Site-fidelity and spatial movements of western North Pacific gray whales on their summer range off Sakhalin, Russia

Abstract

The Western North-Pacific (WNP) gray whale feeding grounds are off the northeastern coast of Sakhalin Island, Russia and is comprised of a nearshore and offshore component that can be distinguished by both depth and location. Spatial movements of gray whales within their foraging grounds were examined based on 13 years of opportunistic vessel and shore-based photo-identification surveys. Site fidelity was assessed by examining annual return and resighting rates. Lagged Identification Rates (LIR) analyses were conducted to estimate the residency and transitional movement patterns within the two components of their feeding grounds. In total 243 individuals were identified from 2002-2014, among these were 94 calves. The annual return rate over the period 2002-2014 was 72%, excluding 35 calves only seen one year. Approximately 20% of the individuals identified from 2002-2010 were seen every year after their initial sighting (including eight individuals that returned for 13 consecutive years). The majority (239) of the WNP whales were observed in the nearshore area while only half (122) were found in the deeper offshore area. Within a foraging season, there was a significantly higher probability of gray whales moving from the nearshore to the offshore area. No mother-calf pairs, calves or yearlings were observed in the offshore area, which was increasingly used by mature animals. The annual return rates, and population growth rates that are primarily a result of calf production with little evidence of immigration, suggest that this population is demographically self-contained and that both the nearshore and offshore Sakhalin feeding grounds are critically important areas for their summer annual foraging activities. The nearshore habitat is also important for mother-calf pairs, younger individuals, and recently weaned calves. Nearshore feeding could also be energetically less costly compared to foraging in the deeper offshore habitat and provide more protection from predators, such as killer whales.

Gray whale density during seismic surveys near their Sakhalin feeding ground

“…Western gray whales migrate over 10000 km to feed within a spatially small foraging ground off Sakhalin Island (Mate et al, 2015 ). Gray whales observed off Sakhalin have high site fidelity and annual return rate to the two known feeding areas (Bröker et al, 2020 ). Other foraging areas within the Sea of Okhotsk have not been identified to date.…”

“…Similarly, mothers with calves have never been observed in the offshore area during the weaning period. Younger individuals (< 5 years of age) also tend to have a higher affinity for the nearshore feeding area (Bröker et al, 2020 ; Schwarz et al, 2022a ). However, the amphipod-rich offshore feeding area (Blanchard et al, 2019 ; Demchenko et al, 2016 ) may be critical for pregnant females (McHuron et al, 2021 ).…”

Gray whale habitat use and reproductive success during seismic surveys near their feeding grounds: comparing state-dependent life history models and field data

“…Their two known feeding areas are small compared to that of eastern gray whales and many other baleen whales (Mate et al, 2011;Moore et al, 2003), and they exhibit a high degree of site fidelity to these areas (Fig. 1) (Bröker et al, 2020). In addition, the estimated population size is small for a K-selected species, with 175 identified non-calf individuals, of which 33 are known reproducing females (Cooke et al, 2017).…”

Population structure of North Pacific gray whales in light of trans‐Pacific movements

“…In 1995, concern for the potential impact of such activities on gray whales feeding off SI led to the initiation of a long-term monitoring effort by Russian and U.S. scientists (Weller et al, 1999). Photo-identification data collected as part of these efforts, which have been conducted annually since 1997 and continue to date as part of the Russian Gray Whale Project, as well as data collected by industry-funded efforts since 2002, indicate that a small number of whales demonstrate strong fidelity to this feeding area both within and between years (Bradford, 2011;Bradford et al, 2008;Bröker et al, 2020;Burdin et al, 2020;Weller et al, 1999Weller et al, , 2002b). Use of the SI feeding ground is driven in part by matrilineal fidelity, as many of the whales first identified as calves with their mothers have been sighted on the SI feeding ground in subsequent seasons.…”

“…Initially, WNP whales were presumed to feed only off the coast of SI, in a nearshore and an offshore area (Bröker et al, 2020;Meier et al, 2007;Weller et al, 2002aWeller et al, , 2003. However, photo-identification studies off southern and eastern Kamchatka indicated that approximately half of the whales identified in that region are whales known to use the SI feeding ground (Burdin et al, 2011;Tyurneva et al, 2010;Vertyankin et al, 2004).…”