Body Size and Growth Rate Influence Emigration Timing of Oncorhynchus mykiss

18 Juvenile Oncorhynchus mykiss migrate extensively in freshwater during fall. We used 19 individual tagging to study the spatial origin, influences, and outcomes of fall migration on fish 20 that emigrated from summer rearing tributaries during fall (early-emigrants) and those that did 21 not (late-emigrants) in the South Fork John Day River, Oregon. Fall migration amplified body 22 size differences between early and late-emigrants. There were more early-emigrants from a 23 lower gradient stream than from a higher gradient stream. Early-emigration was positively 24 related to individual summer growth rate and fall body size. Oncorhynchus mykiss dispersed 25 downstream into higher order streams during fall. Early-emigrants shifted to an alternative 26 location and experienced significantly greater winter growth compared to late-emigrants that 27 remained in tributaries. Early-emigrants initiated smolt migration sooner the following spring 28 than late-emigrants. Early and late-emigration from the South Fork John Day was associated 29 with asynchronous emigrant-to-adult survival rates. 30