Habitat use of the East Asian finless porpoise in the northern Aki Nada of the western Seto Inland Sea, Japan

Résumé

The East Asian finless porpoise is a small cetacean found in shallow coastal habitats in Asian waters, including those around Japan. While it has been designated an endangered species, recent reports have shown possible subpopulation recoveries, including in the Seto Inland Sea. To update the field data on its occurrence patterns in the northern Aki Nada region, we conducted boat surveys from April 2022 to December 2023 using drones to document habitat use, group composition and behaviour. Our surveys resulted in 0.02 ± 0.03 sightings per km (n = 52) and an estimated density (individuals per km) of 0.08 ± 0.11 porpoises per km (n = 52). Finless porpoises occurred throughout the survey period, except in July and August 2022. The average group size was 2.75 ± 2.53 individuals (range = 1–21), one of the highest records for this species in Japan. Most subgroup sightings were made of pairs (37.1%, n = 63/170), mostly mother‐calf pairs. These pairs were mainly observed from February to July and September to November. The GAM analysis resulted in identifying the model with substrate, depth, distance from shore, and sea surface temperature as factors having significant influences on porpoise presence and absence, while location, substrate, depth, distance from shore, and sea surface temperature were identified as factors having significant influences on porpoise group size. The considerably high occurrence of calves suggests that northern Aki Nada is an important breeding, calving, and nursing area for finless porpoises.