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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.

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.

© 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.

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.