Predictive Models for Differentiating Habitat Use of Coastal Cutthroat Trout and Steelhead at the Reach and Landscape Scale

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.

Received June 18, 2012; accepted July 17, 2013

ACKNOWLEDGMENTS

I am grateful to the many fisheries professionals that have conducted, reported on, and archived fish collections and stream inventories to Resources Information Standards Committee standards or better in the Province of British Columbia. This also applies to field biologists in the USA and the foundation of the validation data set. I am indebted to outstanding field support from Brian Clark, Poul Bech, Tom Wilkinson, Mike Ramsay, and the Fisheries Improvement Unit staff of the Provincial Fisheries Program. I appreciate the valuable insights and manuscript improvements made by the reviewers including Tom Northcote, Gordon Hartman, Art Tautz, Juanita Ptolemy, Jordan Rosenfeld, Stephen Riley, and two anonymous reviewers. The work was funded by the Province of British Columbia.