Immigration of the common cormorant Phalacrocorax carbo hanedae into inland areas of the northern part of Nagano Prefecture, eastern Japan, inferred from stable isotopes of carbon, nitrogen and sulphur

Immigration of common cormorants Phalacrocorax carbo hanedae into inland areas might result in substantial loss of inland fish stocks due to predation. We estimated the immigration and habitat of piscivorous common cormorants into inland areas using 3 stable isotope values (δ 13 C, δ 15 N and δ 34 S) collected from 24 cormorant specimens and 9 grey heron ( Ardea cinerea) specimens for comparison; Samples were collected (shot) during the winter and spring of 2007–2008 in the northern part of Nagano Prefecture, an inland area of eastern Japan. Variation in carbon and nitrogen isotope values between cormorant individuals was low in the liver and in the pectoralis major muscle, with a rapid rate of turnover. Relatively higher variation in the primary and secondary feathers, the values of which reflect a moulting period, suggest inter-individual variation in habitat use and prey items during and after summer moulting. Predicted mean isotope values of cormorant prey, calculated from fractionation values of the primary feathers, did not support littoral feeding of the cormorants after moulting. Based on empirical measurement of δ 34 S thresholds, with >10 ‰ being indicative of tissue grown in a marine environment, the percentage of individuals feeding in seawater- dominated areas after moulting was only 16.7 % in cormorants and 0 % in grey herons, suggesting the majority of cormorants examined inhabited inland areas and preyed upon inland fish stock after summer moulting.