Riverine Species Research Articles

Importance of multi-dimensional morphodynamics for habitat evolution: Danube River 1715–2006

Human-unimpaired braided and anabranched river systems are characterized by manifold multi-dimensional exchange processes. The intensity of hydrological surface/subsurface connectivity of riverine habitats depends on more than regular or episodic water level fluctuations due to the hydrological regime. Morphodynamic changes are also a basic underlying factor. In order to provide new insights into the long-term habitat configuration of large rivers prior to channelization, this study discusses the hydromorphological alterations of an alluvial section of the Austrian Danube based on historical records from 1715 to 2006. The study combines the analysis of habitat patterns and intensity of hydrological connectivity over the long term with the reconstruction of short-term morphodynamic processes between 1812 and 1821. The main research questions are (1) whether the intensive morphodynamics prior to channelization are reflected by a marked variation in habitat patterns or whether the variation remained within a small range, and (2) which fluvial processes contributed to the evolution of the habitat configuration identified.The study reveals that the mean variations in the habitat patterns and the intensity of hydrological connectivity were only between 3% and 10% before 1821, although the river landscape was subject to intensive fluvial disturbances. An exception was the expansion of aquatic habitats between low and mean flow, which deviated by 15%. Habitat evolution was affected by morphodynamic processes occurring across different temporal scales. Both gradual channel changes such as incision or migration and sudden processes such as avulsions (cut-offs) contributed to the patterns identified. Locally, sudden channel changes extensively altered the habitat conditions with regard to hydrological surface/subsurface connectivity. Such alterations foster or restrain the potential evolution and the ecological succession of the riparian vegetation at the respective sites. On a larger spatial and temporal scale, however, the changes in the intensity of hydrological connectivity were largely balanced. The results support the hypothesis that a resilient “shifting mosaic steady-state” existed over the long term as long as the framework conditions (e.g. climate) did not significantly change. The habitat mosaic representing different types and different ages potentially allowed many riverine species to co-exist in an environment with frequent perturbations. From 1821 onwards, river engineering measures significantly altered habitat patterns and severely truncated the potential of the system to recover from disturbances.

PATTERNS OF FISH PASSAGE IN THE UPPER MISSISSIPPI RIVER

Dams have been implicated in the alteration of natural river processes. Quantifying spatial and temporal movement and passage patterns of large river fishes are critical for determining the extent of restricted passage and the needs for fish passage improvements. However, limited information regarding this topic exists because of the inherent difficulties associated with large river systems and assumptions associated with movement studies. Because of this lack of information, we investigated broad scale passage patterns of several riverine fish species through seven locks and dams complexes of the Upper Mississippi River using telemetry. Over the course of our 5-year evaluation, we observed species-specific movement and passage patterns, and how these trends were affected by factors such as water level and lock and dam management. Stationary receivers placed in a monitoring array detected a total of 1036 passage events. Eighty-four percent of the passage occurred through all but one of the lock and dam structures during both open and closed river conditions. While 70% of the passage occurred during open river conditions, further investigation of passages that occurred during closed river conditions (when gates are extended into the water column at some level) revealed that the majority of passage occurred when the average opening for all gates ranged from 0.6 to 1.2 m. Lock usage was also quantified, and most species were not routinely using the lock chambers for passage. Ultimately, these data have shown that individuals of each study species were able to negotiate most of the locks and dams during open and closed river conditions in both directions. Copyright © 2013 John Wiley & Sons, Ltd.

Status of the freshwater mussel (Unionidae) communities of the mainstem of the Leon River, Texas

ABSTRACTThe Leon River drainage, located in the Brazos River basin, has not been extensively surveyed for freshwater mussels (Family Unionidae). This is problematic given that three state‐threatened species, Quadrula houstonensis, Quadrula mitchelli, and Truncilla macrodon, have historically occurred in this drainage and two are now candidates for protection under the US Endangered Species Act.Mussels were sampled qualitatively at 44 sites in the summer and fall of 2011 to determine whether these species were still extant in the Leon River. The distributions and abundances of species at present considered common were also examined. Shell length data were assessed to determine the overall viability of the mussel fauna within the Leon River drainage.In total, 2081 live mussels were collected representing 12 species, including the federal candidate species Quadrula houstonensis, but Lampsilis hydiana, Quadrula mitchelli and Truncilla macrodon were not collected. Overall mussel abundance and species richness was low and community composition was highly fragmented with riverine species largely occurring in the middle portion of the Leon River. There was evidence that population recruitment is occurring, but only for a few species.River impoundment, inadequate instream flows, and agricultural practices are probable causes of the changes in mussel species composition. Further studies are needed to evaluate the impacts of reservoir releases on mussel persistence within this basin and in areas where droughts and low stream flow are commonplace. More information is needed on how agricultural practices affect mussel communities; the information that is currently available does little in the way of identifying factors that can be managed at site or reach scales. Studies that address these knowledge gaps will help resource managers to design more effective strategies to protect mussel populations within and outside this basin. Copyright © 2013 John Wiley & Sons, Ltd.

Using anglers' records in Poland as a tool for assessing changes in fish body weight caused by fluctuations of inland water quality

Summary Anglers' records of fish weight (ARFW) were analysed in Poland for the periods 1965–1989 (successive increases in inland water pollution), and 1990–2010 (considerable improvements in water quality). For the initial ARFW analysis, the entire study was divided into nine intervals of 5 years each; the period that followed was divided into three catchments: the Vistula, Oder and Pomeranian rivers, whereby 10 species were chosen to represent obligatory riverine and facultative species. With a few exceptions, the investigated species showed unexpected and varying reactions to the changes in water quality. The ARFW of obligatory riverine barbel Barbus barbus (L.) decreased during the first periods (1965–1989), then increased significantly from 1990 to 2010. For obligatory riverine ide Leuciscus idus (L.), tendencies were less clear but as significant. However, other obligatory riverine species have not shown a relationship between ARFW and water quality. Discrepancies were also recorded in the facultative riverine pike (Esox lucius L.) and brown trout (Salmo trutta L.). Considering the number of medals recorded, there were some interesting tendencies related to the decline or improvement in the water quality, although not in all investigated species. The species displaying a decrease in medals awarded from 1966 to 1989 and increasing after 1990, was chub Leuciscus cephalus (L.), but only until 2005. Species which showed successive increases in awards from 1989 to 2010 were: pike, perch (Perca fluviatilis L.), ide, and wels (Silurus glanis L.). However, for species in the period when the water quality actually deteriorated (1966–1990), the number of medals awarded increased: barbel, brown trout (S. trutta L.), and grayling Thymallus thymallus (L.).

Fish Assemblages in Borrow‐Pit Lakes of the Lower Mississippi River

AbstractBorrow‐pit lakes encompass about a third of the lentic water habitats (by area) in the active floodplain of the Lower Mississippi River, yet little is known about their fish assemblages. We investigated whether fish assemblages supported by borrow‐pit lakes resembled those in oxbow lakes to help place the ecological relevance of borrow‐pit lakes in context with that of natural floodplain lakes. In all, we collected 75 fish species, including 65 species in eight borrow‐pit lakes, 52 species in four riverside oxbow lakes, and 44 species in eight landside oxbow lakes. Significant differences in several species richness metrics were evident between borrow‐pit lakes and landside oxbow lakes but not between borrow‐pit lakes and riverside oxbow lakes. All three lake types differed in fish assemblage composition. Borrow‐pit lakes and riverside oxbow lakes tended to include a greater representation of fish species that require access to diverse environments, including lentic, lotic, and palustrine habitats; fish assemblages in landside oxbow lakes included a higher representation of lacustrine species. None of the fish species collected in borrow‐pit lakes was federally listed as threatened or endangered, but several were listed as species of special concern by state governments in the region, suggesting that borrow‐pit lakes provide habitat for sensitive riverine and wetland fish species. Differences in fish assemblages among borrow‐pit lakes were linked to engineered morphologic features, suggesting that diversity in engineering can contribute to diversity in fish assemblages; however, more research is needed to match engineering designs with fish assemblage structures that best meet conservation needs.

Effects of habitat size and isolation on species immigration–extinction dynamics and community nestedness in a desert river system

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.

Dynamics of tsetse natural infection rates in the Mouhoun river, Burkina Faso, in relation with environmental factors

In Burkina Faso, the cyclical vectors of African animal trypanosomoses (AAT) are riverine tsetse species, namely Glossina palpalis gambiensis Vanderplank (G.p.g.) and Glossina tachinoides Westwood (G.t.) (Diptera: Glossinidae). Experimental work demonstrated that environmental stress can increase the sensitivity of tsetse to trypanosome infection. Seasonal variations of the tsetse infection rates were monitored monthly over 17 months (May 2006–September 2007) in two sites (Douroula and Kadomba). In total, 1423 flies were dissected and the infection of the proboscis, middle intestine and salivary glands was noted. All the positive organs were analyzed using monospecific polymerase chain reaction (PCR) primers. To investigate the role of different environmental factors, fly infection rates were analyzed using generalized linear mixed binomial models using the species, sex, and monthly averages of the maximum, minimum and mean daily temperatures, rainfalls, Land Surface Temperature day (LSTd) and night (LSTn) as fixed effects and the trap position as a random effect. The overall infection rate was 10% from which the predominant species was T. congolense (7.6% of the flies), followed by T. vivax (2.2% of the flies). The best model (lowest AICc) for the global infection rates was the one with the maximum daily temperature only as fixed effect (p < 0.001). For T. congolense, the best model was the one with the tsetse species, sex, maximum daily temperature and rainfalls as fixed effect, where the maximum daily temperature was the main effect (p < 0.001). The number of T. vivax infections was too low to allow the models to converge. The maturation rate of T. congolense was very high (94%), and G. t. harbored a higher maturation rate (p = 0.03). The results are discussed in view of former laboratory studies showing that temperature stress can increase the susceptibility of tsetse to trypanosomes, as well as the possibility to improve AAT risk mapping using satellite images.

The ecology of the riverine Garra species (Teleostei, Cypriniformes) in reservoirs of the semi-arid highlands of northern Ethiopia: temporal dynamics of feeding activity

Garra species are among the most abundant fish in small rivers of northern Ethiopia. Many manmade reservoirs in the region have been colonized by Garra, which often are the only fish species present and have become very abundant. Little is known about the ecology of these reservoir populations of riverine species. In this study we investigated the distribution patterns and gut fullness of 2 dominant species, G. blanfordii and G. geba, in 3 recently created reservoirs (Gereb Awso, Tsinkanet, and Mai Gassa I) in Tigray, northern Ethiopia. Species composition differed among reservoirs. Our data on fish catch densities and the fullness of the foregut suggest that the ecology of the Garra populations in the reservoirs is likely influenced by the avoidance of predation by birds. G. blanfordii, and to a lesser extent G. geba, foraged most actively after sunset.

Demonstration Restoration Measures in Tributaries of the Vindel River Catchment

Gardeström, J., D. Holmqvist, L. E. Polvi, and C. Nilsson. 2013. Demonstration restoration measures in tributaries of the Vindel River catchment. Ecology and Society 18(3): 8. https://doi.org/10.5751/ES-05609-180308

Effects of flow on lateral interactions of fish and shrimps with off-channel habitats in a large river-floodplain system

Off-channel habitats play a crucial role in the life-cycles of many riverine fish species, but lateral movements of fish into these habitats are poorly understood. We tested how flow dynamics affects the movement of fish and shrimps between the main river channel and different types of off-channel habitats: a riverine lake and a wetland. Our study site was the lower Waikato River, North Island, New Zealand, where there are numerous off-channel habitats. Fish were sampled using directional fyke nets. Shortfin eel (Anguilla australis) migrated mostly into the wetland at night, particularly during high river flows. Common bullies (Gobiomorphus cotidianus) were most abundant during the day and in low-discharge conditions, moving mostly into the riverine lake, whereas freshwater shrimp (Paratya curvirostris) moved mostly into the wetland. High numbers of non-native larval common carp (Cyprinus carpio) moved out of the wetland with retreating flood water. This study emphasises the importance of lateral connectivity and flooding in functioning of this river system where numerous native fish, but also exotic fish, used off-channel habitats. Floodplain management strategies should promote ‘controlled connectivity’ measures that provide access for native species at key times while limiting opportunities for introduced species to utilise their favoured off-channel habitats.

Site-Occupancy Monitoring of an Ecosystem Indicator: Linking Characteristics of Riparian Vegetation to Beaver Occurrence

Establishment of sampling frameworks to monitor the occurrence of ecological indicators and to identify the covariates that influence occurrence is a high-priority need for natural resource restoration and management efforts. We utilized occupancy modeling to identify patterns of beaver occurrence and factors influencing these patterns (i.e., type and amount of vegetation cover) in the Grand Canyon of the Colorado River ecosystem. We used rafts and kayaks to access a stratified random sample of sites (i.e., 100-m-long sections of riverbank) and used repeated sampling proce- dures to sample for beaver sign (i.e., lodges, cuttings, tracks, and beaver sightings). We quantified the type and amount of vegetation cover at each sampled section by using a GIS database of remotely sensed information on the riparian vege tation in the Grand Canyon. We first modeled occurrence of beaver sign as a function of the total amount of vegeta- tion cover (summed across classes) and then determined the relative importance score for each of the 7 vegetation classes. Detection probability (p) was 2 times higher when observers traveled in kayaks (0.61) than when they traveled in rafts (0.29). Occurrence of beaver sign (ψ) in sampled transects was widespread throughout the Grand Canyon (ψ = 0.74, SE = 0.06) and positively associated with total vegetation. The relative importance scores for Tamarix and Pluchea vegetation classes were 1.5-2.5 times larger than those for all other vegetation classes, indicating that occurrence of beaver sign was most strongly associated with the cover of these 2 vegetation classes. Our results imply that quantifying the amount of riparian vegetation in close proximity to a river helps determine the occurrence of an important ecological indicator in riparian systems. The results also demonstrate a useful and cost-effective method for monitoring riverine species' usage patterns by explicitly accounting for detectability.

Semi-natural river system maintains functional connectivity and gene flow of the critically endangered gravel grasshopper (Chorthippus pullus)

River engineering and the subsequent loss and fragmentation of riparian habitats during recent centuries have had serious impacts on the distribution and population size of many riverine species. Thus it is important to identify the characteristics of semi-natural river systems that still support functional connectivity for vulnerable species. Chorthippus pullus is a critically endangered grasshopper that inhabits patches of gravel banks along Alpine river systems. The species’ distribution conceptually resembles a metapopulation. We analysed the patch occupancy and gene flow of a C. pullus population in a river system that had been historically drained in 1923 and improved 67years later by allowing residual water flow. In 2010, we surveyed a total of 43 patches for occupancy, marked and recaptured 299 individuals and analysed DNA samples of 16 local populations using five microsatellite markers. We further correlated a measure of gene flow to landscape resistance matrices using least-cost-path modelling of the contemporary landscape. Our results show that occupancy patterns are positively related to habitat area and connectivity. Gene flow was restricted by forests and large river streams in the contemporary semi-natural situation. The observed high level of genetic diversity is potentially the result of the historical management over several decades. To preserve functional connectivity and gene flow among local populations of critically endangered gravel grasshoppers, habitat availability and suitability have to be maintained by ensuring appropriate temporal and spatial fluctuations in water level.

Extensive Clonal Assemblies in Populus alba and Populus x canescens from the Iberian Peninsula

Riparian vegetation in temperate zones holds great environmental importance and thus its conservation should be a priority. Among riverine tree species, genus Populus stands out, being also a model organism in research. In this work, we present a genetic analysis of the hybrid Populus x canescens and its parent species Populus alba and Populus tremula, with special emphasis in P. alba with which backcrosses frequently occur. This study focuses on the river Douro basin (Spain) where the presence of hybrids has been previously reported. Nuclear microsatellite markers and Bayesian statistical analysis have been used for the detection of hybrids and purebred stands. This methodology has also made possible the study of clonality in the taxonomic continuum P. alba–P. x canescens. Our results highlight the existence of a small number of genotypes accounting for most of the individual trees and stands. Possible causes leading to this situation are discussed. Also, the presence of individuals genetically close to ornamental cultivars is reported for the first time. Finally, concern about the present situation of P. alba genetic resources is brought up.

Incorporating ecological principles into statistical models for the prediction of species’ distribution and abundance

Understanding the determinants of species’ distributions and abundances is a central theme in ecology. The development of statistical models to achieve this has a long history and the notion that the model should closely reflect underlying scientific understanding has encouraged ecologists to adopt complex statistical methods as they arise. In this paper we describe a Bayesian hierarchical model that reflects a conceptual ecological model of multi‐scaled environmental determinants of riverine fish species’ distributions and abundances. We illustrate this with distribution and abundance data of a small‐bodied fish species, the Empire gudgeon Hypseleotris galii, in the Mary and Albert Rivers, Queensland, Australia. Specifically, the model sought to address; 1) the extent that landscape‐scale abiotic variables can explain the species’ distribution compared to local‐scale variables, 2) how local‐scale abiotic variables can explain species’ abundances, and 3) how are these local‐scale relationships mediated by landscape‐scale variables. Overall, the model accounted for around 60% of variation in the distribution and abundance of H. galii. The findings show that the landscape‐scale variables explain much of the distribution of the species; however, there was considerable improvement in estimating the species’ distribution with the addition of local‐scale variables. There were many strong relationships between abundance and local‐scale abiotic variables; however, several of these relationships were mediated by some of the landscape‐scale variables. The extent of spatial autocorrelation in the data was relatively low compared to the distances among sampling reaches. Our findings exemplify that Bayesian statistical modelling provides a robust framework for statistical modelling that reflects our ecological understanding. This allows ecologists to address a range of ecological questions with a single unified probability model rather than a series of disconnected analyses.

STREAM HABITAT MODELLING FOR CONSERVING A THREATENED HEADWATER FISH IN THE UPPER CUMBERLAND RIVER, KENTUCKY

ABSTRACTThe conservation of stream biodiversity requires more explicit knowledge on the distribution of aquatic species within the context of their specific environmental settings and stresses. Although species distribution models (SDMs) have been widely used for organisms occupying contiguous spatial extents, the implementation of SDMs in relatively complex and segmented riverine networks is still at its early stage. In this study, we explicitly modelled the headwater stream habitat for the threatened blackside dace (Phoxinus cumberlandensis) endemic to the upper Cumberland River, Kentucky, USA. An occurrence record data set, along with variables describing stream properties and land use impacts, was used to predict the fish habitat suitability at the stream segment level. An approach combining geographic information systems and the maximum entropy species distribution modelling (MaxEnt) was adopted. Results demonstrated that natural conditions and land use disturbances, respectively, form the primary and secondary environmental constraints on the species' habitat. We generated regional‐scale management‐friendly maps showing subwatershed habitat suitability and locations of the clustered suitable habitats (hotspots) and thus set an example for spatially explicit management of threatened and endangered riverine species. This study demonstrates the usefulness of SDMs for stream network–based environments in the facilitation of biogeographic conservation efforts and studies. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Modelling the habitat requirement of riverine fish species at the European scale: sensitivity to temperature and precipitation and associated uncertainty

Abstract – Species distribution models (SDMs) have been widely used for different purposes such as studying species niche or forecasting the effects of global change on species distribution. Nevertheless, these models are often calibrated on datasets that only cover a fraction of the species’ realised niches, which could lead to unrealistic results. The aim of this study was to model the habitat requirements of 21 freshwater fish species that are native to Europe, using a dataset that accurately reflects their realised niches. Both temperature and precipitation were used as climatic factors to model the habitat requirements of the species, and the uncertainty associated with the fitted environment–occurrence relationships was examined. The results demonstrated the importance of accounting for these two climatic components when estimating the habitat requirements of riverine fish species and whether the uncertainty associated with model expectations varies with the species and the environmental factor considered. These results are discussed with regard to the known ecology of the 21 riverine fish species and within the perspective of climate change.

Ecology of Unique Lentic Populations of Roundtail Chub,Gila robusta

ABSTRACT. Although roundtail chub (Gila robusta) is generally considered a riverine species, 6 natural lakes in the upper Green River basin, Wyoming, have resident populations of this fish. In 2 of the lakes, Halfmoon and Little Halfmoon, we investigated the ecology of resident roundtail chub, including their habitat use, diet, weight—length relationships, growth, and reproduction. Lentic roundtail chub used littoral and mid-depth benthic habitats most often, with the highest catch rates in littoral habitats. Roundtail chub were rarely caught in pelagic or deep benthic habitats. Opportunistic foraging was observed in both lakes, with roundtail chub consuming terrestrial and aquatic insects, vegetation, and fish. Roundtail chub from both lakes had weight—length relationships similar to those reported for lotic populations but slower annual growth rates. Fish in spawning condition were captured from mid-June to late July when water temperatures ranged from 8.7 to 18.3 °C and snowmelt runoff was at its maxim...

Assessing the range of newly established invasive species in rivers using probabilistic methods

Data on the presence of invasive species are often scarce, especially during the initial period following their introduction, when population abundance and species detectability are both still very low. In such cases, sporadic sightings in their newly occupied territory are often the only available information about their presence and distribution. Here, we demonstrate the potential of using probabilistic models for the assessment of the distribution of newly established invasive species in rivers, based on their sighting records. We have applied Solow’s equation to the sighting record of the Amur sleeper (Perccottus glenii) in the Danube River, to assess the probable range of its population. The method applied indicated that, at the time when the sightings have been made (2003–2008), the Amur sleeper population in the Danube most probably extended between the 704.5 and 1,178.5 river km, while it was highly improbable (P < 0.05) that it was present downstream from the 606 river km or upstream from the 1,277 river km. The application of this approach has, however, to be tested further on a larger number of scenarios and case studies in order to evaluate its reliability. What this method lacks in complexity, compared to more advanced species distribution assessments, it makes up by its ability to be easily used and quickly applied for preliminary assessments of the ranges of riverine species. As a result, the proposed approach might be of interest for scientists and managers dealing with invasive species.

Larval Surveys Indicate Low Levels of Endangered Pallid Sturgeon Reproduction in the Middle Mississippi River

AbstractThe pallid sturgeon Scaphirhynchus albus is an endangered riverine species that is less abundant than the sympatric shovelnose sturgeon S. platorynchus with which it hybridizes. The two species are morphologically similar, and due to morphological variation within species, allometry, and the occurrence of morphological intermediates, morphological identification of specimens can be problematic. When the pallid sturgeon was listed under the U.S. Endangered Species Act in 1990, perceived threats included habitat loss due to habitat alteration (dams and channelization), hybridization, and commercial harvest. Previous studies estimated the ratio of adult pallid sturgeon to adult shovelnose sturgeon to be approximately 1:77 in the middle Mississippi River (MMR; the reach between the Missouri River confluence and the Ohio River confluence). One study using morphological identification estimated that 8–26% of Scaphirhynchus larvae collected in the MMR during May and June were pallid sturgeon. Given that larval pallid sturgeon, shovelnose sturgeon, and their hybrids have overlapping morphologies, DNA markers may provide more accurate species identification. The present study aimed to estimate relative larval abundances of pallid sturgeon, shovelnose sturgeon, and hybrids in the MMR. We genotyped 583 Scaphirhynchus larvae collected from the MMR at 16 DNA microsatellite loci and determined that 581 larvae were shovelnose sturgeon, one was a hybrid, and one was a pallid sturgeon. This study is the first to provide genetically verified larval abundance estimates of Scaphirhynchus species in the MMR. The present data indicate that the rate of pallid sturgeon reproduction relative to shovelnose sturgeon reproduction is an order of magnitude lower than expectations based on adult ratios and that hybrid sturgeon larvae are as common as pure pallid sturgeon.

Reproductive plasticity of Hypostomus affinis (Siluriformes: Loricariidae) as a mechanism to adapt to a reservoir with poor habitat complexity

In this work we describe the gonad morphology and spawning season of Hypostomus affinis (Steindachner, 1877) in a tropical reservoir based on 55 males and 125 females. Our aim was to assess eventual adaptations in reproductive tactics developed by this riverine species inhabiting an oligotrophic reservoir with low habitat complexity, few rocks and few other preferred consolidated substrata. We described the stages of cells of reproductive lineage, gonadal development and some reproductive traits which were compared with information in the available literature. Cells from the spermatogenic lineage were spermatogonia, spermatocytes, spermatids and spermatozoa, and cells from the oocytarian lineage were primary oocytes, previtelogenic oocytes, cortical vesicle oocytes and yolk globules or vitellogenic. Five stages were described for the males/females according to the distribution of oocytes and spermatogenic lineage cells: resting; initial maturation; advanced maturation; partially spent/spawned; totally spent/spawn. Females outnumbered males and reached larger size. Synchronic ovary development in two groups was found with diameter of mature oocytes ranging from 2 to 3.35 mm. Indication of early maturation, a longer reproductive period and the production of smaller eggs in small clutches seems to be features of the reservoir population not found in riverine systems. Such changes in tactics may indicate a shift to an opportunistic strategy, helping the population to withstand environmental constraints and to succeed in this oligotrophic and poorly structured reservoir.