A STUDY OF TWIN LAKES, COLORADO. 173 



T.Scott (1901) notes that the smaller crustaceans (schizopods, amphipods, and 

 copepods) form a very important part of the food of the young marine fishes which 

 he studied. Steuer ( 1901 ) found that 39 specimens of fresh-water fishes, varying in 

 length from 5.5 to 13.5 centimeters and belonging to five different species, had 

 partaken more or less freely of plankton crustacea. He found that a specimen of 

 Seardinius erythrophthalmus 7 centimeters long had eaten 2,000 to 3,000 Chydorus 

 sphsericus, and a slightly larger Carassiw (Karausche) about 5,000 Acroperus haipae. 

 Needham (1903) notes the presence of Daphnia in the stomachs of two brook trout 

 out of 25 which he examined. Kofoid (l'.'o:;) found that tin- annual production of 



plankton and the output of the fisheries of the Illinois River sho^ so correlation 



in their changes from year to year. 



Considered with respect to whether plankton crustacea form any part of their 

 food supply at any stage of their existence, trout readilj fall into inn groups, 

 resulting from differences in habitat. The one group includes the trout that live in 

 mountain streams. They ran not depend upon these minute crustaceans for a supply 

 of food because the waters of these streams are generally too wild and rapid for the 

 development of plankton life. < Occasionally, of course, there may be favored localities 

 where pools are quiet enough for the growth of such organisms, but these arc rare 

 The fry of trout which regularly inhabit lakes, but which ascend streams to spawn, 

 must depend upon some other source of food while they are in the stream-. This 

 was well illustrated by the fry obtained in Lake Creek, whose food consisted entirely 

 of insect-. The other group is composed of trout that inhabit lake-. Here the 

 conditions are favorable tor the development of plankton organisms, and the trout 

 may draw upon the plankton crustacea for food. That these crustaceans may form an 

 (dement of the food of some trout for a considerable period of time i- shown b\ t he 

 fad tint they were found in stomach- of specimens that had reached a length of 12 to 

 15 inches (30.5 to 38 centimeters). Ii seems a little strange that trout of thi- size 

 should bi' abl to obtain such large numbers of t hese -mall crust iceans with an appa- 

 ratus apparently -o very poorly adapted to procuring such minute organisms. 



The investigations relating to the food of marine fishes -how that copepods 

 are by far the mo-t important (dement of the entomostracan part of their food, the 

 Cladocera being negligible. Almost the reverse of thi-. however, wa- found to be 

 true of the trout of Twin Lake-. Out of 31 specimens that had eaten Entomostraca, 

 2t> had eaten only Cladocera {Daphnia), I only copepods, and 4 both Daphnia and 

 copepods. In the four instances in which both had been eaten, the Daphnia consti- 

 tuted thirteen times as much of the stomach content-, on an average, a- 1 he copepods. 



Huitfeldt-Kaas (1898) says concerning his results, "Im dan/en ge imen die Daph- 



nien vie! haufiger in dem Fischemagen anzutreffen als die Copepoden." He thinks 

 the explanation of this lies in the fact that the Daphnida?, with their slow, regular 

 movements, are more easily obtained than the copepods with their irregular, jumping 

 movements. Steuer (1901), however, thinks that the predominance of Cladocera is 

 due chiefly to their greater size, and secondly to their greater abundance. It is true, 

 of course, that the copepods found in Twin Lakes are more powerful swimmers than 

 the f><ij>Itiu,t, and are more irregular in their movements, so that it would be more 

 difficult for the trout to capture them. It is also true that Daphnia are slightly larger 

 than the copepods, and for this reason could be more easily obtained, while, also, the 



