Evidence of Food Limitation of Rainbow and Brown Trout in Southern Appalachian Soft‐Water Streams

Seasonal patterns of age‐specific growth rates and condition factors of rainbow trout Salmo gairdneri and brown trout S. trutta were studied in relation to the available food resources in five streams of the southern Appalachian mountains. Standing crops of benthic invertebrates were low relative to streams of similar size in other geographic areas. Although terrestrial organisms contributed substantially to the invertebrate drift, total drift rates were also relatively low. Numbers of prey items per trout stomach were small and were directly related to drift rate. These results reflected the limited food base. Condition factors (weight˙length −3 ) of age‐ 1 trout declined during summer, and growth rates among age‐1 and older trout were generally lower in summer than in winter, despite favorable summer water temperatures. This “inverted” seasonal pattern of growth was likely due to an inadequate food base. We believe that growth rates were relatively low in summer because much of the limited energy intake was devoted to metabolism and little energy was left for growth. Higher growth rates occurred in winter because energy requirements for metabolism were reduced at lower water temperatures. An important function of habitat in food‐limited streams may be to partition overall fish production among age classes by providing energy‐efficient feeding sites for different sizes of fish.