Seasonal migration, growth, and exposed water temperature of immature giant octopus Enteroctopus dofleini using mark and recapture experiment and seasonal change in bottom water temperature in the fishing ground

The migration, growth, and exposed water temperature of immature giant octopuses (Enteroctopus dofleini) were investigated in the coastal Okhotsk Sea area of Hokkaido using the mark and recapture method. Disk tags were used to document the migration and growth patterns, and data loggers were used to measure exposed temperature. Data loggers were also placed on the sea bottom off Notoro Cape, Abashiri City, which is one of the main giant octopus fishing grounds in this area, to document the spatial and temporal changes in water temperature during the summer and winter. The daily growth of the immature giant octopuses changed seasonally, and the daily growth of marked individuals was faster during summer and autumn than during winter and spring. A major portion of the released individuals (86%) were recaptured from the area where they were initially released, and the migration distance of 76% of the octopuses was <= 10 km, suggesting a small scale of migration in the area. However, a seasonal pattern of migration between deep and shallow areas was evident, with more individuals migrating toward deep areas during July-September and toward shallow areas starting in October. In the summer of 2013, sea-bottom temperature was higher in shallower areas, particularly in August and September, when the water temperature of 20- to 40-m deep areas exceeded 20degC. This suggests that the driving factor of summer migration to deeper waters is related to the avoidance of overheating. Highest exposed water temperature was 20.1degC and mean temperature of 4 individuals (50%) was >= 15.0degC. The present study indicates the water temperature of the giant octopus fishing ground changes rapidly in response to both the sea currents and the passing of strong low pressure. It also reports, for the first time, information on the movement, growth, and exposed temperature of immature octopuses in the area.