MARINE FISHES IN SOUTH EAST ASIAN WATERS 459 



proper and upstreams only to a very limited extent; whereas, they too 

 occur far out at sea. The existence of this fauna is probably condi- 

 tioned in the first place by a lowered salinity, though one should not 

 think in too low percentages. A salinity of about 30 Yoo is probably the 

 upper limit, almost true seawater therefore. Further the amount of silt 

 seems to have an influence. At least according to my experience the 

 muddier the river water is, the farther the aestuarine conditions reach. 

 There is a coordination between the somewhat lowered salinity and the 

 amount of silt resulting maybe in a high fertility of the water as shown 

 by a rich diatom plankton. Such aestuarine fauna we find in front of 

 the Indus and on the other side of the Indian Peninsula in front of the 

 Ganges and Brahmaputra mouths. From there eastwards, there is a 

 gap where the Burmese mountains reach the coast. Again such an area 

 appears before the Salween and Irawaddy. The narrow part of the 

 Straits of Malacca may be considered as one large aestuary. An aestua- 

 rine fauna fringes the whole coast of Borneo and the whole East coast 

 of Sumatra. Farther North on the Asian mainland, we find isolated 

 stretches of aestuarine waters of a great extent in the Gulf of Thailand 

 and in front of the Mekhong and the big Chinese rivers. This aestua- 

 rine fauna is petering out to the North as far as the Yellow Sea. 



Subject to this, it is to be noted that on the South coast of Java 

 we have a rather large aestuary where the aestuarine fauna as such is 

 lacking except Gohioides and Eleutheronema tetradactylus. It seems 

 that elements of this fauna have not been able to cross the coastal 

 stretches of the Indian Ocean from East Sumatra. The more remark- 

 able is it, therefore, that on the south coast of New Guinea, where a 

 system of big rivers empty into the sea, a similar aestuarine fishfauna 

 appears again, partly composed of the same species like Eleutheronema 

 tetradactylus and Setipin7ia, partly, composed of other new local species. 

 A great field of investigation is still waiting for the ichthyologist. This 

 isolated aestuarine fauna of New Guinea is separated by hundreds of 

 miles of open deep sea from Borneo, the nearest coast where aestuarine 

 conditions are found. Here a local aestuarine fauna, partly composed 

 of other elements like another Herpodon species as on the Asian main- 

 land, has come into being. 



Stretches of coast along the open ocean often show no coastal zone 

 in the water. The open sea, the oceanic conditions, come up to the 

 shore itself. Thus it is possible that tuna's and related forms are caught 

 there in traps, a few meters out from land, whereas these fishes are true 

 oj>en sea forms often even avoiding narrow -channels between islands. 



Seas on continental shelves are often influenced by the nearby land. 

 Here, as for instance, in the Java Sea we may distinguish between a 

 narrow stretch along the coast and the waters farther away. It is in- 



