A STLTDY OF TWIN LAKES. COLORADO. 171 



Thirty-three specimens of Twin Lakes trout in ail had eaten Crustacea. Of tliis 

 number, 26 had eaten onh' Duphnin^ 1 had eaten only copepods, i had eaten both 

 DapJmia and copepods, and 2 had eaten only OainmaruH. The insigniticance of 

 Gainmarus as an element of food was rather surprising, as it was abundant in both 

 lakes. 



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, Potamogeton leaves, and algai. It seems probable that most of the sand and 

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

 of some of the trichoptor larva\ Judging from the difference in the percentages of 

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

 the bottom much more extensi\'ely than either the greenback or the brook trout. 



PLANKTON CRUSTACEA AS FISH FOOD. 



It is frequently stated that the Entomostraca are very important factors in the 

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

 relative proportions, or the numbers of these small crustaceans eaten l)v the different 

 fishes. It is known, of course, that plankton Crustacea are an imi^ortant element 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 as to the importance of these minute crusta- 

 ceans in the diet of fishes that are not regarded as plardvton feeders; and in spite of 

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

 fish fry, here, too, definite information is by no means as extensive as it should be. 

 A brief summary of some of the results obtained by several observers is given below. 



Knox (1831:) observed that the \-endace {Cori-tj<muj<) fed exclusively on micro- 

 scopic animals of the class Entomostraca. Baird (18.57) mentions the fact that 

 "delicate little crustaceans" were found in the stomachs of Loch Lcven trout 

 {Sahiiofarlo levenensls), ot the charr Sal/no .fali'elhiafi, and of thevendace {Coregonus 

 'willughhtt). The crustaceans mentioned in this connection are Daphnin, Bosmina 

 coregoni^ and Cypri>i. 



Jardine (18.57) states that stomachs of the vendace {C()r<jg(>inis<) were found 

 filled with Entomostraca. In speaking of the food of the trout in Loch Leven, 

 Francis (1868) says that large quantities of Entomostraca were often found in their 

 stomachs. Smith (1874) found DajiJinia in the stomachs of six specimens of Coregonus 

 alljus. Barfurth (1871) found microscopic Entomostraca in the stomachs of Alosa 

 vulgaris. 



Between 1878 and 1888 Forbes made a careful study of the food of many Illinois 

 fishes. He examined j'oung 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 16 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- 

 'phemus pediculus and Daphnla pulex. 



