A STUDY OF TWIN LAKES, COLORADO. 171 



Thirty-three specimens of Twin Lakes trout in all had eaten ( Irustacea. Of this 

 number. 26 had eaten only Daphnia, 1 had oaten only copepods, 4 had eaten both 

 Daphnia and copepods, and 2 had eaten only Gammarus. The insignificance of 

 Gammarus as an (dement of food was rather surprising, as it was abundant in both 

 lake-. 



The vegetable matter that was found in the various stomachs consisted of such 

 things as small pieces of wood, spruce leaves, seeds and seedpods of land plants, 

 roots. Potarnogeton leaves, and alga?. It seems probable that mosl of the sand and 

 gravel was taken by accident. Much of it was. no doubt, derived from the cases 

 of some of the trichopter larvae. Judging from the difference in the pex-centages of 

 sand and gravel, il seems that the landlocked salmon and the rainbow trout feed on 

 the bottom much more extensively than either the greenback or the brook trout. 



PLANKTON CRUSTACEA AS FISH FOOD. 



It is frequently stated thai the Entomostraca are very important factors in the' 

 food of many fishes, but we have very little definite information as to the kind-, the 

 relative proportions, or the numbers of these small crustaceans eaten b\ the different 

 fishes. It is known, of course, that plankton Crustacea are an important (dement of 

 the food of some fishes, such as the whitefish and herring, but here, even, we know- 

 little of the relative importance of the different Entomostraca. There is likewise 

 great need of extending our knowledge a- to the importance of these minute crusta- 

 ceans in the diet of fishes that are not regarded as plankton feeder-: and in spite of 

 the emphasis that has been laid on the fact that thej are of supreme importance to 

 fish fry, here, too, definite information i- l>\ no mean- as extensive as it should be. 

 A brief summary of some id' the results obtained bj several obsen ers is given below. 



Kno\ (1834) observed that the vendace (Coregonus) fed exclusively on micro- 

 scopic animals of the (das- Entomostraca. Hair. I ils.",7i mention- the fact that 

 "delicate little crustaceans " were found in the stomachs id' Loch Leven trout 

 (Salmofario l> vt ic nsis), of the chart- Salmo&alvt Units, and of the vendace ( ( 'oregonus 

 willughbii). The crustaceans mentioned in thi- connection are Daphnia, Bosmina 

 coregoni, and < ypris. 



Jardine (1857) states that stomach- of the vendace (Cwegonus) were found 

 tilled with Entomostraca. In speaking of the food of the trout in Loch Levin. 

 Fraiu-is (IStJS) says that large quantities of Entomostraca were of ten found in their 

 stomachs. Smith (1874) found Daphnia in the stomach- of six specimens of < 'oregonw 

 alius. Barfurth (1874) found microscopic Entomostraca in the stomach- of Alosa 

 vulgaris. 



Between lsTs and L888 Forbes made a careful study of the food of many Illinois 

 fishes. He examined young belonging to 12 families and 26 genera, and found that 

 the representatives of only one genus had not been feeding more or less extensively 

 on Entomostraca. In examining both young and adults, he found that specimens 

 belonging to more than 50 genera, representing Hi families, partook of Entomostraca 

 during some period of their lives. He also states ( 1893) that two specimens of young 

 trout taken at the mouth of Bridge Creek, Wyoming, had been feeding on Poly- 

 phemux ped icuhis and Daphnia pulex. 



