In this paper observations are summarized, dealing with the occurrence, movements and part of the life-history of the Cetaceans Tursiops truncatus (the bottlenosed dolphin) and Phocoena phocoena (the harbour porpoise or common porpoise). The observations were made in the neighbourhood of Den Helder, Holland, chiefly in the years 1933-1938 and later. The value of the paper lies in the relatively great number of in itself little important facts. The behaviour of the two species studied is, as far as possible, compared with that of other cetacean species, whereby social behaviour (group formation and other behaviour traits), (vertical and horizontal) jumping, respiration and diving, floating at the water surface, eventual basking, and sleeping, are especially dealt with. The data show how scarce details on several of these behaviour traits are. The data on the movements of Tursiops and Phocoena in the course of the year show that observations on annual periodicity in the appearance of the living animals give a picture quite different from that on numbers of animals stranded. In so far Tursiops is concerned, there may be a low mortality near Den Helder in spring, when numbers of living Tursiops are high; as to Phocoena annual periodicity is probably chiefly due to movements towards the coast in spring and movements away from coastal areas in autumn and winter, whereas stranded animals show a maximum in summer, when the animals are chiefly to be found in warm coastal water. In this part of the paper migrations of Cetaceans generally are also shortly dealt with. |
