34 Records of the Indian Museum. [Vol,. XXIV, 



CONDITIONS AFFECTING FISH IN RAPID WATERS. 



The conditions that influence the fauna of hill-streams are the 

 following : — 



(i) The chief factor is the strength of the current, and all 

 the remaining conditions are due to it. The adaptations which 

 are dealt with further on are all due primarily or secondarily to 

 this one cause. The rate of flow of water varies considerably 

 according to the season, but throughout the year its average 

 flow is much higher than that of any stream in level countr)'. 

 This rapid flow of water would render life impossible to many 

 animals if they did not possess special organs of adhesion or other 

 appliances to counteract its influence. In places like Cherrapunji 

 (Khasi Hills), where 458 inches of rain falls in a comparatively 

 short time, the rate of flow of the water must at times be ex- 

 tremely rapid, and at such times some even of the most powerful 

 fish cannot withstand it for more than a few minutes. It is unfor- 

 tunate that I have not been able to collect any precise data to 

 compare the rate of flow of water and the fauna inhabiting it. 

 This is a case in which co-operation between a zoologist and a 

 physicist is called for. 



(ii) Next in importance are two factors on which the very 

 existence of the animals depends — food and shelter. In a hill- 

 stream there is always a sufficient quantity of lood, but the only 

 type usually available consists of algal slime covering stones and 

 rocks. There is no opportunity for any other type of vegeta- 

 tion to grow, as it is liable to be uprooted and carried away by the 

 strength of the current. In pools and ditches that are sometimes 

 formed on the bank of the streams, there is generally a growth of 

 water-weeds, but these cannot be referred to as rapid streams. 

 Certain fishes such as the species of Nemachilus feed on May-fly 

 and Dragon-fly larvae, but this type of food is usually scarce. 



As regards shelter, there is plenty of it in a hill-stream for 

 little fish. The species of Nemachilus, on the slightest provoca- 

 tion, hide themselves underneath stones. Those who have made 

 collections in the hiil-streams know how advantageous it is to run 

 the net among small stones and sometimes to pick up stones in 

 the net, because in this way all those forms which rest underneath 

 stones are netted. 



(iii) Hill-streams are never very deep, and their water is 

 usually very clear. Consequently during the day-time the ani- 

 mals have to withstand intense light. 



(iv) The water is well aerated as it is constantly in motion. 



These conditions do not apply to pools that occur in the 

 course of hill-streams, and the fish-fauna of these pools is very 

 different from that of the rapid current. It is possible that those 

 forms which live in rapid waters are sometimes carried into these 

 pools, but I have never come across any instance in which the 

 typical sluggish-water forms have been found in rapid waters. 

 Species of Danio, Lepidocephalichthys, Barbus, Barilius are generally 



