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1. Determining the comparative impacts of increased intra- versus inter-specific competition is important in freshwater ecosystems for understanding the ecological changes that can result from activities such as fish stocking events (using alien and/ or native fish species), as well as from natural processes that elevate population abundances (e.g increased annual recruitment success). While increased inter-specific competition can result in slower growth rates and/ or reduced population density in the weaker or less abundant competitor, it is important that this is assessed in relation to the impacts of increased intra-specific competition. 2. We tested how the strength of inter-specific competition from a co-existing species varies with abundance, and how this compares with increased intra-specific competition. Fish were the model taxa, as their growth rates strongly correlate with competitive success. Replicated pond mesocosms (150 days) used chub Squalius cephalus in an allopatric control (n=5; C5) and allopatric treatment (n=10; C10), and in sympatric treatments (n=5) with European barbel Barbus barbus (n=5 (T1), 10 (T2) and 15 (T3)). Treatment effects were tested on fish specific growth rates (SGR), and the size and position of the trophic and isotopic niche (stomach contents and stable isotope analyses (SIA) respectively). 3. Chub SGRs were significantly higher in C5 versus all other treatments, but did not differ among the other allopatric and sympatric treatments. Chub trophic niche sizes in T1 to T3 were significantly smaller than C5, indicating more specialised diets in the presence of barbel. Chub trophic niche size in C10 was, however, larger than C5 and T1, indicating a shift to a more generalised diet as intra-specific competition increased. 4. As SGRs reduced in treatments, so did the predicted extent of fish stable isotope turnover, with SI data in T1 to T3 not at isotopic equilibrium with their diet in the mesocosms at the experiment’s end. Following conversion of fish SI data to represent values at 95% isotopic turnover, chub isotopic niches also revealed shifts to a more general diet as intra-specific competition increased, but to more specialised diets as inter-specific competition increased. 5. Increased intra- and inter-specific competition impacts on the trophic and isotopic niches were contrasting; both metrics indicated niche constrictions in sympatry but niche expansions in allopatry. Impacts on fish growth were evident from both. These results emphasise that the trophic consequences of competition in freshwater fish can differ between stocking events involving the release of conspecifics or other species, with this having important considerations for how freshwater fish communities are managed for angling exploitation.

Spawning strategies of lowland river fishes include single spawning, where reproduction generally occurs in early spring to provide 0+ fish with an extended growth season through the summer, but with a high risk of stochastic mortality events occurring, such as early summer floods. This risk can be reduced by multiple or protracted spawning strategies, where 0+ fish are produced over an extended period, often into mid-summer, but with the trade-off being a shorter growth season. The spawning strategies of cypriniform fish were explored in the River Teme, a spate river in Western England, which has non-indigenous European barbel Barbus barbus present. Sampling 0+ fish in spring and summer and across three spawning periods, B. barbus, chub Squalius cephalus and minnow Phoxinus phoxinus always revealed multiple spawning events, with 0+ fish of < 20 mm present in samples collected from June to August. Fish below 20 mm in August remained relatively small by the end of their growth season (October). For dace Leuciscus leuciscus, only single spawning events were evident, but with 0+ dace always being relatively large. Therefore, multiple spawning appears to be a common strategy that provides resilience in 0+ fish against stochastic mortality events in lowland rivers.

Ecological studies on the critically endangered European eel Anguilla anguilla often incorporate stable isotope analysis that typically uses dorsal muscle sampled from euthanised eels. To minimise the lethal sampling of imperilled populations, fin tissue and/or epidermal mucus can provide non-lethal alternatives to muscle. The results here indicate that δ13 C and δ15 N values of both eel fin and mucus are not significantly different to those of muscle and can be applied directly in comparative SI studies.

Concern is growing about ecosystem collapse, namely the abrupt decline or loss of an ecosystem resulting from human activities. While efforts to assess the risk of ecosystem collapse have developed at large spatial scales, less attention has been given to the local scales at which conservation management decisions are typically made. Development of appropriate management responses to ecosystem collapse has been limited by uncertainty regarding how collapse may best be identified, together with its underlying causes. Here we operationalise ecosystem collapse for conservation practice by providing a robust definition of collapse, in a form that is relevant to the scale of conservation decision-making. We provide an overview of different causes of collapse, and then explore the implications of this understanding for conservation practice, by examining potential management responses. This is achieved through development of a decision tree, which we illustrate through a series of case studies. We also explore the role of indicators for the early detection of collapse and for monitoring the effectiveness of management responses. Ecosystem collapse represents a significant challenge to conservation practice, as abrupt changes in ecosystem structure, function and composition can occur with little warning, leading to profound impacts on both biodiversity and human society. The risks of ecosystem collapse are likely to increase in future, as multiple forms of environmental change continue to intensify. We suggest that selection of management responses should be based on an understanding of the causal mechanisms responsible for collapse, which can be identified through appropriate monitoring and research activities.

1. Anadromous fishes can be important prey resources for piscivorous fauna in lowland rivers. Freshwater anglers exploiting large-bodied cypriniform fishes use high quantities of pelletized marine fishmeal baits that can contribute substantially to fish diets. This marine-derived energy pathway also potentially provides a marine prey resource for freshwater piscivores. However, large-bodied cypriniform fishes are often in a size refuge against predation due to their large sizes. 2. Stable isotope (δ15N and δ13C) analysis assessed how novel marine prey resources influenced the diet of a freshwater apex predator, Northern pike Esox lucius, in an impounded river basin (lower River Severn, Western England). Up to three groups of prey resources were present: anadromous European shad (Alosa spp.), cypriniform fishes with dietary specialisms based on marine fishmeal baits, and freshwater prey. The availability of these prey resources to E. lucius varied according to river connectivity and levels of angling exploitation in different river reaches. 3. Where the three prey groups were present, E. lucius were more enriched in δ13C values (range: -24.74 to -16.34 ‰) compared to river reaches where aspects of the marine prey groups were absent. (range: -28.30 to -21.47) In all reaches, δ13C increased as E. lucius length increased. In the reach where all prey groups were present, the isotopic niches of three E. lucius size classes were strongly partitioned; this was not apparent in reaches where the marine pathways were unavailable. 4. Stable isotope mixing models suggested that freshwater prey were the most important prey item, contributing between 42 and 96 % to the diet of individual E. lucius. However, where present, anadromous fishes and cypriniform fishes specialising on marine fishmeal baits were also important prey items, contributing substantially to the diet of larger E. lucius (length > 650 mm). The total dietary contributions of the marine resources varied considerably among the individual larger fish (22 to 58 % of total diet). 5. The presence of two marine resource pathways in a lowland river thus strongly influenced the diet of an apex predator, but with contributions being a function of their spatial availability, E. lucius body size and individual trophic specialisations. These results emphasise how the anthropogenic activities of river engineering and human subsidies can affect the trophic dynamics of apex predators.

1. Populations of the European shads Alosa alosa (Linnaeus, 1758) and Alosa fallax Lacépède, 1800 (Alosa spp.) are protected under legislation because of their vulnerability to human disturbances. In particular, river impoundments block their upstream migration, preventing access to spawning areas. Knowledge on the spatial extent of their spawning is important for informing conservation and river management plans. 2. Determining the spatial extent of Alosa spp. spawning is challenging. They enter rivers over a 2‐3‐month period and the species potentially migrate different distances upstream. Capture and handling can be problematic, spawning events generally occur at night, and kick‐sampling for eggs is limited to shallow water. Assessing their spatial extent of spawning could, however, incorporate non‐invasive sampling tools, such as environmental DNA (eDNA). 3. An eDNA assay for Alosa spp. was successfully developed, based on the cytochrome c oxidase subunit I gene segment and quantitative polymerase chain reaction (qPCR). Application in spring 2017 to the River Teme (River Severn catchment, western England) revealed high sensitivity in both laboratory and field trials. Field data indicated Alosa spp. spawning between May and June, with migrants mainly restricted to areas downstream of the final impoundment. 4. eDNA can thus be used as a non‐invasive sampling tool to determine the freshwater distribution of these fishes in Europe, enhancing their conservation at local and regional scales.

Genetic introgression with native species is recognized as a detrimental impact resulting from biological invasions involving taxonomically similar invaders. Whilst the underlying genetic mechanisms are increasingly understood, the ecological consequences of introgression are relatively less studied, despite their utility for increasing knowledge on how invasion impacts can manifest. Here, the ecological consequences of genetic introgression from an invasive congener were tested using the endemic barbel populations of central Italy, where the invader was the European barbel Barbus barbus. Four populations of native Barbus species (B. plebejus and B. tyberinus) were studied: two purebred and two completely introgressed with alien B. barbus. Across the four populations, differences in their biological traits (growth, body condition and population demographic structure) and trophic ecology (gut content analysis and stable isotope analysis) were tested. While all populations had similar body condition and were dominated by fish up to 2 years of age, the introgressed fish had substantially greater lengths at the same age, with maximum lengths 410–460 mm in hybrids versus 340–360 mm in native purebred barbel. The population characterized by the highest number of introgressed B. barbus alleles (81%) had the largest trophic niche and a substantially lower trophic position than the other populations through their exploitation of a wider range of resources (e.g. small fishes and plants). These results attest that the genetic introgression of an invasive congener with native species can results in substantial ecological consequences, including potential cascading effects.

Globally, inland waters emit over 2 Pg of carbon per year as carbon dioxide, of which the majority originates from streams and rivers. Despite the global significance of fluvial carbon dioxide emissions, little is known about their diel dynamics. Here we present a large-scale assessment of day- and night-time carbon dioxide fluxes at the water-air interface across 34 European streams. We directly measured fluxes four times between October 2016 and July 2017 using drifting chambers. Median fluxes are 1.4 and 2.1mmolm(-2) h(-1) at midday and midnight, respectively, with night fluxes exceeding those during the day by 39%. We attribute diel carbon dioxide flux variability mainly to changes in the water partial pressure of carbon dioxide. However, no consistent drivers could be identified across sites. Our findings highlight widespread day-night changes in fluvial carbon dioxide fluxes and suggest that the time of day greatly influences measured carbon dioxide fluxes across European streams. Diel patterns can greatly impact total stream carbon dioxide emissions, with 39% greater carbon dioxide flux during the night-time relative to the day-time, according to a study of 34 streams across Europe.

Background: Globally, streams emit significant amounts of methane, a highly potent greenhouse gas. However, little is known about the stream sediment microbial communities that control the net methane balance in these systems, and in particular about their distribution and composition at large spatial scales. This study investigated the diversity and abundance of methanogenic archaea and methane-oxidizing microorganisms across 16 European streams (from northern Spain to northern Sweden and from western Ireland to western Bulgaria) via 16S rRNA gene sequencing and qPCR. Furthermore, it examined environmental factors influencing both abundance and community composition and explored the link to measured potential methane production and oxidation rates of the respective sediments. Results: Our results demonstrated that the methanogenic and methanotrophic microbiomes of the studied European streams were linked to both the temperature and degree of anthropogenic alteration. The microbiomes could be separated into two to three groups according to environmental factors at both stream and catchment scales. Main methanogenic taxa found within more anthropogenically-altered, warm, and oxygen-poor environments were either Methanospirillum spp. or members of the families Methanosarcinaceae and Methanobacteriaceae . Within such environments, methane oxidizing communities were strongly characterized by members of the family Methylobacteriaceae ( Meganema spp. and Microvirga spp.). Contrastingly, communities in colder environments rich in oxygen and with relatively little anthropogenic impact at the catchment scale were characterized by the methanogenic Methanosaetaceae , Methanocellaceae and Methanoregulaceae and the methanotrophic Methyloglobulus spp ., members of the CABC2E06 group (all Methylococcaceae ) and by various Candidatus Methanoperedens. Overall, diversity of methanogenic archaea increased with increasing water temperature. Methane oxidizing communities showed higher diversities in southern sampling sites and in streams with larger stream areas and widths. Potential methane production rates significantly increased with increasing abundance of methanogenic archaea, while potential methane oxidation rates did not show significant correlations with abundances of methane oxidizing bacteria, presumably due to the more diverse physiological capabilities of this group. Conclusions: We present the first large scale overview of the large-scale microbial biogeography of two microbial groups driving the methane cycle dynamics within stream sediments and deduce the impact that future anthropogenic alterations may cause.

© 2019 The Authors. River Research and Applications published by Wiley Periodicals, Inc. Suitable gravel availability is critical for the spawning success of lithophilous fishes, including redd builders. Redd construction during spawning can alter substrate characteristics, thereby influencing hydraulic conditions and sediment transport, highlighting the importance of spawning as a zoogeomorphic activity. Here, interactions between redd-building fish and their spawning environment were investigated for European barbel Barbus barbus with a comparative approach across three English rivers: Teme (western), Great Ouse (eastern) and Idle (central). Sediment characteristics of spawning habitats were similar across the rivers, including subsurface fine sediment (

Following their invasion, non-indigenous fish can potentially exclude native fishes from their original niches through competition, or can partition in their resource use with native species to facilitate co-existence. Here, using stable isotope analysis, the trophic interactions of invasive European barbel Barbus barbus and other fishes were tested in an invaded river of relatively low fish species diversity and where no other Barbus species were present. Testing was over three distinct life stages: age 0 + ( 386 mm). There were strong patterns of isotopic niche partitioning between the juvenile fishes, with some inter-specific niche differences also apparent in 0 + fishes. For adult B. barbus and chub Squalius cephalus, however, niche convergence was evident. Within the B. barbus population, the niches of the adult fish differed significantly from the 0+ and juvenile fish, indicating considerable dietary changes with development. These results suggested that niche partitioning at the most abundant life stages were facilitating the co-existence of invasive B. barbus with other fishes in the community, with this most likely driven by inter-specific differences in functional morphology and habitat use.

© 2019, The Author(s). Riverine fish are often adversely affected by impoundments that reduce longitudinal connectivity and impede movements, while river fishery performance is often enhanced by introductions of non-indigenous fishes that diversify angling opportunities. The influence of factors (including impoundment) on the movements of European barbel Barbus barbus, a fish frequently introduced for angling, was tested in the continuous reach of the lower River Teme and Severn, western England. Following capture by electric fishing and angling, the movements of 18 fish were followed for 12 months using acoustic telemetry. Weirs at the upstream end of both river reaches impeded fish movements; only three fish traversed the Teme weir and no fish traversed the Severn weir. Net movements were mainly upstream in spring and downstream in autumn, and the daily movements of fish were asynchronous between individuals. Total (home) ranges varied considerably between individuals; those of fish initially captured by angling were significantly smaller than those captured by electric fishing. This potentially relates to fish with smaller total ranges being more vulnerable to angling via higher spatial encounters. The results reveal considerable individual variation in the movement of B. barbus and emphasises how river regulation can impact the dispersal and behaviour of river fishes.

International audience; Large-scale studies are needed to identify the drivers of total mercury (THg) and monomethyl-mercury (MeHg) concentrations in aquatic ecosystems. Studies attempting to link dissolved organic matter (DOM) to levels of THg or MeHg are few and geographically constrained. Additionally, stream and river systems have been understudied as compared to lakes. Hence, the aim of this study was to examine the influence of DOM concentration and composition, morphological descriptors, land uses and water chemistry on THg and MeHg concentrations and the percentage of THg as MeHg (%MeHg) in 29 streams across Europe spanning from 41°N to 64°N. THg concentrations (0.06e2.78 ng L-1) were highest in streams characterized by DOM with a high terrestrial soil signature and low nutrient content. MeHg concentrations (7.8 e159 pg L-1) varied non-systematically across systems. Relationships between DOM bulk characteristics and THg and MeHg suggest that while soil derived DOM inputs control THg concentrations, autochthonous DOM (aquatically produced) and the availability of electron acceptors for Hg methylatingmicroorganisms (e.g. sulfate) drive %MeHg and potentially MeHg concentration. Overall, these results highlight the large spatial variability in THg and MeHg concentrations at the European scale, and underscore the importance of DOM composition on mercury cycling in fluvial systems.

Ecological theory on the trophic impacts of invasive fauna on native competitors is equivocal. Whilst increased interspecific competition can result in coexisting species having constricted and diverged trophic niches, the competing species might instead increase their niche sizes to maintain energy intakes. Empirical experiments can test invasion theory on competitive interactions and niche sizes across different spatial scales and complexity. The consequences of increased interspecific competition from a model alien fish Leuciscus idus were tested on two taxonomically and trophically similar native fishes, Squalius cephalus and Barbus barbus. Competitive interactions were tested in tank aquaria using comparative functional responses (CFRs) and cohabitation trials. The consequences of these competitive interactions for the trophic niche sizes and positions of the fishes were tested in pond mesocosms. Comparative functional responses revealed that compared to B. barbus, L. idus had significantly higher attack and consumption rates; cohabitation trials revealed B. barbus growth rates were depressed in sympatry with L. idus. For L. idus and S. cephalus, differences in their functional response parameters and growth rates were not significant. Pond mesocosms used stable isotope metrics to quantify shifts in the trophic niche sizes of the fishes between allopatry and sympatry using a substitutive experimental design. Isotopic niches were smaller and more divergent in sympatric paired species than predicted by their allopatric treatments, suggesting trophic impacts from interspecific competition. However, an all-species sympatric treatment revealed similar niche sizes with allopatry. This maintenance of niche sizes in the presence of all species potentially resulted from the buffering of direct competitive effects of the species pairs by indirect effects. Experimental predictions from tank aquaria assisted the interpretation of the constricted and diverged trophic niches detected in the paired-species sympatric treatments of the pond mesocosms. However, the all-species sympatric treatment of this experiment revealed greater complexity in the outcomes of the competitive interactions within and between the species. These results have important implications for understanding how alien species integrate into food webs and influence the trophic relationships between native species.

© 2018 John Wiley & Sons A/S. Quantifying feeding interactions between nonindigenous and indigenous fishes in invaded fish communities is important for determining how introduced species integrate into native food webs. Here, the trophic interactions of invasive 0+ European barbel Barbus barbus (L.) and the three other principal 0+ fishes in the community, Squalius cephalus (L.), Leuciscus leuciscus (L.) and Phoxinus phoxinus (L.), were investigated in the River Teme, a River Severn tributary in Western England. B. barbus has been present in the River Teme for approximately 40 years. Analyses of stomach contents from samples collected from three sites between June and September 2015 revealed that, overall, fishes displayed a generalist feeding strategy, with most prey having low frequency of selection. Relationships of diet composition versus body length and gape height were species-specific, with increasing dietary specialisms apparent as the 0+ fishes increased in length and gape height. The trophic niche size of invasive B. barbus was always significantly smaller than S. cephalus and L. leuciscus and was significantly smaller than P. phoxinus at two sites. This was primarily due to differences in the functional morphology of the fishes; 0+ B. barbus were generally restricted to foraging on the benthos, whereas the other fishes were able to forage on prey present throughout the water column. Nevertheless, the invasive B. barbus were exploiting very similar prey items to populations in their native range, suggesting these invaders were strongly pre-adapted to the River Teme and this arguably facilitated their establishment and invasion.

Life history trait analyses of non-native fishes help identify how novel populations respond to different habitat typologies. Here, using electric fishing and anglers as citizen scientists, scales were collected from the invasive barbel Barbus barbus population from four reaches of the River Severn and Teme, western England. Angler samples were biased towards larger fish, with the smallest fish captured being 410 mm, whereas electric fishing sampled fish down to 60 mm. Scale ageing revealed fish present to over 20 years old in both rivers. Juvenile growth rates were similar across all reaches. Lengths at the last annulus and L infinity of the von Bertalanffy growth model revealed, however, that fish grew to significantly larger body sizes in a relatively deep and highly impounded reach of the River Severn. Anglers thus supplemented the scale collection and although samples remained limited in number, they provided considerable insights into the spatial demographics of this invasive B . barbus population.

1. The crossing of freshwater ecosystem boundaries by marine derived nutrients (MDN) is usually associated with migratory salmonid fishes returning to natal rivers. An alternative source of MDN in freshwaters is the widespread use of pelletized marine fishmeal (‘pellets’) by freshwater anglers as they target large bodied cyprinid fishes, such as European barbel Barbus barbus. 2. Here, the trophic consequences of MDN from pellets for riverine cyprinid fishes were tested. Approaches used stable isotope analyses in controlled and wild scenarios, using B. barbus and chub Squalius cephalus as model species. The isotopic niche, measured as standard ellipse area, was used to assess trophic niche size, and mixing models predicted the extent to which MDN contributed to fish diet. 3. In experimental mesocosms, B. barbus fed low volumes of pellets (approximately 3 per fish) for 130 days had isotopic niche sizes that were up to four times larger than a control and ‘medium’ (6 per fish) and ‘high’ pellet (12 per fish) treatments. Somatic growth rates were significantly higher in the ‘medium’ and ‘high’ treatments. In pond enclosure experiments, when juvenile B. barbus and S. cephalus were fed pellets daily for 100 days, there was a substantial and significant shift in the position of their isotopic niche compared to controls with no pellets fed. However, for each species, there were no significant differences in their somatic growth rates in the presence/ absence of pellets. 4. In a lowland river, high proportions of MDN contributed to the diet of B. barbus and S. cephalus captured by angling, but with substantial individual variability in those captured by electric fishing. Across all B. barbus > 400 mm, MDN dietary contributions ranged between 9 and 71%. This suggested some individual diet specialisations within their population that was associated with feeding on this angler subsidy and that also resulted in a significant increase in the size of their population isotopic niche. 5. These results suggested that when pellets containing MDN are used in freshwater angling, they are consumed and assimilated by cyprinid fishes, influencing individual and population trophic positions, and isotopic niche sizes and dietary specialisations. The results also suggested that the extent to which individuals specialise in feeding on pellets potentially influences their vulnerability to capture by anglers.