Visual Prey Detection and Foraging in Sympatric Cutthroat Trout (Salmo clarki clarki) and Dolly Varden (Salvelinus malma)

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Abstract
In sympatric lacustrine cutthroat trout (S. clarki) and Dolly Varden (S. malma), reaction distance to artificial and natural prey increased with increases in irradiance from a visual irradiance threshold (VIT, the maximum quantity of irradiance resulting in zero reaction distance to prey) to a saturation irradiance threshold (SIT, the minimum quantity of irradiance that maximizes reaction distance to prey). The VIT and SIT for Dolly Varden were approximately 1 and 2 orders of magnitude lower, respectively, than those for trout. At all irradiance levels > 4.2 .times. 1017 photons .cntdot. m-2 .cntdot. s-1 the reaction distance of trout exceeded that of Dolly Varden. Reaction distance to prey in both species was greatest for red irradiance followed in descending order by green, yellow, and blue irradiance. The midsummer ambient irradiance regime in a lake cohabited by trout (concentrated in surface waters) and Dolly Varden (inhabiting deeper waters) showed that during daylight hours a much larger portion of the water column had irradiance levels .gtoreq. the SIT of Dolly Varden than the SIT of trout and that irradiance levels were always .gtoreq. the VII of Dolly Varden although only in the upper 5m at night. Estimates of reaction distance to Diaptomus kenai and foraging velocity at several irradiance levels showed that trout have the potential to visually search a volume of water 7 times greater than that searched by Dolly Varden for D. kenai on a summer day.