Diverse portfolios: Investing in tributaries for restoration of large river fishes in the Anthropocene

Abstract

Rehabilitation of large Anthropocene rivers requires engagement of diverse stakeholders across a broad range of sociopolitical boundaries. Competing objectives often constrain options for ecological restoration of large rivers whereas fewer competing objectives may exist in a subset of tributaries. Further, tributaries contribute toward building a “portfolio” of river ecosystem assets through physical and biological processes that may present opportunities to enhance the resilience of large river fishes. Our goal is to review roles of tributaries in enhancing mainstem large river fish populations. We present case histories from two greatly altered and distinct large-river tributary systems that highlight how tributaries contribute four portfolio assets to support large-river fish populations: 1) habitat diversity, 2) connectivity, 3) ecological asynchrony, and 4) density-dependent processes. Finally, we identify future research directions to advance our understanding of tributary roles and inform conservation actions. In the Missouri River United States, we focus on conservation efforts for the state endangered lake sturgeon, which inhabits large rivers and tributaries in the Midwest and Eastern United States. In the Colorado River, Grand Canyon United States, we focus on conservation efforts for recovery of the federally threatened humpback chub. In the Missouri River, habitat diversity focused on physical habitats such as substrate for reproduction, and deep-water habitats for refuge, whereas augmenting habitat diversity for Colorado River fishes focused on managing populations in tributaries with minimally impaired thermal and flow regimes. Connectivity enhancements in the Missouri River focused on increasing habitat accessibility that may require removal of physical structures like low-head dams; whereas in the Colorado River, the lack of connectivity may benefit native fishes as the disconnection provides refuge from non-native fish predation. Hydrologic variability among tributaries was present in both systems, likely underscoring ecological asynchrony. These case studies also described density dependent processes that could influence success of restoration actions. Although actions to restore populations varied by river system, these examples show that these four portfolio assets can help guide restoration activities across a diverse range of mainstem rivers and their tributaries. Using these assets as a guide, we suggest these can be transferable to other large river-tributary systems.

The effect of temperature on the dynamics of common bream Abramis brama migrations between the reservoir and its tributary

“…The drive to find suitable feeding habitats corresponds with the findings that, compared to stationary individuals, migratory individuals had lower somatic conditions (Brodersen et al, 2019;Grift et al, 2001). Moreover, reservoirs may represent a more competitive environment with lower food availability (Šmejkal et al, 2023) than the inflow area of tributaries and/or structured habitats within tributaries (Benda et al, 2004;Bouska et al, 2023;Pander & Geist, 2010).…”

Quantifying the contributions of tributaries to large‐river fish populations through mark‐recapture modeling

“…Tributaries in river networks may play a vital role in maintaining populations of large-river fishes (Pracheil et al 2013;Laub et al 2018). Although a complete understanding of the functional role of tributary habitat use in species' life history is lacking in many river systems (Pracheil et al 2009;Dunn and Paukert 2021;Bouska et al 2023), the use of small tributaries by large-river fishes for spawning or rearing has been noted in multiple river systems (Webber et al 2013;Moser et al 2016;Phillis et al 2018;Cathcart et al 2019;Dunn and Paukert 2021). Contributions of main-stem habitats or connected tributaries toward recruitment of large-river fish populations are not often quantified (Bouska et al 2023).…”

“…Although a complete understanding of the functional role of tributary habitat use in species' life history is lacking in many river systems (Pracheil et al 2009;Dunn and Paukert 2021;Bouska et al 2023), the use of small tributaries by large-river fishes for spawning or rearing has been noted in multiple river systems (Webber et al 2013;Moser et al 2016;Phillis et al 2018;Cathcart et al 2019;Dunn and Paukert 2021). Contributions of main-stem habitats or connected tributaries toward recruitment of large-river fish populations are not often quantified (Bouska et al 2023). For instance, evidence of spawning by endangered Razorback Sucker Xyrauchen texaus was found in a small Colorado River tributary in Utah, USA, but the fate of larvae and contribution of tributary recruits to the overall population is unknown (Webber et al 2013).…”

“…For instance, evidence of spawning by endangered Razorback Sucker Xyrauchen texaus was found in a small Colorado River tributary in Utah, USA, but the fate of larvae and contribution of tributary recruits to the overall population is unknown (Webber et al 2013). Thus, quantifying spatial and temporal variation in sources of recruitment in tributarymain-stem habitats and their contributions toward overall metapopulation maintenance could assist managers in prioritizing tributaries for large-river fish reintroductions or habitat restoration (reviewed in Bouska et al 2023).…”

Migration timing and tributary use of spawning flannelmouth sucker (Catostomus latipinnis)

“…For example, tributaries can be easily fragmented, isolated, degraded, or dewatered (Flanigan & Haas, 2008;Udall, 2020;Whipple, 2007), which could prevent fish migration. Tributaries are often excluded from critical habitat designations (Bottcher et al, 2013), but they could be an important component of large river fish conservation through portfolio effects (Bouska et al, 2023).…”