Flow-related patterns in abundance and composition of the fish fauna of a degraded Australian lowland river

1. A 7‐year study was conducted in three hydrologically distinct sections within the highly regulated, lowland Campaspe River to investigate the influence of hydrology on temporal and spatial patterns in fish composition, abundance and recruitment. One section had 6 months, one section 2 months and one section no months of increased flow due to storage releases. The fish fauna of the less regulated, nearby Broken River served as a reference to which that of the Campaspe River was compared for the last 3 years of the study to allow insight into the relative effects of hydrology, barriers to movement and other environmental characteristics. The study included one high‐flow year, a moderate‐flow year and five low‐flow years. 2. A total of 16 fish species – 10 native and six alien – were caught in the Campaspe River, although of the native species, only three are considered to have self‐sustaining populations. The remaining species are either itinerants or a result of stocking. Alien species comprised approximately 64% of the total biomass of all fish caught. 3. Overall composition of the fish fauna did not differ significantly by year, but did by section of river. Species richness and the abundance of most of the dominant species also differed significantly by river section, but there was little inter‐annual variation in the abundance of any species, except for European perch and for common carp; the latter showing an increase in abundance following a high‐flow event during the spring of 2000 as a result of recruitment. 4. Overall faunal composition was not influenced by hydrology. However, multiple regression indicated that species richness, abundance of the dominant species and abundance of young‐of‐year (YOY) of golden perch, European perch and common carp all were influenced significantly by hydrological variables. The nature of the relationships was dependent on river section and hydrological season (‘winter’ or ‘spring/summer’). Of note was the result that the total abundance of fish and that of YOY common carp were significantly positively related to the number of spells above the threshold for movement upstream through the lower two weirs in the Campaspe River. Only one significant relationship between hydrological and fish‐related variables was found for the upper river section, whereas seven and five were found for the lower and middle sections respectively. 5. Comparisons with fish collected in the Broken River over 3 years suggest that the fauna of the Broken River is in a more natural state than that of the Campaspe River. Since the two rivers do not differ substantially in water quality, and since both contain significant weirs, which act as barriers to movement of fish, flow regulation is most likely to be the major reason for the poor state of the fauna in the Campaspe River.     

Changes in fish assemblages in catchments in north‐eastern Spain: biodiversity,conservation status and introduced species

1. North‐eastern Spain is a hot spot for the introduction of alien fish species, and its native fish fauna is one of the most endangered worldwide. We used an extensive data set from 2002 to 2003 and historical information from the area to characterize fish diversity and establish conservation priorities in river catchments. 2. Diversity indices were used to characterize fish diversity at the basin scale. An index of conservation status was applied for each species, which considers the occurrence, abundance and endemicity of each taxon. We used indirect ordination methods to test the relationship among basin features and to identify those variables most correlated with each other. To identify physical, biotic and environmental characteristics that seem to make a basin particularly susceptible to invasion, we performed a step‐wise multiple regression to examine the relationship between the number of native, translocated and introduced fish species (including the original native species richness of each basin), and landscape variables. 3. Over a period of approximately 50 years, the mean range size of native fish species has decreased by 60%. The greatest decline occurred in Gasterosteus gymnurus, Anguilla anguilla and Salaria fluviatilis, for which species over 75% of the original distribution area has been lost. The species with the highest conservation index were Gasterosteus gymnurus and Salaria fluviatilis. 4. Basin area and the catchment type explained 70% of variation in native species richness, whereas the number of dams and basin area accounted for more than 80% of variation in the number of introduced species. 5. The original native species richness and the number of introduced species at basin scale were not related, and thus there was no evidence of “biotic resistance” to invasion. The restoration of natural hydrologic processes and the development of specific management tools to protect native species, such as the prioritization of areas for fish conservation and the eradication of local populations of exotic species, are required to restore native fish fauna in these catchments.     

怒江流域鱼类多样性及空间分布格局

怒江是我国西南地区重要的国际河流, 也是全球生物多样性热点区域之一。受地理位置和地形条件限制, 目前尚缺乏怒江流域鱼类物种多样性及其分布格局的系统研究。本研究利用近5年的全流域实地采样数据, 结合文献资料, 系统整理了怒江流域鱼类物种组成信息; 利用聚类和排序方法分析了怒江流域鱼类空间分布格局。结果表明, 怒江流域共有土著鱼类85种, 隶属于5目12科47属; 外来鱼类18种, 隶属于3目8科16属, 另外实地调查发现新记录外来种2种。怒江流域鱼类多样性从上游至下游呈明显的递增趋势。根据土著鱼类组成的聚类和排序分析结果, 可以将怒江25个亚流域分为3个部分: (1)西藏自治区内的上游河段, 分布有鱼类15种, 组成以裂腹鱼类、高原鳅类为主, 具有明显青藏高原冷水性鱼类区系特征。(2)贡山至泸水的中游河段, 分布有鱼类36种, 组成兼有上游冷水性鱼类和下游喜温的鲃亚科、野鲮亚科鱼类。(3)泸水以下的下游河段, 分布有鱼类74种。下游河段干流广泛分布有喜温的鲃亚科、野鲮亚科和 亚科鱼类, 支流则分布有数量众多的南鳅属(Schistura)、纹胸鮡属(Glyptothorax)鱼类, 鱼类组成以适应流水、激流环境的热带亚热带鱼类为主。2017-2021年的多次调查仅发现土著鱼类43种(占历史记录的50.6%), 土著鱼类资源衰退明显。过度捕捞、支流小水电开发、外来鱼类入侵等是主要威胁因素。随着人类活动的急剧增多, 怒江流域鱼类将面临更大的威胁, 亟需从流域层面开展系统的鱼类资源管理和保护规划。     

Decline of Native Freshwater Fishes in a Mediterranean Watershed on the Iberian Peninsula: A Quantitative Assessment

We examined changes in the distribution of 9 native and 18 introduced freshwater fishes in the south-eastern Pyrenees watershed, Iberian Peninsula, using data from 1996, 1984–1988 and historical information. This region suffers many modifications to its freshwater ecosystems that are linked to human activity in the Mediterranean regions. Fish communities, stream physical habitat and environmental degradation were assessed at 168 sites from 11 basins in 1996. Seven native species (78%) showed decline from previous data, one of which became extirpated in the first half of the 20th century. On the other hand, introduced species are expanding. As a consequence, intact native communities are increasingly rare, declining from presence in 22% of river courses in 1984–1988 to 15% in 1996. The most typical community type is a mixture of native and introduced species occupying 30% of river courses. Stream degradation seems to be the main cause of this process because fish communities differed between degraded streams and streams suffering less impact. A principal component analysis showed that water pollution and modifications to the habitat were the two anthropogenic factors that accounted for most changes in the fish community integrity. Habitat alteration, primarily through construction of dams and water diversions, has fragmented habitats and isolated native fish communities in headwater streams. Current protection measures do not offer effective conservation of threatened species and communities. A global conservation and restoration programme from an ecosystem-based approach is essential to reverse the trend affecting native freshwater fishes in this Mediterranean region.     

Distribution pattern, threats and conservation of fish biodiversity in the East Tiaoxi, China

East Tiaoxi River is one of the largest inflowing rivers into Taihu Lake, and the fish fauna in the river is poorly understood. In the present study, an extensive survey of fish was conducted in October and November 2009, May and September 2010 and May 2011 covering a total of 55 sites along the whole river. A total of 84 freshwater fish species belonging to 8 orders, 18 families and 52 genera have been recorded. Among these are 35 species endemic to China, and 3 newly recorded exotic species. The fish composition varies greatly from headwaters to downstream. Based on cluster analysis with presence-absence data, the East Tiaoxi River is divided into four regions, specifically, the upper reach, middle-up reach, middle reach and lower reach. It is observed that species richness and the proportion of omnivorous species increased from upstream to downstream while the proportion of invertivorous species decreased consequently. Habitat alteration, overfishing, pollution and inland navigation adversely affect the fish diversity and ecosystem functioning in the East Tiaoxi River. To protect fish diversity more effectively in the area, the conservation of fish biodiversity in the North Tiaoxi River and Middle Tiaoxi River should be considered as a priority. Meanwhile, shallow zones or backwater areas should be created in the middle-lower reaches. Furthermore, river restoration, in terms of habitat creation, should be considered to protect the structure and diversity of fish communities, halt the progressive deterioration of freshwater ecosystems and sustain a valuable ecological resource for humans.     

Distribution of fishes in the Río Guayalejo-Río Tamesí system and relationships with environmental factors in northeastern Mexico

A priority issue in ecology and biogeography is understanding the patterns in species diversity and the causal factors of their distribution, which allows the generation of information for conservation strategies. The longitudinal distribution of fishes and their relationships with environmental variables were studied in the Guayalejo-Río Tamesí system (northeastern Mexico) from February 2000 to July 2001. A total of 5918 fish were caught in 27 collections along an altitudinal gradient in the main river course, from high mountain (1500 masl) to coastal plain near Tampico. Forty-three native and five exotic species, belonging to 35 genera in 23 families, were identified. Cluster analyses identified four major fish habitats in the river system. A distinctive euryhaline marine fish habitat (1) occurs near the mouth with native and two exotic species. Two other habitats consist essentially of freshwater fish species that are distributed along the longitudinal gradient. One of these habitats (habitat 4) shows greater diversity, as per the Shannon index value, and also includes amphidromous fish, in addition to two exotic freshwater fish; the other (habitat 2) includes freshwater, euryhaline and three exotic species. The changes in the frequency of occurrence and the abundance of Gambusia vittata, Astyanax mexicanus, and Xiphophorus variatus contribute to explaining differences between these habitats. Another habitat (3) is represented by two sampling sites located near the mouth and consist of freshwater and euryhaline fish and three exotic cyprinids with broad salinity tolerance. The low abundance and richness of exotic species suggest little impact on native fish fauna in this river. The fish assemblage of the Guayalejo-Tamesí river system species changes along a longitudinal gradient with the addition, replacement and presence of indicator species. Upstream fish fauna is mostly composed of freshwater species, some of them generalists that inhabit the entire longitudinal gradient, others that are restricted to certain sites, and the remainder of species is an assemblage composed of a mixture of euryhaline freshwater and marine species near the mouth.     

Are alien fish a reliable indicator of river health?

1. The ability of many introduced fish species to thrive in degraded aquatic habitats and their potential to impact on aquatic ecosystem structure and function suggest that introduced fish may represent both a symptom and a cause of decline in river health and the integrity of native aquatic communities. 2. The varying sensitivities of many commonly introduced fish species to degraded stream conditions, the mechanism and reason for their introduction and the differential susceptibility of local stream habitats to invasion because of the environmental and biological characteristics of the receiving water body, are all confounding factors that may obscure the interpretation of patterns of introduced fish species distribution and abundance and therefore their reliability as indicators of river health. 3. In the present study, we address the question of whether alien fish (i.e. those species introduced from other countries) are a reliable indicator of the health of streams and rivers in south‐eastern Queensland, Australia. We examine the relationships of alien fish species distributions and indices of abundance and biomass with the natural environmental features, the biotic characteristics of the local native fish assemblages and indicators of anthropogenic disturbance at a large number of sites subject to varying sources and intensities of human impact. 4. Alien fish species were found to be widespread and often abundant in south‐eastern Queensland rivers and streams, and the five species collected were considered to be relatively tolerant to river degradation, making them good candidate indicators of river health. Variation in alien species indices was unrelated to the size of the study sites, the sampling effort expended or natural environmental gradients. The biological resistance of the native fish fauna was not concluded to be an important factor mediating invasion success by alien species. Variation in alien fish indices was, however, strongly related to indicators of disturbance intensity describing local in‐stream habitat and riparian degradation, water quality and surrounding land use, particularly the amount of urban development in the catchment. 5. Potential confounding factors that may influence the likelihood of introduction and successful establishment of an alien species and the implications of these factors for river bioassessment are discussed. We conclude that the potentially strong impact that many alien fish species can have on the biological integrity of natural aquatic ecosystems, together with their potential to be used as an initial basis to find out other forms of human disturbance impacts, suggest that some alien species (particularly species from the family Poeciliidae) can represent a reliable ‘first cut’ indicator of river health.     

Null model of biotic homogenization: a test with the European freshwater fish fauna

In recent years, there has been growing concern about how species invasions and extinctions could change the distinctiveness of formerly disparate fauna and flora, a process called biotic homogenization. In the present study, a null model of biotic of homogenization was developed and applied to the European freshwater fish fauna. We found that non-native fish species led to the greatest homogenization in south-western Europe and greatest differentiation in north-eastern Europe. Comparing these observed patterns to those expected by our null model empirically demonstrated that biotic homogenization is a non-random ecological pattern, providing evidence for previous assumptions. The place of origin of non-native species was also considered by distinguishing between exotic (originating from outside Europe) and translocated species (originating from within Europe). We showed that exotic and translocated species generated distinct geographical patterns of biotic homogenization across Europe because of their contrasting effects on the changes in community similarity among river basins. Translocated species promoted homogenization among basins, whereas exotic species tended to decrease their compositional similarity. Quantifying the individual effect of exotic and translocated species is therefore an absolute prerequisite to accurately assess the spatial dynamics of biotic homogenization.     

Assessing fish metrics and biotic indices in a Mediterranean stream: effects of uncertain native status of fish

Implementation of the Water Framework Directive requires tools for measuring and monitoring the ecological status of aquatic ecosystems. Several indices are in use in the Iberian Peninsula, although there has been little comparison among them. We sampled the fish assemblage and limnological features of the Tordera stream (NE Spain) quarterly from September 2001 to May 2003 to evaluate the usefulness of several fish metrics and to compare habitat quality and biotic indices currently in use. Data for eight biotic and abiotic indices for this and three other Catalan river basins were also compiled in order to analyse the relationships among indices. In the Tordera stream, fish abundance and richness increased with stream order except in the last sampling site that had the lowest fish abundance owing to the effects of drought and water abstraction. Although most indices were positively correlated, some displayed low or null correlations particularly for the Tordera basin which is more affected by water abstraction and less by pollution; a commonly used physico-chemical index (ISQA) was the least correlated. In a regional fish index (IBICAT) under development, the brown trout (Salmo trutta) has been previously considered as introduced in the Tordera basin. Here, we report an old published record that demonstrates that trout was present before 1845 and we argue that its status should be considered as uncertain given the current information available. Whether brown trout is treated as native or introduced to this river basin has profound effects on the results of fish metrics because of its dominance in the upper reaches. We briefly discuss the role of introduced species, particularly in headwater streams, in the development of fish indices. Our study exemplifies the need for careful, basin-specific assessment of native/introduced status in the development of fish metrics. Handling editor: K. Martens     

塔里木盆地鱼类入侵及区系演变趋势

鱼类入侵是塔里木盆地土著鱼类濒危的关键性影响因素之一,结合历史文献数据及近年调查数据,系统综述了塔里木盆地鱼类入侵现状,并就鱼类入侵引起的鱼类区系均一化问题进行了分析。结果显示,该地区记录鱼类63种,其中土著种19种,入侵种44种。尽管入侵鱼类提升了塔里木盆地鱼类群落属级及科级的多样性水平,但相应的G-F多样性指数却明显下降,表明该区鱼类群落从物种组成简单、分化明显的特点演变为物种成分复杂、分化贫乏的特点。入侵种中源于长江和额尔齐斯河分别有30种(68.2%)和7种(15.9%),源于美欧和亚洲其他区域的有7种(15.9%)。入侵鱼类的进入引起塔里木盆地与其相关区域鱼类区系相似性显著上升(P0.01),并导致该区鱼类区系从单一的中亚高原山区复合体演变为多区系混合体。进一步利用鱼类个体生态矩阵分析方法研究显示,外来种在塔里木盆地适应性广泛,能够占据水域各种栖息生境,显著压缩了土著种的栖息空间,进而通过捕食、食物竞争及其他作用途径严重危及土著鱼类种群的续存,提示须采取有力措施限制入侵种的进一步扩散及对濒危土著物种迅速开展人工保育工作。     

Long-term Developments in Ecological Rehabilitation of the Main Distributaries in the Rhine Delta: Fish and Macroinvertebrates

Significant anthropogenic pressure in the Dutch part of the River Rhine is present from the 12th century. River engineering and water pollution were the main stress factors for flora and fauna. From the middle of the 20th century measures were taken to reduce water pollution. Recently, from 1987 onwards, these activities were put into a wider context of ecological river rehabilitation. Effects of improvements on fish and macroinvertebrates in the main distributaries in the Rhine delta are reviewed. The conclusions are that (a) most of the alterations in the Rhine delta are irreversible due to hard socio-economic boundary conditions (e.g. safety, navigation); (b) chances for the development of riverine biotopes have therefore to be found in the forelands and not in the main channels of the Rhine delta; (c) further reduction of pollutants, especially thermal pollution, is needed to help original species to colonise the Rhine delta again; (d) non-indigenous species clearly leave a mark on recolonisation possibilities of original species.     

Effect of non-native species on taxonomic and functional diversity of fish communities in different river types

Recent researches suggest that functional diversity represents the response of communities to environmental alterations better than taxonomic diversity. However, there is scarce information about how the functional diversity of freshwater fishes is affected by habitat type and the dominance of non-native species. To address this question, we analysed a large database containing 15 morpho-functional traits of 61 fish species from the Pannon Biogeographic region (Hungary). Based on a fish faunistic list and relative abundance of taxa, we quantified the taxonomic and functional diversity of riverine communities for?>?700 sites of six habitat types. We asked how non-native fishes affected the taxonomic and functional diversity in different river types and at the local scale (i.e. at the site level), and how the diversity measures of native fauna elements changes along the invasion gradient. Our results showed that both functional and taxonomic richness increases with habitat complexity, from small headwater streams to large rivers. Therefore taxonomic diversity served as a good proxy for functional diversity along the environmental gradient of river types. Non-natives showed considerable functional diversity relative to their species number in each habitat type. Diversity values of native fauna elements initially increased, and then showed a major decrease along the invasion gradient. River type-specific evaluations highlighted the importance of considering the proliferation of invasive species based on both taxonomic and functional diversity indices. We argue that type-specific action plans are needed in conservation management to preserve the taxonomic and functional diversity of native fishes in Hungary, but also elsewhere.

     

Changes in sediment seed‐bank composition of invaded macrophyte communities in a thermal river

相似文献   

Fish community structure and environmental correlates in the highly altered southern Sacramento-San Joaquin Delta

We sampled 11 sites in the southern Sacramento-San Joaquin Delta from 1992–1999, to characterize fish communities and their associations with environmental variables. Riparian habitats were dominated by rock-reinforced levees, and large water diversion facilities greatly influenced local hydrodynamics and water quality. We captured 33 different taxa, only eight of which were native. None of the native species represented more than 0.5% of the total number of individuals collected. The abundance of native species was consistently low but typically peaked during high outflow periods. Fish communities were predominantly structured along environmental gradients of water temperature and river flow. Native species (tule perch, Hysterocarpus traski, & Sacramento sucker, Catostomus occidentalis) were associated with conditions of high river flow and turbidity, while the majority of the non-native species were associated with either warm water temperature or low river flow conditions. The exceptions were the non-native striped bass, Morone saxatilis, and white catfish, Ameiurus catus, which were positively associated with relatively high river flow. Variation in fish community structure was greater among river locations within years than within river locations among years, thus fish communities at each river location were consistently different each year. Differences in fish communities among river locations were correlated with river flow and turbidity. We predict that the fish communities of this region will remain numerically dominated by non-native species if the environmental conditions we observed persist in the future.     

Presence and absence of non‐native fish species in the Wet Tropics region,Australia

Distributional records of non‐native fish species were identified in the Wet Tropics region, Far North Queensland, Australia, through a compilation of published records and expert knowledge. A total of 1106 records were identified comprising 346 presence and four uncertain records for at least 13 species, and 756 absence records. All current presence records consist of six species from the families Cichlidae and Poeciliidae with established self‐sustaining populations in the region, probably affecting the highly diverse native fish fauna.     

The benthic biological conditions of Los Angeles-Long Beach Harbors: Results of 28 years of investigations and monitoring

Los Angeles-Long Beach Harbors were a grossly polluted body of water at the time of the initiation of benthic biological studies in 1951. Industrial, domestic and storm wastes were emptied into these waters with little or no treatment. The inner harbor waters contained little or no dissolved oxygen and much of the benthos was azoic or possessed a stressed community. However, the outer harbor water mass contained adequate dissolved oxygen, and the benthos supported a rich fauna dominated by many species of polychaetes. A pollution abatement program was initiated in 1968, and today many former wastes have been eliminated or are being diverted to treatment plants for processing. The benthic fauna changed markedly and rapidly following this environmental clean-up. Peaks in population were reached throughout the harbor in 1973–1974 which was followed by a slight decline then stability. An oil tanker explosion, pre-treatment of fish-cannery wastes then diversion of these wastes to a sewage treatment plant, and a change from primary to secondary treatment of sewage brought about changes in the benthic fauna. In the latter instance, benthic population of polychaetes, fish populations and the number of birds feeding within the area decreased significantly.     

不同类型人类活动干扰对河岸带外来植物群落的影响——以北京永定河为例

强烈的人类活动对自然生境的扰动促进了外来植物侵入河岸带,但不同类型的人类活动干扰对外来植物的影响有所不同。按照不同人类活动干扰类型设置了山峡段、平原段和城市段,通过比较河段间河岸带外来植物群落特征的差异,探讨不同人类活动干扰类型对河岸带外来植物群落的影响。结果表明:(1)永定河河岸带共有维管束植物27科72属101种,外来植物13科28属29种。外来种比例高达28.7%,其中82.4%的国外外来种来自美洲和亚洲。(2)不同河段间的外来植物群落的物种构成和优势度呈现出显著性差异。外来种比例由山峡段的20.9%,上升至平原段和城市段的30.2%和25.5%;而优势度由12.1%,分别上升至13.4%和17.5%。(3)不同河段间的外来植物群落生活型结构有显著性差异。多年生草本植物在山峡段比例最高,达到66.7%,在平原段和城市段较低,分别为46.2%和30.8%;而一年生草本植物在城市段比例最高,达到69.2%,在平原段和山峡段较低,分别为53.8%和33.3%。(4)平原段属于农业干扰类型,其河岸带外来植物主要以农业类杂草为主,如禾本科的假稻(Leersia japonica)和菊科的钻叶紫菀(Aster subulatu);城市段是城市干扰类型,外来植物中入侵种比例和优势度较高,典型入侵种为反枝苋(Amaranthus retroflexus)和牛筋草(Eleusine indica)等。北京永定河不同河段河岸带外来植物的种类构成特点,反映了河岸带外来植物受快速城市化、农业活动等不同人类活动干扰的影响呈逐渐扩大的趋势。     

Alien Fishes in Natural Streams: Fish Distribution, Assemblage Structure, and Conservation in the Cosumnes River, California, U.S.A.

The Cosumnes River is the largest stream without a major dam on its mainstem in the Sacramento–San Joaquin drainage, central California, U.S.A. We studied its fishes over a 3-year period to answer the following questions: (1) Was the native fish fauna still present? (2) Why were alien fishes so abundant in a river system with a 'natural' flow regime, which elsewhere has been shown to favor native fishes? (3) Were there assemblages of fishes that reflected environmental differences created by the underlying geology? (4) Were there features of the watershed that consistently favored native fishes or that could be managed to favor native fishes? Of the 25 species collected, 17 were alien species; 14 species (five native) were abundant or widely distributed enough to use in detailed analyses. Of the native species, only rainbow trout, Oncorhynchus mykiss, still occupied much of its native range in headwater streams. Other native species have been extirpated or persisted mainly above barriers to alien invasions. The most widely distributed alien species was redeye bass, Micropterus coosae, previously unknown from the river, whose abundance was associated with low-numbers of native species. Other aliens were found primarily in low-land habitats on the valley floor or foothills. Canonical Correspondence Analysis indicated that both native and alien species located on environmental gradients determined largely by elevation, temperature, flow, and emergent vegetation, but the associations with these variables were not strong. While most alien fishes were found in lowland sections of river flowing through agricultural regions, the general relationships between species abundance and landscape-level variables were weak. Assemblages of fishes were poorly defined mixtures of native and alien species. The strikingly distinct geological regions of the basin no longer supported distinct fish assemblages. Species distributions were highly individualistic, reflecting dynamic patterns of introductions, invasions, and local extinctions, as well as physiological tolerances and life history patterns. Most native fishes are likely to persist in the Cosumnes River only if summer flows are increased and if populations above natural barriers are protected from further invasions by alien species, especially redeye bass. General conclusions from this study include: (1) altered habitats can support native species under some circumstances; (2) new fish assemblages with characteristics of 'natural' communities are likely to develop in invaded systems; (3) restoring flow regimes to favor native fishes may require restoring minimum summer flows as well as high channel-forming flows. However, reversing or even reducing, the impact of the predatory redeye bass, pre-adapted for California streams, is probably not possible.     

Influences of dams with different levels of river connectivity on the fish community structure along a tropical river in Southeastern Brazil

The influence of three types of dams with different degrees of river connectivity on the structure of fish communities along the Paraíba do Sul River was studied: total blockage (Dam 1); partial blockage allowed by a permanent lateral channel (Dam 2); and total blockage, but with a mechanism operating in the summer for fish passage (Dam 3). The tested hypothesis is that the degree of river connectivity influences the fish community. The largest differences in fish fauna were expected between the reservoir and downriver stretch in Dam 1 with total blockage; an intermediate difference in Dam 3 with total blockage but with a fish passage; and the least difference in Dam 2 with partial blockage. Fish were caught by gill nets between January and March 2010 and between January and February 2011 (wet seasons) in two zones: the reservoir and the downriver stretch from the dam. A total of 43 fish species was recorded, including eight non‐native and two marine species. The 13 most abundant species (n > 100; frequency of occurrence >20%) occurred in all three stretches of the river. The community structure changed significantly between the reservoir and associated downriver stretch, with higher richness downriver compared to the reservoir zone. A trend for higher occurrence of migratory fishes (e.g. Pimelodus maculatus, Pimelodus fur, Leporinus copelandii and Prochilodus lineatus) was found in the downriver zone, suggesting the influence of dams on their upriver migration. The predictions were not fully matched. Although the most significant difference in the fish community structure between the reservoir and the downriver was found for Dam 1, the partial blockage in Dam 2 showed broader differences in the fish fauna than the total blockage in Dam 3 with the fish ladder, which may indicate that the latter is not a guarantee that species and genetic flux will be interchanged, since the water velocity may be a constraint to upriver fish migration.     

Contrasting patterns and mechanisms of spatial turnover for native and exotic freshwater fish in Europe

Aim We compare the distribution patterns of native and exotic freshwater fish in Europe, and test whether the same mechanisms (environmental filtering and/or dispersal limitation) govern patterns of decrease in similarity of native and exotic species composition over geographical distance (spatial species turnover). Locations Major river basins of Europe. Methods Data related to geography, habitat diversity, regional climate and species composition of native and exotic freshwater fish were collated for 26 major European river basins. We explored the degree of nestedness in native and exotic species composition, and quantified compositional similarity between river basins according to the beta‐sim (independent of richness gradient) and Jaccard (dependent of richness gradient) indices of similarity. Multiple regression on distance matrices and variation‐partitioning approaches were used to quantify the relative roles of environmental filtering and dispersal limitation in shaping patterns of decreasing compositional similarity over geographical distance. Results Native and exotic species exhibited significant nested patterns of species composition, indicating that differences in fish species composition between river basins are primarily the result of species loss, rather than species replacement. Both native and exotic compositional similarity decreased significantly with increasing geographical distance between river basins. However, gradual changes in species composition with geographical distance were found only for exotic species. In addition, exotic species displayed a higher rate of similarity decay (higher species turnover rate) with geographical distance, compared with native species. Lastly, the majority of explained variation in exotic compositional similarity was uniquely related to geography, whereas native compositional similarity was either uniquely explained by geography or jointly explained by environment and geography. Main conclusions Our study suggests that large‐scale patterns of spatial turnover for exotic freshwater fish in Europe are generated by human‐mediated dispersal limitation, whereas patterns of spatial turnover for native fish result from both dispersal limitation relative to historical events (isolation by mountain ranges, glacial history) and environmental filtering.