Patternizing of impoundment impact (1985–2002) on fish assemblages in a lowland river using the Kohonen algorithm

Impoundment impact on fish assemblage structure was investigated in the dammed middle course of the Warta River. A backwater site (AB) was located 2 km upstream of the Jeziorsko Reservoir, and a tailwater site (CD) 1.5 km downstream of the dam. Both sites were studied for 3 years in the pre‐impoundment period (1985–1987) and 15 years after damming (1988–2002). Quantitative electrofishing in spring and autumn assured obtaining yearly average biomass for each population. Most of the data analysis aimed to assess the dam impact on the fish assemblage structure but other accompanying impacts such as discharge manipulations, revetment, different forms of engineering, and water quality improvement in the tailwater and backwater reaches were also discussed. The Kohonen algorithm (self‐organizing map, SOM) was used for the analysis, and perfectly separated AB and CD samples into two clusters. Samples from the backwater (AB) proved that this reach of the Warta River had maintained its almost natural character and that fish assemblages had changed moderately, now occupying only five neighbouring hexagons out of a total of 16. In the tailwater (CD), however, because of considerable fluctuations in fish assemblages the SOM produced three subclusters, which engaged nine hexagons: (i) the pre‐impoundment period (1985–1987, two hexagons); (ii) 7 years after the definite closure of dam sluices (1988–1994, five hexagons); and (iii) the past 8 years of sampling (1995–2002, two hexagons), when stabilization in the assemblage was observed. The SOM also definitely proved profound changes in fish assemblage composition: most lithophilous species declined and many phytolithophilous and phytophilous species became dominants, particularly in the tailwater site where downstream migration of 0+ of successfully spawned species from the reservoir took place.     

Longitudinal and Vertical Spatial Gradients in the Distribution of Fish within a Canyon‐shaped Reservoir

The large‐scale spatial distribution of fish was investigated within a morphometrically simple canyon‐shaped reservoir with a single major tributary and a longitudinal trophic gradient (Římov Reservoir, Czech Republic). Samples of fish were taken by Nordic survey gill nets (several mesh sizes from 8 to 70 mm knot to knot) installed as surface nets at several offshore areas located along the longitudinal axis of the reservoir. Surveys were carried out in late summer during 1999–2003. An obvious distribution gradient of fish was revealed along the longitudinal axis of the Římov Reservoir. The total relative fish abundance and biomass (catch per unit effort) decreased considerably from the upstream end of the reservoir toward the dam. Roach (Rutilus rutilus), bleak (Alburnus alburnus) and bream (Abramis brama) comprised the bulk of catches at all areas. Enhanced dominance of bream was observed in the fish assemblage at the uppermost, more eutrophic area of the reservoir. The highest number of fish species and the highest abundance of young‐of‐the‐year fish were also observed in the tributary area. In the downstream part of the reservoir, gill net surveys along the vertical depth profiles indicated that offshore fish occupied mostly the epilimnion. Extreme flood events affected the Římov Reservoir, however, it seemed they had no significant impact on the gradients described. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A Bioenergetic Analysis of Factors Limiting Brown Trout Growth in an Ozark Tailwater River

We examined prey utilization and energy consumption by brown trout, Salmo trutta, in a cold tailwater (Little Red River, Arkansas, USA; LRR) having low biodiversity and low availability of fish as prey. Stomach content analysis and age estimation were performed on thirty brown trout (10 each of three size classes for a total of 710 trout) collected monthly from an upstream and downstream site over a 1-year period. Diet diversity was low at both sites, as 80% and 70% of all prey consumed by upstream and downstream brown trout, respectively, were isopods. Piscivory (<0.5% of individuals sampled) and consumption of terrestrial invertebrates were rare. There was no relation between diet diversity and trout age, and a very small ontogenetic shift in brown trout diet. Second, we investigated brown trout growth rates relative to prey consumption and temperature. Temperatures and availability of prey were less than required for maximal trout growth. However, prey availability limited trout growth directly, but sub-optimal temperatures probably buffered the effect of this reduced energy consumption by reducing metabolic energy expenditures. Brown trout growth was 54.8–57.0% of the maximum predicted by a bioenergetics model. Instantaneous growth rates for age 1 and adult brown trout were slightly higher for those downstream (0.195) versus those upstream (0.152). Although isopods are abundant within this tailwater to serve as a forage base, the displacement of native fish fauna and subsequent lack of establishment of cold-tolerant forage fish species due to the thermal regime of hypolimnetic release from Greers Ferry Reservoir probably serves as a major barrier to brown trout growth.     

A long-term study (1985–1995) of fish populations in the impounded Warta River,Poland

Three sites, two in the tailwater and one upstream of the backwater of a large man-made reservoir in the middle course of the 808 km long lowland Warta River (Oder catchment), were studied. The pre-impoundment study lasted one (1985), the post-impoundment one 10 yr (1986–1995). The dam contributed to the extinction of the anadromous Vimba vimba, and of the rheophilous Chondrostoma nasus, both cyprinids. A number of directly or indirectly negative changes related to the construction of the reservoir were recorded in the tailwater. These changes affected the species number, spawning groups, diversity, density, standing crop, mean body weight, ABC index, and proportional stock density – PSD. Numerous impacts related to engineering, water chemistry and climate were more negative for fish at the tailwater than at the backwater sites. The hydrology of the site upstream of the backwater continued to be dependent on natural factors. Fish populations there suffered only from accidental impacts of the construction of the dam, such as bank revetment or the clearance of trees and shrubs. This revised version was published online in July 2006 with corrections to the Cover Date.     

Patterning of impoundment impact on chironomid assemblages and their environment with use of the self-organizing map (SOM)

The paper assesses the impact of the Jeziorsko dam reservoir on chironomid assemblages and selected environmental factors in the Warta River, Poland, by means of patterns recognized with the self-organizing map (SOM, Kohonen unsupervised artificial neural network). Over 1988–1996, in four annual cycles, a total of 233 monthly samples were collected in a seven order section of the river at two sites: WAA (backwater) located about 2 km upstream from the Jeziorsko Reservoir, and WAB (tailwater) located about 1.5 km downstream from the reservoir's dam. At each site three habitats were selected: H₁, H₂ and H₃ at WAA, and H₁₁, H₁₂ and H₁₃ at WAB. H₁ and H₁₁ were located in the depositional area close to the banks, H₂ and H₁₂ about 6–7 m towards the mid-river and H₃ and H₁₃ in the mid-river. SOM effectively vertically separated H₁ and H₁₁ (bank habitats) from H₃ and H₁₃ (the mid-river zone of both sites) and H₂ (the transition zone of the upstream site). The H₁₂ samples were scattered all over SOM but still exhibited a slight temporal gradient. At the end of the study the water discharge, especially in summers, stabilized at WAB at a level lower than natural and as a result submerged macrophytes appeared at H₁₂ making the abundance of macroinvertebrates increase very quickly. Moreover, a weaker horizontal grouping of samples by season and by site of collection (upstream or downstream from the reservoir) was observed over SOM: 1) bank upstream habitat H₁, with hydrological regime resembling natural, was separated from the downstream H₁₁, which enlarged and contracted in response to dam operation, 2) deeper habitats were less dependent on water level and this is why they underwent seasonal fluctuations. To sum up, the deepest habitats were most resistant to water level fluctuations, while the formerly most productive habitat at the tailwater WAB site, H₁₁, became the most negatively impacted. Nevertheless, the reservoir has not negatively influenced chironomid density, because the latter increased closer the mid-river, at H₁₂, where large patches of macrophytes developed.     

Patterning of impoundment impact on chironomid assemblages and their environment with use of the self-organizing map (SOM)

The paper assesses the impact of the Jeziorsko dam reservoir on chironomid assemblages and selected environmental factors in the Warta River, Poland, by means of patterns recognized with the self-organizing map (SOM, Kohonen unsupervised artificial neural network). Over 1988–1996, in four annual cycles, a total of 233 monthly samples were collected in a seven order section of the river at two sites: WAA (backwater) located about 2 km upstream from the Jeziorsko Reservoir, and WAB (tailwater) located about 1.5 km downstream from the reservoir's dam. At each site three habitats were selected: H₁, H₂ and H₃ at WAA, and H₁₁, H₁₂ and H₁₃ at WAB. H₁ and H₁₁ were located in the depositional area close to the banks, H₂ and H₁₂ about 6–7 m towards the mid-river and H₃ and H₁₃ in the mid-river. SOM effectively vertically separated H₁ and H₁₁ (bank habitats) from H₃ and H₁₃ (the mid-river zone of both sites) and H₂ (the transition zone of the upstream site). The H₁₂ samples were scattered all over SOM but still exhibited a slight temporal gradient. At the end of the study the water discharge, especially in summers, stabilized at WAB at a level lower than natural and as a result submerged macrophytes appeared at H₁₂ making the abundance of macroinvertebrates increase very quickly. Moreover, a weaker horizontal grouping of samples by season and by site of collection (upstream or downstream from the reservoir) was observed over SOM: 1) bank upstream habitat H₁, with hydrological regime resembling natural, was separated from the downstream H₁₁, which enlarged and contracted in response to dam operation, 2) deeper habitats were less dependent on water level and this is why they underwent seasonal fluctuations. To sum up, the deepest habitats were most resistant to water level fluctuations, while the formerly most productive habitat at the tailwater WAB site, H₁₁, became the most negatively impacted. Nevertheless, the reservoir has not negatively influenced chironomid density, because the latter increased closer the mid-river, at H₁₂, where large patches of macrophytes developed.     

Distribution and Diet of 0+ Fish within a Canyon‐Shaped European Reservoir in Late Summer

The distribution and diet of age 0+ fish were studied in the deep canyon‐shaped Římov Reservoir (Czech Republic), which is characterized by a longitudinal trophic gradient. During late summer of two years, 0+ fish were sampled from inshore and offshore habitats along the longitudinal reservoir axis. Offshore catches of 0+ fish from the surface layer were dominated by roach (Rutilus rutilus ), bream (Abramis brama ) and perch (Perca fluviatilis ), whereas in the deeper open water perch predominated. Inshore catches of 0+ fish were constituted mainly by perch and roach. The proportions of roach in the inshore catches were highest at the upper and most eutrophic part of the reservoir, whereas the proportions of perch in the inshore catches were higher at the downstream areas. Total catches of both inshore and offshore 0+ fish increased upstream in the reservoir. Offshore 0+ perch were of consistently smaller size than inshore 0+ perch. Inshore 0+ perch had significantly smaller size at the upstream reservoir part than at the downstream, more lacustrine regions. The diet of both inshore and offshore 0+ fish consisted predominantly of crustacean zooplankton. Perch diet was generally dominated both by cladocerans and copepods, whereas roach diet consisted chiefly of cladocerans. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Seasonal strengths of the abiotic and biotic drivers of a zooplankton community

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Upstream and downstream responses of fish assemblages to an eastern Amazonian hydroelectric dam

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Is Niagara Falls a barrier to gene flow in riverine fishes? A test using genome‐wide SNP data from seven native species

Since the early Holocene, fish population genetics in the Laurentian Great Lakes have been shaped by the dual influences of habitat structure and post‐glacial dispersal. Riverscape genetics theory predicts that longitudinal habitat corridors and unidirectional downstream water‐flow drive the downstream accumulation of genetic diversity, whereas post‐glacial dispersal theory predicts that fish genetic diversity should decrease with increasing distance from glacial refugia. This study examines populations of seven native fish species codistributed above and below the 58 m high Niagara Falls – a hypothesized barrier to gene flow in aquatic species. A better understanding of Niagara Falls’ role as a barrier to gene flow and dispersal is needed to identify drivers of Great Lakes genetic diversity and guide strategies to limit exotic species invasions. We used genome‐wide SNPs and coalescent models to test whether populations are: (a) genetically distinct, consistent with the Niagara Falls barrier hypothesis; (b) more genetically diverse upstream, consistent with post‐glacial expansion theory, or downstream, consistent with the riverscape habitat theory; and (c) have migrated either upstream or downstream past Niagara Falls. We found that genetic diversity is consistently greater below Niagara Falls and the falls are an effective barrier to migration, but two species have probably dispersed upstream past the falls after glacial retreat yet before opening of the Welland Canal. Models restricting migration to after opening of the Welland Canal were generally rejected. These results help explain how river habitat features affect aquatic species’ genetic diversity and highlight the need to better understand post‐glacial dispersal pathways.     

Migratory movements of pacu, Piaractus mesopotamicus, in the highly impounded Paraná River

A mark‐recapture study was conducted in 1997–2005 to investigate movements of stocked pacu, Piaractus mesopotamicus, in the Paraná River Basin of Brazil, Paraguay, and Argentina. Fish raised in cages within the Itaipu Reservoir and in ponds were tagged externally (n = 2976) and released in the Itaipu Reservoir (53.2%) and bays of its major tributaries (46.8%). In total, 367 fish (12.3%) were recaptured. In all, 91% of the pacu moved away from the release site; upstream movements were more extensive than downstream movements. Pacu traveled upstream a maximum of 422 km (average of 41.3 km) at a maximum rate of 26.4 km day⁻¹ (av. 0.8). Downstream movements were limited in terms of number of individuals and distance moved. Fish released during the wet season moved farther than those released during the dry season, and feeding rather than spawning might have been the compelling reason for movement. Although fish passed downstream through dams, none of the marked fish were detected to have moved upstream through the passage facilities. Pacu showed movement patterns not radically different from those of other neotropical migratory species, but their migratory movements may not be as extensive as those of other large migratory species in the basin.     

The relationship between marine fronts and fish diversity in the Patagonian Shelf Large Marine Ecosystem

Aim To evaluate the relationship between marine fronts and (1) fish diversity patterns, (2) fish biomass distribution, and (3) fish assemblage structure. Location Patagonian Shelf, Southwestern Atlantic Ocean. Methods Three main frontal systems and eight nearby zones (frontal vs. non‐frontal areas) were compared. An extensive fishery database, obtained during an a priori sampling scheme (spatial resolution 1 × 1) from 1978 to 1979, was employed. Analyses of 248 trawling stations were performed using a combination of diversity measures (species richness, Shannon diversity and evenness), biomass analyses and multivariate analyses [nonparametric analysis of similarity (ANOSIM), non‐metric multidimensional scaling ordination (NMDS) and similarity percentages (SIMPER)] to evaluate the effect of fronts on fish communities. Results In total, 46 fish taxa were identified. Demersal fish diversity increased at the tidal front of Península Valdés but decreased in the frontal zones of the Southern Shelf‐Break and Magellan frontal systems. The richness and biomass of pelagic fish were higher at the Península Valdés Tidal Front, while the biomass of demersal fish was higher at its non‐frontal counterpart. Fish diversity did not differ consistently along the Northern Shelf‐Break Front. However, a higher fish biomass was registered at the non‐frontal zone. Demersal and pelagic fish assemblages differed between frontal and non‐frontal zones. Main conclusions The Patagonian Shelf Large Marine Ecosystem has diverse frontal types that appear to have different effects on fish diversity and assemblage structure. Seasonal (tidal) or permanent fronts have different effects on the parameters analysed, which could be due to productivity–diversity relationships. The Península Valdés Tidal Front, a relatively short temporal scale system, demonstrates a stronger influence on fish diversity, biomass and assemblage structure than the other frontal areas studied. The seasonality of nutrient input owing to frontogenesis, which begins during the austral spring, may play an important role in determining local diversity. At permanent frontal features, diversity decreases when the nutrient availability is high. This is potentially because, when food resources increase, a few species become dominant.     

Distribution patterns of fish biomass by acoustic survey in three Tunisian man‐made lakes

In order to test the hypothesis that fish distribution is related to water depth, three exploited reservoirs were sampled at day and at night using a Simrad EK60 echosounder. Acoustic surveys were carried out between February and June 2008, which focused on the fish size, species density (fish per ha) and biomass (kg ha^?1) along vertical and longitudinal gradients. It was evident in the surface layer (0–3 m) during daytime that the larger fish sizes occupy waters near the dam or the middle of Lakhmess and Sejnane reservoirs. In the upper layer at nighttime, a gradient of fish size is shown proportional to the depth at Lakhmess Reservoir and inversely proportional at the man‐made Sejnane Lake. At Lakhmess and Laabid reservoirs, fish density was significantly higher in deep water (3926 ± 1409 and 624 ± 258 fish per ha) rather than in the upper layer (988 ± 2 and 8 ± 2 fish per ha in daytime, respectively), while at Sejnane Reservoir the number of fish per area was similar along the vertical gradient. The biomass was significantly higher in waters deeper than 3 m at Sejnane Reservoir whereas at Lakhmess it was higher in the surface layer. Fish biomass increased from upstream (0.16 ± 0.05 kg ha^?1) to middle (3.07 ± 2.96 kg ha^?1) and downstream (3.82 ± 2.30 kg ha^?1 at night) areas in the man‐made Laabid Lake while a similar longitudinal gradient occurred in the deepest Sejnane Reservoir. The vertical gradient in fish distribution is discussed. From the dam to the tributary of the entire water column, it was concluded that fish biomass distribution was governed by depth and was most abundant in areas with deep waters.     

Temporal changes in fish diversity in lotic and lentic environments along a reservoir cascade

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Disturbance accumulation hampers fish assemblage recovery long after the worst mining spill in the Iberian Peninsula

The influence of environmental variables on native and exotic fish species richness and diversity was analysed 8 years after one of the most environmentally harmful toxic spills worldwide. Environment‐diversity relationships were addressed on different scales, and values were also compared with those of six similar basins not affected by the spill, with the aim of determining whether this disturbance was still exerting an influence on the fish assemblage. Results showed higher native species richness in environments with low human influence, no reservoirs upstream, a large drainage area and coarse substrate reaches. For native fish, variables at both the catchment and site were equally relevant. Exotic fish were mainly favoured by site‐scale factors such as valley width downstream from the reservoir, where the alteration of the river channel and accumulated disturbances give them an advantage vs native species. Overall, 8 years after the accident, richness and diversity of the Guadiamar fish assemblage seemed more affected by anthropogenic impacts than by the long‐term influence of the toxic spill. This work highlights that the potentially synergic effects of anthropogenic factors must be taken into account when monitoring the long‐term effects of pollution events.     

Species-specific schooling behaviour of fish in the freshwater pelagic habitat: an observational study

Social living of animals is a broadly occurring phenomenon, although poorly studied in freshwater systems, fish schooling behaviour is an excellent example. The composition of fish schools, species-specific schooling tendencies and preferences of adult fish were studied in the pelagic habitat of the Římov Reservoir, Czech Republic. Video recordings captured over a total of 34 days (16 h per day) in the clear water period of three seasons were analysed. From four species identified as school-forming species – bream, bleak, roach and perch, 40% of the individuals observed formed schools of 3–36 individuals. Although conspecific schools prevailed, 20% of individuals formed heterospecific schools, except bleak that schooled strictly with conspecifics. Schools were composed of individuals of similar body size and life strategy. Heterospecific schools were significantly larger than conspecific schools and showed uneven proportion among species, that is, one species being more abundant when the school dimension increased. Probability of encounter in bleak was lowest and proved highest inclination for schooling. Gregarianism levels depended on species morphology and body size, with larger and morphologically advanced fish tending less to sociability. This indicates that the antipredator function of schooling behaviour is intensified with increasing vulnerability of the species.     

Littoral age 0+ fish distribution in relation to multi-scale spatial heterogeneity of a deep-valley reservoir

Littoral age 0+ fish were studied with respect to spatio-temporal heterogeneity in the deep-valley ?ímov Reservoir (Czech Republic) from June to October 2007 using point abundance sampling by electrofishing. The abundance and diversity of age 0+ fish in different types of littoral habitats were examined along the longitudinal gradient of the reservoir during daytime. The impact of some physical attributes of the studied littoral habitats, e.g. slope steepness and substrate size, along with the season was the main factors affecting the distribution of age 0+ fish. The level of structural complexity was not a strong determinant, because the overall diversity and structural complexity of the available littoral habitats were relatively too low to have greater impact on the age 0+ fish distribution. The physical factors markedly influenced the spatial segregation between the two most important taxa in the reservoir—percids and cyprinids. Perch was the only representative of age 0+ percids in the littoral zone, which occupied steep-sloped habitats early in the season. In contrast, gently sloped habitats were mainly occupied by cyprinids later in the season. Species diversity was reflected in the occurrence of age 0+ cyprinids, achieving a maximum in gently sloped habitats in October. The effect of the longitudinal gradients on age 0+ fish distribution and diversity was generally far less significant, but was rather decisive during the period of a pronounced longitudinal trophic gradient during the summer season.     

Fish migration between a temperate reservoir and its main tributary

Hydrobiologia - Upstream- and downstream-directed migration between the Římov Reservoir and its only tributary, the Malše River (Czech Republic) was studied using two giant traps,...     

Sequential fishways reconnect a coastal river reflecting restored migratory pathways for an entire fish community

River regulation infrastructure has been implicated in worldwide aquatic biodiversity loss. In‐stream barriers such as weirs prevent fish migration and the impact can be particularly severe for diadromous species. Fishways are frequently installed on in‐stream barriers to reconnect migratory pathways and rehabilitate diadromous fish populations. Sequential fishways may be necessary in rivers where multiple barriers prevent migration. We monitored a coastal fish community's response to fish passage restoration at 10 predominantly low‐level weirs in the freshwater reaches of the Nepean River in southeastern Australia. Few studies have simultaneously assessed fish community passage at successive fishways within a river system. Prior to the installation of the fishways, there was a gradient of reduced species diversity in an upstream direction including the absence of many diadromous species, despite the regular inundation frequency of most weirs. Following the installation of the fishways, species diversity was still greater in the downstream monitoring sites; however, there was evidence of a positive change in fish community structure in middle and upstream sites. Most notably, three diadromous species rapidly expanded their distribution upstream and one amphidromous species expanded its downstream distribution. This study demonstrates appropriately designed successive fishways can successfully reconnect river systems for an entire fish community, encompassing species with a broad range of swimming abilities and diverse life histories.     

To migrate, or not to migrate: partial diel horizontal migration of fish in a temperate freshwater reservoir

The diel horizontal migration (DHM) of fish between the inshore and offshore zones of the ?ímov Reservoir (Czech Republic, deep, stratified, meso-eutrophic) was investigated by a combination of horizontal and vertical hydroacoustic surveys at 3-h intervals over 48 h and day/night purse seining in August 2007. An overwhelming majority of fish were aggregated within the epilimnetic layer. Considering only the horizontal surveys, cyclic diel fish movements between inshore and offshore habitats were apparent, while the total fish biomass remained constant between day and night. A higher fish biomass was detected in the offshore zone during daytime by both hydroacoustics and purse seining. In contrast, a higher fish biomass was recorded at night in the inshore zone. Bream Abramis brama, roach Rutilus rutilus, and perch Perca fluviatilis dominated the daytime offshore fish assemblage whereas bleak Alburnus alburnus prevailed at night. Bream and roach decreased in abundance at night while perch completely disappeared from the offshore habitat. The diel differences in size distributions and direct catches suggested the population-wide horizontal offshore migration of bleak and inshore migration of all perch during dusk. On the other hand, partial inshore migration of bream and roach adults was observed during dusk (52 and 80%, respectively). The different proportions of offshore residents among species and size classes suggested that differences in size, and, therefore, predation vulnerability, contributed to the observed migration patterns.