Differential Marking of Embryos by Location and Date of Release Reveals Within‐River Natal Homing and Parental Influence on Progeny Return Timing in Sockeye Salmon

Abstract

AbstractThe tendencies of salmon and trout to spawn in proximity to their natal sites and follow the parental timing of breeding are fundamental to their population structure and also affect the performance of enhancement facilities. Specifically, an understanding of how parental spawning date and offspring release location affect the timing and place of breeding by returning adults is important for the evaluation of hatchery supplementation programs and their integration with sympatric naturally spawning populations. We collected Sockeye Salmon Oncorhynchus nerka carcasses in the Cedar River, Washington, over three spawning seasons to evaluate spawn timing and distribution. Thermal marks on the otoliths of hatchery‐origin carcasses allowed assignment of fish to parents that spawned in the early, middle, and late parts of the season and to fry release locations in the lower, middle, and upper parts of the river. The early portion of the spawning run was composed of a higher proportion of hatchery‐origin fish than was the case later in the season. Hatchery‐origin adults that were released early in the spring as fry (from early spawning parents) were recovered early in the season, and those released later in the spring as fry were recovered later in the season. The natural‐origin spawn timing most closely matched the spawn timing of middle and late hatchery fry release groups in all years. Our data also demonstrated a correspondence between hatchery fry release location and adult recovery location; fry released at river kilometer (rkm) 1.0 were recovered 7.0 km lower (median value) in the river than fry released at rkm 21.7 and were recovered 10.0 km lower than fry released at rkm 35.1. These data contribute to the substantial body of evidence that population characteristics of anadromous salmonids are sensitive to influence from fish culture practices—even under benign regimes—because of the strong connections between parental and progeny timing and the fine‐scale homing of salmon to release sites within a river.