FRESH-WATER FISHES OF SIAM, OR THAILAND 373 



age character and whether teeth may not exist in examples of, say, 

 half a meter length. 



The absence of mandibulary barbels might likewise be associated 

 with excessive growth or great age. Chevey himself raised the ques- 

 tion whether minute barbels may not perhaps exist in the young and be 

 by degrees overwhelmed in the enormous fatty layer that develops on 

 the jaw. 



From the foregoing it would seem that the final determination of 

 the status of Pangasianodon must await the examination of small indi- 

 viduals with special reference to their possible possession of teeth and 

 mandibulary barbels. 



PANGASIANODON GIGAS Chevey 



Plate 7 

 Pangasiimodon gigas Chevey, 1930, p. 536, figs. 1, 2 (Mekong basin). 



In Thai territory this fish occurs and is caught throughout the 

 Mekong where it forms the international boundary, and it also enters 

 the major streams tributary to the Mekong in Eastern Thailand, no- 

 tably the Songkram, although in only limited numbers. It has a range 

 of several thousand miles in the Mekong, coming at various times in 

 its migratory movements under the jurisdiction of French Indo-China, 

 Thailand, Burma, and China. There is no evidence that it ever enters 

 the sea, and Chevey stated that in the protracted trawling done by the 

 de Lanessan in the mouths of the Mekong and the Bassac this fish has 

 never been taken. 



During the period of flood water the fish remains in the lower 

 Mekong and is caught in the Pnom-Penh area, a favorite resort and 

 place of capture being the Quatre-Bras, near the lower end of the 

 Tonle Sap in Cambodia. After the end of the rainy season and the 

 subsidence of the flood water, the fish begins a well-marked upstream 

 migration apparently for spawning purposes. In February the fish 

 reaches Luang Prabang, in Laos, and there formerly gave rise to an 

 extensive fishery. The fish pushes its way farther upstream, and is 

 known from Chiengsen, Thailand, opposite the Shan States, and 

 apparently traverses the entire length of the Shan States and enters 

 the Chinese province of Yunnan. According to Pavie (1904) the fish 

 spawns in Lake Tali, which, according to a special communication 

 from the National Geographic Society, of Washington, D. C, is just 

 east of the town of Tali in Yunnan Province, is about 30 miles long 

 and 3 to T miles wide, and is connected with the Mekong by the 

 Yangpi River. 



At Luang Prabang the fish on its downstream movement after the 

 spawning season used to be intercepted by nets until June (Pavie, 

 1904). 



590087 — 45 25 



