No abstract

Steelhead Oncorhynchus mykiss and Coastal Cutthroat Trout Oncorhynchus clarkii clarkii are closely related species that are difficult to differentiate as juveniles and are commonly sympatric at the watershed scale. If Cutthroat Trout spawning and early rearing occur in small streams, it is often difficult to assess which parts of a stream network are dominated by each species. In this study I used catch data from 649 sites in coastal British Columbia to develop quantitative models of species presence and relative dominance as a function of stream size. An independent data set of 561 streams from the USA was used for the cross validation of models developed with data from British Columbia. The relative dominance of Cutthroat Trout or steelhead was predicted using logistic regression with stream and channel width, stream order, watershed area, unit runoff, ecoregion placement, and long-term mean annual discharge (LT mad) as predictor variables. The LT mad was the best predictor of Cutthroat Trout and steelhead dominance, with a correct classification rate of 98% for the entire species range. Costal Cutthroat Trout dominated in reaches or streams where LT mad was ≤630 L/s, and steelhead dominated in reaches where LT mad was >1,000 L/s. The models have practical application for predicting stream-bearing length and area used primarily by each species at the landscape scale of productive capacity relative to habitat threats. *

For the management of Coastal Cutthroat Trout Oncorhynchus clarkii clarkii, resource managers require abundance estimates and demographic structure of juvenile emigrants. However, Coastal Cutthroat Trout research and monitoring have been confounded by the variability in assessment approaches and ecological plasticity of the subspecies. The primary purpose of this study is to help address this current Coastal Cutthroat Trout fisheries management issue by conducting a synthesis of juvenile emigration studies and proposing solutions that should be considered by resource managers when designing juvenile emigration studies. A total of 100 studies conducted in British Columbia and Washington since the 1970s, representing 1,100 annual estimates, were reviewed. The results of this review highlighted inconsistencies in study timing, biological data collection, juvenile emigrant life stage assignments, species identification, and ecotype assumptions. Solutions to address these inconsistencies include increased recognition of juvenile emigrant studies targeting Coastal Cutthroat Trout, strengthened partnerships and data sharing between management agencies, implementation of a life stage assignment standard for juvenile emigration studies, implementation of a field-based and genetic classification model to quantify study-specific identification error rates of juvenile emigrants, and genetic research to determine ecotype expression at the population and individual scales. We highlight the practical issues related to juvenile emigration studies and provide recommendations for management actions to account for the great plasticity in Coastal Cutthroat Trout behavior across the subspecies' range.