![]() Partial migration in salmonids provides access to freshwater and marine feeding environments varying in productivity. The accelerating pace of dam removals and the paucity of data for fishes other than salmonids, other vertebrates, invertebrates, and tropical and southern hemisphere organisms highlights the urgent need for more studies on the rapid evolutionary effects of dams. Fish passage structures can restore migratory populations but also create artificial selection pressures on body size and migration. Studies show that impoundments can cause rapid adaptive evolution in migration timing, behavior, life history, temperature tolerance, and morphology, as well as reduce phenotypic variance, which can alter adaptive potential and ecological roles. Genetic diversity loss was more commonly observed for small populations impounded in small habitat patches for many generations behind low-passability barriers. In the present article, we synthesize 307 studies in a systematic review of contemporary evolution following reduced connectivity and habitat alteration on freshwater fishes. We highlight that partial barriers and landscape permeability can be temporally dynamic, and this effect can be observed through changing genotype frequencies in migratory animals.ĭams and other anthropogenic barriers have caused global ecological and hydrological upheaval in the blink of the geological eye. In years when rain events occurred during the peak breeding season, migratory allele frequency was high (60-68%), and otherwise it was low (30% in two years). In Fox Creek, the primary barrier is at the mouth, and we found that the migratory allele frequency varied with the annual timing high flow events. We also observed a strong relationship between distance upstream and proportion of migratory alleles. In this stream, the overall spatial pattern, with fewer migratory genotypes above the waterfall, remained true across dry and wet years (67-76% of migratory alleles were downstream of the waterfall). In Elder Creek the largest waterfall was passable for up-river migrating adults 4-39 days/year. Across four years of study (2014-2017), the permeability of partial barriers varied across dry and wet years. We genotyped >4,000 individuals using RAD-capture and classified individuals as resident, heterozygous, or migratory-genotypes using life history-associated loci. We explore the influence of partial barriers on the spatial and temporal distribution of migration-linked genotypes of Oncorhynchus mykiss, a salmonid fish with co-occurring resident and migratory forms, in tributaries to the South Fork Eel River, California, USA, Elder and Fox Creek. Partial barriers are common in rivers where barrier passage varies with streamflow. Landscape permeability, including partial barriers, influences migratory animals who move across the landscape. Landscape permeability is often explored spatially, but may also vary temporally. ![]()
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