Here's a snippet from the introduction:
Nearly all rivers in the world have been impacted to some degree by dams. While providing benefits to society such as hydropower, flood control and water supply, dams alter rivers on many levels by fragmenting, inundating, and thermally or hydrologically altering habitats, often to the detriment of river fish populations and other lotic fauna (e.g. Cooper et al., 2017; Wang, Infante, Lyons, Stewart, & Cooper, 2010). However, much of this infrastructure is ageing, with nearly 75% of 49,000 large dams in the United States being more than 50 years old by 2020 (Cooper et al., 2017). As many dams ap-proach the end of their useful lives, and with considerable costs of maintaining old dams, dam removal opportunities presently abound and will continue to increase into the foreseeable future (Poff & Hart, 2002).Opportunities to remove dams or increase connectivity within river systems will often lead to changes in fish communities that should be considered prior to taking management action. These may include increased occurrence and production of downstream fishes in upstream reaches, and positive or negative responses of resident fishes to these changes. For example, by providing access to additional spawning habitat, increasing the connectivity of trib-utaries to North America's Great Lakes (hereafter Great Lakes) may result in greater reproduction for invasive species such as sea lam-prey Petromyzon marinus and round goby Neogobius melanostomus, a negative effect, but also increased reproduction of highly valued introduced Pacific salmonids currently stocked into the Great Lakes, namely Chinook salmon Oncorhynchus tshawytscha, coho salmon Oncorhynchus kisutch, and adfluvial rainbow trout or steelhead Oncorhynchus mykiss. Eggs and carcasses of migratory Pacific salmo-nids can provide an additional food resource for resident trout pop-ulations (Ivan, Rutherford, & Johengen, 2011), though the degree to which they produce a measurable increase in growth is less certain (Gerig, Weber, Chaloner, McGill, & Lamberti, 2018). Regardless of the geographic location, understanding how resident fish popu-lations will respond to dam removals or increased fish passage is needed to inform management decisions on dam removals and river connectivity, and for informing anglers on how fish populations might respond to river connectivity changes. For coldwater rivers in the Great Lakes region, the trout angling community's support for such actions may hinge on how they think resident trout populations will respond to these changes.Evaluations of the effects of river connectivity changes on res-ident trout populations are limited, hampering the ability of fishery managers to draw broad conclusions on effects of increased connec-tivity on resident trout populations. One study of juvenile salmonids in the Great Lakes region identified coho salmon as competitively superior to brown trout Salmo trutta and brook trout Salvelinus fon-tinalis (Fausch & White, 1986). Another could not detect an effect of steelhead on behaviour, growth or survival of brown trout (Kocik & Taylor, 1995, 1996), which contrasts with results from a long-term study by Nuhfer, Wills, and Zorn (2014) showing a 46% decline in survival of brown trout from age 0 to age 1 when steelhead were introduced into a low-gradient Michigan stream. Earlier studies im-plicated coho salmon and steelhead with declines in brook trout and brown trout populations at index sites (population estimation reaches) in Lake Superior tributaries (Peck, 2001; Stauffer, 1977). Such conclusions have been supported by later authors (Huckins, Baker, Fausch, & Leonard, 2008). Data from multiple locations and over longer time periods would provide a more robust characterisa-tion of resident trout population responses to Great Lakes salmo-nids that would have access to reaches following dam removal or fish passage installation, providing important information to fisher-ies managers or others needing to weigh trade-offs with regard to re-establishing river connectivity.Typically, the most downstream dam in a river system prevents migratory fish from accessing the system, but upstream dams can have similar negative hydrologic, geomorphic and biological effects. For example, dam removal in the Great Lakes region may provide non-native brown trout with access to reaches where native or resi-dent brook trout are the sole salmonid species present, with poten-tially negative consequences for brook trout populations. Previous research has often identified brown trout as competitively superior to brook trout (e.g. Fausch & White, 1981; Waters, 1983; Zorn & Wiley, 2010). Brown trout have been identified as one of the most invasive fish species, interacting with salmonids and native fishes in portions of North America, South America, Europe, New Zealand and other regions of the world (e.g. Budy & Gaeta, 2018; Casalinuovo, Alonso, Macchi, & Kuroda, 2018; Jones & Closs, 2018). The potential for negative effects of brown trout on native salmonid species has led to consideration or use of dams as management tools for isolating upstream populations of native salmonids from downstream non-na-tive populations (Fausch, Rieman, Dunham, Young, & Peterson, 2009; Rahel, 2013; Kirk, Rosswog, Ressel, & Wissinger, 2018). Long-term data from multiple locations would enable more robust estimates of brown trout effects on brook trout populations and the potential trade-offs of such management actions.We evaluated effects of increased salmonid species co-occur-rence (associated with fish passage) on resident trout in a broad array of streams to provide supportive data for fisheries management de-cisions on dam removal or fish passage in the Great Lakes region. Lacking multiple before-after studies, we focused on comparing attributes of resident trout populations between sets of streams where populations were or were not interacting with salmonid spe-cies that might be present if a dam was removed. More specifically, we compared resident trout populations between streams with and without Great Lakes access, and brook trout populations in Great Lakes inaccessible (land-locked) streams where brown trout were or were not present. We accomplished this by analysing electrofishing data from sites on 34 streams throughout Michigan that have been repeatedly sampled since 2002 under the Michigan Department of Natural Resources’ (MDNR) Status and Trends Program. Our ob-jectives were as follows: (1) to compare age-specific abundance, growth, and survival of resident brook trout and brown trout be-tween stream reaches with and without Great Lakes access to as-sess effects of Pacific salmonid species passage on resident trout; (2) | 3ZORN etal.to compare age-specific abundance, growth, and survival of brook trout between index reaches where brown trout did or did not occur to estimate potential effects of allowing brown trout access into brook trout streams; and (3) to compare overall abundance and bio-mass of salmonids between reaches without versus with Great Lakes access, and between land-locked brook trout streams without ver-sus with brown trout, to evaluate the hypothesis that total salmonid abundance would be higher in the latter scenarios where salmonid species richness was higher. The resident trout species we studied have wide-ranging distributions, with brook trout being present in 44 countries and brown trout in 59 countries (Fishbase, 2019). By quantifying potential effects of increased stream connectivity (and salmonid species richness) on these important and widely distrib-uted resident trout species, our findings provide managers and ecol-ogists useful information on effects of dam removal or fish passage actions for streams within the Great Lakes region and beyond.
The paper continues in the pdf file linked above.
