Corey G. Dunn

Freshwater species are increasingly threatened by climate change, yet our ability to assess their vulnerability remains incomplete. Typically, climate change vulnerability assessments (CCVAs) evaluate three components: exposure, sensitivity, and adaptive capacity. Adaptive capacity, defined as the ability of a species to adjust to changing conditions, provides critical insight into how species may persist under future scenarios and can strengthen conservation planning by highlighting opportunities for resilience and targeted management strategies. Trait‐based approaches offer a promising path for managers to operationalize adaptive capacity by identifying measurable biological and ecological traits that influence climate change response strategies. However, these insights are rarely integrated into broader vulnerability frameworks that support conservation decision making. We build on previous research to synthesize current understanding of adaptive capacity for three freshwater taxa in North America: fishes, mussels, and crayfishes. Our objectives were to: (1) assess the relevance of adaptive capacity factors for fishes, mussels, and crayfishes; (2) identify key opportunities and gaps in linking trait‐based information into adaptive capacity assessments; and (3) illustrate how incorporating adaptive capacity can enhance management decisions for freshwater species under climate change. We used an expert workshop, literature review, and case studies to identify relevant adaptive capacity factors, assess available information, and evaluate inclusion in management contexts. We found that all three taxa had sufficient information to inform adaptive capacity assessments. In addition to existing adaptive capacity factors, we identified Morphology as an important yet underutilized cross‐cutting diagnostic category when information was limited. By explicitly linking trait‐based approaches with adaptive capacity frameworks, we offer practical guidance for improving climate adaptation strategies and prioritizing management actions for freshwater biodiversity under accelerating global change.

Hydrologic connectivity is a crucial determinant of aquatic ecosystem services, governing the exchange of nutrients, sediments, chemicals, and biota. Various indices and metrics exist for quantifying hydrologic connectivity across diverse environments and scales. However, existing methodologies often fail to adequately capture lateral connectivity between lakes and streams across vast, low‐relief, multi‐lake floodplain systems. This study introduces a novel approach for quantifying lateral hydrologic connectivity specifically tailored for floodplain lakes connecting to streams within the expansive floodplain of the Lower Mississippi River. This approach centers on the spatial and temporal intersection of lakes and streams, leveraging remote sensing and GIS data to estimate nine distinct metrics of hydrologic connectivity. To assess the reliability of the method, the study estimated connectivity metrics for 92 randomly selected lakes, comprising 53 lakes connected to large streams, 13 lakes connected to medium streams, and 26 lakes connected to small streams. As expected, there was significant variability in hydrologic connectivity across different stream size classes. The outlined approach contributes valuable insights into the hydrologic connectivity of floodplain lakes and offers a generalizable framework applicable to other floodplains. Its versatility makes it a practical tool for understanding connectivity requirements for biota and facilitating applications in conservation and water resources management. Thus, this work represents a meaningful step toward advancing our understanding of lateral hydrologic connectivity dynamics in complex aquatic ecosystems.

Conservation prioritization frameworks are used worldwide to identify species at greatest risk of extinction and to allocate limited resources across regions, species, and populations. Conservation prioritization can be impeded by ecological knowledge gaps and data deficiency, especially in freshwater species inhabiting highly complex aquatic ecosystems. Therefore, we developed a flexible approach that calculates a species' imperilment risk based on the conservation principles of resiliency, redundancy, and representation (i.e., the “three R's”). Our approach organizes data on species traits, distributions, population connectivity, and threats within a Bayesian belief network capable of predicting resiliency and redundancy within representative ecological settings. Empirical data and expert judgment inform the model to provide robust and repeatable risk assessments for rare and data‐deficient species. The model calculates resiliency at hierarchical spatial scales from distributional trends and population strength. Redundancy is estimated from the connectivity and quantities of extant populations. Resiliency, redundancy, and species' inherent vulnerability based on species traits collectively estimate extirpation risk within each unique ecological setting. Extirpation risks across ecological settings characterize representation and are aggregated to estimate global imperilment risk. We demonstrate the model's utility with Piebald Madtom (Noturus gladiator), a species petitioned for listing under the U.S. Endangered Species Act. Our results revealed that resiliency, redundancy, and extirpation risks can vary spatially across the species' range while identifying populations where additional sampling could disproportionally reduce uncertainty in estimated global imperilment risk. Our approach could standardize and expedite conservation status assessments, identify opportunities for early management intervention of at‐risk species and populations, and strategically reduce uncertainty by focusing monitoring and research on priority information gaps.

The transformations of fish assemblages caused by reservoir cascades can be severe at the reach scale, but basin‐scale effects are less clear. However, prevailing river concepts provide a framework for predicting basin‐scale effects. To determine if predictions made by the River Continuum Concept relative to the function of fish assemblages are sustained in a temperate river transformed into a reservoir cascade, we examined longitudinal trends in the distribution of fish functional traits over 23 reservoirs of the Tennessee River, U.S.A. In all, 115 species were recorded representing 62 traits, with trait richness increasing longitudinally in a downstream direction. Trophic, reproductive, and habitat traits showed various increasing and decreasing patterns up and down the reservoir cascade. The observed gradients in trait richness and trait distributions were generally consistent with those expected in unregulated rivers, with few unexpected results. The transformation of lotic systems into lentic ones has changed habitats and sources of food and encouraged the proliferation of certain feeding (e.g., detritivores, planktivores, invertivores, piscivores), reproduction (e.g., nest spawners polyphils, broadcast spawners phytolithophils), and habitat (slow current, lacustrine, large river) traits. In essence, reservoirs have expanded downstream habitats in an upstream direction, and thus allowed upstream expansion of species and traits that would have normally not been well represented in upper reaches of the Tennessee River basin. Nevertheless, the impounded Tennessee River has maintained much of its functional integrity, despite extensive alterations to the riverscape. We suggest that, while reservoirs have been shown to have major local‐scale effects on riverine fish assemblages, with access to riverine habitats, and with proactive conservation strategies, fish functional richness can remain remarkably high at the basin scale.

Conservation practitioners increasingly recognize the conservation value of tributaries for supporting mainstem, large‐river specialist fishes. A tributary’s discharge at its mouth is a coarse indicator of the richness of large‐river specialists found within the tributary, but the relative influences of regional dispersal and local habitat underpinning this species–discharge relationship are often unknown. We sampled large‐river specialist fishes at sites within two nonwadeable tributaries of the Missouri and Mississippi rivers with contrasting prairie (Grand River) vs. upland (Meramec River) habitats to address four research questions: (1) Do alpha diversity (mean site‐level species richness) and beta diversity (among‐site species compositional differences) vary between tributaries? (2) Does mean annual discharge correlate with local habitat and downstream distance to mainstem rivers (i.e., mainstem connectivity)? (3) Are slopes of species–discharge relationships consistent between tributaries? And (4) Do local habitat and downstream distance explain residual richness at sites beyond variation already explained by species–discharge relationships? We detected 30 of 42 potential large‐river specialist fishes, demonstrating most mainstem species use tributaries. Mean site richness was higher in the Grand River (12.5 species vs. 9.8 species in Meramec River), but partitioning of lower reaches (sites < 116 km from river mouth) and dispersal limitation in upper reaches (sites ≥ 145 km from river mouth) caused Meramec River beta diversity to be three times higher. Mean annual discharge correlated with habitat availability at sites and downstream distance to a mainstem. Although site‐level alpha richness generally increased with discharge in both tributaries, slopes of species–discharge relationships varied between tributaries. Analyzing species–discharge residuals revealed downstream distance explained additional variation in site‐level richness not accounted for by local discharge. For example, discharge alone underrepresented richness in lower‐discharge sites accessible to dispersers (maximum underestimate = 8.7 species) and overrepresented richness in isolated sites (maximum overestimate = 5.8 species). Thus, predictive performance of species–discharge relationships can be improved by accounting for varying habitats among tributaries and downstream distance of sites to mainstems when valuing tributaries for fishes with dispersal‐dependent life cycles.

Aim: Conserving stream biota could require strategies that preserve habitats conveying resistance to ecological impacts of changing land use and climate. Retrospective analyses of species’ responses to anthropogenic disturbances can inform such strategies. We developed a hierarchical framework to contrast environmental conditions underlying persistence versus extirpation of an imperilled stream fish, Candy Darter (Etheostoma osburni), over decades of changing land use. The decline of E. osburni may broadly represent the challenge of conserving sensitive freshwater species in intensively used upland environments.Location: New River drainage, Appalachian Mountains, USA.Methods: We surveyed fish and habitat in historically occupied sites to identify population refugia, and used multivariate and spatial analyses to address three questions: (a) what are the environmental correlates of refugia? (b) are the pathways by which land use impacts instream habitat constrained by catchment‐ and/or segment‐scale features? and (c) are E. osburni distributional dynamics spatially structured and explained by fine sediment and warm stream temperatures?Results: We confirmed a recently localized distribution similar to other upland species, marked by at least seven extirpations from streams throughout E. osburni's southern range. Catchment‐scale features primarily constrained land use and finer‐scale habitat, leading to either extirpations or population‐supporting refugia defined by features at multiple scales. Refugium habitats contained cooler temperatures and less fine sediment. Rare mismatches between persistence and habitat suitability were explained by network location, suggesting unmeasured environmental gradients and/or dispersal contributed to distributional dynamics.Main conclusions: We provided insight at multiple spatial scales into how aquatic species’ distributions become fragmented and localized. Our results demonstrate that natural landscape heterogeneity imparts spatially variable resistance of sensitive species to intensive land uses. By recognizing the scale‐specific features that buffer populations from extirpation, conservation strategies could be tailored to protect naturally occurring refugium habitats and focus restoration in systems where such habitats are broadly lacking.

Large tributaries may help sustain large-river fish populations by mitigating fish-habitat losses within the highly modified great rivers of the Mississippi River basin. These tributaries are likely most beneficial for fish species specializing on non-degraded large-river habitat for some portion of their life histories. Few great-river tributaries, however, have been surveyed using methods that comprehensively target all fish species, resulting in uncertainty or bias in the reported composition of many tributary fish assemblages. We report important distributional records, including 23 new accounts, for 12 large-river specialist fishes in Missouri—Alosa alabamae (Alabama Shad), Cycleptus elongatus (Blue Sucker), Pimephales vigilax (Bullhead Minnow), Notropis wickliffi (Channel Shiner), Polyodon spathula (Paddlefish), Hybognathus placitus (Plains Minnow), N. blennius (River Shiner), Macrhybopsis hyostoma (Shoal Chub), Scaphirhynchus platorynchus (Shovelnose Sturgeon), M. storeriana (Silver Chub),...

Understanding relationships between habitat associations for individuals and habitat factors that limit populations is a primary challenge for managers of stream fishes. Although habitat use by individuals can provide insight into the adaptive significance of selected microhabitats, not all habitat parameters will be significant at the population level, particularly when distributional patterns partially result from habitat degradation. We used underwater observation to quantify microhabitat selection by an imperiled stream fish, the Candy Darter Etheostoma osburni, in two streams with robust populations. We developed multiple‐variable and multiple‐life‐stage habitat suitability indices (HSIs) from microhabitat selection patterns and used them to assess the suitability of available habitat in streams where Candy Darter populations were extirpated, localized, or robust. Next, we used a comparative framework to examine relationships among (1) habitat availability across streams, (2) projected habitat suitability of each stream, and (3) a rank for the likely long‐term viability (robustness) of the population inhabiting each stream. Habitat selection was characterized by ontogenetic shifts from the low‐velocity, slightly embedded areas used by age‐0 Candy Darters to the swift, shallow areas with little fine sediment and complex substrate, which were used by adults. Overall, HSIs were strongly correlated with population rank. However, we observed weak or inverse relationships between predicted individual habitat suitability and population robustness for multiple life stages and variables. The results demonstrated that microhabitat selection by individuals does not always reflect population robustness, particularly when based on a single life stage or season, which highlights the risk of generalizing habitat selection that is observed during nonstressful periods or for noncritical resources. These findings suggest that stream fish managers may need to be cautious when implementing conservation measures based solely on observations of habitat selection by individuals and that detailed study at the individual and population levels may be necessary to identify habitat that limits populations.Received March 30, 2016; accepted July 19, 2016Published online October 7, 2016

The covert habits of Cryptobranchus alleganiensis alleganiensis (Eastern Hellbender) make direct field observations of their behavior difficult. Here I provide the first video documentation of in situ predation by a hellbender on a spawning aggregation of stream fishes. Both targeting fish and diurnal activity are behaviors rarely documented among hellbenders. The present observation, however, supports previously described patterns of elevated diurnal activity in late spring hypothesized to be associated with increased foraging-activity to meet higher energy-demands. The observation demonstrates that Eastern Hellbenders will modify behavior to exploit stream fishes when prey are easily captured.