Anadromous salmonids migrate seaward to exploit feeding and growth opportunities in marine habitats, yet how smolt biological characteristics influence their marine migratory behavior remains poorly understood. This study used 9 years of trout (Salmo trutta) population monitoring data from 15,595 tagged age-0+ parr, 1033 smolts detected migrating downstream in spring, and 99 adults detected returning from their first marine migration to the River Frome (Dorset, UK) to investigate the influence of smolt biological characteristics on their migration timing and maiden marine sojourn duration. Age-specific differences in the influence of smolt length on migration timing were found, with longer 1-year-old smolts emigrating later than their shorter counterparts within the same age class, but the opposite association existed for 2-year-old smolts. A bespoke integrated statistical model quantified the effects of smolt emigration day of year, age, sex, and length on the probability of first-time migrants returning to the river after one or more sea winters. Younger, later migrating smolts had a longer marine sojourn duration than their older, earlier migrating counterparts, and females remained at sea for longer periods than males. Although the statistical model was designed to maximize the use of information available in the data, it revealed only weak effects of smolt biological characteristics on the maiden marine sojourn duration. A complementary simulation study suggested that detecting more spring migrating smolts and analyzing longer time series of trout population monitoring data would increase the ability to detect statistically significant effects. Therefore, a strategic review of the trout population monitoring program, including more long-term biological data collection, is recommended. The modelling work presented here can provide guidance on the size of the required dataset and how to maximize the power of imperfect data.

The challenge of managing aquatic connectivity in a changing climate is exacerbated in the presence of additional anthropogenic stressors, social factors, and economic drivers. Here we discuss these issues in the context of structural and functional connectivity for aquatic biodiversity, specifically fish, in both the freshwater and marine realms. We posit that adaptive management strategies that consider shifting baselines and the socio-ecological implications of climate change will be required to achieve management objectives. The role of renewable energy expansion, particularly hydropower, is critically examined for its impact on connectivity. We advocate for strategic spatial planning that incorporates nature-positive solutions, ensuring climate mitigation efforts are harmonized with biodiversity conservation. We underscore the urgency of integrating robust scientific modelling with stakeholder values to define clear, adaptive management objectives. Finally, we call for innovative monitoring and predictive decision-making tools to navigate the uncertainties inherent in a changing climate, with the goal of ensuring the resilience and sustainability of aquatic ecosystems.

In recent decades, the population of European eel Anguilla anguilla has strongly declined and the stock is outside safe biological limits. Freshwater habitat degradation has been cited as a key causal factor in the European eel decline, but there are limited studies assessing the responses of this species to river habitat restoration efforts. This study utilized mark-and-recapture data from annual electrofishing surveys conducted between 2009 and 2014 to describe European eel population density and size structure (length, weight) in the River Glaven − a chalk stream in eastern England. Short-term effects of river restoration on European eel were assessed via a Before-After-Control-Impact experimental design. Of the recaptured individuals, 73% were sedentary and the rest mobile. Despite re-meandering work increasing habitat heterogeneity in the restoration reach relative to the control reach, no change in European eel density or size structure was detected across treatments and time. While length and weight increased in the downstream control reach over the study period, density declined. This can be attributed to various local stressors such as barriers to European eel migration, as well as broader range-scale causes including climatic and oceanic factors. Although further research is ideally necessary to ensure adequate sample sizes, as well as to provide long-term monitoring of eel responses to river restoration, this study emphasizes the need for whole-catchment efforts in European eel conservation that combine river–floodplain restoration with greatly improved fish passage.

Eel Management Plans demand European silver eel (Anguilla anguilla) escapement to the sea of at least 40% of that expected historically in the absence of human impacts. Landlocked lentic waterbodies, such as drinking water reservoirs, host substantial numbers of eel, which could represent a significant contribution to catchment-based conservation targets. To optimize netting strategies for eel management policies, information on their aggregation behaviour is currently needed but lacking. We performed a fine-scale acoustic tracking study to monitor the movements of 86 European eel in a UK reservoir. Social network sampling and sensitivity analyses were used to assess whether eel aggregate at scales relevant for current capture techniques. European eel were found to aggregate at spatial and temporal scales of 50 m and 2 days, respectively, which complements current capture methodologies and recommendations. Aggregations were not driven by fixed resources, indicating that other factors, such as sociality, may drive aggregation behaviour. Results also show that current netting practices could be optimized by increasing netting lengths from 50 to 80 m. In addition to aiding conservation and management protocols, these results provide an ecological foundation for exploring the role of social behaviour in this Critically Endangered species.

Historical and existing environmental and human induced pressures have negatively impacted diadromous species, as well as benefits to humans, derived from the ecosystem services these species contribute to. As species move across national boundaries, successful management is rendered a complex process. Future climate change scenarios are likely to change species distributions, further impacting management and the benefits available to people across each species range. To provide an evidence base on the contribution of eleven diadromous fish populations to ecosystem services, and so gains and losses of ecosystem service benefits in the European Atlantic Area, we completed an evidence review of available literature identified within the search strategy. We also gathered expert opinion of diadromous species contributions to ecosystem services for individual catchments across Europe, to gather the full extent of contributions. Evidence was arranged in a matrix, documenting each species ecosystem service contribution to aid communication of results and application to policy and decision makers. The evidence base provided by the literature review identified that each diadromous species contributed to provision of multiple ecosystem services. Volume of evidence available to compile the matrix was far greater for Atlantic salmon Salmo salar and brown trout Salmo trutta. All species had historically contributed to provisioning ecosystem services. However, in recent decades, declines in certain species populations has reduced opportunity for commercial fisheries, leading to contributions to cultural services such as recreational activities and cultural heritage becoming comparatively greater. Evidence of contribution to regulating ecosystem services is growing in recent years, especially transfer of nutrients from marine to freshwater environments. Inclusion of local expert knowledge enabled verification of evidence from literature, in addition to identification of important contributions of lesser studied species, to culturally important commercial fisheries and cultural heritage in European regions. Collecting expert knowledge also aided communication of ecosystem service approaches.

Atlantic salmon Salmo salar is a socio-economically important anadromous fish species that has suffered synchronous population declines around the North Atlantic over the last five decades. Reduced marine survival has been implicated as a key driver of the declines, yet the relative importance of different stressors causing mortality at sea is not well understood. This review presents a synopsis of the principal stressors impacting Atlantic salmon in estuarine and marine environments. It also applies a semi-quantitative 2-D classification system to assess the relative effects of these stressors on English salmon stocks and their likely development over the next decade. Climate change and predation were identified as the biggest threats at present and over the next decade. Poor water quality and bycatch were classified as relatively high impact stressors, but with a lower likelihood of becoming more prevalent in the future due to available mitigation measures. Other, less influential, stressors included tidal barrages, artificial light at night, impingement in power-station cooling waters and thermal discharges, pile-driving noise pollution, invasive non-native species, electromagnetic fields, salmon mariculture, and tidal lagoons. Salmon fisheries exploitation was not regarded as an important stressor currently because effective exploitation rate controls have been implemented to substantially reduce fishing pressure. Future research priorities include addressing knowledge gaps on expanding stressor impacts from climate change, predation, renewable energy developments, and artificial light at night. Local management actions directed towards improving freshwater and estuarine habitats to maximise ecosystem resilience to stressors and minimise their cumulative impacts are recommended.

Fish somatic growth is indeterminate and can be influenced by a range of abiotic and biotic variables. With climate change forecast to increase the frequency of warming and unusual discharge events, it is thus important to understand how these variables currently influence somatic growth and how that might differ for specific age-classes and/ or life stages. Here, we used a 17-year dataset from a chalk stream in southern England to identify the abiotic and biotic influences on the growth of juvenile, sub-adult and adult life stages of European grayling ( Thymallus thymallus ), a cold-water riverine salmonid. The results revealed that interannual variations in grayling growth were well described by annual- and site-specific abiotic and biotic explanatory variables. We found divergent responses between life stages to increased temperature and unusual discharge during the main growth period with, for example, elevated temperatures related to increased juvenile growth but reduced sub-adult growth, and high discharge events related to increased sub-adult growth yet reduced juvenile growth. Conversely, stage-specific grayling abundance negatively influenced growth at each life stage, though only juvenile growth was impacted by the abundance of a competitor species, brown trout ( Salmo trutta ). These results emphasise the merits of testing a wide range of environmental and biological explanatory variables on fish growth, and across life stages. They also reveal the importance of maintaining high habitat heterogeneity in rivers to ensure all life stages can reduce their competitive interactions and have access to adequate flow and thermal refugia during periods of elevated environmental stress.

Globally, inland waters emit over 2 Pg of carbon (C) per year as carbon dioxide (CO2), of which the majority originates from streams and rivers. Despite the global significance of fluvial CO2 emissions, little is known about their diel dynamics. We present the first large-scale assessment of day- and night-time CO2 fluxes at the water-air interface across European streams. Fluxes were directly measured four times throughout one year using drifting chambers. Median CO2 fluxes amounted to 1.4 and 2.1 mmol m-2 h-1 at midday and midnight, respectively, with night fluxes exceeding those during the day by 39%. Diel CO2 flux variability was mainly attributed to changes in the water partial pressure of CO2 (pCO2) but no consistent drivers could be identified across sites. Our results highlight widespread day-night changes in fluvial CO2 fluxes and that the time of day greatly influences measured CO2 fluxes across European streams.

Human‐generated pressures are continuing to have a detrimental effect on diadromous fishes, resulting in legislative initiatives to conserve and manage these species. Field studies to inform these initiatives focus almost exclusively on larger marine, estuarine, and inland waters, neglecting the role of small coastal streams and tributaries in population recovery. As an example of the potential contribution of very small coastal streams to diadromous fish recruitment, we report here on the densities and distribution of European eel Anguilla anguilla L. and flounder Platichthys flesus (L.) in one such stream, “La Warenne,” in northeast France in October 1998 and 1999. Both species were found mainly in downstream sites in both years—flounder were captured in low numbers only, but mean CPUE values for eel were similar to the highest densities reported in the ICES WGEEL database for larger rivers of Great Britain, France and Spain in the Interreg Atlantic and North Sea areas. This small dataset illustrates that small coastal streams may represent an important, but overlooked, role in conservation strategies for European eel, and possibly flounder, indicating the need for further field studies and investigations of existing data sources to evaluate the contributions to diadromous fish recruitment of these poorly studied water courses.

Environmental changes due to non-native species introductions and translocations are a global concern. Whilst understanding the causes of bioinvasions is important, there is need for decision-support tools that facilitate effective communication of the potential risks of invasive non-native species to stakeholders. Decision-support tools have been developed mostly in English language only, which increases linguistic uncertainty associated with risk assessments undertaken by assessors not of English mother tongue and who need to communicate outcomes to local stakeholders. To reduce language-based uncertainty, the 'ecology-of-language' paradigm was applied when developing the Aquatic Species Invasiveness Screening Kit (AS-ISK), a decision-support tool that offers 32 languages in which to carry out screenings and communicate outcomes to stakeholders. Topics discussed include uncertainty related to language-specific issues encountered during the AS-ISK translation and the potential benefits of a multilingual decision-support tool for reducing linguistic uncertainty and enhancing communication between scientists, environmental managers, and policy and decision makers.

Assessments of the trophic consequences of invasive fishes are important for quantifying their ecological impacts on native species more generally. A small-bodied cyprinid fish native to continental Europe and introduced in the 1970s to the U.K, the sunbleak Leuciscus delineatus, has been shown previously to establish closer social associations with native species of similar size than do native species amongst themselves. To assess the potential detrimental trophic consequences of native species associations with L. delineatus, a field-based experiment was undertaken in summer 2015 in six outdoor, artificial ponds containing three native cyprinid species (rudd Scardinius erthrophthalamus, gudgeon Gobio gobio, tench Tinca tinca). Three ponds were controls (no L. delineatus) and three were treatments (L. delineatus present). The results of stable isotope analysis (SIA) of fish tissue samples provided strong evidence that the isotopic niches of both native benthic fishes were reduced in the presence of L. delineatus, although there were no significant effects on the trophic position, body size or condition of two of the three native fish species. Introduced L. delineatus maintained a core isotopic niche that was distinct from the two native benthic fishes, with no overlap detected between native and non-native fishes when including 40% and 95% of the data. These results indicate that the response of the native fishes to the introduction of L. delineatus was niche constriction via trophic specialisation, with this response sufficient to maintain their growth rates and condition. This result is similar to studies on a range of small-bodied invasive fishes, suggesting the trophic impacts of these invaders are relatively consistent across species and systems.

The spawning success of lithophilic salmonids is strongly influenced by the fine sediment content ("fines") of spawning substrates, yet knowledge on the impacts of fines on the spawning of non-salmonid lithophiles remains limited, despite their ecological and socio-economic importance in European rivers. Consequently, the aim here was to use an ex-situ experiment to investigate the impact of sand content on egg survival and timing of larval emergence of the surface-spawning cyprinid European barbel Barbus barbus. Thirty incubator boxes within a recirculating system were filled with one of five experimental sediment mixtures (0%-40% sand by mass) that each contained 300 fertilised eggs at a depth of 50 mm. Emerged, free-swimming larvae were captured and counted daily to assess grain-size effects on larval survival and emergence. Specifically, total proportion of emerged larvae, cumulative daily proportion of emerged larvae and time required to reach 50% emergence were measured during the study. Whilst the proportion of sand in the sediments did not have a significant impact on egg-to-emergence survival (mean survival per treatment 75%-79%), it significantly affected the timing of larval emergence to the water column; early emergence was detected in treatments with elevated sand content (on average, 50% emergence after 12-13 days versus 19 days in the control). Similar to findings from salmonid studies, these results suggest high sand content in spawning gravels can influence timing of larval emergence and potentially cyprinid lithophilic fish survival.

Conservation of the critically endangered European eel Anguilla anguilla (Linnaeus, 1758) requires knowledge of silver eel escapement across all eel-producing habitats. Describing eel production from deep waters is especially challenging, thus telemetry studies can be used to detect specific behavioural patterns and aid in designing cost-effective survey methodologies. Here, 36 and 68 European eels were monitored via acoustic telemetry in Hanningfield reservoir, UK in 2015 and 2016, respectively. The aim of the study was to assess activity rates and home range sizes of eels in relation to explanatory variables: fish length, stage, lipid level, time of day, water temperature, night duration, lunar phase, water depth and distance to shore. In addition, site fidelity and return time to previously visited locations across the study area were investigated. Time of day, water temperature and lunar phase significantly affected eel behaviour, with higher activities observed during increased temperatures and dark periods. Furthermore, their monthly displacements rates were higher over medium-ranged depths (4–7 m), with depth use affected by water temperature, time of day and lunar phase. In general, eels had large home ranges in this study, which increased further during warmer and darker periods. Despite travelling great distances, with long return times to previously visited locations, they displayed high home fidelity between months. The results of this study provide greater insight into eels’ temporal and spatial utilisation of a lacustrine habitat, and survey design to collect abundance data for the assessment.

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 (<2 mm) content (≈20% dry weight), but elevated subsurface silt content and coarser surface sediments were found in the river Teme. Water velocities were similar at spawning sites despite differences in channel width and depth. Redds were characterized by a pit and tailspill, with no differences in surface grain‐size characteristics between these and the surrounding riverbed, but with topographic alteration (dimensions and tailspill amplitude) in line with those of salmonids. Estimates of the fraction of the bed that spawning barbel were capable of moving exceeded 97% in all rivers. Estimated reproductive potential varied significantly between the rivers Idle and Teme (3,098 to 9,715 eggs/m2), which was largely due to differences in barbel lengths affecting fecundity. Larger barbel, capable of producing and depositing more eggs, but in more spatially extensive redds, meaning fewer redds per given surface area of riverbed. Predictions of barbel egg mortality based on sand content were low across both rivers. The effects of silt on barbel egg and larvae development are unknown, but the levels detected here would significantly impact salmon egg mortality. Similarities in fish length to redd area and the size of moveable grains by spawning barbel and salmon suggest they have similar geomorphic effects on sediments, although fine sediment tolerance is highly divergent.

Recruitment of salmonids is a result of density-dependent factors, specifically egg production in the previous year, and density-independent environmental processes driven by discharge and temperature. With the plethora of knowledge on major drivers of Atlantic salmon Salmo salar and brown trout Salmo trutta recruitment, there is a requirement to explore less known species, such as European grayling Thymallus thymallus, whose postemergence time coincides with period of increasing temperature and low discharge. This study assessed drivers of grayling recruitment in a southern English chalk stream, a system vulnerable to discharge and temperature alterations under future climate change predictions. The analyses explored age 0+ grayling survival in relation to conspecific and heterospecific densities and discharge- and temperature-derived factors. The final mixed-effects model revealed a positive relationship between age 0+ grayling survival and incubation temperature anomaly and age 0+ trout abundance. Similarly, postincubation temperature anomaly had a positive effect on 0+ grayling survival, but only up to a threshold temperature of 13.5 degrees C, beyond which it had a negative effect. In contrast, increasing number of days with low discharge postincubation negatively influenced age 0+ grayling survival, with no evidence of an effect of elevated discharges following spawning. Our results emphasise the importance of maintaining natural discharge regimes in salmonid rivers by tackling multiple stressors operating at the catchment scale, including land and water use to mitigate for predicted climate driven changes. In addition, further research on recruitment drivers in less stable, rain-fed systems, is required.

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 fresh waters is the widespread use of pelletised marine fishmeal ("pellet") 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 (c. 3 per fish) for 130days 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 100days, 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.

Fine sediments can impact river biota, with egg and larval stages of lithophilic fish particularly sensitive to deposition of sand- to clay-sized particles (‘fines’) in spawning gravels. Mitigation and restoration methods include jetting to cleanse gravels of fines. Despite wide application, impacts of jetting on gravel composition and quality have rarely been quantified. Here, gravel jetting impacts on sediment composition in the River Great Ouse (UK), were tested during an in-situ experiment completed at riffle (55.6±13.4m2) and patch (0.3m2) scales to determine its magnitude and persistence on surface and subsurface substrate conditions. Before-after (riffle) and control-impact (patch) designs were used, with bedload sediment traps installed downstream of experimental patches to investigate the sediments mobilised during jetting. At the riffle scale, surface grain size was significantly altered; fines were removed resulting in coarser and better-sorted sediments. Similar patterns were detected at the patch scale, although sediment sorting was not significantly altered. Despite reduced fine sediment content of subsurface gravels at the riffle scale, the overall grain size composition was not significantly altered. At the patch scale, no subsurface improvements were detected. Temporally, at the riffle scale, no changes in surface or subsurface sediments lasted more than 12 months; patch scale changes generally persisted for less than 3 months. Thus, whilst gravel jetting could improve spawning gravel quality for surface spawning fishes, including European barbel Barbus barbus, its effects are short-lived. Because subsurface sediments are not affected by gravel jetting, the benefits are limited for redd-building fishes, such as salmonids. Consequently, reducing fine sediment delivery to rivers, such as by changes in agricultural practices, is more sustainable for managing excessive river sedimentation.

AbstractHatchery‐reared fish are commonly stocked into freshwaters to enhance recreational angling. As these fishes are often of high trophic position and attain relatively large sizes, they potentially interact with functionally similar resident fishes and modify food‐web structure. Hatchery‐reared barbel Barbus barbus are frequently stocked to enhance riverine cyprinid fish communities in Europe; these fish can survive for over 20 years and exceed 8 kg. Here, their trophic consequences for resident fish communities were tested using cohabitation studies, mainly involving chub Squalius cephalus, a similarly large‐bodied, omnivorous and long‐lived species. These studies were completed over three spatial scales: pond mesocosms, two streams and three lowland rivers, and used stable isotope analysis. Experiments in mesocosms over 100 days revealed rapid formation of dietary specializations and discrete trophic niches in juvenile B. barbus and S. cephalus. This niche partitioning between the species was also apparent in the streams over 2 years. In the lowland rivers, where fish were mature individuals within established populations, this pattern was also generally apparent in fishes of much larger body sizes. Thus, the stocking of these hatchery‐reared fish only incurred minor consequences for the trophic ecology of resident fish, with strong patterns of trophic niche partitioning and diet specialization. Application of these results to decision‐making frameworks should enable managers to make objective decisions on whether cyprinid fish should be stocked into lowland rivers according to ecological risk.