Abstract
The coast of Japan is a migratory corridor for the western stock of the gray whales (Eschrichtius robustus), which was once considered as extinct and remains endangered. According to the historical records, from 1955 to 2014, only 21 gray whales occurrence has been recorded in 59 years. However, from 2015 to 2016, intensive occurrence including the seven sightings and the two strandings were noted. In this paper, we found that those sightings were re-sightings of the same individual, which was initially sighted off Sakhalin during August, 2014. On 4 March, 2016, a young female gray whale (8.9 m in body length) was stranded at Wadaura beach, Chiba prefecture. We also conducted research on this animal including taking pictures and external measurements. In addition, we flensed this animal to observe the internal organs and collect a skeletal specimen. The reason for the death of this animal remains unclear; however, from its external characteristics, we identified that this animal was not an identical one, sighted off Sakhalin and the coast of Japan from 2014 to 2016. On 5 April, 2016, another young female gray whale (7 m in body length) was stranded at Arai beach, Shizuoka prefecture. We concluded that from 2015 to 2016, at least three distinct gray whales have migrated along the coast of Japan.
Highlights
From 2015 to 2016, intensive occurrence including the seven sightings and the two strandings were noted. We found that those sightings were re-sightings of the same individual, which was initially sighted off Sakhalin during August, 2014
Eastern gray whales were depleted by historical American whaling as well as modern whaling, it was dramatically recovered after 1960s, with individuals estimated at approximately 20,990 (95% Highest Posterior Density Interval (HDPI) 12,230 22,900), almost reaching the limit of the carrying capacity of their habitat [4]
Sightings made at Kozushima and Niijima Islands, and Miho beach have been identified as the same animal, which was sighted off Sakhalin Island [12]
Summary
Eastern gray whales (California stock) breed near the coastal lagoons of Baja California, Mexico, during winter, whereas they migrate to the Bering and Chukchi seas during summer for feeding [2]. Western gray whales (Asian stock) are believed to breed in the western part of the Hainan Island and migrate to the coast of Sakhalin in the Okhotsk Sea for feeding [3]. These two stocks show distinct population dynamics. The population of the western gray whales is estimated to be only 121 (95% CV 112 - 130) individuals by IWC scientific committee; concerns of extinction [9]
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