A STUDY OF TWIN LAKES, COLORADO. 173 



T. Scott (11)01) notes tluit tho siiialloi- crustaceans (scliizopods. aniphipods, and 

 copepod.'!i) form a very important part of the food of the young marine ti.shes whicli 

 he studied. Steuer (1901) found tiiut 39 specimens of fresh-water tishes. varying in 

 longtli from 5. .5 to 1.S.5 centimeters and belonging to five different species, had 

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

 Scdrdiiiiiis ei'i/i/uyt/Jif/iith/iiis 7 fBntlmctei'n long had eaten ii,()Oi) to 3,000 Cliydonis 

 isph.xfieuft^ and a slightlv largei Carannlus (Karausche) about 5,000 Acroperns luirpn'. 

 Needham (l'.>0.3) notes the presence of Daphnia in the stomachs of two brook trout 

 out of 2.) which he examined. Kofoid (1903) found that the annual production of 

 jilankton and the output of the fisiieries of the Illinois lli\'er show some 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 readily fall into two groups, 

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

 moiuitain streams. They can not depend upon these minute crustaceans for a suppl}' 

 of fot)d 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 ai'e rare. 

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

 uuist depend upon some other source of food while they are in the streams. This 

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

 of insects. The other group is composed of tiout that inhabit lakes. Here the 

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

 ma}' draw upon the ]3lankton crustacea for food. That these crustaceans may form an 

 element of the food of some trout for a considerable period of time is shown by the 

 fact that they were found in stomachs of specimens that had reached a length of li^ to 

 15 inches (30.5 to 38 centimetei's). It seems a little strange that trout of this size 

 should be able to obtain such large numbers of these small crustaceans with an appa- 

 ratus apparently so very poorly adapted to procuring such minute organisms. 



The investigations relating to the food of marine tishes show that copepods 

 are by far the most important element of the entomostracan part of their food, the 

 Cladocera being negligible. Almost the rev^erse of this, however, was found to be 

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

 26 had eaten only Cladocera {Dajihnui), 1 only copepods, and -i both JJaphnia and 

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

 tuted thirteen times as much of the stomach contents, on an average, as the copepods. 

 Huitfeldt-Kaas (LS9S)says concerning his results. *'Im (fanzen genommon die Daph- 

 nien viel hauliger in dem Fischemagen anzutrefl'en als die Copepodcn." He thinks 

 the ('xj)lanation of this lies in the fact that the Daphnid;e, with tlieii- slow, regular 

 movements, are moi'e easily obtained than the copepods with their irregular, jumping 

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

 due chiefl}- 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 Daphnia, 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 DajiJuiiu are slightly larger 

 than the copepods, and for this reason could l)e more easih' obtained, while, also, the 



