26 U. S. BUREAU OF FISHERIES. 



January had eaten five and seven lar^e Caledonia shrimp, respec- 

 tively, notwithstanding the great abundance of Hyalella and E. 

 gracilis in the same stream. 



Shira * states that Hyalella has been found to be a secondary host 

 for one of the parasitic roundworms that infests the young of the 

 smallrnouth black bass. For that reason the feeding of this species 

 to the bass is not to be recommended. 



SYNOPSIS AND DISCUSSION. 



Pearse (1918) stated that most fishes are not indiscriminate 

 feeders, but that they select specific objects from the available food 

 supply, that in some instances the powers of selecting and rejecting 

 are remarkable, and that a perch may have its whole alimentary 

 canal packed full of Daphnias, when the surrounding water con- 

 tains Daphnia mixed with greater quantities of algse. "Only the 

 animal planlrton is taken." He also cited the fact that young suckers 

 are able to reject fine particles of silt, retaining only the food or 

 organisms, and stated that even large adult perch often have noth- 

 ing in their alimentary canal excepting great numbers of cladocerans. 



The quantity of any food, particularly material found in the ali- 

 mentary tracts of individual fishes, is not indicative of the amount 

 necessary to sustain life, of the general supply, or even of the fish's 

 preference. It is merely indicative of immediate availability. That 

 perch have been found gorged with Daphnia or trout with other 

 forms of Crustacea might be considered presumptive evidence of 

 exclusive selection or discrimination, although it is quite possible 

 that of the food ever utilized by the fishes it is the only form con- 

 veniently available at the time. It is a matter of common knowl- 

 edge of those who have observed various fishes while feeding that 

 any selection or rejection made is often after having taken the object 

 into its mouth. 



The fact that Mather (1900) found only 5 per cent crustaceans in 

 the stomachs of certain individual trout does not indicate that those 

 trout would not have eaten more, even to the exclusion of ever^i:hing 

 else, had they been able to get them. Nor does the fact that Em- 

 body's (1911) observations showed that certain trout had eaten a few 

 Gam/marus limncvus^ while Eucrangonyx and Hyalella were also abun- 

 dant in the brook, indicate a pieference for that particular crusta- 

 cean. Nor could the fact be regarded as positively demonstrating 

 a power of discrimination, other than that produced perhaps by the 

 larger and possibly more active, and therefore more conspicuous, 

 Caledonia shrimp. Therefore absence, scarcity, or apparent dis- 

 crimination exhibited by stomach contents in trout are, as a rule, 

 no arguments against the use of crustaceans as trout food, for it 

 has been shown that trout thrive and attain the pink of condition 

 (literally and figuratively) upon food consisting largely of crus- 

 taceans. 



On the other hand, what holds true for one species of fish might 

 not for another. The fact that Pearse (1918) found a large black 

 crappie containing 75 per cent pelagic entomostracans is no evidence 

 that crappies could be fed exclusively upon Entomostraca. Pearse 



* Unpublished manuscript, 1918. 



I 



