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Experimental study of feeding behavior and interaction of coastal cutthroat trout (Salmo clarki clarki) and dolly varden (salvelinus malma) Schutz, David C.


Differences in food habits and spatial distribution of sympatric Dolly Varden (Salvelinus malma) and cutthroat trout (Salmo clarki clarki) in a small coastal lake were documented by Andrusak (MS 1968). Segregation was inferred to be of the interactive type hypothesized by Nilsson (1965, 1967). The object of this study was to describe feeding behavior of individuals from these sympatric populations, and to evaluate the importance of food exploitation to the segregation process. Individual and paired fish were studied in the laboratory throughout the spring, summer and autumn. The different food habits were found to be due to a number of basic behavioral and morphological differences between the species. Dolly Varden oriented to and rested on the bottom. Cutthroat rested in the water column and were frequently surface oriented. Searching behavior differed between the species. Dolly Varden swam faster and at relatively constant rates. They sampled "mouthfuls" of substrate as they searched. Trout alternately hovered and cruised, sampling specific items. At low light intensities they were much less successful than the char at finding benthic food items. The mouth of the Dolly Varden is small and "scoop-like" compared to that of the cutthroat, and seems particularly well adapted for benthic feeding. Dolly Varden searched persistently for benthic organisms in the absence and presence of surface insects. Cutthroat rapidly switched from bottom to surface feeding if insects were presented there. The observed differences between species were fully expressed in isolated individuals. There was no evidence of the differences being magnified through interspecific competition. These differences, believed to be inherent, were considered sufficient to keep the species segregated without the involvement of competition. Segregation was concluded not to be of the interactive, type, even though the populations still retained considerable plasticity enabling them to switch diets or habitats when necessary or advantageous. The period of intense competition and food exploitation was considered to have occurred and ended during earlier stages of the coexistence.

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