Journal of Fish Biology (2014) doi:10.1111/jfb.12286, available online at wileyonlinelibrary.com The effects of riverine physical complexity on anadromy and genetic diversity in steelhead or rainbow trout Oncorhynchus mykiss around the Pacific Rim M. V. McPhee*†, D. C. Whited*, K. V. Kuzishchin‡ and J. A. Stanford* *Flathead Lake Biological Station, The University of Montana, 32125 Bio Station Lane, Polson, MT 56860, U.S.A. and ‡Department of Ichthyology, Moscow State University, Vorob’evy Gory, Moscow 119899, Russian Federation This study explored the relationship between riverine physical complexity, as determined from remotely sensed metrics, and anadromy and genetic diversity in steelhead or rainbow trout Oncorhynchus mykiss . The proportion of anadromy (estimated fraction of individuals within a drainage that are anadromous) was correlated with riverine complexity, but this correlation appeared to be driven largely by a confounding negative relationship between drainage area and the proportion of anadromy. Genetic diversity decreased with latitude, was lower in rivers with only non-anadromous individuals and also decreased with an increasing ratio of floodplain area to total drainage area. Anadromy may be less frequent in larger drainages due to the higher cost of migration associated with reaches farther from the ocean, and the negative relationship between genetic diversity and floodplain area may be due to lower effective population size resulting from greater population fluctuations associated with higher rates of habitat turnover. Ultimately, the relationships between riverine physical complexity and migratory life history or genetic diversity probably depend on the spatial scale of analysis.  2014 The Fisheries Society of the British Isles Key words: biocomplexity; habitat turnover; migratory life history; remote sensing; salmonids. INTRODUCTION Recent advances in salmonid ecology indicate that maintaining or restoring phe- notypic and genetic diversity should be prioritized in management and conserva- tion efforts (Carlson & Satterthwaite, 2011). The ‘portfolio effect’ is an emerg- ing paradigm in the conservation and management of salmonids, whereby genetic and phenotypic diversity within and among populations lends greater population resilience at the regional scale (Hilborn et al., 2003; Schindler et al., 2010). While there has been substantial effort in cataloguing genetic diversity across salmonid populations (Beacham et al., 2006; Seeb et al., 2011), this effort is generally limited to commercially important species. Documenting life-history variation within popu- lations is costly and labour intensive, and is therefore often limited to populations †Author to whom correspondence should be addressed at present address: Fisheries Division, University of Alaska Fairbanks, 17101 Point Lena Loop Road, Juneau, AK 99801, U.S.A. Tel.: +1 907 796 5464; email: mvmcphee@alaska.edu 1  2014 The Fisheries Society of the British Isles