A STUDY OF TWIN LAKES, COLORADO. 



165 



that is digestible, as well as many things that are not digestible. Trout are regarded 

 as carnivorous from choice, but omnivorous in cases of necessity. 



EXAMINATIONS OF TROUT STOMACHS. 



Dufine the investigation at Twin Lakes the stomachs of Hit-t trout were obtained 

 and the contents studied. Twenty-six specimens were fry that were too small to be 

 positively identified, and the other 308 belonged to six different species. The 

 specimens were obtained during duly and August. L902, and between the middle of 

 dune and the 10th of September, L903. They were caught by various methods, such 

 as by trolling with a spoon-hook or baited hook, by fly-casting, with a seine, in gill- 

 nets, and on trout-lines or set-lines. The fry, of course, were taken with a dip net. 

 They were caught at various hours of the day. and under as favorable conditions as 

 possible, so that the results might represent the natural food of the specimens. 



Only a very general classification of the different elements of the stomach 

 contents was attempted. They are recorded under twenty two different heads. 

 ChtroiKiiinix and Simulium were noted separately from the other Diptera, because 

 they were represented chiefly by their aquatic larva'. The term "insect fragments" 

 includes all fragments of insects that were too small to be identified. The other 

 terms are self-explanatorj . 



In recording the contents of a stomach an estimate was first made as to the 

 relative amount of food it contained; that is, whether it was well tilled, half, or a 

 quarter full, or contained only a little. Then the various constituents were sorted 

 out, and the percentage of each in the entire quantity was carefully estimated. In 

 the first six of the following tables the first column shows the number of specimens 

 in which the respective elements appeared, and the second column shows the average 

 percent of each element in all the specimens that contained it. An explanation of 

 the last table is given below. 



Landlocked salmon (Safono sebago). —Twenty-four specimens of landlocked salmon 

 wore examined. Twenty-three were obtained from Lower Lake and one from the 

 pool just below the gates, or dam. in the outlet canal. The length of the specimens 

 varied from 8 to -j.\\.'-< inches (20 to 70 centimeters). Two stomachs were entirely 

 empty, another contained only a grain of oats, while two others contained only a few 

 grains of coarse sand. Thus nineteen are accounted for in the following table: 



Contents of stomachs of 19 landlocked salmon. 



Food elements. 



Fish remains 



Trichoptera (larvae and pupse i 



Lepidoptera 



Diptera 



Chironomus (larvae and pupsej 



Simulium (larvse) 



Coleoptera 



Number 

 oi speci- 

 al ens 

 in which 

 found. 



Average 

 percent 



of 



element 



ion. a 



95.0 

 40.0 

 52. 5 

 2.0 

 100.0 

 73.0 



F I elements. 



Hymenoptera (ants) . . 



Insect fragments 



i Irustacea (Qammaru.0 



Mollusca 



Vegetable debris 



Sand and gravel 



Number 

 "i speci- 

 mens 

 in which 



I. mil. I 



Average 

 percent 



of 



element. 



20.0 

 20.0 

 5.0 

 l.n 

 62. o 

 61.0 



The specimen containing Sim ulium larva? was the one caught in the pool below t he 

 dam. The intestines of several specimens contained a great deal of sand and mud 

 and the shell of a small bivalve mollusk was found in one. 



