Spatial and Temporal Variation in Risk to Piscivory of Age-0 Rainbow Trout: Patterns and Population Level Consequences

Abstract

We used tethering to measure spatial and temporal patterns of relative risk of predation to age-0 rainbow trout Oncorhynchus mykiss over 2 years in five lakes that varied widely in the abundance of piscivorous rainbow trout. During the day, the most risky habitats were just above the thermocline near the substrate and in open water at the same depth. Risk was intermediate at the surface in the littoral zone and at the surface over the intersection of the thermocline and the lake bottom. The least risky habitat was at the surface in the open water. The intensity and spatial pattern of relative predation risk changed at dusk. The most risky habitat was still at the thermocline–lake bottom intersection, but it was more than twice as risky as during the day. At dusk, relative predation risk increased in the littoral zone and at the surface above the thermocline–lake bottom intersection. The open-water habitats, both at the surface and at the thermocline, had a low relative predation risk. Age-0 and older rainbow trout in the experimental populations segregated in their use of habitat, with the majority of age-0 trout in the littoral zone and the majority of older conspecifics in the pelagic zone. Survival rate of age-0 rainbow trout over the growing season was inversely correlated with instantaneous measures of relative predation risk but approached an asymptote at high relative predation risk, suggesting that the littoral zone provides a refuge that prevents the elimination of age-0 fish by piscivory.