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Aim We test the hypothesis that the North Pacific coastline from British Columbia to the Kuril Islands forms a broad region of admixture between two divergent mitochondrial‐genome (mtDNA) lineages in three‐spined stickleback (Gasterosteus aculeatus): the Euro‐North American (ENA) and Trans‐North‐Pacific (TNP or Japanese) clades. We test whether distance is the primary determinant of geographical patterns of haplotype distributions and whether deep‐water trenches in the Aleutian and Kuril archipelagos impede gene flow. Location Coastal marine and freshwater sites from the Kuril Islands (north‐western Pacific Ocean) to Oregon (north‐eastern Pacific Ocean). Methods We determined the mtDNA clade for 1327 individuals from 67 locations across 8000 km of the North Pacific using restriction fragment length polymorphism assays of the cytochrome b mitochondrial gene. We supplemented this with published clade designations from coastal Pacific populations and applied generalized linear modelling and Mantel tests. We used analysis of molecular variation (AMOVA) to test for significant partitioning of genetic variation by deep‐water trenches. Results The western boundary of the ENA clade was Simushir (Kuril Islands) and the eastern boundary of the TNP clade was British Columbia. Coastline distance from Japan was a significant predictor of TNP abundance. Clade composition variance was high at small geographical scales, in apparent discordance with the broader‐scale pattern of admixture. Deep‐water trenches were not found to significantly partition genetic variation in the Aleutian and Kuril island chains. Main conclusions The North Pacific coastline forms a broad region of secondary contact between divergent mitochondrial lineages, but with clear western and eastern boundaries. Patterns of clade abundance in the Kuril and Aleutian islands are similar to those observed in other taxa, suggesting shared biogeographical histories and common barriers shaping species distributions along the North Pacific coast. Overall patterns of clade distributions are likely to be driven by a combination of factors, including geomorphological impediments to migration, ecology, asymmetrical dispersal patterns and differences in timing of population expansions.

The Aleutian Archipelago is an isolated arc of over 300 volcanic islands stretching 1,600 km across the interface of the Bering Sea and North Pacific Ocean. Although remote, some Aleutian Islands were heavily impacted by military activities from World War II until recently and were exposed to anthropogenic contaminants, including mercury (Hg). Mercury is also delivered to these islands via global atmospheric transport, prevailing ocean currents, and biotransport by migratory species. Mercury contamination of freshwater ecosystems is poorly understood in this region. Total Hg (THg) concentrations were measured in threespine stickleback fish (Gasterosteus aculeatus) collected from eight lakes at Adak Island, an island in the center of the archipelago with a long military history. Mean THg concentrations for fish whole-body homogenates for all lakes ranged from 0.314 to 0.560 mg/kg dry weight. Stickleback collected from seabird-associated lakes had significantly higher concentrations of THg compared to non-seabird lakes, including all military lakes. The δ13C stable isotope ratios of stickleback collected from seabird lakes suggest an input of marine-derived nutrients and/or marine-derived Hg.

We assessed temporal variation in mercury (Hg) concentrations of threespine stickleback (Gasterosteus aculeatus) from Agattu Island, Aleutian Archipelago, Alaska. Total Hg concentrations in whole-bodied stickleback were measured at two-week intervals from two sites in each of two lakes from June 1 to August 10, 2011 during the time period when lakes were ice-free. Across all sites and sampling events, stickleback Hg concentrations ranged from 0.37–1.07 µg/g dry weight (dw), with a mean (± SE) of 0.55±0.01 µg/g dw. Mean fish Hg concentrations declined by 9% during the study period, from 0.57±0.01 µg/g dw in early June to 0.52±0.01 µg/g dw in mid-August. Mean fish Hg concentrations were 6% higher in Loon Lake (0.56±0.01 µg/g dw) than in Lake 696 (0.53±0.01 µg/g dw), and 4% higher in males (0.56±0.01 µg/g dw) than in females (0.54±0.01 µg/g dw). Loon Lake was distinguished from Lake 696 by the presence of piscivorous waterbirds during the breeding season. Mercury concentrations in stickleback from Agattu Island were higher than would be expected for an area without known point sources of Hg pollution, and high enough to be of concern to the health of piscivorous wildlife.

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