CALIFORNIA FISH AND GAME 119 



NOTES ON THE FOOD OF TROUT 



By P. R. Needpiam, Ph.D., Associate Aquatic Biologist, United States 



Bureau of Fisheries 



With seven photographs by the author 



Trout foods are normally derived from two sources, the water 

 itself, and the adjacent land. Any angler who has lifted stones or 

 sticks from the bed of a permanent stream has seen the great array of 

 organisms present. Underwater foods include a host of aquatic insects 

 such as caddis-fly, mayfly, or stonefly larvae and nymphs, beside numer- 

 ous shrimp, snails, and fishes. Counts of submerged trout foods from 

 given areas have shown insect foods to average over 7835 in number 

 per square yard in certain coastal streams in California. On the other 

 hand, land foods eaten by trout are mostly insects or other terrestrial 

 animals that are blown or fall into streams accidentally. Water living 

 food organisms are available to trout the year round, while in localities 

 where winter is severe, land foods will be abundant only during the 

 warm summer months. 



Many anglers open stomachs to see what trout have been eating 

 and use their observations as a guide in the selection of lures. The 

 varied assortment of bugs in most stomachs and the difficulty of identi- 

 fying them properly due to their small size usually leads anglers to 

 conclude that the trout are bottom feeding and hence, they should turn 

 to bait or sunken fly to stimulate their gastronomic interests. 



In the course of the work of the California Trout Investigations 

 we have had occasion to examine microscopically many stomachs ** of 

 rainbow and Loch Leven trout and below are presented a few of our 

 observations along this line that may prove of interest to anglers. In 

 no sense is this data intended to show how to overcome a trout's critical 

 taste for our feathered offerings, but instead, may serve to point out a 

 few interesting dietary idiosyncrasies along with some observations on 

 the food organisms themselves. 



In Table 1, it is shown that the dominant food of young steelhead 

 trout from the coastal stream, Waddell Creek, were caddis-flies, over 

 50 per cent of all items eaten belonging to this group. Of the 557 

 eaten, only one was eaten in the adult stage at the surface of the water. 

 All the rest were taken as larvae or pupae below the w^ater surface 

 where they normally live in their immature stages. 



* Published by permission of the U. S. Commissioner of Fisheries. 



** Thanks are due Francis Sumner of the staff of the Trout Investigations for 

 his help in both field and laboratory work herein reported. Permission to collect 

 trout for scientific purposes at all times was granted by the Division of Fish and 

 Game. 



