The permeability of landscapes is often explored spatially, but may also vary across time. For example, highways are more passable with low traffic, and mountain landscapes are more passable once snow-free. Landscape permeability especially influences migratory animals. Partial barriers are features that are passable in some conditions. Partial barriers are common in rivers due to the two-dimensional nature of river networks: aquatic organisms cannot circumvent in-river barriers. Barriers in river networks include waterfalls, logjams, or culverts, all of which vary in permeability depending on streamflow. We explore the influence of partial barriers on the spatial and temporal distribution of migration-linked genotypes of fish in tributaries to the South Fork Eel River, California, USA. We study Oncorhynchus mykiss, a salmonid fish with co-occurring resident and migratory forms. We genotyped >3,000 individuals using RAD-capture and classified individuals as resident, heterozygous, or migratory-genotypes using life history-associated loci. Across four years of study (2014-2017), the permeability of partial barriers varied with streamflow, and the largest waterfall was passable 4-39 days/year. In one stream, partial barriers limited the up-river movement of migratory adult fish, decreasing the number of migratory juveniles found upstream of barriers (67-76% of the total migratory alleles in the watershed were downstream of the largest barrier). In another stream, migratory allele frequencies varied from 30-68%. Our work highlights the temporal dynamics of landscape permeability, which influences animal movements and genotype frequencies. While temporal variation in landscape permeability is likely common, migratory animals may be especially affected in their movement and genetic structure.

See methods in associated paper, Kelson et al. Temporal dynamics of migration-linked genetic variation are driven by streamflows and riverscape permeability.

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