Combining biometrics and genetics to distinguish two subspecies of the Great Cormorant Phalacrocorax carbo carbo and P. c. sinensis at an inland lake in southeast Norway

Abstract

Cover photo: Adult Great Cormorant of the subspecies Phalacrocorax carbo sinensis in breeding plumage. Photo: Frode Falkenberg. Great Cormorants Phalacrocorax carbo are now regularly seen in inland watercourses in southeast Norway, after the P. c. sinensis subspecies first established breeding colonies in coastal south Norway in 1996. Although both the sinensis and carbo subspecies occur, the ratio between them is unknown. We tested the accuracy of subspecific identification based on biometrics and genetic analyses in 75 Great Cormorants that drowned in fishing nets in a lake in southeast Norway during the period 2009-2020. Primarily based on the gular pouch angle (GPA), we classified 40 individuals to the carbo subspecies and 35 individuals to the sinensis subspecies. Eight of the carbo individuals and 14 of the sinensis individuals were within the overlapping range of GPA and were therefore classified to subspecies by supplementary measurements (bill depth minimum and bill length). Genetic analyses were based on seven polymorphic microsatellite markers. Assuming one panmictic population, we performed Structure analyses to separate the genotypes into two assumed genetic clusters. The two clusters, carbo and sinensis, could only be separated when adding information about morphological subspecies identities for the carbo, sinensis and carbo/sinensis groups. The sinensis and the carbo/sinensis groups belonged almost entirely to one genetic cluster, whereas the carbo group consisted of individuals with varying proportions of mixed ancestry. Furthermore, we found significant genetic differentiation between the carbo and sinensis groups, and between the carbo and the carbo/sinensis groups, but no significant differentiation between the sinensis and the carbo/sinensis groups. Our results suggest that gene flow is more common from sinensis into carbo than vice-versa, and that the use of GPA < 73° has limitations for the identification of Great Cormorant subspecies in areas where both forms occur. We conclude that the numbers of carbo vs. sinensis individuals in our sample were 32 and 43, respectively.