i9 T 5] Fauna of the Chilka Lake : Introduction. 7 



gravities varying from 1-02625 to 1 -02650. l It is clear that in this region, during 

 March, the water was for all practical purposes as salt as the sea. 



In September the conditions were markedly different. The level of the water 

 was some five feet higher than in March and many of the low-lying islands in the 

 channel were almost or entirely submerged, a strong current was flowing out of the lake 

 and the water throughout the length of the channel was entirely fresh up to the point 

 where it entered the sea. Ebb and flow at this period made no alteration in salinity 

 and the maximum effect even of a high spring tide could only have been a slight 

 banking of the water at the mouth. The specific gravity of the sea a little to the 

 south of the entrance to the lake was at this period 1 01675, a reading considerably 

 lower than those obtained in March of the same year. 



By December the freshwater floods had in a large measure subsided and samples 

 taken in the early part of this month at Satpara and near Manikpatna gave readings 

 respectively of 1 00325 and 1-01250. At this time a small outflow from the lake 

 probably still persisted, salt water entering the channel only at high tide or under 

 specially favourable conditions of wind. 



In the outer channel, then, the range of salinity is the greatest possible, and 

 animals that live permanently in this region are able to exist for some eight months 

 in water almost or quite as salt as the neighbouring sea (sp. gr. 1-0270) and for at 

 least three months in water that is entirely fresh. 



The change from salt to fresh water that takes place annually towards the close 

 of the monsoon season is probably effected gradually. The discharge from the rivers 

 at the northern end of the lake must in the first place drive before it the saline water 

 with which the main area was previously filled, and there can be no doubt that the 

 first slow currents that pass down the outer channel have a comparatively high 

 salinity, which slowly decreases with the augmenting volume of the flood. The 

 change from fresh to salt water, on the other hand, probably takes place more 

 suddenly. After the floods have subsided and the head of water in the lake has 

 disappeared, there must, under suitable conditions of tide and perhaps also of wind, 

 come a time when a volume of salt water enters the sea-mouth and it is possible that 

 far-reaching alterations take place in the channel in the course of a single day. 



Both periods of change must have marked effects on the fauna of the outer 

 channel and on each occasion there is probably a high mortality ; freshwater forms 

 must be largely exterminated on the entrance of salt water, while many marine 

 species that have established themselves during the salt-water period must succumb 

 in the flood season. We have direct evidence that this occurs. 



Though less extensive than is the case with the outer channel, the changes of 



density to which the waters of the main area are subject 

 Salinity of the main area. .-. 



are nevertheless great ; the specific gravity varying , 



according to our observations, from i - ooo to 1-0150. 



1 A sample taken in a swamp south-east of the northern extremity of Barhampur Id. and separated 

 by a bar from the main channel gave a reading 1 02376. The water in this place was probably mixed 

 with a certain amount of surface drainage from land in the vicinity. 



