Use of Winter Concealment Cover by Juvenile Cutthroat and Brown Trout in the South Fork of the Snake River, Idaho

Abstract
During February-April 1990 the number of age-0 cutthroat trout Oncorhynchus clarki and brown trout Salrno trutta concealed during the day in several types of cover was assessed in a large river, the South Fork of the Snake River in southeastern Idaho. Fish typically were concealed along the edge of the wetted perimeter at water depths shallower than 0.5 m. Population estimates for age-0 cutthroat trout ranged from 0 in rounded cobble (<20 cm in diameter) to 4.56 fish per meter of bank in clean boulder substrate. Abundance of age-0 brown trout varied with substrate in a similar manner but ranged from 0 to 0.50 fish per meter of bank. Cobble and boulder habitat that was heavily embedded with fine sediment contained fewer juvenile trout of either species. The first electrofishing pass extracted 78% of the age-0 cutthroat trout and 76% of the age-0 brown trout estimated to be present in concealment cover. The fraction offish emerging from concealment to swim in the water column at night was 61–66% of the numbers estimated to be in concealment during the day.

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