Reservoir fish assemblage structure across an aquatic ecotone: Can river‐reservoir interfaces provide conservation and management opportunities?

Abstract

River‐reservoir interfaces have been described as aquatic ecotones and contain strong environmental gradients of depth, turbidity and trophic resource abundance. These transitional habitats have traditionally been excluded by riverine and reservoir management schemes despite their prevalence in modern riverscapes. By systematically sampling shoreline habitats along a river‐reservoir interface gradient from riverine to lacustrine zones with trammel nets in 2018–2019, strong patterns were identified in total species captured and individuals captured in the San Juan River‐Lake Powell inflow, USA. Changes in assemblage structure were driven mainly by increases in relative abundance of benthic omnivores towards the riverine zone, including imperiled razorback sucker, Xyrauchen texanus (Abbott), but also by increases in predatory species, such as striped bass, Morone saxatilis (Walbaum). Inter‐annual variation in species distributions along the river‐reservoir interface gradient was likely influenced by variation in reservoir water level that differed by nearly 12 m between years. River‐reservoir interfaces provide high‐quality feeding and potentially spawning areas for both benthic omnivores and piscivores, and these areas should be considered in management and conservation efforts for species using these habitats.

Trophic niches of native and nonnative fishes along a river-reservoir continuum

“…This study adds to a growing body of research suggesting adult native fishes may be able to coexist with some nonnative fishes that also have an opportunistic feeding strategy. In addition, we demonstrate some native fishes might be able to successfully utilize highly modified river-reservoir inflow and reservoir habitat 51,95,96 , habitats similar to lotic-lentic habitats that have been present across their evolutionary history.…”

“…Human transformation of riverscapes has restructured fish assemblages with consequences for the conservation of native species. Artificial habitats, such as reservoirs and isolated river fragments, are now common 92 , and native and nonnative species co-occur in these habitats 93 , with nonnative species often thriving in more modified habitat 51 , 94 , 95 . In the Colorado River basin, native fish assemblages have endured habitat loss and degradation alongside the introduction and establishment of nonnative species, but recovery efforts are hindered by a full accounting of limiting factors, particularly the importance of biotic interactions.…”

“…This habitat consisted of lower gradient (0.6 m km −1 ), slowing velocity, and turbid lotic habitats at the upper end of the reservoir (the riverine zone; sensu 26 ). The last, and most downstream, habitat also occurred in the San Juan River–Lake Powell inflow across 15 rkm, and encompassed gradients of decreasing turbidity and increasing channel width and depth (here after reservoir) 51 .

Figure 1 Fish tissues for isotope analyses were collected over 135 river kilometers (rkm) from three habitats along the lower San Juan River and into Lake Powell in the Colorado River Basin, southeastern Utah, USA.

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“…In the river, we sampled fish in 0.1–3.2 km increments depending on fish densities to reduce handling stress on native fishes. We sampled fish in the reservoir by placing a trammel net in every 0.5 km segment of shoreline along approximately 20 km of the historic river channel 51 . The US Fish and Wildlife Service stocks PIT-tagged X. texanus every fall in the San Juan River, and therefore we removed individuals stocked the previous fall from analyses to avoid influence of residual hatchery diets on isotope signatures.…”

“…Nonnative species introductions and habitat alteration are the leading hypothesized drivers for native fish imperilment in the Colorado River basin in the American Southwest 35 , 49 . Novel habitats in the basin, such as large water-storage reservoirs, serve as sources for nonnative species, but some native species also appear to be using river-reservoir inflow habitats 50 , 51 . Previous research identified overlap in resource use between younger life stages of native (e.g., Flannelmouth Sucker Catostomus latipinnis ) and nonnative species (e.g., Common Carp Cyprinus carpio ) in the upper Colorado basin 9 , 52 , 53 , but data on large-river, federally endangered species, such as Colorado Pikeminnow ( Ptychocheilus lucius ) and Razorback Sucker ( Xyrauchen texanus ) are more limited, likely because of their rarity in the system.…”