Prioritizing native migratory fish passage restoration while limiting the spread of invasive species: A case study in the Upper Mississippi River

Fish community dynamics following the low-head dam removal and newly installed fish passage in a headstream tributary of Jinsha River, Southwest China

Low-head dams and small hydropower developments are widely distributed in global rivers because of their high cost-efficiency and smooth implementation. However, these installations have great ecological impacts on native fish communities due to the loss of longitudinal river connectivity. Although dam removal and fishway construction are effective measures to mitigate river fragmentation, research on their effects on fish communities remains limited to date. In this study, we investigated fish community dynamics in a mountainous tributary of the Jinsha River-Heishui River following dam removal, fishway construction, and fishway renovation. Our fish sampling data from 2018 to 2023 yielded 6137 fish belonging to 25 species, 12 families, and 3 orders. The native fish community predominantly represented the typical fish fauna of the Qinghai-Tibetan Plateau, primarily including the genus Schizothorax and the families Sisoridae and Nemacheilidae. Representative fish species in the Heishui River demonstrate notable adaptations to cold water, high altitude, and rapid flow environments. After removing the Laomuhe Dam, upstream fish abundance increased sharply, but species richness slightly decreased in the short term. Fishway construction significantly enhanced species richness and abundance and reduced the difference between the upstream and downstream fish communities from the Songxin Dam. However, the subsequent fishway renovation altered the dominance of fish species, with marginal changes in community structure and abundance. Variations in fish community dynamics in the river channel and fishway structure can be attributed to differences in ecological guilds (e.g., flow preference and thermal regime) and environmental factors (e.g., flow discharge, water temperature, and water depth). The present study provides insight into the significance of mitigation measures for the impact of dams on mountainous rivers in southwest China through ecological assessment and guides for decision-making in the conservation and restoration of specific fish communities.

Flooding and dam operations facilitate rapid upstream migrations of native and invasive fish species on a regulated large river

Dams commonly restrict fish movements in large rivers but can also help curtail the spread of invasive species, such as invasive bigheaded carps (Hypophthalmichthys spp). To determine how dams in the upper Mississippi River (UMR) affect large-scale invasive and native fish migrations, we tracked American paddlefish (Polyodon spathula) and bigheaded carp across > 600 river km (rkm) and 16 navigation locks and dams (LD) of the UMR during 2 years with contrasting water levels. In 2022, a low-water year, both native paddlefish and invasive bigheaded carp had low passage rates (4% and 0.6% respectively) through LD15, a movement bottleneck being studied for invasive carp control. In contrast, flooding in 2023 led to open-river conditions across multiple dams simultaneously, allowing 53% of paddlefish and 46% of bigheaded carp detected in Pool 16 to move upstream through LD15. Bigheaded carp passed upstream through LD15 rapidly (μ = 32 rkm per day) a maximum of 381 rkm, whereas paddlefish moved an average of 9 upstream rkm per day (maximum of 337 rkm). Our results can inform managers examining trade-offs between actions that enhance native fish passage or deter movements of invasive species. This understanding is critical because current climate change models project increases in flooding events like that observed during 2023.

Whether including exotic species alters conservation prioritization: a case study in the Min River in southeastern China

Conservation practices from the perspective of functional diversity (FD) and conservation prioritization need to account for the impacts of exotic species in freshwater ecosystems. This work first simulated the influence of exotic species on the values of FD in a schemed mechanistic model, and then a practical case study of conservation prioritization was performed in the Min River, the largest river in southeastern China, to discuss whether including exotic species alters prioritization. The mechanistic model revealed that exotic species significantly altered the expected FD if the number of exotic species occupied 2% of the community. Joint species distribution modelling indicated that the highest FD occurred in the west, northwest and north upstreams of the Min River. Values of FD in 64.69% of the basin decreased after the exotic species were removed from calculation. Conservation prioritization with the Zonation software proved that if first the habitats of exotic species were removed during prioritization, 62.75% of the highest prioritized areas were shifted, average species representation of the endemic species was improved and mean conservation efficiency was increased by 7.53%. Existence of exotic species will significantly alter the metrics of biodiversity and the solution for conservation prioritization, and negatively weighting exotic species in the scope of conservation prioritization is suggested to better protect endemic species. This work advocates a thorough estimate of the impacts of exotic species on FD and conservation prioritization, providing complementary evidence for conservation biology and valuable implications for local freshwater fish conservation.

The effects of longitudinal fragmentation on riverine beta diversity are modulated by fragmentation intensity

The loss of longitudinal connectivity affects river systems globally, being one of the leading causes of the freshwater biodiversity crisis. Barriers alter the dispersal of aquatic organisms and limit the exchange of species between local communities, disrupting metacommunity dynamics. However, the interplay between connectivity losses due to dams and other drivers of metacommunity structure, such as the configuration of the river network, needs to be explored. In this paper, we analyzed the response of fish communities to the network position and the fragmentation induced by dams while controlling for human pressures and environmental gradients. We studied three large European catchments covering a fragmentation gradient: Upper Danube (Austrian section), Ebro (Spain), and Odra/Oder (Poland). We quantified fragmentation through reach-scaled connectivity indices that account for the position of barriers along the dendritic network and the dispersal capacity of the organisms. We used generalized linear models to explain species richness and Local Contributions to Beta Diversity (LCBD) and multilinear regressions on the distance matrix to describe Beta Diversity and its Replacement and Richness Difference components. Results show that species richness was not affected by fragmentation. Network centrality metrics were relevant drivers of beta diversity for catchments with lower fragmentation (Ebro, Odra), and fragmentation indices were strong beta diversity predictors for the catchment with higher fragmentation (Danube). We conclude that in highly fragmented catchments, the effects of network centrality/isolation on biodiversity could be masked by the effects of dam fragmentation. In such catchments, metapopulation and metacommunity dynamics can be strongly altered by barriers, and the restoration of longitudinal connectivity (i.e. the natural centrality/isolation gradient) is urgent to prevent local extinctions.

Resisting-Accepting-Directing: Ecosystem Management Guided by an Ecological Resilience Assessment

As anthropogenic influences push ecosystems past tipping points and into new regimes, complex management decisions are complicated by rapid ecosystem changes that may be difficult to reverse. For managers who grapple with how to manage ecosystems under novel conditions and heightened uncertainty, advancing our understanding of regime shifts is paramount. As part of an ecological resilience assessment, researchers and managers have collaborated to identify alternate regimes and build an understanding of the thresholds and factors that govern regime shifts in the Upper Mississippi River System. To describe the management implications of our assessment, we integrate our findings with the recently developed resist-accept-direct (RAD) framework that explicitly acknowledges ecosystem regime change and outlines management approaches of resisting change, accepting change, or directing change. More specifically, we developed guidance for using knowledge of desirability of current conditions, distance to thresholds, and general resilience (that is, an ecosystem's capacity to cope with uncertain disturbances) to navigate the RAD framework. We applied this guidance to outline strategies that resist, accept, or direct change in the context of management of aquatic vegetation, floodplain vegetation, and fish communities across nearly 2000 river kilometers. We provide a case study for how knowledge of ecological dynamics can aid in assessing which management approach(es) are likely to be most ecologically feasible in a changing world. Continued learning from management decisions will be critical to advance our understanding of how ecosystems respond and inform the management of ecosystems for desirable and resilient outcomes.