Body Size and Growth Rate Influence Emigration Timing of Oncorhynchus mykiss

Juvenile Oncorhynchus mykiss migrate extensively in freshwater during the fall. We used individual tagging to study the spatial origin, influences, and outcomes of fall migration on fish that emigrated from summer rearing tributaries during the fall (early emigrants) and those that did not (late emigrants) in the South Fork John Day River, Oregon. Fall migration amplified body size differences between early and late emigrants. There were more early emigrants from a lower‐gradient stream than from a higher‐gradient stream. Early emigration was positively related to individual summer growth rate and fall body size. Oncorhynchus mykiss dispersed downstream into higher‐order streams during the fall. Early emigrants shifted to an alternative location and experienced significantly greater winter growth than did late emigrants that remained in tributaries. Early emigrants initiated smolt migration sooner the following spring than did late emigrants. Early and late emigration from the South Fork John Day River was associated with asynchronous emigrant‐to‐adult survival rates.