The complementary use of otolith chemistry and radiotelemetry demonstrates that bull trout Salvelinus confluentus from the Hoh River, Washington, exhibit considerable life history variability. Adult bull trout lived exclusively in the river, inhabited freshwater for prolonged periods and later became anadromous, or were anadromous and made multiple migrations between freshwater and salt water. Twenty of 40 radio‐tagged juvenile bull trout emigrated to the ocean at lengths ranging from 243 to 360 mm (mean, 287 mm), which is the first published verification of anadromy at this life history stage. Otolith chemistry analyses of 105 bull trout that were incidentally killed in commercial gill‐net fisheries revealed that 85% had migrated from freshwater to the sea at least once and that 75% had migrated multiple times. Anadromous females produced 95% of all individuals examined, but both anadromous and nonanadromous females produced progeny that were anadromous. Age at first seaward migration ranged from 3 to 6 years, 88% first emigrating to sea in their third or fourth growth year. For ages 3 and 4, anadromous individuals were larger than those that remained in freshwater. A wide size range (287–760 mm, 0.2–4.9 kg) of bull trout were killed in commercial fisheries; ages 3–5 composed 88% of the total bycatch. Relocation data from radio‐tagged juvenile and adult bull trout provided important insights on anadromous movements that helped to validate inferences drawn from widely oscillating strontium levels in otolith chemistry. In view of the direct mortality in gill‐net fisheries, an understanding of the age‐specific movements and life history variability of anadromous bull trout will be crucial to future conservation efforts, which should focus on improved monitoring of recreational and commercial bycatch in Pacific salmon fisheries.
Dam removal has been increasingly proposed as a river restoration technique. In 2011, two large hydroelectric dams will be removed from Washington State's Elwha River. Ten anadromous fish populations are expected to recolonise historical habitats after dam removal. A key to understanding watershed recolonisation is the collection of spatially continuous information on fish and aquatic habitats. A riverscape approach with an emphasis on biological data has rarely been applied in mid-sized, wilderness rivers, particularly in consecutive years prior to dam removal. Concurrent snorkel and habitat surveys were conducted from the headwaters to the mouth (rkm 65-0) of the Elwha River in 2007 and 2008. This riverscape approach characterised the spatial extent, assemblage structure and patterns of relative density of Pacific salmonids. The presence of dams influenced the longitudinal patterns of fish assemblages, and species richness was the highest downstream of the dams, where anadromous salmonids still have access. The percent composition of salmonids was similar in both years for rainbow trout, Oncorhynchus mykiss (Walbaum), coastal cutthroat trout, Oncorhynchus clarkii clarkii (Richardson) (89%; 88%), Chinook salmon, Oncorhynchus tshawytscha (Walbaum) (8%; 9%), and bull trout, Salvelinus confluentus (Suckley) (3% in both years). Spatial patterns of abundance for rainbow and cutthroat trout This article is a U.S. Government work and is in the public domain in the USA. 36
Fisheries Management and EcologyThis article is a U.S. government work, and is not subject to copyright in the United States.(r = 0.76) and bull trout (r = 0.70) were also consistent between years. Multivariate and univariate methods detected differences in habitat structure along the river profile caused by natural and anthropogenic factors. The riverscape view highlighted species-specific biological hotspots and revealed that 60-69% of federally threatened bull trout occurred near or below the dams. Spatially continuous surveys will be vital in evaluating the effectiveness of upcoming dam removal projects at restoring anadromous salmonids.
Environmental DNA (eDNA) has emerged as a potentially powerful tool for use in conservation and resource management, including for tracking the recolonization dynamics of fish populations. We used eDNA to assess the effectiveness of dam removal to restore fish passage on the Elwha River in Washington State (USA). Using a suite of 11 species‐specific eDNA polymerase chain reaction (PCR) assays, we showed that most targeted anadromous species (five Pacific Salmon species and Pacific Lamprey) were able to pass upstream of both former dam sites. Multiscale occupancy modeling showed that the timing and spatial extent of recolonization differed among species during the four years of post‐dam removal monitoring. More abundant species like Chinook Salmon and Coho Salmon migrated farther into the upper portions of the watershed than less abundant species like Pink Salmon and Chum Salmon. Sampling also allowed assessment of potamodromous fish species. Bull Trout and Rainbow Trout, ubiquitous species in the watershed, were detected at all sampling locations. Environmental DNA from Brook Trout, a non‐native species isolated between the dams prior to dam removal, was detected downstream of Elwha dam but rarely upstream of the Glines Canyon Dam suggested that the species has not expanded its range appreciably in the watershed following dam removal. We found that eDNA was an effective tool to assess the response of fish populations to large‐scale dam removal on the Elwha River.
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