Interannual and seasonal variations in the frequency of occurrence of harbor porpoises, Phocoena phocoena (Linnaeus, 1758) (Cetacea: Phocoenidae), off the northeastern Sakhalin Coast, Sea of Okhotsk, in 2008–2021

The frequency of occurrence of harbor porpoises, Phocoena phocoena (Linnaeus, 1758), off the northeastern coast of Sakhalin Island (Sea of Okhotsk) was studied from 2008 to 2021. As the results showed, the frequency of occurrence of individuals of this cetacean species in the study area was subject to significant seasonal and interannual variations. The median seasonal frequency of occurrence of harbor porpoises was the highest in October (0.16 individuals per observation hour, ind./h) and was significantly different from the frequencies observed in the other study months (0.09 ind./h). The high frequency of occurrence was presumably related to the seasonal southward migration of porpoises. In 2014, 2016, and 2017, the frequency of occurrence reached the statistically highest level (0.19 ind./h), while the lowest values were recorded in 2008, 2010, 2013, and 2019 (0.07 ind./h). In the remaining seven study years, the frequency of occurrence was stable at 0.12 ind./h.