Habitat selection and longitudinal distribution patterns of sympatric sub-adult trout (Salmo trutta) and grayling (Thymallus thymallus) in two large northern rivers
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Compared with wadeable streams there is little knowledge on habitat selection and distribution patterns of sub-adult fish in large rivers, because fish sampling and habitat measurements in large rivers are methodically challenging and require a high effort in order to reduce bias. In my thesis I studied habitat selection and longitudinal distribution patterns of sub-adult (<3 years, sorted by: 0+, 1+ and 2+) trout (Salmo trutta) and grayling (Thymallus thymallus). Fish sampling and habitat measurements were conducted with transects (approx. 500 m in length) in two large river sections (>10 km) that differed in valley confinement and gradient. Habitat selection were analyzed with AICc model selection, while longitudinal distribution patterns were analyzed by testing if fish abundance had a linear or non-linear response to distance from the upmost spawning ground. Segregation in habitat preference between sub-adult trout age-classes indicated intraspecific competition, whereas trout 0+ had a low abundance in areas with high substrate coarseness in contrast to trout 2+. Abundance of sub-adult grayling was associated with areas of reduced valley confinement and river gradient. This caused grayling 0+ and grayling 1+ to have an increasing non-linear relation to distance from the upmost spawning ground in the confined and high gradient river Otta. A similar distribution pattern was observed for trout 0+ in Otta, in contrast to trout 2+ which displayed a simple linear decrease in abundance from the upmost spawning ground. In the un-confined low gradient river Lagen, none of the species age-classes expressed a distribution pattern related to distance from the upmost spawning ground, indicating that the effects of a high valley confinement and river gradient in Otta was the predominant drivers of longitudinal distribution patterns.