I cannot find much on record concerning the habits of this dolphin, and my own acquaintance with it is too limited for me to afford much original information.
[NO. 259. ORCELLA FLUMINALIS (Anderson).]
The Fresh-water Round-headed Dolphin.
HABITAT.—The Irrawaddy river; Burmah.
DESCRIPTION.—This differs from the last in a "rather smaller, lower, and more falcate dorsal fin, its more pointed and less anteriorly bulging head, and rather shorter and broader pectoral fins" (Anderson). The colour is a pale bluish above, and white underneath, with numerous streaks, as in Risso's grampus.
SIZE.—From 7 to 7½ feet from snout to fork of tail.
Dr. Anderson, who has fully described this species, says that he has "never observed it in tidal waters, so that it is even more strictly fluviatile than the Gangetic dolphin. From a little below Prome to as far up as Bhamo, which is about 550 miles, as the crow flies, from the sea, these animals abound. It is asserted by the Shans of Upper Burmah that these dolphins are not to be found beyond a point thirty miles above Bhamo, where the course of the river is interrupted by rocks, and which they style Labine or Dolphin Point, from the circumstance that, according to them, it is the residence of certain Nâts, who there impose so heavy a toll on dolphins as to deter them from proceeding upwards."
This dolphin is somewhat like its marine cousins, being fond of gambolling round the river steamers. Solitary ones are seldom met with, usually two or three being together. When they rise to breathe the blow-hole is first seen; then, after respiration, the head goes down, and the back as far as the dorsal fin is seen, but rarely the tail flippers. They rise to breathe every 70 to 150 seconds, and the respiratory act is so rapid that it requires a very expert marksman to take aim and fire before the animal disappears.
Dr. Anderson says: "I have observed some of them disporting themselves in a way that has never yet been recorded of Cetacea, as far as I am aware. They swam with a rolling motion near the surface, with their heads half out of the water, and every now and then nearly fully exposed, when they ejected great volumes of water out of their mouths—generally straight before them; but sometimes nearly vertically. The sight of this curious habit at once recalled to me an incident in my voyage up the river, when I had been quite baffled to explain an exactly similar appearance seen at a distance, so that this remarkable habit would appear to be not uncommonly manifested. On one occasion I noticed an individual standing upright in the water, so much so that one-half of its pectoral fins was exposed, producing the appearance against the background as if the animal was supported on its flippers. It suddenly disappeared, and again, a little in advance of its former position, it bobbed up in the same attitude, and this it frequently repeated. The Shan boatmen who were with me seemed to connect these curious movements with the season—spring—in which the dolphins breed."
A similar thing has been noticed in the case of marine dolphins off the coast of Ceylon by Mr. E. W. H. Holdsworth, whose observations confirm the opinion of the Shan boatmen. (See 'P. Z. S.' 1872, p. 586.)
"The food of the Irrawady dolphin is apparently exclusively fish. The fishermen believe that the dolphin purposely draws fish to their nets, and each fishing village has its particular guardian dolphin, which receives a name common to all the fellows of his school, and it is this supposition that makes it so difficult to obtain specimens of this cetacean. Colonel Sladen has told me that suits are not unfrequently brought into the native courts to recover a share in the capture of fish in which a plaintiff's dolphin has been held to have filled the nets of a rival fisherman" (Anderson). This reminds me that in the surveying voyage of the Herald, as related by Mr. H. Lee, the natives of Moreton Bay entreated the seamen not to shoot their tame porpoises, which helped them in their fishing.