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. 2020 Aug 14;15(8):e0236649.
doi: 10.1371/journal.pone.0236649. eCollection 2020.

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

Koen C A Bröker  1   2 , Glenn Gailey  3 , Olga Yu Tyurneva  4 , Yuri M Yakovlev  4 , Olga Sychenko  3 , Jennifer M Dupont  5 , Vladimir V Vertyankin  6 , Evgeny Shevtsov  4 , Konstantin A Drozdov  7
Affiliations

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

Koen C A Bröker et al. PLoS One. .

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.

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Conflict of interest statement

First author Koen C.A. Bröker was employed by Sakhalin Energy Investment Company Ltd. from 2006 –2010 and by Sakhalin Energy Investment Company Ltd.’s shareholder Shell from 2010 onwards. Jennifer M. Dupont was employed by Exxon Neftegas Ltd.’s shareholder ExxonMobil from 2010 onwards. This does not alter our adherence to PLOS ONE policies on sharing data and materials.

Figures

Fig 1
Fig 1. Photo-identification study areas off Sakhalin Island, Russia.
Locations of gray whale sightings in the study area are designated as brown dots. Top right inset shows a regional view of the northeast Sakhalin Island.
Fig 2
Fig 2. A discovery curve representing the total number of new individuals, number of new non-calves and new calves identified as function of the total cumulative number of identifications made from 2002–2014 in the Sakhalin feeding grounds.
No distinction was made between calves and non-calves during the first year of the study (2002).
Fig 3
Fig 3
(a) The annual number of new and resighted gray whales and (b) the annual resighting rate (resighted individuals / total number of sighted individuals (new + resight)).
Fig 4
Fig 4
LIR for a single season (top) and for the total study duration (2002–2014) (bottom). The green circles provide the LIR of different lagged time bins.
Fig 5
Fig 5. LIR and selected models within and between the offshore and nearshore area within a season.
Red stars and green circles are the LIR of different lagged time bins.
Fig 6
Fig 6. LIR within and between offshore and nearshore area over the duration of the study period (2002–2014).
Red stars and green circles are the LIR of different lagged time bins.
See this image and copyright information in PMC

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