A STUDY OF TWIN LAKES, COLORADO. 



]65 



tliat is digestible, as well as inaiiy thinos that are not digestible. Trout are regarded 

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



EXAMINATIONS OF TROUT STOMACHS. 



During the investigation at Twin Lakes the stomachs of SO-i trout were obtained 

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

 positively identified, and the other 368 belonged to six different species. The 

 specimens were obtained during July and August, 190'i, and l)etween the middle of 

 June and the 10th of Septoml)er, V.HKi. They were caught b\' various methods, such 

 as by trolling with a spoon-hook or baited hook, bj- 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 favora1)le 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 diflerent heads. 

 Chiro)iomus and Sin/tdmiii. 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-explanatoiy. 



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 filled, 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 cohunn shows the number of specimens 

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

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

 the last table is given below. 



Landlocked salmon (Salrao sehago). — Twenty -four specimens of landlocked salmon 

 were 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 23. .5 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: 



Contfriln nf xloinarlis of 19 landlocked salmon. 



Food elements. 



Fish remains 



Triehoptera (larvEe and pupse) 



Lepidoptera 



Diptera 



Chironomus (larvae and pupje) 



Simuliuni (larvEe) 



Coleoptera 



Number 

 of speci- 

 mens 

 in which 

 found. 



Average 

 per cent 



of 

 element. 



100.0 



95.0 



40.0 



52.5 



2.0 



100.0 

 73.0 



Food elements. 



Hj'menoptera (ants; . . 



Insect fragments 



Crustacea (Gammaru.-^) 



Mollusca 



Vegetable debris 



Sandandgravel 



Number 

 of speci- 

 men.'* 

 in which 

 found. 



.Average 

 percent 



of 

 element. 



20.0 

 20.0 

 5.0 

 1.0 

 62.0 

 61.0 



The specimen containing Simulium larvse was the one caught in the pool below the 

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

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



