1 / 4 BULLETIN OF THE BUREAU OF FISHERIES. 



hitter, being long- and slender, would l>e more likely to pass through the straining 

 apparatus of the trout. The differences in size and shape would be particularly 

 effective in permitting the copepods to escape from the larger trout, which have a 

 rather coarse straining apparatus. 



With respect to size, however, it may be said that the only Entomostraca repre- 

 sented in the food of some specimens of Sal/m.o henshawi from Lake Tahoe were 

 two species of Daphnia, while the large copepod Epischura ra< vadt nsis was found to 

 be more abundant in the lake at the time than Daphnia. So far as size is concerned, 

 therefore, it would seem that this copepod could be obtained by the trout as easily as 

 the Daphnia. As to the point that the Cladocera are more abundant, it is only nec- 

 essary to say that adult copepods were nearly as abundant in Twin Lakes in 1902 as 

 Daphnia, and were more abundant in 1903. Also, at the time of the above observa- 

 tion on Lake Tahoe in 1904, the copepods were much more abundant than Daphnia. 



There is some doubt, however, as to whether the very great predominance of 

 Daphnia over copepods in these instances is fully accounted for by the three factors 

 that have been considered — namely, differences in movement, size, and shape. But 

 no other factors were apparent. It was thought, at first, that possibly small swarms 

 of I><i/>Iini<i might contribute to the explanation, but no evidence whatever of any 

 unusual aggregations of these was found. A single catch of trout will serve to show. 

 still further, the striking difference in the role played by these two forms of Crus- 

 tacea as sources of food. Ten rainbow trout whose stomachs contained Entomostraca 

 were caught in the pool below the dam in the outlet canal of Twin Lakes within an 

 hour one afternoon. Of this number '•' had eaten only l><ij>]iiiin . while the tenth had 

 eaten both copepods and l)<i/>ltiti<t. The specimens were all about the same in size — 

 6 to S inches (15 to 20 cm.) — so that their straining apparatus should have been 

 equally effective. Since one trout was able to secure copepods. and it was one of the 

 larger specimens, it hardly seems probable that the other nine were unable to secure 

 them also, either on account of the irregular movements, the small size, the slender 

 form, or the scarcity of the copepods, as these were more abundant than Daphnia in 

 the water that was flowing into the pool. 



From what has already been said, it will be seen that many species of fish are 

 important factors in the destruction of plankton Crustacea. Hence, this fact must 

 be taken into account in a quantitive consideration of this element of the plankton of 

 a body of water inhabited by these fishes. Immense numbers of minute crustaceans 

 are destroyed by the trout in Twin Lakes. One greenback trout l~2 inches (30 centi- 

 meters) long had eaten 4,500 Daphnia/ another 1-1 inches (35 centimeters), 2,250, 

 and the stomach of a rainbow trout 15 inches (38 centimeters) long contained 1,300. 

 Over 16 per cent of the greenback and rainbow trouts whose stomachs contained 

 food, had eaten Daphnia, the number eaten by each individual varying from about 

 ■"in or 75 up to 4.500. When it is considered that these figures represenl the number 

 consumed by each trout at a single meal, and also that the lakes were well populated 

 with these two species of trout, we can see how enormously they affect the numbers 

 of Daphnias. In this connection it is interesting to note also that the stomach of a 

 l.vineh Tahoe trout (Salmo henshawi) which was examined during the summer of 

 1904 contained 1,700 Daphnias. about two-thirds of which were 1). hyalina and one- 

 third />. pulex. The above numbers, moreover, do not represent the entire drain 



