Distribution and abundance of dalli-type Dall’s porpoises Phocoenoides dalli migrating into waters off southeastern Hokkaido, Japan, during summer: results of 2014–2016 aerial surveys

Abstract

A series of systematic aerial sighting surveys were conducted in the coastal waters off southeastern Hokkaido to reveal distribution patterns and estimate the abundance of Dall’s porpoises migrating into those waters during summers. A total of 6805.5 km was flown, and 98 porpoise schools were detected in 2014–2016. Dalli-type animals were discriminated from truei-type by logistic discriminant functions constructed based on previous sighting data, and the abundance of the dalli-type porpoise was estimated using a line-transect methodology. The estimated abundance of dalli-type Dall’s porpoises migrating into the main study area (S-block) was 134 (coefficient of variation = 0.91) in 2014, 766 (0.77) in 2015, and 720 (0.87) in 2016. For these estimates, correction of availability bias was performed using data from archival tags attached to Dall’s porpoises. Those estimates were 239 (0.96) in 2014, 1363 (0.83) in 2015, and 1281 (0.91) in 2016. Sightings of porpoises were concentrated in shallow coastal waters in 2015 and 2016, while porpoise schools tended to be distributed offshore in 2014. Cold and high-productivity waters shrank to the coastal region in 2015 and 2016, corresponding well with high-density concentration of porpoises in coastal waters in these years.