Abstract

Two species of dolphin inhabit the Changjiang (Yangtze): Lipotes vexillifer MILLER, 1918 (known as Baiji in Chinese) and the Finless porpoise, Neophocaena asiaeorientalis PILLERI and GIHR, 1972. Although quite a lot is known about the anatomy of the two species, until now no data were available on ecology and behaviour or distribution and size of the population. The research ship Ke-Hu was used on a number of expeditions to explore the stretch of river between Nanjing and Taiyangzhou where a census was made of the dolphins, and in so far as possible their behaviour was observed. The study contains data on the chemical composition of the water, the degree of visibility and the temperature at different times of the year. Lipotes usually occurs in pairs or in small loose groups. The calves are born in March – April. The Finless porpoise is usually seen in groups of five or six individuals. Sometimes mixed schools of Lipotes and Neophocaena are to be seen swimming in the same area. The blow lasts 0.2-0.6 sec in Lipotes and 0.2-0.5 sec in Neophocaena. The diving time in Lipotes is 10-20 sec for a short dive, and up to 1-2 min for a long dive (average value 31 sec). In Neophocaena a long dive lasts a maximum of 65 sec with an average of 17.5 sec. The apnoea of Lipotes is therefore considerably longer than in Neophocaena. The Changjiang is considerably less turbid than the Indus, and unlike Platanista indi and gangetica but like Inia, Lipotes vexillifer has a functional eye. The selective secondary atrophy of a number of eye muscles and nerves, as well as the position of the eyes in the head, correspond with the physical conditions of the environment, so that the field of vision is directed forwards and upwards. When pursued by motor boat, both species react with a long dive and a change of direction under water. The result is that Lipotes often suffers fatal injuries from the propeller of the ship. This seems to occur much less often with Neophocaena. The population of Lipotes is at present very small. Compared with Platanista indi, with a population in the protected area of the lower Indus of three individuals per 2 kilometres of river, in the stretch of river we explored we found roughly only 1 dolphin per 4 kilometres or even less. The most common cause of the decimation in numbers, according to Nanjing records, are accidental catches and wounds from propellers. This suggests that Neophocaena can be seen as less endangered in association with man whereas Lipotes seems threatened by civilization.