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Mark-recapture methods were used to examine watershed-scale survival rates of coastal cutthroat trout (Oncorhynchus clarkii clarkii) from two headwater stream networks located in the foothills of the Cascade Mountain Range, Oregon. Differences in survival were explored among spatial (stream segment, stream network [main stem or tributaries], and watershed) and temporal (season and year) analytical scales and assessed among specific abiotic (discharge, temperature, and cover) and biotic (length, growth, condition, density, and movement) factors. A total of 1,725 adult coastal cutthroat trout (>100 mm, FL) were implanted with half-duplex PIT (passive integrated transponder) tags and monitored seasonally over a 3-year period using a combination of electrofishing, portable remote tracking antennas, and stationary antennas. The effects of watershed, stream network, season, year, and fish length were the most important factors among the candidate survival models. The
greatest source of variation in survival was associated with year-dependent differences among seasons. Seasonal survival was consistently lowest and least variable (years combined) during autumn (September 16–December 15). Among all season and year combinations, there was evidence suggesting that survival was negatively associated with periods of low stream discharge and with individual fish length. In addition, low (-) and high (+) extreme stream temperatures and boulder cover (+) were weakly associated with survival. Watershed-scale seasonal abiotic conditions structured the adult cutthroat trout population in these watersheds, and low-discharge periods (e.g., autumn), when turnover of aquatic food resources declined, cover decreased, and predation success increased, were annual survival bottlenecks. Results emphasize the importance of watershed-scale processes to the understanding of population-level survival. |
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