Morphometric comparisons of skulls of harbour porpoises (Phocoena phocoena) from the Baltic, Kattegat, and Skagerrak seas

The population structure of harbour porpoises (Phocoena phocoena) in Swedish waters was studied by means of morphometric comparisons of skulls from 103 specimens. The study was designed to investigate whether harbour porpoises in the Baltic Sea form a distinct population separate from animals found in the Kattegat and Skagerrak seas. The Kattegat–Skagerrak (K-S) sample was divided into summer and winter samples to account for possible migrational movements by Baltic Sea animals between breeding seasons. Both multivariate and univariate statistical analyses showed significant differences between three female samples from the Baltic Sea and the Kattegat and Skagerrak seas. Pairwise comparisons showed differences between the Baltic sample and the K-S summer sample in five characters and between the Baltic sample and K-S winter sample in one character. No differences were found between the K-S summer and winter samples. The multivariate analysis did not show separation of male specimens and only one character was significantly different in the univariate analyses. Pairwise comparisons of adjusted means of this character did not reveal significant differences between the samples. The results show that female harbour porpoises in the Baltic Sea are morphologically distinct from females in the Kattegat and Skagerrak seas and these differences probably would not exist if animals from the two areas interbreed. Harbour porpoises in the Baltic Sea should therefore be managed separately and measures should be taken to prevent bycatches of this likely endangered population.