Abstract
We investigated the diet and chick body mass of Rhinoceros Auklets Cerorhinca monocerata at four breeding sites in Hokkaido in 2016 and 2017. Matsumae-kojima, Teuri Island and Todo Island are in the southern, middle and northern Tsushima Warm Current system, respectively, although Todo Island is located also beside the colder water mass of the Sea of Okhotsk. Daikoku Island is in the Coastal Oyashio and Oyashio Cold Current system in the North Pacific Ocean. The main prey species delivered in chicks were 0-yr-old (0 +) Arabesque Greenling Pleurogrammus azonus at Teuri and Todo Islnads, 0 + Walleye Pollock Gadus chalcogrammus at Matsumae-kojima and 0 + Chum Salmon Oncorhynchus keta at Daikoku Island. In addition some 0 + and > 0-yr-old sandlance Ammodytes spp. Were delivered at all sites. Parent auklets at Todo Island brought back heavier food-loads and fish species of greater energy density than those at other sites, resulting in greater developmental-stage adjusted chick body mass. Sea water temperature differences might explain the dissimilarity in the availability of 0 + greenling between Todo and Teuri Islands and the inter-year difference in the availability of 0 + salmon at Daikoku Island. The more active Chum Salmon hatchery program in the Pacific Ocean than in the Sea of Japan may also explain the regional differences in the importance of this species in auklet food-loads. In addition, sandlance are likely available in all coastal waters around Hokkaido. This study suggests that ocean environment and anthropogenic factors affect regional prey availability, and explain the regional differences in the diet and chick growth of the Rhinoceros Auklet.
