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Abstract Lotic systems in mountain regions have historically provided secure habitat for native fish populations because of their relative isolation from human settlement and lack of upstream disturbances. However, rivers of mountain ecoregions are currently experiencing heightened levels of disturbance due to the introduction of nonnative species impacting endemic fishes in these areas. We compared the fish assemblages and diets of mountain steppe fishes of the stocked rivers in Wyoming with rivers in northern Mongolia where stocking is absent. Using gut content analysis, we quantified the selectivity and diets of fishes collected in these systems. Nonnative species had more generalist diets with lower levels of selectivity than most native species and native species had high levels of dietary specificity and selectivity. High abundances of nonnative species and high levels of dietary overlaps in our Wyoming sites is a cause of concern for native Cutthroat Trout and overall system stability. In contrast, fish assemblages characterizing Mongolia mountain steppe rivers were composed of only native species with diverse diets and higher selectivity values, suggesting low probability for interspecific competition.

Abstract Stream fishes are restricted to specific environments with appropriate habitats for feeding and reproduction. Interactions between streams and surrounding landscapes influence the availability and type of fish habitat, nutrient concentrations, suspended solids, and substrate composition. Valley width and gradient are geomorphological variables that influence the frequency and intensity that a stream interacts with the surrounding landscape. For example, in constrained valleys, canyon walls are steeply sloped and valleys are narrow, limiting the movement of water into riparian zones. Wide valleys have long, flat floodplains that are inundated with high discharge. We tested for differences in fish assemblages with geomorphology variation among stream sites. We selected rivers in similar forested and endorheic ecoregion types of the United States and Mongolia. Sites where we collected were defined as geomorphologically unique river segments (i.e., functional process zones; FPZs) using an automated ArcGIS‐based tool. This tool extracts geomorphic variables at the valley and catchment scales and uses them to cluster stream segments based on their similarity. We collected a representative fish sample from replicates of FPZs. Then, we used constrained ordinations to determine whether river geomorphology could predict fish assemblage variation. Our constrained ordination approach using geomorphology to predict fish assemblages resulted in significance using fish taxonomy and traits in several watersheds. The watersheds where constrained ordinations were not successful were next analyzed with unconstrained ordinations to examine patterns among fish taxonomy and traits with geomorphology variables. Common geomorphology variables as predictors for taxonomic fish assemblages were river gradient, valley width, and valley slope. Significant geomorphology predictors of functional traits were valley width‐to‐floor width ratio, elevation, gradient, and channel sinuosity. These results provide evidence that fish assemblages respond similarly and strongly to geomorphic variables on two continents.

Abstract River hydrogeomorphology is a major driver shaping biodiversity and community composition. Here, we examine how hydrogeomorphic heterogeneity expressed by Functional Process Zones (FPZs) in river networks is associated with fish assemblage variation. We examined this association in two distinct ecoregions in Mongolia expected to display different gradients of river network hydrogeomorphic heterogeneity. We delineated FPZs by extracting valley‐scale hydrogeomorphic variables at 10 km sample intervals in forest steppe (FS) and in grassland (G) river networks. We sampled fish assemblages and examined variation associated with changes in gradients of hydrogeomorphology as expressed by the FPZs. Thus, we examined assemblage variation as patterns of occurrence‐ and abundance‐based beta diversities for the taxonomic composition of assemblages and as functional beta diversity. Overall, we delineated 5 and 6 FPZs in river networks of the FS and G, respectively. Eight fish species were found in the FS river network and seventeen in the G, four of them common to both ecoregions. Functional richness was correspondingly higher in the G river network. Variation in the taxonomic composition of assemblages was driven by species turnover and was only significant in the G river network. Abundance‐based taxonomic variation was significant in river networks of both ecoregions, while the functional beta diversity results were inconclusive. We show that valley‐scale hydrogeomorphology is a significant driver of variation in fish assemblages at a macrosystem scale. Both changes in the composition of fish assemblages and the carrying capacity of the river network were driven by valley‐scale hydrogeomorphic variables. River network hydrogeomorphology as accounted for in the study has, therefore, the potential to inform macrosystem scale community ecology research and conservation efforts.

We investigated the Gyrodactylus ectoparasites on relict fishes in the isolated endorheic Central Asian Internal drainage basin in Mongolia (The Hollow) and placed them into the global phylogenetic framework based on internal transcribed spacer regions of the nuclear ribosomal DNA (ITS). Much of the rich Pliocene lacustrine ichthyofauna is extinct. We sampled five riverine survivors: Altai osmans Oreoleuciscus humilis and O. potanini (Leuciscidae), Mongolian grayling Thymallus brevirostris (Salmonidae), and stone loaches Barbatula conilobus and B. cobdonensis (Nemacheilidae). We found eight species of the subgenus Gyrodactylus (Limnonephrotus) and four of G. (Gyrodactylus). Nine species were identified as taxa described earlier, and three were described as new. The endemic Mongolian grayling carried four species, only one of wageneri group typical to salmonids (Gyrodactylus radimi sp. nov.), two of nemachili group (G. zavkhanensis sp. nov., G. pseudonemachili Ergens and Bychowsky, 1967), and G. amurensis Akhmerov, 1952 of subgenus G. (Gyrodactylus). G. pseudonemachili was also found on osman and loach. A parasite clade typical for Nemacheilidae was overrepresented by five species (G. tayshirensis sp. nov. on Barbatula conilobus, G. mongolicus Ergens and Dulmaa, 1970, G. nemachili Bychowsky, 1936). Relaxed host specificity mentioned already by Ergens and Dulmaa was evident. In the updated global ITS phylogenies of the two freshwater-restricted subgenera, the parasites from the Mongolian relict populations assumed positions concordant with a hypothesis of multiple ancient introductions from the Euro-Siberian fauna, strong rarefaction and three cases of endemic divergence.

Freshwater ecosystems of Mongolia belong to the three major Central Asian drainage systems: Arctic drainage, Amur River drainage, and Central Asian Internal drainage. The Great Lakes Depression takes considerable part of the latter. One of the few fish species living there, Oreoleuciscus potanini, is the most common and widespread endemic species in the Central Asian region. Parasitological surveys on O. potanini from the Great Lakes Depression have not occurred regularly in recent years. Consequently, this study was conducted to explore the species diversity of helminth parasites in O. potanini in the three lakes of Khar Us Nuur National Park. In total, 52 O. potanini specimens were collected from the lakes Khar, Khar Us and Durgen and sampled in August 2012. In addition to the parasites, the age and diet spectrum of the fish species were studied. The fish were examined for infection with ecto- and endo- macroparasites. The species composition of Diplostomum spp. metacercariae from the eyes of fish was investigated using a molecular approach. In the lakes Khar and Khar Us the fish age ranged from four to 32 years old and from eight to 35 years old, respectively. Fish of four age groups (9, 10, 11 and 14 years old) were in Lake Durgen. Algae and various insects, including Chironomida larvae were the main food of the O. potanini in all three lakes. When the diet was investigated we found that in Lake Durgen Cladocera crustaceans were the significant component in the fish diet, and no fish was found in the O. potanini diet there. During the study 26 parasites species were identified from the three lakes. The total fauna is represented by four phyla, including Acanthocephala (one species), Annelida (one species), Nematoda (four species), Platyhelminthes (20 species). The last group was the most numerous and diverse, including three species of Cestoda, six species of Monogenea and 11 species of Trematoda. Five species of parasites were specific to O. potanini only (monogeneans Gyrodactylus mongolicus, G. oreoleucisci, G. pewsowi, nematodes Philometra oreoleucisci and Rhabdochona humili). The other helminthes exhibited a low host-specificity, infecting a wide range of fishes in the Holarctic. The larvae of endohelminthes predominate in the parasite fauna. The majority of these parasites enter the fish via various invertebrate species during O. potanini's feeding. Our study provided novel data on the species diversity of Diplostomum spp. metacercaria in O. potanini's eyes. They were represented by three species: Diplostomum spathaceum, D. pseudospathaceum and Diplostomum sp. LIN2. The species richness and diversity of parasites were much higher for O. potanini from the lakes Khar and Khar Us (26 species), than that for O. potanini of Lake Durgen (15 species). The possible reason for the absence of some parasites in Lake Durgan is an increased water salinity. It is likely that, despite the potential for the fish to exist under conditions of an elevated salinity, parasites (or their invertebrate hosts) are not adapted to it. Our findings clarified the known parasite diversity in lakes of the Khar Us Nuur National Park, indicating that the Great Lakes Depression ecosystem may be characterised by many unique parasite assemblages. The results of investigation on the parasitic fauna of O. potanini confirm a review of the evolutionary youth of this fish group.

The species composition of trematode metacercariae from the eyes of fish (Oreoleuciscus potanini and Thymallus brevirostris) was studied in the Taishir and Durgun reservoirs in 2011, four and three years respectively after their formation. Seven species of trematodes of the genera Diplostomum, Tylodelphys, Ichthyocotylurus, Posthodiplostomum were identified. The species composition of the parasites is compared with that from fish eyes in the reservoirs of the Great Lakes Depression.

In the channel areas of the large Tayshir Reservoir valley, zooplankton is unevenly distributed along the longitudinal profile. The number and biomass of communities increases from the upper to lower rea­ ches. In the Durgun Reservoir, which has features of a canyon­type reservoir, the greatest abundance and biomass of zooplankton are recorded in the upper reaches, while the greatest number of species is recorded in the lower reaches. In the Taishir Reservoir, an increase in precipitation and a raised water level have a stimula­ ting effect on the crustaceans in the middle part of the reservoir, the area of the greatest accumulation of substances brought from the catchment area, contributing to the food supply of the invertebrates. In the upper reaches of the reservoir, with increased precipitation, high flowage prevents the development of zooplankton, and in the lower reaches, due to minimal accumulation of organic and biogenic substances and the impossibility of their intensive diffusion from bottom sediments due to maximum depth and width, the communities display the effect of “dilution” (reduced abundance). For interannual changes in the zooplankton of the channel of the upper part of the Durgun reservoir, precipitation fluctuations are of major importance. In the lower reaches of the reservoir, in the deeper areas, the temperature regime and fluctuations of the water level, depending on the work of hydroelectric power plants, are of decisive importance.

The interannual differences in the zooplankton of the coastal zone of two different types of reservoirs were analyzed, depending on the location along the longitudinal profile of water bodies, the amount of atmospheric precipitation, air temperature, the level and electrical conductivity of the water, and the morphometry of the water areas. The zooplankton of the coastal area of the large Tayshir Reservoir differed markedly along the longitudinal profile of the reservoir. In the upstream zone, the state of the zooplankton was estimated by totalling the effects from changes in water mineralization, in the supply of organic and nutrient substances brought by the river, in the surrounding landscape, and also the effects of the transfer of nutrients from the sediments to the water. In the middle zone, the following effects were taken into account: external intake of substances and diffusion of nutrients from bottom sediments. In the downstream zone these were: an increase in the amount of precipitation and water level that led to a dilution of water with already impoverished zooplankton content. In the Durgun reservoir, which is short and narrow, the quantity and content of zooplankton in the coastal areas of the upstream and downstream zones were not much different: in the downstream zone the number of species was somewhat higher, as well as the proportion of Cladocera in the total biomass, and the values of Shannon diversity indices calculated for the numbers and the biomass. In general, in the Durgun reservoir, the quantitative characteristics and structure of zooplankton depended on the connection with the feeding eutrophic Har-­Us Lake, which was determined by the amount of precipitation. An increase in precipitation led to an increase in the number and biomass of rotifers. An increase in the water level of the reservoir associated with a reduction in the discharge of hydroelectric power stations and a general decrease in the amount of precipitation, when the littoral zone and floods were nearly entirely absent, was correlated with an increase in the numbers and biomass of Copepoda.