174 BULLETIN OF THE BUREAU OF FISHERIES. 



latter. ht>in,>;' loiii^- iiiid sloiuler, would 1)0 inorr likely to pass through the straining' 

 apparatus of the trout. The dift'ereiiees in size and shape would be particularly 

 etl'eetive in pernutting the copepods to escape from the larger trout, which have a 

 rather coarse straining apparatus. 



\Vith respect to size, however, it may he said that the only Entomostraca repre- 

 siMited in the food of some specimens of S((hii<i Jieniilumu from Lake Tahoe were 

 two species of Daplinia, while the largo copepod Episclinni. nevadensls was found to 

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

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

 the I>itplin'K(. 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 

 Itiiphiiiit. and wore more abundant in lOOo. Also, at the time of the above observa- 

 tion on Lake Tahoe in IIHU, the copepods were nuich more abundant than Daphnla. 



There is some doul)t, however, as to whether the very great predominance of 

 Daplinia over copepods in these instances is fully accounted for by the three factoi's 

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

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

 of Ditp>Iinia might contribute to the explanation, but no evidence whatever of any 

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

 still further, the striking difl'erence in the role played l)y 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 i< had eaten onlj^ Dujilui/n, while the tenth had 

 eaten both copepods and Dn/iluiJa. The specimens were all aliont the same in size — 

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

 equally effective. Since t)no 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 Daplinia. in 

 the water that was flowing into the pool. 



From what has already boon said, it will be seen that many species of tish are 

 important factoi's in the destruction of plankton Crustacea. Hence, this fact must 

 be taken into account in a (juantitive 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 12 inches (30 centi- 

 meters) l^ong had eaten 4,.")00 Baphnia; another 1-t inches (35 centimeters), 2,250, 

 and the stomach of a rainbow trout 15 inches (3S centimeters) long contained 1,300. 

 0\er 16 per cent of the greenback and rainbow trouts whose stomachs contained 

 food, had eaten l>,t jilmiii. the number eaten by each individual varying from about 

 5(1 or 75 up to 4,5u(). When it is considered that these figures represent the number 

 consumed l)y each trout at a single meal, and also that tlie lakes were well populated 

 with these two species of trout, we can see how enormously they affect the nuinl)ers 

 of l)a])luuas. In this connection it is interesting to note also that the stomach of a 

 15-inch Tahoe trout {Subno heiixliairi) \vhich was examined during the summer of 

 I'.tOl contained 1,700 Daphnias, about two-thirds of which were D. hi/alina and one- 

 tliiid Ik piili.r. The above numbers, moreover, do not represent the entire drain 



