1915] Fauna of the Chilka Lake : Introduction. 17 



animals which would otherwise have escaped, and compensated in some measure for the 

 large mesh of the main bag: none the less it was frequently drawn up half full of mud. 



The second type of net employed for bottom work was a Q-net, that is to say 

 a light frame of f in. iron (shaped in the form of a Q and towed by three bridles) to 

 which by means of brass rings a long bag of coarse-meshed canvas was attached. 

 This net produced excallent results; but it was only possible to make very short 

 hauls as the bag rapidly filled with mud. 



Mud we dealt with by means of a series of large rectangular sieves with brass 

 meshing, fitted in a frame to keep them above the level of the deck. 



A larger net, an otter-trawl with head-rope 28 ft. in length and 3 in. mesh at the 

 cod-end, was also employed occasionally and was successful in obtaining large fish 

 that were able to avoid the smaller nets, especially in thickets of Potamogeton near 

 the shore. 



The larger free-swimming organisms were obtained by towing the Q-net in 

 midwater and at the surface; but for many of the fish we were dependent on indigen- 

 ous methods, which will be described in a special paper in this volume. Plancton 

 we collected in silk tow-nets of the ordinary type supplied by the Marine Biological 

 Station at Plymouth. Hand-nets were of course employed in shore-collecting, in 

 which we found a hammer and chisel an essential part of our outfit. 



As regards determinations of salinities it seemed unnecessary, in view of the 

 enormous seasonal changes, to employ the elaborate titration method advocated by 

 the Bureau International pour l'Exploration de la Mer, a method designed to demon- 

 strate extremely slight differences in oceanic and coastal waters. We realized at the 

 outset that to obtain a complete or even an approximate knowledge of the varied 

 physical conditions that affect the salinity of the lake would be beyond our powers 

 and that it was improbable that observations carried out in a single year, however 

 complete, would render possible a true account of the actual changes that take place. 

 Variations in rainfall, temperature, wind, tide and possibly other factors must all 

 produce different effects in different years 



In making our observations on the density of the water we used a hydrometer 

 kindly lent us by Capt. R B. Seymour Sewell, Surgeon-Naturalist to the Marine 

 Survey of India, and our results are therefore expressed in the form of specific 

 gravities. The scale of the instrument, which is calibrated for I5°C, is about 7 cms. 

 in length and is graduated .'rom roo to 1-04 in 40 divisions. Readings were taken to 

 the nearest 0-00025. I* 1 order to give corrected readings of specific gravities of 

 1-0015 and under it was necessary, at the temperature at which we were working, 

 that the hydrometer should be scaled below 1 000. This unfortunately was not the 

 case and we are in consequence unable to insert the line representing sp. gr. 1/001 in 

 the chart reproduced in fig. 2 on p. 9. 



Water-samples were collected in bottles provided with a spring top and rubber 

 washer and were, as a rule, tested the day they were taken. The determinations 

 quoted are in every case reduced to i5°C. by the use of a correction table. This 

 table is based on a series of laboratory experiments made with the same instrument 



