Beluga dive behavior relative to fronts and stratified layers near Barrow Canyon, Alaska

Highlights (DSRI-D-20-00113)

• We attached an oceanographic instrument to a beluga whale (Delphinapterus leucas) of the Eastern Chukchi Sea stock.

• The instrument was a Satellite Relay Data Logger (SRDL) and included a Conductivity, Temperature, and Depth (CTD) sensor.

• The beluga targeted fronts and stratified layers between water masses or colder water masses of Pacific origin.

• In Barrow Canyon, the beluga targeted the front between Alaskan Coastal Water, and colder, saltier water of Pacific origin.

Abstract

We report on the first beluga whale (Delphinapterus leucas), an adult female, from the Eastern Chukchi Sea (ECS) stock to be tagged with a Satellite Relay Data Logger (SRDL) that included a miniaturized Conductivity, Temperature, and Depth (CTD) sensor capable of collecting temperature (T) and salinity (S) profiles as the beluga dove. Prior work speculated that ECS belugas may target fronts and stratified regimes between water masses, because these boundaries may aggregate zooplankton, which in turn may attract fish that belugas prey upon. However, the SRDLs used in those studies were not capable of collecting CTD data necessary to identify water masses. The CTD-SRDL transmitted for 71 days, 8 July–17 September 2019, providing 492 time-depth profiles, indicating the shape and duration of beluga dives, and 255 CTD profiles, providing information on the location of water masses and ocean stratification used by the animal. We found that this beluga typically targeted fronts and stratified layers between water masses or colder water masses of Pacific origin. Of particular significance, we found that within the main summering area in Barrow Canyon, dives were co-located with the frontal zone between relatively warm and fresh Alaskan Coastal Water (derived from river discharge), and colder, saltier water of Pacific origin.

Introduction

Beluga whales (Delphinapterus leucas), also known as white whales, are small (≤5.5 m) odontocetes (i.e., toothed whales) that inhabit seasonally ice-covered waters in the circumpolar Arctic and sub-Arctic. There are currently 22 recognized stocks (i.e., populations) of belugas worldwide (Hobbs et al., 2020). Some beluga stocks migrate seasonally, with distinct winter and summer ranges, while others largely remain in specific bays or rivers year-round (e.g., Hobbs et al., 2020; Lowry et al., 2020). Beluga whales of the Eastern Chukchi Sea (ECS) stock are migratory and winter in the northern Bering Sea (Citta et al., 2016) and summer throughout the eastern Chukchi and Beaufort seas (Suydam, 2009; Hauser et al., 2014). Barrow Canyon is an important use area for ECS belugas in summer (Moore et al., 2000; Hauser et al., 2014; Stafford et al., 2018; Clarke et al., 2018); the canyon is one of the main circulation pathways for Pacific-origin waters exiting the shelf in the Chukchi Sea and entering the Beaufort Sea and Arctic Ocean (Fig. 1a). Other areas with high use in summer by ECS belugas include the shelf break in the Beaufort Sea and, to a lesser extent, the Canada Basin and shelf waters in both the Chukchi and Beaufort seas.

Beluga habitat use is expected to correlate with distinct water masses and sometimes be focused along hydrographic fronts and stratifications. This is because water masses have temperature and salinity characteristics (described below) that reflect their temporal and/or spatial origins, thereby defining different habitats for zooplankton and fish (e.g., Eisner et al., 2013). Furthermore, the boundaries between water masses may have strong salinity and/or temperature gradients which promote the aggregation of zooplankton and potentially larval fish, which in turn may attract higher-order predators such as fish (e.g., Olson and Backus, 1985) or whales (Moore et al., 1995; Citta et al., 2015).

In summer, the dominant circulation feature in the eastern Chukchi Sea is the Alaskan Coastal Current (ACC; Fig. 1a) which carries relatively warm (T > 3 °C), fresh (30 psu < S < 32 psu) Alaskan Coastal Water (ACW; mainly river discharge) into the Canada Basin (Paquette and Bourke, 1974). Various other water masses of Pacific origin also exist in the Chukchi and Beaufort seas, typically occurring farther offshore or beneath ACW. Pacific Winter Water (WW) forms under sea ice in the northern Bering Sea and is near the freezing point of seawater (T < −1.6 °C). In summer, WW typically occurs near the seafloor in shelf waters. Remnant Winter Water (RWW) is WW that has warmed throughout the summer season (−1.6 °C < T < 0 °C). Anadyr Water and Bering Shelf Water combine north of Bering Strait to form Bering Summer Water (BSW; Corlett and Pickart, 2017), which has a similar signature to RWW that has warmed during the summer, and is sometimes known as Chukchi Summer Water (Gong and Pickart, 2015). In the Canada Basin, warmer, saltier water of Atlantic origin (T > −1.26 °C, S > 33.6 psu; Atlantic Water; AW) generally resides below 200 m (Corlett and Pickart, 2017), although upwelling may bring AW closer to the surface. Last, meltwater (MW) occupies the surface layer in summer and is derived from ice melt and precipitation (Gong and Pickart, 2015; Corlett and Pickart, 2017).

The movements and dive behavior of ECS belugas are thought to be related to these water masses and stratified regions between them. In the Canada Basin and the adjacent continental slope, beluga habitat use is thought to be associated with AW, because adult Arctic cod (also known as polar cod, Boreogadus saida) are known to associate with AW (Parker-Stetter et al., 2011). Citta et al. (2013) showed that satellite-tagged belugas of the ECS stock had a modal dive depth of 200–300 m and speculated that zooplankton and beluga prey were aggregating within or near the polar halocline (T < ~0 °C, S > ~32.5 psu) where water of Pacific origin meets the top of the AW layer. Using the same data set of beluga dives as Citta et al. (2013), Hauser et al. (2015) related ECS beluga dive depths to a 2008 hydroacoustic survey of Arctic cod (Parker-Stetter et al., 2011) and found that belugas commonly dove to depths (200–300 m) associated with elevated densities of cod.

Within Barrow Canyon, ACW occurs above and typically inshore of WW/RWW thereby creating a front between these two water masses (Okkonen et al., 2019; Gong and Pickart, 2015). Stafford et al. (2013) showed that when along-canyon winds are from the west-southwest (WSW), the ACC is intensified and use of Barrow Canyon and the adjacent shelf break by belugas increases. Specifically, Stafford et al. (2013) documented increases in aerial sightings, group sizes, and hours with vocalizations during periods of WSW winds. Using the same satellite telemetry dataset referenced above (i.e., Citta et al., 2013; Hauser et al., 2015), Stafford et al. (2016) showed that when the ACC is intensified, belugas respond by targeting shallower depths (10–100 m) associated with the expected location of the front. In contrast, winds from the east-northeast (ENE) displace the ACC westward and cause the frontal feature in Barrow Canyon to weaken or dissipate. Under this wind regime, belugas are more likely to target depths >200 m, within the polar halocline and upper Atlantic layer, and beluga distribution shifts towards the Canada Basin (Stafford et al., 2013, 2016).

The satellite transmitters used in the studies described above did not collect salinity data, thereby precluding identification of water masses used by belugas. Although these tags collected temperature data, software on-board the tag was designed to summarize data for easy transmission to a satellite. Temperature data, summarized as the percentage of time falling within temperature categories (typically 0.5 or 1 °C), was not paired with depth. As such, the oceanographic features associated with beluga dive behavior were taken from other sources and, although reasonable, were largely speculative. In 2019, we attached the first Satellite Relay Data Logger (SRDL) that included a miniaturized, integrated Conductivity, Temperature, and Depth (CTD) sensor (Boehme et al., 2009; Photopoulou et al., 2015) to an adult female beluga from the ECS stock; although CTD-SRDL tags have been attached to belugas from other stocks (e.g., Lydersen et al., 2002). Here we describe the dive behavior of this ECS beluga in relation to water mass characteristics, including stratification, fronts, and wind patterns in Alaskan waters.