The lower Rio Grande watershed below Falcon Dam has been 95% developed for agriculture, urban, and industrial uses. The river has been extremely altered to make this possible, with the addition of two more downstream dams, a series of five rock weirs, and numerous water diversions. This regulation has resulted in an extremely altered flow regime and fish fauna since the early 1950s. There has been a significant general retreat and decline of primary freshwater fishes over time, and we identified three significantly different faunal groups across the fragmented watercourse. However, the overall species richness of the region did not change significantly over time, likely due to an ongoing upstream intrusion of estuarine and marine-derived taxa, as well as the increase in the number and spread of non-native taxa. Despite no overall change in species richness within the region, we identified a significant trend in the species richness of the two most diverse primary freshwater fish families, Leuciscidae and Centrarchidae. Leuciscid richness significantly declined and centrarchid richness significantly increased over the 68-year period. Fluvial native leuciscid species that require a natural flow regime became extirpated or extinct, while lentic-adapted native and introduced centrarchids have thrived. The flow regime of the lower Rio Grande has been severely altered since impoundment of Falcon Reservoir. Median monthly flows have declined for all months, maximum flows and high flow pulses have declined, and base flows have increased. Also noteworthy were the increased number of hydrograph reversals post-impoundment. The streamflow regime is of central importance in sustaining the ecological integrity of rivers, and its disruption in the lower Rio Grande corresponds to a vastly different contemporary fish fauna than what historically occurred.

Seagrass meadows in far South Texas are dominated by turtle grass (Thalassia testudinum), and juvenile pinfish (Lagodon rhomboides) are abundant grazers of seagrass epifauna. As they grow, pinfish exhibit an ontogenetic dietary shift and begin consuming increasing amounts of plant material. In this study, contrasting environmental regimes in two seagrass meadows were examined to identify how they might affect pinfish diets, diet shifts, and growth across the growing season. The eastern site was sheltered from prevailing southeasterly winds by a barrier island, while the western site was shallower and subjected to higher wind energy and turbidity. Gut contents from 290 pinfish were quantified from May through October 2016 and multivariate analyses showed strong but overlapping separation of diets and temporal change in feeding habits. Although there were spatial and temporal differences between the two sites, a site by time interaction indicated different temporal trends. Nonetheless, ooze (amorphous detritus) and aquatic invertebrates (primarily amphipods) were the primary diet items consumed at both localities across time. Pinfish at both sites consumed seagrass, which increased in importance as the growing season progressed, although volume and consumption rate did not differ between sites. Growth rates of pinfish were similar at both sites, but pinfish from the western site were larger and fewer in number across the study. Although pinfish diets differed under different environmental regimes, they were generalist consumers in South Texas seagrass meadows with high diet variability that trended towards herbivory in late summer and fall.

The Mobile Basin has undergone extensive channel modification with corresponding declines in the distribution and abundance of native aquatic fauna. However, many of the declining aquatic species of the Mobile Basin may persist within unmodified sub-basins. The Noxubee River is a sub-basin of the Mobile Basin that is relatively unaltered throughout its watershed, and we hypothesized that the system could serve as a refugium for declining riverine species of the Mobile Basin. We characterized species richness and assemblage structure of fishes in the Noxubee River and its tributaries to determine whether or not contemporary fish assemblages resembled the historical assemblages as described before construction of the Tennessee-Tombigbee Waterway. We found that fish assemblages of the contemporary river are similar to those of the historical river, and many species now absent from another portion of the Mobile Basin are still present and maintain similar distribution and abundance patterns. Our results are informative because there is little existing information available regarding fish assemblages in the Noxubee River since major alterations were imposed upon the Mobile Basin through construction of the waterway. These results are discussed relative to findings from another Mobile Basin tributary that exhibited a marked change in many fish assemblage properties following construction of the waterway. Variable resiliency among tributaries to altered main-channel rivers illustrates the need for predictive tools that can be employed to prioritize conservation efforts.

We investigated how stream fragmentation affects local fish species persistence and extinction from three Rio Grande systems (Texas, USA) stream reaches with different levels of natural fragmentation. We examined species-volume (SV) relationships of fish assemblages in 42 pools across the watersheds and predicted greater fragmentation would correspond to an increase in the slope of the SV relationship due to decreased within-reach rescue effects. In addition, we examined relationships among tributary-specific nested subset patterns, local habitat features and spatial position of the reaches relative to the Rio Grande mainstem to better understand the importance of local and regional processes on fish species richness patterns in the stream reaches. Slopes of the SV curves did not differ among the stream reaches, but the intercepts of the SV curves were significantly different. These results indicated rescue effects among habitats within a stream reach were not apparent; however, rescue effects from the mainstem largely determined the species richness of a given stream reach. The nested subset patterns in all stream reaches were related to several local environmental factors, and large, deep pools provided important aquatic refugia in all three systems. We suggest declines in mainstem and tributary flows will likely continue to impact local and regional fish assemblage attributes. High flow events are important for dispersal and can reset tributary fish communities for the next extinction-driven, successional cycle.

River systems throughout arid regions worldwide have been heavily impacted by human activities, resulting in long-term ecological consequences. The lower Pecos River in the Trans-Pecos region of Texas is no exception, having undergone anthropogenic changes that include decreased flow, elevated salinity, species loss and species invasion. We compared historical and contemporary fish assemblage attributes from the Pecos River at local (site-specific) and regional (Trans-Pecos region) scales across a 24-year time period. Fish assemblage data were collected in October 1987 and 2011, by seining at 15 sites spanning 430km of the river in Texas. Additionally, we examined contemporary environmental conditions to determine species-environment relationships. We found that fish assemblages were significantly different between time periods, likely due to increased salinisation in the upper half of the study region. Decreased species richness, species replacement and increases in euryhaline species were documented in the upstream sites. Freshwater springs lower the salinity and maintain flows in the downstream reach, allowing for maintenance of the native fish fauna. Careful management of regional aquifers, irrigation practices and petroleum waste water will be necessary for protecting biodiversity and environmental flows in the lower Pecos River.

Ordination, correlation, and regression analyses were used to identify trophic and taxonomic distributional patterns of fishes in the Tilostoc River (located west of Valle de Bravo Lake in the Middle Balsas Basin, central Mexico) and associations of these patterns with environmental variables. Nonmetric multidimensional scaling analysis indicated that trophic and taxonomic assemblages corresponded similarly to measured environmental gradients, especially elevation and velocity of currents. We also compared the trophic structure of assemblages of fishes of the Tilostoc River to that of Terreros Creek, a temperate system of similar size and species richness but with a taxonomically different group of species. The two basins were significantly different, indicating strong historical constraint on trophic structure. Our trophic matrix for Tilostoc River was strongly correlated with the taxonomic matrix.

The ashy darter, Etheostoma cinereum, is an imperiled fish within the Cumberland and Tennessee drainages of the southeast United States. An understanding of habitat associations and the relationship of habitat use across multiple spatial scales are critical elements in its conservation. Our objectives were to quantify habitat associations at the stream reach and microhabitat scales for adult and juvenile darters, and to understand the linkage between the two scales based on gradients of habitat use. We focused our efforts within the Rockcastle River, Kentucky (Cumberland River drainage), because the watershed was known to contain a relatively large ashy darter population. Three hundred twenty-two individuals were collected from 21 reaches. The species was restricted to the mainstem of the river and the lower reaches of the larger tributaries. The distribution and abundance of adult and juvenile darters differed significantly at each spatial scale, and both groups demonstrated non-random use of the available habitat. Gradients of stream size and substrate size were identified as important factors. A threshold of environmental quality was determined based on the habitat use patterns among the two scales. Habitat use between the two scales was independent within the threshold, indicating that the specific quality of the microhabitats did not necessarily matter within a stream reach. However, beyond the threshold, a decrease of at least 48% in adult and juvenile darter abundance was seen, indicating that a sufficient network of suitable microhabitats is needed to support a good population of darters within a stream reach.

The Rio Grande and its tributaries in the Trans-Pecos region of Texas have been impacted by a variety of anthropogenic activities such as dewatering and the introduction of nonnative species. These environmental manipulations have negatively affected the native fishes leading to extirpations and declining populations throughout the region. Campostoma ornatum and Notropis chihuahua inhabit tributary streams of the Rio Grande in the Trans-Pecos region and are considered as state-listed threatened species. Little is known about their status and ecological requirements in the region. We hypothesized that the distribution and abundance of these threatened minnows in these spring-fed habitats can be modeled by two primary factors: local environmental conditions that are maintained by spring flow and the abundance of introduced species such as the plains killifish, Fundulus zebrinus. We used classification and regression trees to analyze variation in abundance and incidence of the target species from Alamito, Terlingua, and Tornillo creeks, as well as the Rio Grande proper based on local environmental factors (e.g., size of stream and water quality), abundance of nonnative species, season, and distance from the Rio Grande. The analyses indicated that distance from the Rio Grande, maximum depth, and composition of substrate were the most important predictors for the abundance and occurrence of the target species in the region.

Summary Habitat fragmentation is one of the major causes of local and regional species extinctions in freshwater ecosystems. To predict future trends in community composition, and the potential sequence of extinctions due to fragmentation of the river continuum, it is important to understand how habitat size and isolation affect the dynamics of species immigrations and extinctions and patterns of abundance and occupancy. We examined fish immigration and extinction rates, and abundance and occupancy patterns, in relation to habitat isolation, size and variability in the Rio Grande and its tributaries in the Trans‐Pecos region of Texas, U.S.A. Our results indicated that as habitat isolation increased in tributaries, fish immigration and extinction rates and riverine species abundances decreased. Fish assemblages showed significant nested subset patterns across the study sites, and the influence of habitat size and isolation on the nested subset patterns varied with spatial scale. Certain non‐native fish taxa were idiosyncratic, reducing the strength of the nested subset pattern. The high temporal species turnover in connected habitats was related to the movement of riverine fish species between the mainstem and mouths of tributaries. High persistence of spring‐adapted species decreased extinction rate in isolated habitats. Increasing the spatial scale of the study system probably resulted in an increased ability to detect dispersal limitation between the mainstem and its tributaries. Large, deep pools decreased local extinction of certain native fish taxa in upstream tributaries. The deviation from the nested subset pattern was attributed to the unique regional occupancy of non‐native fish taxa that could be associated with their artificial introduction into this system. Our results suggest that increased habitat fragmentation by human activities can accelerate the regional extinction of certain native fish taxa and the dominance of ecologically tolerant, possibly non‐native fish, leading to a decline in regional diversity.

The Mississippi silverside (Menidia audens), now common throughout the Tennessee-Tombigbee Waterway (TTW) in Mississippi, apparently invaded this highly modified system from the Tennessee River, concurrent with TTW construction (1972-1985). Subsequent decline in distribution and abundance of the native brook silverside (Labidesthes sicculus) led to speculation that dietary competition with M. audens might be occurring. Therefore, diet data from sympatric and allopatric collections of the two silversides at several sites in TTW were analyzed using multivariate statistics and null models to test for significant dietary differences, overlap and niche shifts potentially attributable to habitat alteration. Diet overlap between the two silversides at sites of co-occurrence was greater than expected under the null model. Moreover, intraspecific food habits did not differ between individuals occurring in sympatry and those occurring in allopatry for either silverside. Thus, significant portions of their diet overlapped and neither species has exhibited dietary shifts that would facilitate coexistence. The two species differed significantly along stream size and current velocity gradients, with M. audens preferring the larger, slower moving habitats of the waterway. Unlike L. sicculus, M. audens included the exotic daphnid Daphnia lumholtzi in its diet. Because M. audens capitalized on a wider variety of prey items in the modified TTW environment and because of their previous interactions in other systems, we conclude that the native silverside, L. sicculus, is likely to be replaced by M. audens in lentic TTW habitats.

We investigated variation in environmental conditions and fish assemblages over space and time in the upper Red River drainage (Oklahoma Texas) using samples from eight sites (three tributaries and five mainstems) collected monthly in 1989-1990 and again in 1998-1999. Fish assemblages varied spatially, with geographically ubiquitous and ecologically tolerant taxa dominating tributaries and obligate species of prairie minnows common in mainstems. At the regional scale, fish assemblages were predictable between time periods. This resulted from strong longitudinal structuring in fishes across sites, which likely was influenced by spatial variation in environmental conditions across the drainage. These patterns suggested that, at the regional scale strong forcing of fish by their environment influenced predictability of fishes between time periods. At the local scale, predictability in fish assemblages between time periods differed across type of habitat. Specifically, assemblages in tributary sites varied randomly over time, whereas mainstem assemblages were predictable. We believe that, local environmental conditions favored different suites of taxa in tributaries between time periods. When we compared variation in fish assemblages to variation in environmental conditions at both regional and local scales, we detected that structure of fish assemblages tracked environmental variability at the regional scale. However, at the local scale, fish assemblages did not track environmental conditions. This suggests that environmental variation may affect fishes in a relative sense across the drainage, whereas, within sites, fishes may be decoupled from environmental structuring-forces. Based on the predictable patterns in fishes within the Red River drainage, we propose that monitoring efforts, aimed at detecting human-induced changes in this system, should include a range of mainstem sites that capture the strong structuring forces that, vary longitudinally within this river system.

The Bayou Darter, Etheostoma rubrum, Is endemic to the Bayou Pierre system of southwestern Mississippi where it occupies swift, shallow riffles with coarse, firm substrata. The Bayou Pierre system has experienced extensive erosion in response to rapid headcutting leading to loss of riffle habitat and degradation of riverine conditions. Due to its high degree of habitat specificity along with ongoing habitat fragmentation and potentially reduced gene flow between Isolated populations, the Bayou Darter is vulnerable to severe population declines and possible extinction. Our objectives were to quantify levels of genetic diversity in the mitochondrial control region and Infer population structure across the range of the species. Sequencing of 106 sampled Individuals revealed only three mtDNA haplotypes with one variable site. Haplotype diversity and nucleotide diversity were low (h <= 0.11 and pi < 0.001), and there was no structure revealed among the four sampled populations. The low genetic diversity in E. rubrum may best be explained by a small range size and recent genetic bottleneck.

Riverine fish assemblages on the North American plains continue to change as native species' distributions shrink and become increasingly fragmented due to impoundment, changes in the quantity and quality of water, and negative interactions of introduced species. To identify important changes to fish assemblages and their environment, long-term data are needed. I used fish assemblage and discharge data from 22 plains river localities across nearly 20 years to identify potential covariation between fish assemblages and the flow regime. I also obtained data on water use at the watershed scale to determine its potential effects on flow regimes and fish assemblages. A modified time series analysis indicated that directional change was occurring for many of the fish assemblages, though the strength of this change was highly variable among localities. Directional change was strongly and positively associated with change in flow regime, and change in flow regime was positively associated with human modification of the landscape, including number of wastewater facilities and returns. These results illustrate how demands on our water resources can ultimately influence riverine fish assemblages, largely by disrupting natural flow regimes. As population growth continues, the integrity of plains fish assemblages and their riverine environment will likely continue to decline, further exacerbating the fragmentation and reduction of species' geographic ranges.

Tadpoles of Cope's Gray Treefrog, Hyla chrysoscelis, were reared in the presence of two concentrations of colloidal clay to evaluate the influence of clay-associated, organic materials on growth. There were no dose-dependent effects of the amount of clay, but total tadpole lengths were 14.1 and 12.1% larger than clear-water controls for low and high clay content treatments, respectively. Similarly, tadpole wet masses were 35.1 (low clay) and 38.5% (high clay) larger than controls. Organic molecules and small organisms are sequestered on the enormous, collective surface area of the negatively charged clay particles, and we attribute the increased growth performances to these potential food sources.

We sampled fishes and aquatic insects monthly (Jun.-Sept. 2002) from intermittent tributaries of the Alum Fork of the Saline River (Arkansas, U.S.A.) to quantify the response of fish and aquatic insect assemblage properties to seasonal desiccation and habitat fragmentation. We collected a total of 4219 individuals, representing 18 species of fishes and 27 families of aquatic insects. Changes in the composition of fish assemblages were significantly related to temporal variability in pool volume and location in the watershed. Smaller, upstream pools varied in volume more than downstream pools. Fish assemblages were significantly more similar through summer in downstream, larger pool habitats. Changes in the composition of aquatic insect families were related to variation in water quality conditions. Highly eutrophied sites were typically dominated by midge larvae (Chironomidae), resulting in highly similar assemblages over time. Our results support findings at larger space and time scales and call attention to the importance of rapid changes in habitat quality, size and connectivity on stream communities.

The upper Tombigbee River in northeastern Mississippi now exists as a fragment, confluencing with and fed by an extensively modified aquatic landscape now called the Tennessee‐Tombigbee Waterway (TTW). We examined the changes to fish assemblages and flow regime after waterway construction based on contemporary comparisons to historical fish collections and discharge data. The river's flow regime has changed markedly since TTW construction. Analysis of discharge data from two stations for 15 years, pre‐ and post‐waterway, indicated significant differences in flow regime including increased minimum and base flows, lower spring and higher late summer‐autumn flows, and lower high flow durations, post‐TTW. These changes corresponded to significantly reduced regional and local species richness, and strong shifts in fish assemblage structure across a 20 yr time span. Post‐waterway fish assemblages were related strongly to measured environmental variables characterizing local habitats. Several lentic‐adapted species increased their abundances in lower reaches of the river, including a recent invader to the TTW system, the Mississippi silverside Menidia audens. Fragmentation of river ecosystems via disruption to hydrologic regimes is a major threat to aquatic biodiversity worldwide. Because the flow regime of this fragmented river is in part controlled by waterway operations via five minimum flow control structures, adaptive conservation and management efforts could be implemented in order to maintain and potentially restore the natural flow regime and the ecological integrity of the system.

Los Terreros Creek, a reservoir-fragmented tributary in the upper Lerma River system of central Mexico, was sampled at four localities in wet (2001) and dry (2002) seasons to evaluate spatial and temporal patterns of fish assemblages, the conservation status of native fishes, and prevalence of exotic species. Two of five species present were non-native; the common carp (Cyprinus carpio) and the shortfin silverside (Chirostoma humboldtianum). Fish assemblages formed a strong gradient largely separating the lentic, reservoir-impacted, locality (downstream-most site) from the remaining lotic sites. The two most-upstream sites were not distinguishable based on structure of fish assemblages. Three of five species showed significant spatial variation in abundance. The Aztec shiner (Aztecula sallaei) occurred only at the lotic sites and the two species of silversides (Chirostoma) occurred only at the lentic site. Temporal trends in assemblages were less evident, but assemblages in the dry season had lower mean species-richness and mean abundance values. Analysis of distance matrices based on assemblages, site-specific environmental conditions, and spatial structure indicated that a strong, spatially structured environmental gradient was significantly associated with fish assemblages in the wet season, but not assemblages in the dry season. Los Terreros Creek still supports an assemblage of native fish that is typical for the upper Lerma River system. However, this relict system remains threatened by anthropogenic activities including changes in land use and further introduction of exotic species.

Non-parametric, multivariate analyses commonly used to characterize spatial and temporal patterns in the taxonomic composition of ecological communities can likewise be useful when applied to data generated from the examination of gut contents. A suite of analyses including non-metric multidimensional scaling (NMS), multi-response permutation procedures (MRPP), and indicator species analysis (ISA) revealed interesting dietary shifts that Occurred across individuals of emerald shiner (Notropis atherinoides) collected from among different habitats and two different time periods (pre- and post-impoundment of the upper Tombigbee River). These patterns were delineated from data that exhibited non-linear distribution and that incorporated measurements made at different scales, two factors that confound traditional parametric analyses. NMS ordination provided clear, graphic representation of the separation that occurred when N. atherinoides diets were viewed across space and time. MRPP analysis validated that the observed separation was statistically significant, and ISA complemented these other two analyses by identifying diet contributors that were indicative of each grouping.

The crystal darter, Crystallaria asprella (Jordan), is a benthic, riverine specialist, rare throughout its range and critically imperiled in the state of Mississippi. Construction of the Tennessee-Tombigbee Waterway has fragmented a once continuous population of this species into several subpopulations in remaining, free-flowing tributaries of the system. In spite of this fragmentation and population subdivision, we collected numerous individuals from the waterway during summer 2004 and 2005. Because of the lentic conditions in the waterway, we questioned whether a dietary shift accompanied darters occupying this new environment. We also obtained museum specimens from the Tombigbee River before waterway construction and quantified and compared diets among the historical, tributary (contemporary) and waterway specimens. We hypothesised that waterway specimens would differ significantly in diet from both historical and tributary specimens, and that the latter two groups would have similar diets. Multiresponse permutation procedures indicated that all three groups were significantly different from each other and indicator species analysis identified significant indicator taxa for waterway and tributary specimens. Using a null model approach, dietary overlap was significantly greater than expected for tributary and waterway specimens, and significantly less than expected for waterway and historical specimens. Dietary plasticity was evident for crystal darters across space and time, but it remains unknown whether waterway individuals represent a population sink or are actively dispersing from nearby population sources.