similar to known marine hotspots in the Pacific (Block et al., 2011).

Recognizing and protecting such areas is crucial for understanding ecosystem resilience and ensuring the conservation of species that depend on them (Víkingsson et al., 2015; Nielsen et al., 2018; Davies et al., 2021). Despite its ecological importance, the Irminger Sea lacks formal conservation measures. It is neither designated as an Ecologically or Biologically Significant Marine Area (EBSA) (Johnson and Barrio Froján, 2021), a Marine Protected Area (MPA), nor identified as an Important Marine Mammal Area (IMMA) (IUCN, 2025). Much of the SPNA lies beyond national jurisdiction, complicating management efforts. While much oceanographic research has focused on the SPG, the Irminger Sea has received comparatively little attention from an ecological perspective. This raises concerns given ongoing environmental changes impacting the North Atlantic (Ramírez et al., 2017; IPCC, 2019), where Atlantification (i.e. increased influence of Atlantic waters) has already caused an ecosystem and regime shift along Greenland’s east coast (Heide-Jørgensen et al., 2022). Future climate projections suggest declining animal biomass over the next century due to global warming, primarily because of stronger ocean stratification and reduced nutrient upwelling (IPCC, 2019). As the northeastern Atlantic is expected to be especially affected—likely due to its sensitivity to convection processes—the Irminger Sea may emerge as the last nutritional stronghold in the region and a critical refuge for pelagic species, given projections of continued weakening of the SPG and environmental shifts (Hátún et al., 2009; IPCC, 2019). Given that marine mammals, including white-sided dolphins, are substantial consumers and a significant part of the food web in the Nordic Seas (Skern-Mauritzen et al., 2022), their presence in the Irminger Sea underscores the area’s ecological significance and the importance of its conservation. This highlights the value of tracking top predators such as the dolphins to inform ecosystem-based management by identifying biologically important areas and improving our understanding of species–environment relationships (Hays et al., 2019; Skern-Mauritzen et al., 2022).

4.1 Data limitations

Several limitations should be considered when interpreting our results. Differences in tag programming—variation in transmission schedules and uplink limits—affected data resolution and continuity, leading to uneven temporal coverage and long data gaps that required interpolation. Only nine dolphins provided tracks longer than ten days, and no data were available from December to May, limiting our ability to capture year-round migration patterns or make population level conclusions. The dive dataset is also constrained, as 55 of the 70 dives were recorded by a single dolphin (ID 22). Behavior messages were transmitted every seven days and given medium priority, which further reduced data availability when large messages failed to transmit.

5 Conclusion

Our study presents new insights into the large-scale movements and habitat use of Atlantic white-sided dolphins, revealing their ability to undertake cross-basin migrations at consistent travel speeds. Tracking data indicate that the Irminger Sea, particularly the Irminger Gyre, is an area of repeated use and a potential feeding hotspot, while also highlighting the significance of the Faroe-Shetland Channel as key habitat.

The species’ fission-fusion social structure, together with evidence of trans-Atlantic connectivity, supports the management of these dolphins as a single stock in the central and eastern North Atlantic. Dive records further suggest flexible foraging strategies, likely targeting mesopelagic prey associated with diel vertical migration.

Our findings can be used as guidance for conservation strategies to protect this highly mobile species. Given the absence of formal conservation measures in key habitats, further research and year-round monitoring are needed to assess potential threats and inform management strategies, especially in the context of environmental change.

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