Seasonal and Diel Distribution and Movement of Cutthroat Trout from Ultrasonic Telemetry

Abstract

Our objective was to track the diel movements and distribution of large Bear Lake cutthroat trout Oncorhynchus clarki utah in relation to vertical environmental gradients and prey distribution in Strawberry Reservoir, Utah, during summer and autumn 1997. Implanted ultrasonic transmitters provided the depth and general location of the fish, while a differentially corrected Global Positioning System recorded boat positions that were then overlaid on a bathymetric map to determine temporal and spatial habitat use patterns. Tagged cutthroat trout used both nearshore and offshore habitats during both summer and autumn, but they were generally more pelagic during summer and used shallower nearshore zones more extensively in autumn. During early August, large cutthroat trout were restricted to the thermocline (metalimnion; 10-15 m deep) between a hypoxic hypolimnion and a warm epilimnion during all diel periods. When hypoxia affected the lower metalimnion in mid-August, the fish moved up into the upper metalimnion and warmer epilimnion. From mid-August through autumn, fish utilized a much broader range of depths, remaining more active and in deeper water during daylight and less active and near the surface at night. Fish depth and swimming speed varied by season and diel period but were consistently highest during daylight, intermediate during crepuscular periods, and lowest at night. At dawn, fish closely tracked the maximum light available in the water column until the level of illumination was well in excess of the threshold for maximum reaction distances to invertebrate and fish prey. The available light never restricted daylight foraging opportunities at the depths occupied by the fish. Widespread movements among habitats and depths by large, cannibalistic cutthroat trout allowed considerable spatial and temporal overlap with juvenile salmonids over most of the year, except when a temporary nearshore refuge for juvenile salmonids formed during the temperature-oxygen squeeze in early August. Cannibalism on stocked cutthroat trout could potentially be minimized by releasing juveniles in nearshore areas in early August, when adults are restricted to the thermocline in deeper waters further offshore.