Population Assessment Update for Sakhalin Gray Whales
J.G. Cooke, O. Sychenko, A.M. Burdin, D.W. Weller, A.L. Bradford, A.R. Lang and R.L. Brownell, Jr.
The population assessment of gray whales Eschrichtius robustus feeding off Sakhalin is updated. An individually based population model, with one summer feeding area and up to two wintering areas, is fit to photo-id data collected off Sakhalin during 1995-2018 (Burdin et al. 2019), sex determinations from biopsies (Lang 2010), tracking of whales from Sakhalin to the eastern North Pacific (Mate et al. 2015), and photo-id matches of gray whales between the Sakhalin and Mexico catalogues (Urban et al. 2019). The results show that the Sakhalin feeding population increased at 3.4-4.8% per year over the 20 years to 2018, but with significant inter-annual fluctuations in calving rates and calf survival. It is not possible to verify with these data whether the increase is still continuing, and recent declines in prey availability in the Piltun feeding ground, the main feeding ground for mother-calf pairs within the population, imply that a continued increase cannot be assumed.
The aged 1+ population size in 2018 of the Sakhalin feeding population is estimated at 191 whales, excluding calves (CL 171-214). The proportion of the population that migrates to the eastern North Pacific is estimated to be 45-80%, therefore it is likely that a western breeding population that migrates through Asian waters still exists.