Northward migration dynamics of skipjack tuna (Katsuwonus pelamis) associated with the lower thermal limit in the western Pacific Ocean

•We documented three major migration routes of skipjack tuna from subtropical to temperate waters off Japan.•The 18 °C isotherm is an appropriate indicator for the lowest thermal limit of skipjack tuna distribution.•Horizontal and vertical habitat of skipjack tuna are strongly driven by the 18 °C thermal limit.•The 18 °C isotherm limited skipjack tuna to surface waters in the northernmost part of their habitat.