Abstract
Fishways have been widely used for upstream passage around human-built structures, but ‘success’ has varied dramatically. Evaluation of fishway success has typically been conducted at local scales using metrics such as fish passage efficiency and passage time, but evaluations are increasingly used in broader assessments of whether passage facilities meet population-specific conservation and management objectives. Over 15 years, we monitored passage effectiveness at eight dams on the Columbia and Snake rivers for 26,886 radio-tagged spring-summer and fall Chinook Salmon O. tshwaytscha, Sockeye Salmon O. nerka, and summer steelhead O. mykiss during their migrations to spawning sites. Almost all fish that entered dam tailraces eventually approached and entered fishways. Tailrace-to-forebay passage efficiency estimates at individual dams were consistently high, averaging 0.966 (SD = 0.035) across 245 run×year×dam combinations. These estimates are among the highest recorded for any migratory species, which we attribute to the scale of evaluation, salmonid life history traits (e.g., philopatry), and a sustained adaptive management approach to fishway design, maintenance, and improvement. Full-dam fish passage times were considerably more variable, with run×year×dam medians ranging from 5–65 h. Evaluation at larger scales provided evidence that fishways were biologically effective, e.g., we observed rapid migration rates (medians = 28–40 km/d) through river reaches with multiple dams and estimated fisheries-adjusted upstream migration survival of 67–69%. However, there were substantive uncertainties regarding effectiveness. Uncertainty about natal origins confounded estimation of population-specific survival and interpretation of apparent dam passage ‘failure’, while lack of post-migration reproductive data precluded analyses of delayed or cumulative effects of passing the impounded system on fish fitness. Although the technical fishways are effective for salmonids in the Columbia-Snake River system, other co-migrating species have lower passage rates, highlighting the need for species-specific design and evaluation wherever passage facilities impact fish management and conservation goals.
Highlights
Fishways and other engineered fish passage structures are widely used to mitigate the negative effects of dams in rivers with diadromous and resident fish populations
A intensive example was the development of fishways for adult Pacific salmonids (Oncorhynchus spp) that coincided with the 20th Century construction of large hydroelectric dams in the Columbia River Basin [CRB; 26]
The extensive set of quantitative fish passage metrics in this study indicate that the adult fishways at CRB dams were generally efficient and were qualitatively effective
Summary
Fishways and other engineered fish passage structures are widely used to mitigate the negative effects of dams in rivers with diadromous and resident fish populations. The many challenges for river-dependent species have spurred a surge in technical reviews of fish passage design [16,17,18,19], fishway and dam passage efficiency [1, 20,21,22], and the broader biological effectiveness of fish passage projects [20, 23,24,25]. The iterative fishway design and evaluation process, combined with the biological traits of the targeted species, has been credited with the success of upstream fish passage in the CRB. Hundreds of thousands of returning adult salmonids have passed dams each year over the last several decades [29]
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