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The European eel is facing extinction—in part, because of trafficking. Spain is not only one of the leading source countries of trafficked eels but also the one accounting for the most seizures. Thus, understanding Spain's role can help us to better understand how to stop this crime. Examining the issue under the umbrella concept of "criminogenic asymmetries" and using interviews, observation, and document analysis, this article explores how these asymmetries interact in Spain and along the global supply chain to identify how the illegal trade in European eels could be prevented, stopped, and deterred. The study finds that it requires a multifaceted approach including robust law enforcement, effective legislation, habitat restoration, community engagement, and consumer education. [ABSTRACT FROM AUTHOR]

Context. The Indo-Pacific eel (Anguilla marmorata) is a widely distributed and commercially valuable species across ecological regions worldwide. Overfishing and habitat loss are leaving the Indo-Pacific eel in a risky situation and raising a high demand for conservation. Previous research has found relationships between the Indo-Pacific eel's migration patterns and environmental factors. However, there is still a need to advance the discovery of its spatial distribution by using diverse environmental and ecological datasets and modelling its growth in terms of different environmental characterizations. Aims & Methods. Here, we compared machine learning (ML) CatBoost (CB) and the multivariate linear model to investigate the relationship between spatial distribution, Indo-Pacific eel development stages, and environmental factors in central Vietnam. Key results. Our results show that CB detected the Indo-Pacific eel at high accuracy (Overall Accuracy (OA) = 0.9, F1 = 0.88, AUC = 0.97) and estimated the total length at different confidence levels (R2 ranging from 0.51 to 0.70), demonstrating superior performance to the multivariate linear model. Conclusions & implications. This study highlights the potential use of ML models in species distribution mapping and modelling growth patterns to support conservation efforts of Indo-Pacific eels in their natural habitats. [ABSTRACT FROM AUTHOR]

The European and Japanese eel populations have declined significantly in recent decades. To effectively manage and conserve them, gaining a better understanding of their migratory life cycles is important. Previous research on the spawning ecology and larval dispersal of European and Japanese eels has led to many significant discoveries and advancements for their migratory life cycles. However, different scholars hold varying views on their migratory life cycles, especially concerning the European eel, therefore this article aims to provide a comprehensive review of research from multiple disciplines concerning the spawning ecology and larval dispersal of European and Japanese eels and to propose migratory life cycles of these two species. The migratory life cycle of the European eel is as follows: European silver eels undertake a year‐long spawning migration from September to January to reach the Sargasso Sea for spawning before the next spawning season, typically between December and May. After hatching, European eel leptocephali are transported by the Gulf Stream, Frontal Countercurrents, North Atlantic Current, and Azores Current and return to Europe and North Africa for growth. Recruitment of European glass eels mainly occurs between October and June of the following year, and the recruiting season is more concentrated in countries closer to the spawning area and more dispersed in countries farther away. The consistent recruitment pattern and the growth rate of leptocephali suggest a larval transport period, also called larval duration, of around 1 year. Understanding the migratory life cycle of European eels can facilitate the evaluation or development of their conservation measures. [ABSTRACT FROM AUTHOR]

Recruitment of the catadromous and critically endangered European eel Anguilla anguilla in Europe has declined substantially since the 1980s, with considerable knowledge gaps remaining in many aspects of their life cycle. The aim was to assess eel migration phenology in three regulated rivers in England between 2009 and 2019 through analyses of eel numbers using passes at their tidal limits, with calculation of the annual timings of migration initiation (10% of all eels passed, T10), peak (50%, T50) and conclusion (90%, T90). Across the three rivers, T10 varied between Julian Day ('Day') 94 and 173. Years of earlier T10 had significantly earlier T50, where T50 varied between Day 105 and 200. The considerable inter-annual variability in migration timings was associated with environmental variables; earlier T10 and T50 occurred in years of warmer river temperatures (RTs) and cooler sea surface temperatures (SST), and in years where RTs were higher than SSTs. No environmental variables were significant predictors of T90. These results indicate that whilst there is annual variability in the timing of eel migration initiation and peak into freshwaters, this variability is predictable according to differences in environmental conditions. As many of these conditions associated with annual variability in temperature and precipitation then climate change has the potential to shift these migration timings. [ABSTRACT FROM AUTHOR]

Aberrations in the otolith can harmfully influence the imperative actions of the fish species. The chief drive of the current investigation is to describe the deformities in the otoliths of the European eel Anguilla anguilla. Out of 76 Anguilla anguilla individuals collected from Bafa Lake in Western Anatolia, Türkiye, four female specimens exhibited abnormal otoliths. Scanning electron microscope was used to determine the deformed otoliths examined. The outcomes of the abnormality analyses in A. anguilla otoliths, deformities were noticed such as numerous prominence structures and bumps of different sizes on the surface of the otolith produced by buildup of the different crystal structures in some parts of the otolith. These deformities were found in the left and/or right otoliths of the female individuals of different total lengths. The percentage of individuals with abnormal otoliths of A. anguilla is calculated as 5.3%. The current study presents for the first‐time abnormal otolith information on left and right otoliths in female A. anguilla. Deformities in the otoliths of European eels are believed to result from a combination of genetic predisposition and stress caused by various environmental factors, such as water pollution, changes in salinity, temperature fluctuations and nutritional imbalances. [ABSTRACT FROM AUTHOR]

Since the 1980s, the European eel (Anguilla anguilla) has declined by over 90% in recruitment across its European and North African distribution area. This diadromous fish spawns at sea and migrates into continental waters, where it grows for three to more than 30 years, depending on habitat conditions and location. During their growth, different habitat use tactics can locally influence the life‐history traits of eels, including their survival rates. Thus, the spatio‐temporal dimension of this species is crucial for management. Based on a rare Mediterranean long‐term survey of more than 20 years (2001–2022) in an artificial drainage canal connected to a vast brackish lagoon (the Vaccarès lagoon), we aimed to study the dynamics of one subpopulation's life‐history traits. We used Bayesian multistate capture‐mark‐recapture (CMR) models to assess the temporal variability in survival and abundance at both seasonal and inter‐annual scales, considering life‐stage structure. High survival rates and low detection probabilities were found for the undifferentiated and female yellow stages. In contrast, female silver eels exhibited lower survival rates and higher capture probabilities. Estimating detection probabilities and survival rates enabled accurate assessment of relative abundance across different life stages and time periods. Our findings indicated a substantial decrease in the abundance of undifferentiated and female yellow eels in the early 2000s, whereas the abundance of female silver eels remained consistently low yet stable throughout the study period. Considering the life stage seemed essential to study the dynamics of the eel during its continental growing period. The present results will provide key elements to propose and implement suitable sustainable environmental management strategies for eel conservation. [ABSTRACT FROM AUTHOR]

The European eel (Anguilla anguilla (L.)) is a unique catadromous euryhaline fish and the only anguillid eel species classified as 'critically endangered'. The Baltic Lakeland area contains naturally recruited and introduced eels. There is great uncertainty regarding a baseline for the location and number of naturally dispersed eels in the region. Little is known about the genetic structure of the European eel populations in Baltic countries. The estimation of population genetic structure is important for the efficient management of naturally recruited and introduced eels. Two mitochondrial regions were used to investigate the genetic structure within and between eel samples from 11 waterbodies. In this study, new, unique, and widely distributed haplotypes were revealed. The studied eel population in the Baltic Lakeland shows high genetic diversity, which is possibly a result of intensive restocking programs. Sequences characterized for Anguilla rostrata were revealed in both mitochondrial regions. Understanding the genetic structure of eel populations worldwide is crucial for conservation efforts. Eel restocking in waterbodies where natural migration is restricted contributes to diversity loss for the world gene pool of eels. [ABSTRACT FROM AUTHOR]

The European eel is critically endangered due to heavy impact of anthropogenic factors, such as habitat fragmentation, overexploitation and climate change. During downstream migration, silver eels may encounter hydropower plants, which often result in delay or mortality from impingement on trash-racks or turbine passage. These problems can be mitigated with downstream passage solutions, such as angled racks that guide downstream-migrating eels to safe passage routes. The importance of bar spacing and phenotypic diversity for passage performance is, however, largely unknown. In this study, we investigated how morphological parameters (body mass, eye and fin indices) and behavioral score (open field test) influenced passage rate at an experimental intake equipped with a bypass and angled racks with either 15 or 30 mm bar spacing. Both racks were efficient in guiding eels into a bypass. There was a strong positive effect of body mass and a weak positive effect of open field test score on passage rate. Other factors such as eye and fin indices played a minor role. These results demonstrate the performance of angled racks with bypasses and form a useful starting point for further research regarding the relationships between individual variation in behavior, morphology and passage solutions for silver eels. [ABSTRACT FROM AUTHOR]

In East Asia, anguillid eels are commercially important. However, unlike other species, they have not been successfully cultivated throughout their lifecycle. Facing population decline due to overharvesting and environmental pressures, the industry is turning to alternatives, such as Anguilla bicolor pacifica (short-finned eel). However, genomic data for short-finned eels are unavailable. Here, we present in-depth whole-genome sequencing results for short-finned eel obtained using two sequencing platforms (PacBio Revio, and Illumina). In this study, we achieved a highly contiguous genome assembly of the short-finned eel, comprising 19 pseudochromosomes encompassing 99.76% of the 1.087 Gb genome sequence with an N50 of 16.88 and 61.07 Mb from contig and scaffold, respectively. Transcripts from four different tissues led to the annotation of 23,095 protein-coding genes in the eel genome, 98.66% of which were functionally annotated. This high-quality genome assembly, along with the annotation data, provides a foundation for future functional genomic studies of short-finned eels. [ABSTRACT FROM AUTHOR]

In fisheries management, accurate stock assessment is pivotal to determine sustainable harvest levels or the scope of conservation measures. When assessment is decentralized and methods differ regionally, adopted approaches must be subjected to rigorous quality-checking, as biased assessments may mislead management decisions. To enable recovery of the critically endangered European eel, EU countries must fulfill a biomass target of potential spawner ("silver eel") escapement, while local eel stock assessment approaches vary widely. We summarize local approaches and results of ground-truthing studies based on direct silver eel monitoring, to evaluate the accuracy of eel stock assessments in retrospect and identify bias sources. A substantial fraction of eel habitat is currently unassessed or assessed by unvalidated approaches. Across assessment models for which validation exists, demographic models frequently overestimated actual escapement, while misestimations of extrapolation ("spatial") models were more balanced, slightly underestimating escapement. Stock size overestimation may lead to overexploitation or insufficient conservation measures, increasing the risk of stock collapse or slow recovery in coordinated frameworks. Underestimations may imply inefficient allocation of conservation efforts or negatively affect socioeconomy. Our work highlights the risks of regionalizing assessment responsibilities along with management decisions, calling for a common assessment toolbox and centralized quality-checking routines for eel. [ABSTRACT FROM AUTHOR]

Globally, catadromous freshwater eels of the genus Anguilla are of conservation concern, including critically endangered European eel (Anguilla anguilla). Pumping stations that move river water to a higher elevation severely impact eels during their seaward spawning migration. Fish-friendly pumps can mitigate fish injury and mortality but here we uniquely rethink a fish-friendly pump as a fish passage solution. In this pluriannual study, the seasonal timing of pump operation was misaligned with the typical silver eel migration period. Eels were almost exclusively nocturnal but night-time pumping represented as little as 5.6% a year. Night-time eel approaches were primarily influenced by pump duration and temperature, but did not align with lunar phase, unlike in unregulated rivers. After reaching the pumping station, eel passage was influenced by weedscreen aperture and increased when the aperture was increased. Passive sensor collision suggested non-pump infrastructure could cause injury and mortality to eels. It is therefore recommended pump operation should align with the timing of silver eel migration, weedscreen and pump entrance efficiencies should be maximised, and non-pump infrastructure must have low fish injury risk. Ultimately, considering the entire structure a fish passage solution will help ensure fish-friendly pumps have high conservation value for anguillid eels globally. [ABSTRACT FROM AUTHOR]

The use of Pop‐up Satellite Archival Tags (PSATs) has provided considerable new information about the behaviour of migrating (silver) eels (Anguilla spp.) at sea, with 9 of 19 recognised species or subspecies tagged to date. However, such studies often reported premature tag detachment and relatively high rates of predation. Since 2000, several PSAT‐tagged New Zealand longfin eels, Anguilla dieffenbachii, were released to understand migratory behaviour and location of potential spawning grounds. Of 20 PSAT‐tagged eels released in 2019, all tags detached prematurely, but 7 (58%) were likely eaten or injured by predators 1–81 days after release. Two eels were probably eaten by sperm whales, whereas other tagged eels were likely eaten by lamniform and non‐lamniform sharks, and southern bluefin tuna. Because of the high rate of predation, we recommend that future releases should be from smaller catchments or offshore. [ABSTRACT FROM AUTHOR]

Objective: Visible implant elastomer (VIE) tags are commonly used as a cost‐effective tool for marking small fish, making them valuable in mark–recapture studies. It is crucial to quantify the impact of tagging procedures on fish survival to address inferential bias in mark–recapture studies. We assessed marking‐related mortality and growth in American Eel Anguilla rostrata elvers in a 40‐day laboratory experiment, following VIE tag application. Methods: There were 500 elvers (80–149 mm) that were divided into four treatment groups and one control group. Treatment groups were tagged with two tags in three body locations (anterior, central, posterior on left bilateral side) or with two tags in all three locations, while the control group remained untagged. Eels were retained in experimental tanks, and mortality rates were compared. Result: The VIE tagging did not significantly affect survival, which was 90.9% across all treatment groups and 92% for the untagged control group; nor did it affect growth. Conclusion: The application of VIE tags on various body parts should be a safe and effective method for marking American Eel elvers. Impact statementThe effect of visible implant elastomer tags on juvenile eel survival has only been assessed for European Eel, and tagging was restricted to tail fins. By testing novel tag locations on American Eel elvers, we demonstrated that this species can be tagged multiple times with visible implant elastomer tags without affecting short‐term survival and growth. [ABSTRACT FROM AUTHOR]

Protecting ocean habitats is critical for international efforts to mitigate climate impacts and ensure food security, but the ecological data upon which policy makers base conservation and restoration targets often reflect ecosystems that have already been deeply impacted by anthropogenic change. The archaeological record is a biomolecular archive offering a temporal scope that cannot be gathered from historical records or contemporary fieldwork. Insights from biogeochemical and osteometric analyses of fish bones, combined with context from contemporary field studies, show how prehistoric fisheries in the western Baltic relied on seagrass meadows. European eels (Anguilla anguilla) harvested by Mesolithic and Neolithic peoples over millennia showed a strong fidelity for eelgrass foraging habitats, an ecological relationship that remains largely overlooked today, demonstrating the value of protecting these habitats. These data open new windows onto ecosystem- and species-level behaviours, highlighting the need for wider incorporation of archaeological data in strategies for protecting our oceans. [ABSTRACT FROM AUTHOR]

The European eel encounters challenges in achieving sexual maturation in captivity, which has been a concern for researchers. This study explores surrogate broodstock technology as an alternative approach for eel production. The present study aimed to evaluate zebrafish and European sea bass as potential recipients for European eel spermatogonia transplantation, given the abundance of eel type A spermatogonia (SPGA). Immature European eel testes were dissected and maintained at 4 °C or cryopreserved. SPGA were obtained by dissociation of fresh or post-thawed tissue, employing an enzymatic solution, and then labelled with fluorescent membrane marker PKH26. SPGA from fresh tissue were transplanted into wild-type zebrafish larvae and triploid European sea bass larvae, while SPGA from cryopreserved testis were transplanted into vasa::egfp transgenic zebrafish larvae. One-and-a-half months post-transplantation (mpt), fluorescent donor cells were not detected in the gonads of zebrafish or European sea bass. Molecular qPCR analyses at 1.5 or 6 mpt did not reveal European eel-specific gene expression in the gonads of any transplanted fish. The findings suggest that the gonadal microenvironments of zebrafish and European sea bass are unsuitable for the development of European eel spermatogonia, highlighting distinctive spermatogonial stem cell migration mechanisms within teleost species [ABSTRACT FROM AUTHOR]

Estuaries are considered as key habitats for the early life stages of fish. However, in the face of massive destruction of many estuarine intertidal areas, management and conservation measures are needed. Fish condition indicators may be used as a proxy of habitat quality and provide valuable information for management of coastal areas. In this study, the larvae of golden mullet (Chelon auratus) and European glass eels (Anguilla anguilla) were sampled in three sites of the Gironde Estuary. Different lipid classes and fatty acids were quantified: phospholipids (globally, phosphatidylethanolamine and phosphatidylcholine), triglycerides, omega‐3 (particularly docosahexaenoic and eicosapentaenoic acids), omega‐6 and C18:1. These biomarkers provide information on the nutritional status of the larvae as well as on prey availability and larvae diet between sites. One site significantly differed from the others as it seemed to offer abundant and better‐quality prey. The very high levels of omega‐3 contained in mullet larvae suggested that this site provided a high amount of diatoms. However, the mullet larvae that colonized this site also showed physiological stress that could be explained by exposure to pollutants through their prey. This work constitutes an essential baseline for developing biomarkers to assess the quality of habitats in a global change context. [ABSTRACT FROM AUTHOR]

The glass eel catch from the international River Minho, western Iberian Peninsula, forms the southernmost Atlantic series in the composite index outside the North Sea assessing European eel recruitment. Here, new experimental yield data from the lower estuary (1981–2022) and fishers´ daily records (1990–2022) were modelled to describe and compare seasonal and interannual trends. Seasonality matched river discharge climatology, possibly a more general feature in the southern range of the species distribution, with the difference between high and low season becoming less marked in recent periods of lower abundance. Glass eel yield showed a sharp decline during the 1980s, in line with the recruitment index outside the North Sea and with total Minho catch, but not with recent local estimates of catch per unit effort. This decline is corroborated by trends in fishers´ daily records, suggesting that the reduction in effort in the Minho was the consequence of a gradual adjustment to recruitment in the catchment mediated by progressively tighter local management measures. The inefficiency of fisheries regulatory measures to guarantee stock recovery alone calls for more comprehensive measures to reverse anthropogenic impacts on continental eel populations, but the task seems Herculean. [ABSTRACT FROM AUTHOR]

Anguillid eel populations are under threat globally. A particularly vulnerable life‐cycle stage is the migration of mature adult eels downstream from freshwater habitats through estuaries into the sea to spawn. This study investigated the factors associated with downstream migration of the short‐finned eel Anguilla australis (Richardson 1841) from a coastal wetland (Lake Condah) in south‐east Australia, using acoustic telemetry. Migration was associated with time of the year, higher water level and river flows, decreasing water temperature, and darker moon phases. Larger individuals and those in better condition were more likely to migrate from the wetland. Downstream migration peaked in spring, in contrast to the typical autumn migration period for other temperate anguillids. Variable responses, in comparison to other studies, highlight how migration cues may not be universal. In south‐east Australia, short‐finned eels may have evolved to migrate in multiple phases by first migrating to the estuary during typical seasonal spring flow pulses (e.g., to avoid being stranded in upland reaches during dry summer periods) and then migrating into the ocean in autumn. More research is needed to unravel these processes and causes, especially considering that the relationship between migration and hydrology may be complex and confounded (e.g., by human‐induced disruptions to migratory pathways). [ABSTRACT FROM AUTHOR]

The present paper is the first document on heavy metal levels in the European eel (Anguilla anguilla) collected from the Gediz Delta in the Eastern Aegean (Türkiye). In this study, concentrations of Cd, Cu, Zn and Pb were determined in the liver, gills, and muscle tissues of A. anguilla eels. Sixty dead eels were obtained between June 2015 and January 2016, and their total size and weight were measured. Heavy metal accumulation levels in edible muscle tissue of the eels were compared with national and international standards, and Estimated Weekly Intake (EWI), Target Hazard Quotient (THQ), and Total Target Hazard Quotient (Σ THQ) were calculated. It was determined that metal concentration levels in A. anguilla follow the sequence of Zn > Cu > Pb > Cd. Zn has the highest concentration in all sampling periods and in all tissue types. It was found that Σ THQ was below 1 and amounted to 0.41. This result shows that there is no carcinogenic risk associated with the consumption of A. anguilla in adults. [ABSTRACT FROM AUTHOR]

The existence of ultraviolet (UV) biofluorescence in nature has been documented in a wide range of flora and fauna. Biofluorescence is utilised throughout biological and ecological functionality, and although invisible to the human visual perception, this spectral range is known to be integral for communication across flocks, swarms, shoals and between individuals. Under UV illumination, European eel juveniles (glass eel) were found to fluoresce bright yellow. Follow‐up experiments using the UV light source on preceding eel life stages such as pigmented glass eels (elvers), 'bootlace eels' (juvenile yellow eels), older yellow eels and migrating silver eels, all found that these stages had lost the capacity for biofluorescence. The information gathered from this study suggests that the use of UV light for presence/absence glass eel arrival surveying could be a useful and effective tool for researchers in the research field. [ABSTRACT FROM AUTHOR]

For the conservation and management of the critically endangered European eel, it is crucial to maintain an overview on the biomass of the eels moving from freshwater feeding areas to the sea for spawning. Rohtla et al. suggested an assessment of the escapement of eels from the Narva River to the Baltic Sea based on their study of origin of yellow eels from Estonian coastal areas and eels from Finnish coastal areas. Silver eels from the Estonian coast have not been included in the study. The number of descending eels in the Narva River has not been investigated. The sampling areas in the sea were located far from the river mouth, ranging from 100 to 500 km away. The article does not include an analysis of how to calculate the number of eels descending from the river based on the samples used. Thus, estimates can be significantly biased. We demonstrate that the approach used to estimate escapement is methodologically incorrect and needs further development before applying in eel management. This comment recommends methodological improvements for estimating eel escapement using the indirect method. [ABSTRACT FROM AUTHOR]

Fish marking is an essential tool for fisheries management, especially for evaluating the stocking of endangered fish species to support conservation and sustainable use of fish stocks. Batch marking of young European eels Anguilla anguilla (L.) prior to stocking is recommended as the benefits of stocking for the spawning stock can be evaluated by recapturing marked fish over time, therefore mass marking of young eels with substances such as alizarin red S (ARS) is becoming increasingly important. To improve the marking method and reduce marking costs when immersing glass eels in an ARS solution, eight laboratory experiments under varying conditions (e.g., temperature, ARS concentration, immersion time, osmotic induction, fish density) and with ARS from different suppliers were carried out. The results show that optimal marking of glass eels can be carried out in the field or during transport by putting approximately 50 g of glass eels per liter in 150 mg L−1 ARS solution for 3 h at 10–15°C. Lower concentrations did not result in reliable marking. Water temperatures of 5°C and below can have a stunning effect on the eels and increase mortality significantly, regardless of the concentration of ARS. Glass eel densities below 50 g L−1 in the marking bath increase marking costs unnecessarily, while a higher density of 100 g L−1 resulted in significantly higher mortality and lower marking success. A somewhat more difficult but less expensive alternative is to bathe the fish in a saline solution of 1% (10 PSU) of 80 mg L−1 ARS for 3 h at 10°C. Costs can also be significantly reduced by choice of supplier for ARS, but care should be taken as the quality of the powder appears to vary (mean percentage of sufficiently marked eels ranged from 59% to 91% among suppliers in the present study) and can lead to marking failure. The optimal marking conditions can help ensure that stocked glass eels can be reliably identified in future studies to assess stocking benefits while reducing costs. [ABSTRACT FROM AUTHOR]

It is well established that non‐native fish can become invasive and outcompete and displace native fish populations. However, little research has explored the potential benefits that non‐native fish may provide to native fish populations. To address this information gap, we examined how the availability of non‐native perch (Perca fluviatilis) as prey could benefit populations of the endemic longfin eel (Anguilla dieffenbachii) and the native shortfin eel (Anguilla australis) in two neighbouring open‐water wetlands in the Rakatu‐Redcliff wetland complex in the Southland region of Aotearoa New Zealand. The Redcliff wetland fish community comprised native fish only, while the Rakatu wetland comprised native fish and non‐native perch. We compared the size, condition, population density, population size structure and diet of eels in these wetlands. While eels were not necessarily larger or better conditioned in Rakatu wetland, their population density was three times higher than the Redcliff wetland, with young‐of‐the‐year perch comprising ca. 40% of their diet. Furthermore, juvenile eel density was four times lower in Redcliff wetland, suggesting that cannibalism may occur at this site to support the existing eel population. Based on our findings, we suggest that freshwater managers consider the predator–prey dynamics of both native and non‐native fish before removing non‐native species, to avoid unintended negative consequences for native predatory fish species. [ABSTRACT FROM AUTHOR]

Eel (Anguilla sp.) is an important freshwater‐cultured species with high economic value in China. Anguillid herpesvirus 1 (AngHV‐1) has been proven to be the pathogen of "mucus sloughing and haemorrhagic septicaemia disease" in eels, resulting in significant mortality and substantial losses to the eel industry. Current diagnostic methods for detecting AngHV‐1 are limited to laboratory‐based tests, for example, conventional end‐point PCR and qPCR. Therefore, there is an urgent need to develop an accurate, rapid, and simple detection method for on‐site diagnosis of AngHV‐1. In this study, we developed a recombinase‐aided amplification combined lateral flow dipstick (RAA‐LFD) assay for the detection of AngHV‐1. The RAA‐LFD assay can be performed within a temperature range of 18–45°C, with a reaction time of just 10 min for amplification. Importantly, the established RAA‐LFD assay exhibited no reactivity with other common aquatic viral pathogens, indicating its high specificity. The limit of detection for this method is 102 copies of AngHV‐1, which is more sensitive than the established conventional end‐point PCR method similarly targeting ORF95. Clinical detection of the diseased samples demonstrated that the accuracy of RAA‐LFD was significantly higher than that of the conventional end‐point PCR. In conclusion, the developed RAA‐LFD assay has proven to be a convenient, rapid, sensitive, and reliable tool for on‐site diagnosis of AngHV‐1. This advancement will be invaluable for the prevention and control of AngHV‐1 in the eel farming industry. [ABSTRACT FROM AUTHOR]

Upstream migration of the Atlantic blue crab Callinectes sapidus is well described in its native range, but it is little known in invaded areas.Here, we report several long‐distance migration events of the blue crab in the Iberian Peninsula and use a global database to calculate the probability of occurrence of the species in relation to distance to sea. We then exemplify the potential of blue crab impacts on freshwaters by overlapping its probability of occurrence with that of the critically endangered European eel Anguilla anguilla. Finally, we evaluate the extent to which dam barriers can halt the upstream migration of the blue crab.Invasive blue crab has been detected at 108 km from the sea in the Guadalquivir River (the farthest distance reported in the non‐native range), at 71 km in the Guadiana River and at 53 km in the Ebro River. The probability of occurrence decreased steeply in the first upstream kilometres, but it is still concerning at a dozen kilometres from the river mouth (50% at 19 km, 25% at 38 km, and 3% at 100 km). The blue crab invasive potential widely overlaps (63%) with the Iberian range of the European eel. The blocking effect of dams on the potential invasion of freshwater systems by the blue crab was limited (13%).Our study highlights the need to study the upstream migration of the blue crab within its non‐native range, since it could involve serious impacts in reachable river and stream habitats. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We aimed to evaluate the efficiency of hard-gelatin and hard-hydroxypropyl methylcellulose (HPMC) capsules as biodegradable alternative containers to plastic straws in European eel (Anguilla anguilla), gilthead seabream (Sparus aurata) and European sea bass (Dicentrarchus labrax) sperm cryopreservation. Sperm samples from each European eel (n = 12) were diluted 1:8:1 (sperm: extender P1+5 % egg yolk: methanol). Gilthead seabream (n = 12) samples were individually diluted in a cryoprotectant solution of 5 % Me 2 SO + NaCl 1 % plus BSA (10 mg mL−1) at a ratio of 1:6 (sperm: cryoprotectant solution). European sea bass (n = 10) sperm from each male was diluted in non-activating medium (NAM) at a ratio of 1:5.7 (sperm: NAM), and 5 % of Me 2 SO was added. The diluted European eel and sea bass sperm aliquots (0.5 mL) were individually filled in plastic straws (0.5 mL), hard-gelatin, and HPMC capsules (0.68 mL). Gilthead seabream diluted sperm (0.25 mL) were filled in plastic straws (0.25 mL) and identical capsules described. All samples were frozen in liquid nitrogen vapor and stored in a liquid nitrogen tank. Sperm kinetic parameters were evaluated by CASA-Mot software. Sperm membrane integrity was performed using a Live and Dead KIT and an epifluorescence microscope. To quantify DNA damage, the alkaline comet assay was performed and TailDNA (TD-%) and Olive Tail Moment (OTM) were evaluated by CaspLab software. Sperm cryopreservation of the three Mediterranean species in straws, gelatin, or HPMC capsules reduced the kinetic parameters and cell membrane integrity. Generally, the post-thawing samples cryopreserved in straws and capsules did not differ for the kinetic parameters and cell membrane integrity, except for European sea bass sperm, where the samples stored in gelatin capsules showed higher velocities (VCL - 100; VSL - 76; VAP - 90 μm s−1) than the sperm stored in HPMC capsules (VCL - 87; VSL - 59; VAP - 73 μm s−1). The cryopreservation process did not damage the sperm DNA of European eel and European sea bass, regardless of the containers used. On the other hand, gilthead seabream sperm cryopreserved in gelatin (TD - 9.8 %; OTM - 9.7) and HPMC (TD - 11.1 %; OTM - 11.2) capsules showed higher DNA damage than fresh samples (TD - 3.6 %; OTM - 2.7) and the sperm stored in straws (TD - 4.4 %; OTM - 5.2). The hard-gelatin and HPMC biodegradable capsules can be used as an alternative to straws for European eel, gilthead seabream, and European sea bass sperm cryopreservation. • Hard-capsules maintain the cryopreserved sperm motility, as well as plastic straws. • Sperm thawing in an extender did not compromise the cell membrane integrity. • Capsules' thawing process can damage the gilthead seabream sperm DNA. • Alkaline comet assay protocol for European eel sperm worked well. [ABSTRACT FROM AUTHOR]

Wildlife trafficking has generally been studied for its harm to humans, to species of nonhuman animals, and to ecosystems. Such approaches fail to recognize the intrinsic value of nonhuman animal life. As discussed by various authors, even biodiversity loss discourses tend to adopt a human-centered perspective which overlooks the suffering of those individual nonhuman animals who are trafficked and traded. Through a case study of the European eel based on data collected through interviews, observation, and document analysis, this paper exposes the demand for European eels and the harms caused to them throughout the supply chain. Moreover, using a visual approach to green criminology, the research employs photographs as evidence of harm. By doing so, the paper makes visible the impact of wildlife trafficking and trade on European eels. As the research shows, European eels suffer along the supply chain not only when they are trafficked, but when they are subjected to a legal trade. [ABSTRACT FROM AUTHOR]

This paper integrates political ecology and green criminology to examine the critical endangerment of the European eel. Using a harms-based approach, our research suggests that the identification of organised crime networks as the central perpetrators of illegal wildlife trade (IWT) and of IWT itself as the main threat to eels, neglects a myriad of practices—many of which are related to legal businesses and activities—that significantly contribute to the endangerment of the species. We suggest that, in order to better protect the European eel, we need more holistic conservation measures that go beyond a focus on fisheries and IWT. [ABSTRACT FROM AUTHOR]

This study investigated the concentrations and profiles of 19 perfluoroalkyl substances (PFASs) in the muscle and liver of four freshwater species from Lake Trasimeno (Italy): Anguilla anguilla (European eel), Carassius auratus (goldfish), Perca fluviatilis (European perch), and Procambarus clarkii (red swamp crayfish). In livers, the amount of PFASs ranged from 3.1 to 10 µg kg−1, significantly higher than that in muscle (0.032–1.7 µg kg−1). The predominant PFASs were perfluorooctane sulfonic acid (PFOS) and long-chain carboxylic acids (C8–C14). Short-chain compounds (C4–C5), as well as the long-chain sulfonic acids (C9–C12), were not quantified. The contamination patterns were similar among species with few differences, suggesting the influence of species-specific accumulation. The PFAS concentrations in livers were comparable among species, while in muscle, the higher values were measured in European eel, followed by goldfish, European perch, and red swamp crayfish. The levels were generally lower than those reported for fish from Northern Italian lakes and rivers. The concentrations of regulated PFASs were lower than the maximum limits set by Regulation EU 2023/915 and did not exceed the Environmental Quality Standards (PFOS in biota). This study provides the first valuable insights on PFASs in freshwater species from Lake Trasimeno. [ABSTRACT FROM AUTHOR]

Abstract Diadromous fish such as the European eel (Anguilla anguilla L.) are hampered by a high density of barriers in estuaries and freshwater systems. Modified and fragmented waterbodies lack tidal flows, and habitat may be less accessible and underutilized compared to free-flowing rivers and estuaries. With rising sea levels and increased occurrence of droughts, the number of barriers may further increase, implying that the need to study migration in such areas may even become more urgent worldwide. To study glass eel migration and behaviour in such highly modified water systems, a mark-recapture study was carried out in the North Sea Canal (NSC) basin, which drains into the North Sea via a large sluice complex. In total, eight uniquely tagged groups (3,797 glass eels) were released near the sluice complex, and 11 groups (2,663 glass eels) were released at inland barriers upstream over a 28 km long stretch in the NSC in spring 2018. The sluice complex attracted 10.3 million glass eel and did not block or delay their immigration. The large and diurnally intensively used coastal ship locks and allowings some saltwater intrusion, efficiently facilitated glass eel migration. Once in the NSC, water outlets from adjacent polders attracted glass eels relative proportional to the discharge of pumping stations. In the NSC, average migration speeds of 0.7 km/day (max. 1.8 km/day) were measured, and this increased with higher temperatures. Redistribution of glass eel from accumulations at inland barriers to other outlet locations was observed in both upstream and downstream directions in the NSC. Passage success and residence time (‘delays’ of 4.1–13.7 days) varied between the different inland barriers. Most of the glass eel, however, appears to settle in the easily accessible habitats within the brackish NSC catchment. This study combined an integral assessment of successive bottlenecks in a modified inland water system.

Abstract Acoustic telemetry provides valuable insights into behavioural patterns of aquatic animals such as downstream migrating European eels (Anguilla anguilla), so called silver eels. The behaviour of silver eels during the migration is known to be influenced by environmental factors, yet so is the performance of acoustic telemetry networks. This study quantifies the impact of these environmental factors on both, migration behaviour and receiver performance to determine possible limiting conditions for detecting tagged eels in tidal areas. A dominance analysis of the selected models describing migration speed, activity and receiver performance was conducted following 234 silver eels that were tagged with acoustic transmitters and observed by a receiver network in the Ems River during two subsequent migration seasons. The results suggest a passive locomotion of silver eels during their downstream migration by taking advantage of selective tidal stream transport (STST). It is further shown that water temperature, salinity, turbidity, precipitation, and especially current velocity were major parameters influencing migration activity and speed. At the same time, analyses of the detection probability of tagged eels under varying environmental conditions indicated a decreased receiver performance during increased current velocities, meaning that high migration activity and -speed coincides with reduced detection probability. Consequently, there is a risk that particularly during phases of increased activity, migration activity may be underestimated due to reduced acoustic telemetry performance. To avoid bias in telemetry studies, it is, therefore, crucial to conduct range tests and adjust the receiver placement in areas and conditions of high current velocities.

Diadromous fish such as the European eel (Anguilla anguilla L.) are hampered by a high density of barriers in estuaries and freshwater systems. Modified and fragmented waterbodies lack tidal flows, and habitat may be less accessible and underutilized compared to free-flowing rivers and estuaries. With rising sea levels and increased occurrence of droughts, the number of barriers may further increase, implying that the need to study migration in such areas may even become more urgent worldwide. To study glass eel migration and behaviour in such highly modified water systems, a mark-recapture study was carried out in the North Sea Canal (NSC) basin, which drains into the North Sea via a large sluice complex. In total, eight uniquely tagged groups (3,797 glass eels) were released near the sluice complex, and 11 groups (2,663 glass eels) were released at inland barriers upstream over a 28 km long stretch in the NSC in spring 2018. The sluice complex attracted 10.3 million glass eel and did not block or delay their immigration. The large and diurnally intensively used coastal ship locks and allowings some saltwater intrusion, efficiently facilitated glass eel migration. Once in the NSC, water outlets from adjacent polders attracted glass eels relative proportional to the discharge of pumping stations. In the NSC, average migration speeds of 0.7 km/day (max. 1.8 km/day) were measured, and this increased with higher temperatures. Redistribution of glass eel from accumulations at inland barriers to other outlet locations was observed in both upstream and downstream directions in the NSC. Passage success and residence time (‘delays’ of 4.1–13.7 days) varied between the different inland barriers. Most of the glass eel, however, appears to settle in the easily accessible habitats within the brackish NSC catchment. This study combined an integral assessment of successive bottlenecks in a modified inland water system. [ABSTRACT FROM AUTHOR]

The aim of this study was to investigate the influence of environmental characteristics and anthropogenic pressures on the abundance of estuarine European eels (Anguilla anguilla L.) during their continental growth phase. European eels were collected with fyke nets from spring to autumn in twenty-nine estuaries along the French English Channel and the Atlantic coast. Eel abundance (catch per unit effort, CPUE) was assessed for all eels and by size class for small (total length < 300 mm), intermediate (≥300 to <450 mm), and large (≥450 mm) eels. The environmental characteristics of the French estuaries were described by twelve descriptor variables, mainly related to hydro-morphological and sedimentary factors. Based on principal component analysis and hierarchical clustering analysis, estuary size was identified as the main explanatory variable and used to compare eel abundance. Eel abundance differed significantly according to estuary size, with higher abundances observed in small estuaries (7.22 to 13.00 ind. fyke nets 24 h−1) compared to large estuaries (0.13 to 0.71 ind. fyke nets 24 h−1). Spatial variation in eel abundance was correlated with differences in estuary size for all eel size classes. The influence of anthropogenic pressures on eel abundance was assessed by nine anthropogenic estuarine pressure indicators. The results indicate that high values of the anthropogenic pressure indicators were correlated with low eel abundance. This study highlights that large French estuaries subject to stronger anthropogenic pressures were less favourable habitats than small estuaries with less anthropogenic pressure. [ABSTRACT FROM AUTHOR]

• An illuminated bridge can create a light barrier when it embodies unnatural light variations with increased and decreased light levels. • The natural heterogeneity of a river can be adversely affected by illuminated bridges that create light barriers. • An illuminated bridge can cause a pitfall effect where fish are bound to a small area, resulting from their responses to light stimuli. • The unnatural presence and absence of light can create a behavioural barrier for migrating fish. Illuminated bridges have become important assets to navigable aquatic systems. However, if artificial light at night (ALAN) from illuminated bridges reaches aquatic habitats, such as rivers, it can threaten the river's natural heterogeneity and alter the behavioural responses of migratory fish. Here, via a pilot study, we quantified levels of ALAN at illuminated bridges that cross a river and, propose a conceptual model to estimate its potential implications on two migrating fish species with contrasting life histories. Night-time light measurements on the river Spree in Berlin were performed continuously along a transect and in detail at seven illuminated bridges. Photometric data of the pilot study showed rapidly increased and decreased light levels at several illuminated bridges from which we derived several model illumination scenarios. These illumination scenarios and their potential effect on migrating Atlantic salmon smolts (Salmo salar) and European silver eel (Anguilla anguilla) are presented as a conceptual model, considering illuminated bridges as behavioural barriers to fish migration. ALAN's adverse effects on freshwater habitats must be better researched, understood, managed, and properly communicated to develop future sustainable lighting practices and policies that preserve riverscapes and their biodiversity. [ABSTRACT FROM AUTHOR]

Production of European eel offspring has become a reality, but liquid diets during larval culture hold new challenges. This study focused on increasing food amounts without compromising well-being or healthy larvae-bacteria interactions. First-feeding larvae were fed two food amounts (Low = 0.5 mL food/L water vs. High = 1.5 mL food/L water) until 30 days post-hatch (dph). Results indicated that ~75% of larvae ingested the diet in both treatments, but upregulation of a stress/repair-related gene (hsp90) on 25 and 30 dph indicated nutritional inadequacy. Larvae fed a High amount of food were 3.68% bigger, while larvae in the Low-food group showed 45.2% lower gut fullness and upregulated expression of the gene encoding the "hunger hormone" ghrelin (ghrl), indicating signs of starvation. The High-food group larvae exhibited a healthier bacteriome with a higher abundance of potentially beneficial orders (Lactobacillales and Bacillales), whereas the Low-food group showed more potentially harmful orders (Vibrionales, Rhodobacterales, and Alteromonadales). While survival was initially lower in the High-food group, both treatments had comparable survival by the end of the experiment. In conclusion, feeding European eel larvae with High food amounts seemed beneficial, supported by increased gut fullness, reduced ghrl expression (no starvation), enhanced growth, and the presence of a healthier bacteriome. [ABSTRACT FROM AUTHOR]

The globally threatened European eel (Anguilla anguilla) is the only freshwater fish native to Macaronesia. Despite being a keystone species to freshwater habitats, little is known about its population, environmental associations and conservation requirements in oceanic islands.The density and geographical‐related factors influencing the distribution of A. anguilla in the subtropical Archipelago of Madeira (Portugal) were examined. Data on the species occurrence was initially collected using an online survey addressed to citizens. Eels were then sampled through electrofishing in 31 sampling sites along 11 streams on Madeira Island, and eel‐habitat associations were investigated using LMMs and GLMMs. One stream was also sampled in Porto Santo Island.The presence of A. anguilla was confirmed in the two islands of the archipelago with streams (Madeira and Porto Santo). Eels are distributed throughout Madeira Island, being more abundant – particularly small size individual– at lower altitudes (mostly below 150 m). The number of weirs was an important predictor of eel density, suggesting that these flood‐control structures limit their upstream migration.This study represents a pioneer sampling effort of the European eel in Madeira and results provide much‐needed baseline information on the species distribution and related environmental factors in oceanic islands. While most eel research has historically focused on mainland habitats where dams pose a significant threat to eel habitats, our study underscores the critical importance of addressing smaller, often overlooked barriers that may also cause habitat fragmentation.The European eel is a panmictic species, thus, the conservation of the Macaronesian subpopulations can have substantial benefits for its global recovery. A comprehensive investigation of the impact of man‐made obstacles on eel habitat is still needed. Similarly, other common threats such as interactions with non‐indigenous species and infection by Anguillicola crassus should be a research priority in future studies. [ABSTRACT FROM AUTHOR]

Zoogeography and ecology of 3 widespread anguillid species (Anguilla marmorata, A. megastoma, A. obscura) in the western South Pacific are reviewed in relation to the freshwater habitats and communities of the many islands of Oceania (island geological ages, from > 40 Ma to < 1 Ma. Anguillid eels show habitat segregation among A. obscura (lower reaches-brackish water), A. marmorata (habitat generalist) and A. megastoma (upper reaches), where they feed on amphidromous snails, Macrobrachium shrimp and gobies, and freshwater insect larvae. Some information has been obtained about possible anguillid spawning areas in the region near New Caledonia and Fiji from larval surveys and satellite transmitting tagging studies. Larval drift simulations suggest separate spawning areas occur in the eastern region of French Polynesia. Little is known about how geography might influence the spawning ecologies and population structures of South Pacific anguillid eels that live on the many tropical islands that are spread across a wide region of ocean in the western Indo-Pacific. [ABSTRACT FROM AUTHOR]

The Scientific Advisory Committee on Fisheries (SAC) of the General Fisheries Commission for the Mediterranean (GFCM) held its twenty-fourth session at the headquarters of the Food and Agriculture Organization of the United Nations (FAO) in Rome, Italy, from 20 to 23 June 2023, with Lebanon as honorary host. The session was attended by delegates from 18 Mediterranean contracting parties, seven observers, as well as representatives of FAO, the GFCM Secretariat and invited experts. The Committee reviewed the work carried out during the 2022–2023 intersession, including in the context of the MedSea4Fish programme, and provided advice on the status of priority stocks and ecosystems and on potential management measures addressing key fisheries and vulnerable species in the Mediterranean. At the regional level, the Committee provided advice on: i) European eel (Anguilla anguilla), red coral (Corallium rubrum) and common dolphinfish (Coryphaena hippurus) fisheries in the Mediterranean; ii) minimum conservation reference size for GFCM priority species, including deep-water red shrimp (Aristaeomorpha foliacea and Aristeus antennatus) and European hake (Merluccius merluccius) at the regional level as well as small pelagics in the Adriatic sea and round sardinella (Sardinella aurita) in the eastern Mediterranean; and iii) the socioeconomic impacts of a potential extension of bottom trawling limits. With regard to small-scale fisheries (SSF), the Committee supported the need to revise the monitoring framework of the Regional Plan of Action Regional Plan of Action for small-scale fisheries in the Mediterranean and the Black Sea (RPOA-SF). It discussed additional work in support of the GFCM, endorsing dedicated research programmes, including on recreational fisheries and on jellyfish (Pelagia noctiluca) in the Alboran Sea, as well as a draft regional plan of action to monitor and mitigate interactions between fisheries and vulnerable species in the Mediterranean and the Black sea and identified further actions towards the implementation of standardized monitoring plans for fisheries restricted area (FRAs) and the development of pilot studies to identify boundaries of known vulnerable marine ecosystem. It also discussed issues related to decarbonization and climate change, estimation of discards and fishing capacity. In line with the subregional approach, the Committee formulated advice on i) blackspot seabream (Pagellus bogaraveo) in the western Mediterranean; ii) small pelagics in the Alboran Sea; iii) a FRA in the Cabliers Coral Mound Province; iv) Norway lobster (Nephrops norvegicus), red mullet (Mullus barbatus) and striped red mullet (Mullus surmuletus) in the central Mediterranean; v) round sardinella, SSF and non-indigenous species in the eastern Mediterranean; and vi) small pelagics and key demersal stocks in the Adriatic Sea. Finally, the Committee agreed upon its workplan for 2023–2025. [ABSTRACT FROM AUTHOR]

Acoustic telemetry provides valuable insights into behavioural patterns of aquatic animals such as downstream migrating European eels (Anguilla anguilla), so called silver eels. The behaviour of silver eels during the migration is known to be influenced by environmental factors, yet so is the performance of acoustic telemetry networks. This study quantifies the impact of these environmental factors on both, migration behaviour and receiver performance to determine possible limiting conditions for detecting tagged eels in tidal areas. A dominance analysis of the selected models describing migration speed, activity and receiver performance was conducted following 234 silver eels that were tagged with acoustic transmitters and observed by a receiver network in the Ems River during two subsequent migration seasons. The results suggest a passive locomotion of silver eels during their downstream migration by taking advantage of selective tidal stream transport (STST). It is further shown that water temperature, salinity, turbidity, precipitation, and especially current velocity were major parameters influencing migration activity and speed. At the same time, analyses of the detection probability of tagged eels under varying environmental conditions indicated a decreased receiver performance during increased current velocities, meaning that high migration activity and -speed coincides with reduced detection probability. Consequently, there is a risk that particularly during phases of increased activity, migration activity may be underestimated due to reduced acoustic telemetry performance. To avoid bias in telemetry studies, it is, therefore, crucial to conduct range tests and adjust the receiver placement in areas and conditions of high current velocities. [ABSTRACT FROM AUTHOR]

European eel (Anguilla anguilla) is a euryhaline species that has adapted to survive in both hyper- and hypo-osmotic conditions. Although substantial research has been carried out on the effect of water salinity on this species' osmoregulation, growth, and survival, no research has been carried out on optimum habitats for better performance. This study evaluated the effect of different water habitats on the water quality, carcass composition, blood parameters, oxidative stress, and immune response of European eel. This study employed sampling from the Nile River, the Mediterranean Sea, Lake Burullus, and private farms. The results show that the fish cultured in the seawater had the best water quality parameters, antioxidant response, blood biochemical parameters, immune status, and less stress response. Conversely, fish grown in pond water showed the highest stress conditions in cortisol levels and, consequently, the lowest antioxidative, biochemical and immunological response. There was no significant effect of water habitats on carcass composition. These results suggest that the performance of Anguilla anguilla can be optimized by culturing in seawater. [ABSTRACT FROM AUTHOR]

Optimising the design of passage facilities to restore fluvial connectivity for juvenile European eel (Anguilla anguilla) is a key priority within conservation efforts for the species, across the majority of its freshwater range. Employing an experimental setup that simulated gravity-fed upstream eel passes, this study demonstrated that novel V-profile passes, which incorporate two lateral slopes (15°), performed better than laterally flat passes over the flow rates tested (0.2–0.6 L s−1). For the small eel size used (60–80 mm length), the bristle substrate consistently outperformed studs, but the lateral slope had a greater effect on passage metrics than the substrate choice. Our findings strongly support the use of V-shaped channels for upstream migrating eel at fish passage facilities, particularly in scenarios where flow rates may be elevated and/or fluctuating, such as for gravity-fed passes. [ABSTRACT FROM AUTHOR]

Morphological similarity between an intruder and a native species can hinder the detection of the intruder. The undetected non-native species will thus have more time to affect the local ecosystem and spread. This paper describes an instance of unnoticed introduction of a fish species morphologically resembling a native endemic species. We report on the first established population of the Rhodes minnow, Ladigesocypris ghigii (Actinopterygii: Leuciscidae), endemic to Rhodes Island, which has been recently introduced to a river near Athens, mainland Greece. When it was first noticed, in 2017, the species was misidentified due to its morphological similarity to Pelasgus marathonicus, the only native leuciscid of the streams and rivers around Athens, until it was subject to genetic and detailed morphological examination. Genetic analyses based on both mitochondrial (cytochrome b) and nuclear (recombination activating gene, RAG1) markers unambiguously identified the collected individuals and provided information on the relationships of Ladigesocypris at both the species and generic level. Our investigation supports that L. ghigii was recently established near Athens, and that the population probably originated from Rhodes Island, an eastern Aegean island. In 2021, a high-density thriving mainland population coexisted solely with the native European eel (Anguilla anguilla) along at least 9.7 km of river habitat. It was assessed as being of high invasive risk for the region where it was established by the Aquatic Species Invasiveness Screening Kit (AS-ISK) screening tool. We propose a feasibility study be undertaken to investigate the application of measures that may include actions to exterminate or contain the population. Special attention is required since the species is considered vulnerable in its native range, while the translocated population coexists with native biota within a stream ecosystem of outstanding local conservation value. [ABSTRACT FROM AUTHOR]

In the area of the Neretva delta in the eastern Adriatic, where the European eel, Anguilla anguilla (Linnaeus, 1758) has been traditionally fished for centuries, a decline in its population has been observed, as in most of Europe. Despite several studies, systematic monitoring was not performed, and the causes of population decline are attributed to anthropogenic stressors, mainly overfishing and interventions that disrupt the migration. With the stock at a low level, there is a need for a detailed assessment of biological data and the determination of the "zero state" of the eel population in the areas where monitoring was not previously performed, such as the Neretva delta. This data would serve as a basis for the development of an appropriate monitoring and eel management plan. One of the under-researched aspects is still the eel's morphology, which is closely related to all basic life functions. The aim of this work was to analyze in detail the morphological parameters of yellow and silver eels from the mouth of the Neretva River in different seasons and the relationships between the measured morphometric parameters and physiological indicators and to compare them with previously published results for different life stages across Europe. The samples were collected during spring, summer and autumn of 2021, and winter of 2022. Yellow eels were present in the catch throughout the sampling period, while silver eels were caught in the autumn and winter. Yellow and silver eels were significantly different regarding 22 morphometric measures that were analyzed. Isometric growth was recorded for yellow eels in the spring and autumn of 2021, and positive allometric growth was recorded for yellow eels in the summer and silver eels in the autumn of 2021 and winter of 2022. PCA showed that the main factor that separates the eels grouped by life stage in different seasons is the intestine length (IL), whereas the rest of the factors (weight—W; intestine weight—IW; liver weight—LW; and total length—TL) affect the groupings almost equally. Seasonal averages of the condition factor (CF) for yellow and silver eels did not differ statistically. Three indicators were used to describe intestine morphology: relative gut weight (RGW), relative gut length (RGL), and Zihler's index (ZHI); and the only statistically significant difference between yellow and silver eels was recorded for the RGW. The hepatosomatic index (HSI) was significantly different between silver eels in winter and yellow eels in spring. In addition to supplementing the already known facts, this paper provides new information on the functional morphology of the European eel. Monitoring of these characteristics is crucial for management of the European eel fisheries as they are directly related to functional performance and affect the ability to maintain sustainable populations in anthropogenically altered environments. [ABSTRACT FROM AUTHOR]

A species of catfish belonging to the Anguilla genus called Anguilla marmorata has long been exploited and grown in Vietnam. Marbled eels are regarded as a specialty seafood species with several therapeutic and nutritive benefits. This study used High-Performance Liquid Chromatography (HPLC) to identify various biochemical and nutritional ingredients in A. marmorata collected in its natural habitat. There was 16 different amino acids analysed in the muscle, skin, and mucus of marbled eels at the rates of 14.01%, 5.48%, and 9.96%, respectively. Twenty-four fatty acids were found in muscle and skin, while only nine fatty acids were detected in the mucus with proportions of 11.93, 25.91, and 0.37 g/100 g, respectively. Six different vitamins (A, B3, B5, D3, C, E) are found in the muscle of the eel; five types of vitamins (A, B3, B5, D3, E) were detected in the skin; and three vitamins (B3, B5, C) are found in mucus. Using HPLC analysis has shown remarkable effectiveness in micronutrient analysis in nutritional research. The results will increase our knowledge of the nutritive benefits of the A. marmorata and point the way toward the creation of functional foods and cosmetics with natural ingredients that may be used on people. [ABSTRACT FROM AUTHOR]

Habitat fragmentation and migration barriers have attributed to the decline of European eel (Anguilla anguilla). The migration of silver eels through pumping stations, ship locks, and migration facilities within the North Sea Canal basin using acoustic telemetry and overall escapement by mark–recapture was studied. A network of 61 acoustic receivers and three PIT‐tag stations was built, and 305 silver eels were tagged with acoustic transmitters and 3923 with PIT‐tags. Of all the silver eels that were detected, 55% passed barriers between the polders and the canal and 46% also passed the barrier complex at IJmuiden to sea, mainly via the ship locks. Overall, silver eel escapement to the North Sea per year averaged 81,629 silver eels and 14.3% suffered mortality in the pumping station at IJmuiden. Migration speed was lower for silver eels that initiated their migration upstream compared with silver eels further downstream, higher for silver eels passing barriers later in the migration period, and highest at sea. This study of silver eel movement and escapement in the North Sea canal basin indicated several bottlenecks. Passage success along barriers varied strongly between sites and types of barriers. While at two smaller locations, silver eels migrated through the pedal valves in the ship lock gates, a small passage facility and the pedal valves in a ship lock at complex IJmuiden did not enhance silver eel migration. The barriers and unnatural canal system caused additional delay during silver eel migrations. Mitigation measures for management could include installing fish‐friendly pumps, using pumping stations only during the day, and in addition opening ship locks and pedal valves at the beginning of the night. [ABSTRACT FROM AUTHOR]