Residency and abundance of sperm whales (Physeter macrocephalus) in Nemuro Strait, Hokkaido, Japan

We examined the trend in residence patterns and abundance of male sperm whales in Nemuro Strait, Japan, based on long-term photo-identification (1,513 survey days, total 2,969 photos) between 2006 and 2017. A total of 225 unique individuals were identified during this study, with an average of 36 (SE = 2.55) new individuals identified in each season. The model chosen by maximum likelihood suggests that residence time around Nemuro Strait is 769 (SE = 372.4) days, with individuals staying in the strait about 48 days (SE = 8.36) per year. While the migration patterns of male sperm whales visiting this area are still unclear, these findings along with previous studies suggest that males move from one breeding area to another neighboring area every several weeks, shifting their home ranges gradually over a period of a few years. The abundance of sperm whales in Nemuro Strait varied greatly from year to year; from 28 (95%CI: 24-44) in 2015 and (95%CI: 22-48) in 2016 to 66 (95%CI: 57-84) in 2011. This study provides important knowledge of abundance and residency for Nemuro Strait, information which will contribute to further research on the social structure and movement pattern of male sperm whales.