Three-dimensional migratory behaviour of European silver eels (Anguilla anguilla) approaching a hydropower plant

The global population of European eel (Anguilla anguilla) is rapidly declining, and migration barriers in rivers are believed to be one of several key causes. While progress has been made in the development of bypass solutions, they are often constructed based on a limited knowledge of swimming behaviour. A bypass close to the stream bed is often recommended at fish passage facilities to accommodate downstream eel migration. The results of this recommendation are poorly studied, and the few studies that exist show varying bypass efficiencies. The current study used acoustic telemetry with depth sensors to explore the three-dimensional migratory behaviour of downstream-migrating silver eels. The eels were tracked as they approached a hydropower plant with a state-of-the-art angled bar rack and full-depth bypass. Downstream and upstream swimming differed in preferred vertical and lateral positions. During periods of local downstream movement, the density of observations was largest in the upper middle section, away from the river boundaries and in higher velocities. Conversely, when moving upstream, eels tended to avoid the upper layers of the middle part of the river, swimming closer to the riverbed and using the bank areas to a greater extent. Downstream-moving fish swam higher in the water column during night and in turbid conditions (high discharge). When approaching the impassable bar rack and the full-depth bypass, the eels searched most intensely but not exclusively along the bottom third of the rack, often exploring at new depths after changing direction. The impediment passage efficiency was 100% when both bypass solutions were considered. The study provides knowledge of the swimming behaviour of silver eels, which is relevant for the design of bypass solutions for eels at migration barriers.

Evaluation of Chemical Elements, Lipid Profiles, Nutritional Indices and Health Risk Assessment of European Eel (Anguilla anguilla L.)

The concentrations of ten elements (K, Na, Ca, Mg, Zn, Fe, Hg, Cu, Mn, and Cd) and fatty acids were analyzed in muscles of the European eel (Anguilla anguilla Linnaeus, 1758). The eels were caught in freshwater lakes connected with the Sawica River (north-eastern Poland). On this basis, it was determined whether the consumption of the fish is beneficial and safe for the health of the consumer. The results showed that the metal concentrations followed this order: K > Na > Ca > Mg > Zn > Fe > Hg > Cu > Mn > Cd. The fatty acids gave rise to the following sequence: MUFAs > SFAs > n-3 PUFAs > n-6 PUFAs. The target hazard quotient (THQ) value was below 1.0. The hazard quotient for the benefit-risk ratio HQ_EFA (0.39) also was below one, indicating that the intake of the recommended dose of EPA + DHA (250 mg/day) and the intake of mercury (0.415 mg/kg) for a person weighing 70 kg does not pose an obvious risk for human health. The lipid quality indices were OFA: 24.69, DFA: 74.36, AI: 0.55, and TI: 0.41. Based on the above statements, the consumption of eel meat is safe from a health point of view. However, the levels of toxic metals in the muscles of eels and their environment should continue to be monitored, as eels occupy a high position in the food chain.

Organochlorine residues in muscle of European eels (Anguilla anguilla) from four Spanish Mediterranean wetlands and coastal lagoons

European eels (Anguilla anguilla) are an endangered species throughout their range, and chlorine organic compounds are some of the most important pollutants for marine species. Data on contaminants in eel stocks remain incomplete, so organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs) in muscle of European eels from four Spanish Mediterranean ecosystems were analyzed. COPs are presents in eels from all areas, but some compounds are not detected: HCH α, β and γ (lindane), endosulfan sulfate, heptachlor, and PCBs 28, 52 and 180. The high percentage of DDT 2,4' in eels from S'Albufera des Grau Natural Park, an ecosystem with good ecological status according to the Water Framework Directive, and the presence of PCBs in S'Albufereta Natural Reserve indicate the need to carry out further studies in the future. The results obtained can improve the management of this species in the studied areas.

Isolation and Identification of a New Isolate of Anguillid Herpesvirus 1 from Farmed American Eels (Anguilla rostrata) in China

Anguillid herpesvirus 1 (AngHV-1) is a pathogen that causes hemorrhagic disease in various farmed and wild freshwater eel species, resulting in significant economic losses. Although AngHV-1 has been detected in the American eel (Anguilla rostrata), its pathogenicity has not been well characterized. In this study, an AngHV-1 isolate, tentatively named AngHV-1-FC, was isolated from diseased American eels with similar symptoms as those observed in AngHV-1-infected European eels and Japanese eels. AngHV-1-FC induced severe cytopathic effects in the European eel spleen cell line (EES), and numerous concentric circular virions were observed in the infected EES cells by transmission electron microscopy. Moreover, AngHV-1-FC caused the same symptoms as the naturally diseased European eels and Japanese eels through experimental infection, resulting in a 100% morbidity rate and 13.3% mortality rate. The whole genome sequence analyses showed that the average nucleotide identity value between AngHV-1-FC and other AngHV-1 isolates ranged from 99.28% to 99.55%. However, phylogenetic analysis revealed that there was a genetic divergence between AngHV-1-FC and other AngHV-1 isolates, suggesting that AngHV-1-FC was a new isolate of AngHV-1. Thus, our results indicated that AngHV-1-FC can infect farmed American eels, with a high pathogenicity, providing new knowledge in regard to the prevalence and prevention of AngHV-1.

Impact of environmental micropollutants and diet composition on the gut microbiota of wild european eels (Anguilla anguilla)

In fish, the gut microbiome plays a crucial role in homeostasis and health and is affected by several organic and inorganic environmental contaminants. Amphidromous fish are sentinel species, particularly exposed to these stressors. We used whole metagenome sequencing to characterize the gut microbiome of wild European eels (Anguilla anguilla) at a juvenile stage captured from three sites with contrasted pollution levels in term of heavy metals and persistent organic pollutants. The objectives were to identify what parameters could alter the gut microbiome of this catadromous fish and to explore the potential use of microbiota as bioindicators of environment quality. We identified a total of 1079 microbial genera. Overall, gut microbiome was dominated by Proteobacteria, Firmicutes and Actinobacteria. Alpha and beta diversity were different amongst sites and could be explained by a reduced number of environmental and biological factors, specifically the relative abundance of fish preys in eels' diet, PCB101, γHCH (lindane), transnonachlor and arsenic. Furthermore, we identified a series of indicator taxa with differential abundance between the three sites. Changes in the microbial communities in the gut caused by environmental pollutants were previously undocumented in European eels. Our results indicate that microbiota might represent another route by which pollutants affect the health of these aquatic sentinel organisms.

Contrasting riverine distribution and habitat use of the Japanese eel, Anguilla japonica, and the giant mottled eel, Anguilla marmorata, in a sympatric river

Electrofishing and visual observations revealed contrasting riverine distribution and habitat use of the Japanese eel, Anguilla japonica, and the giant mottled eel, Anguilla marmorata, in a relatively gentle-slope river of Southern Kyushu, Japan. A. japonica was distributed in the lower and upper reaches. A. marmorata inhabited the middle reaches with relatively fast water velocity, heterogeneous water depth, coarse substrata and no adjacent paddy fields. This may have important conservation implications because a decrease in environmental diversity and/or river connectivity could result in the disappearance of A. marmorata habitats.

Survival, growth and tag retention of juvenile European eel (Anguilla anguilla L.) with implanted 12 mm passive integrated transponder tags and acoustic tags

To evaluate the efficiency of tagging juvenile European eels with implanted 12 mm passive integrated transponder (PIT) tags or Eel/Lamprey acoustic transmitters (ELATs), the authors studied tag retention, survival and growth of eels (7-25 g). Experimental eels were obtained from an eel farm, tagged and then released in a series of shallow dug-out ponds with a surface area of c. 200 m² . Tagged and control eels were distributed evenly, with 50 tagged and 50 control eels in each of four ponds, giving a total of 200 tagged and 200 control eels mixed. After 76 days, the ponds were drained, and eels were sampled and measured. A total of 344 eels (86%) were recaptured, indicating high survival. Tag retention was 99% as only one of the recaptured PIT-tagged eels had lost the tag and none of the ELAT tagged. The results demonstrated that tagging juvenile eels >16 cm with these small tags is indeed feasible. The growth of tagged and control fish was differentiated but generally low in length and negative in mass but did not differ between the three groups.

Slower growth of farmed eels stocked into rivers with higher wild eel density

Farmed anguillid eels are frequently stocked into natural fresh waters to enhance eel resources, but little is known about what happens to these eels or their interactions with wild eels after stocking. A recent study observed a depressed survival and growth rate of farmed Japanese eels when they were reared with wild eels, which indicated that wild eels might interfere with the survival and growth of farmed-and-stocked eels through intraspecific competition. To contribute to improving eel stocking efficiency, the growth of farmed-and-stocked Japanese eels was compared among four rivers with different wild eel densities using mark-and-recapture studies. Based on the 2-year recapture survey after stocking, it was found that the density of the farmed-and-stocked eels was not significantly different among rivers. The daily growth rates of farmed-and-stocked eels in the rivers with lower wild eel density were significantly higher than those of the eels stocked into the rivers with higher wild eel density. The farmed-and-stocked eels moved significantly greater distances downstream than wild eels that showed sedentary behaviour. This and previous studies indicate that significant questions remain about the effectiveness of stocking farmed eels into water bodies where naturally recruited wild eels are present.

Identification of virus infections of European eels intended for stocking measures

The spread of viral diseases in eels is suggested to severely affect the European eel (Anguilla anguilla) panmictic population. The European Commission has initiated the Eel Recovery Plan (Council Regulation No. 1100/2007) to try to return the European eel stock to more sustainable levels within that measures eel restocking. However, scientific evidence evaluating the efficacy of stocking remains scarce. In addition, knowledge about the impact and contribution of eel stocking on the distribution of infectious diseases is insufficient. In this study, we aimed to investigate virus infections in batches of eels intended for restocking. We analysed samples of glass eels from certified fisheries and farmed European eels from different aquaculture farms. All analysed eels were purchased within a North Rhine Westphalian conservation program. Via a combination of cell culture and qPCR-based techniques, we detected infections of glass eels with the rhabdovirus Eel Virus European X and anguillid herpesvirus 1 infections in farmed eels (10-15 cm).

Microplastics across biomes in diadromous species. Insights from the critically endangered Anguilla anguilla

Microplastic pollution affects freshwater and marine biota worldwide, microplastics occurring even inside the organisms. With highly variable effects, from physical damage to toxicity of plastic compounds, microplastics are a potential threat to the biodiversity, community composition and organisms' health. This emerging pollutant could overstress diadromous species, which are exposed to both sea and river water in their life cycle. Here we have quantified microplastics in young European eel Anguilla anguilla, a critically endangered catadromous fish, entering three rivers in southwestern Bay of Biscay. River water, sediments and seawater were also analysed for microplastics. The microplastic type was identified using Fournier-Transform Infrared spectroscopy and then searched for their hazard potential at the European Chemical Agency site. Both riverine and sea microplastic pollution were predictors of eels' microplastic profile (types of microplastics by shape and colour): A. anguilla juveniles entering European rivers already carry some marine microplastics and acquire more from river water. Potentially hazardous plastic materials were found from eels, some of them dangerous for aquatic life following the European Chemical Agency. This confirms microplastics as a potential threat for the species. Between-rivers differences for microplastics profiles persistent over years highlight the convenience of analysing and preventing microplastics at a local spatial scale, to save diadromous species from this stressor. Since the origin of microplastics present in glass eels seems to be dual (continental + seawater), new policies should be promoted to limit the entry of microplastics in sea and river waters.

Role of estuarine habitats for the feeding ecology of the European eel (Anguilla anguilla L.)

This study aims to characterize and compare the feeding ecology of the European eels (Anguilla anguilla L.) during the continental phase (i.e. yellow and silver) along a salinity gradient (i.e. lower, middle and upper) in six northern France estuaries (i.e. brackish water). The diet and stable isotopic (i.e. δ15N and δ13C values) compositions of eels collected with a fyke net in six estuaries (Slack, Wimereux, Liane, Canche, Authie and Somme estuaries) located along the French coast of the eastern English Channel per season over a year were described by combining gut content and stable isotope analyses. Eel guts were dominated by typical BW prey, Malacostraca and Actinopterygii (54% and 40%, respectively), with the gammare Gammarus zaddachi and the green crab Carcinus maenas (38% and 14%, respectively), and smaller yellow eels of A. anguilla and juvenile European flounder, Platichthys flesus (19% and 14%, respectively) being the most frequently found in their guts. The δ13C values of a majority of eels confirmed the sea- and brackish water-specific carbon resources. Dietary and isotopic niche revealed no clear change between total length, silvering stages and seasons, but a significant difference between salinity gradients and estuaries. Eels δ13C values showed significant enrichment from upper to lower along the estuaries while the δ15N values showed an inverse effect, with the lowest values in the lower part and highest in the upper part. Higher variability in δ13C values in larger estuaries suggested that eels feed on a wide range of food sources than in smaller estuaries. While eels in the smaller estuaries fed mainly on Actinopterygii prey, eels in the larger ones had a lower trophic level (i.e. δ15N values) and fed mainly on Malacostraca prey. This spatial difference in dietary and isotopic niche is discussed in relation to biological structure of eel and environmental variables.

Agonistic behaviour of wild eels and depressed survival and growth of farmed eels in mixed rearing experiments

To contribute to improving eel stocking procedures, the survival, growth and behaviour of farmed Japanese eels reared together with wild individuals were evaluated to learn about possible encounters of farmed- and stocked eels with wild eels. Two experiments were conducted to evaluate the (a) effect of the presence of wild eels on survival and growth of farmed eels in experimental ponds and (b) behavioural differences between wild and farmed eels placed in a small tank. In the survival and growth experiment, significantly lower survival and growth rates of farmed eels reared with wild eels were observed compared with farmed eels reared without wild eels (control). In the behavioural observations, the authors observed significantly higher occupation rates of a refuge and more frequent biting for wild eels, indicating a dominance of wild eels over farmed eels. Because wild and farmed eels used in the current experiments belong to the same genetic population, these differences resulted within one generation through the aquaculture rearing process.

CO2 induced seawater acidification impacts survival and development of European eel embryos

Fish embryos may be vulnerable to seawater acidification resulting from anthropogenic carbon dioxide (CO₂) emissions or from excessive biological CO₂ production in aquaculture systems. This study investigated CO₂ effects on embryos of the European eel (Anguilla anguilla), a catadromous fish that is considered at risk from climate change and that is targeted for hatchery production to sustain aquaculture of the species. Eel embryos were reared in three independent recirculation systems with different pH/CO₂ levels representing “control” (pH 8.1, 300 μatm CO₂), end-of-century climate change (“intermediate”, pH 7.6, 900 μatm CO₂) and “extreme” aquaculture conditions (pH 7.1, 3000 μatm CO₂). Sensitivity analyses were conducted at 4, 24, and 48 hours post-fertilization (hpf) by focusing on development, survival, and expression of genes related to acute stress response (crhr1, crfr2), stress/repair response (hsp70, hsp90), water and solute transport (aqp1, aqp3), acid-base regulation (nkcc1a, ncc, car15), and inhibitory neurotransmission (GABAAα6b, Gabra1). Results revealed that embryos developing at intermediate pH showed similar survival rates to the control, but egg swelling was impaired, resulting in a reduction in egg size with decreasing pH. Embryos exposed to extreme pH had 0.6-fold decrease in survival at 24 hpf and a 0.3-fold change at 48 compared to the control. These observed effects of acidification were not reflected by changes in expression of any of the here studied genes. On the contrary, differential expression was observed along embryonic development independent of treatment, indicating that the underlying regulating systems are under development and that embryos are limited in their ability to regulate molecular responses to acidification. In conclusion, exposure to predicted end-of-century ocean pCO₂ conditions may affect normal development of this species in nature during sensitive early life history stages with limited physiological response capacities, while extreme acidification will negatively influence embryonic survival and development under hatchery conditions.

Observation of the feeding behaviour of reared Japanese eel Anguilla japonica leptocephali fed picocyanobacteria Synechococcus spp

The authors observed the feeding behaviour of artificially reared Japanese eel Anguilla japonica leptocephali, 7.5-19 mm total length (10-61 days post-hatch), fed Synechococcus sp., which is considered a potential food source of anguilliform larvae. Three strains of Synechococcus sp. (NIES-972, 976 and 979) were tested as the food material. Larvae across the entire length range could effectively ingest a suspension of pico-sized cyanobacteria (1-3 μm in diameter). Video observations of the mid-hindgut of larvae under an epifluorescence microscope confirmed that the movement of microvilli of the intestinal epithelium allowed the cell particles to circulate in the mid-hindgut, before becoming solidified in the anal region. Significant differences in food intake were observed between larvae fed two strains of Synechococcus (NIES-972 and 976), and among different cell densities, which suggests feeding selectivity and density dependence. Comparisons of feeding behaviour under the light group (9L:15D) and the dark group (24D) showed significantly higher food intake (measured as an index of intestinal fullness) in the light group, although substantial and continuous ingestion was observed in the dark group, indicating continuous feeding by swallowing sea water. The authors hypothesise that the feeding ecology of anguilliform leptocephali is based on a survival strategy whereby the larvae do not compete with various higher-trophic-level fishes for food in an oligotrophic environment.

The relevance of European Biota Quality Standards on the ecological water quality as determined by the multimetric macro-invertebrate index: A Flemish case study

European Biota Quality Standards (EQS_biota), for compounds with low water solubility and high biomagnification, were created to sustain water quality and protect top predators and humans from secondary poisoning. In reality, for multiple compounds, an exceedance of these standards is often reported in literature without a decrease in ecological water quality determined by biotic indices. In the present study, threshold concentrations were defined in biota (from 44 sampling locations throughout Flanders (Belgium)), above which a good ecological water quality, assessed by the Multimetric Macroinvertebrate Index Flanders (MMIF), was never reached. Threshold values were compared to current EQS_biota. Accumulated perfluoroctane sulfonate (PFOS), mercury (Hg), hexabromocyclododecane (HBCD), polybrominated diphenyl ethers (PBDEs), dioxins and polychlorinated biphenyls (PCBs) concentrations were measured in muscle tissue of European yellow eel (Anguilla anguilla) and perch (Perca fluviatilis). Fluoranthene and benzo(a)pyrene were also analyzed in translocated mussels (Dreissena bugensis, D. polymorpha and Corbicula fluminea). Threshold values could only be calculated using a 90th quantile regression model for PFOS (in perch; 12 µg/kg ww), PCBs (in eel; 328 µg/kg ww) and benzo(a)pyrene (in mussels: 4.35 µg/kg ww). The lack of a significant regression model for the other compounds indicated an effective threshold value higher than the concentrations measured in the present study. Alternatively, the 95th percentile of concentrations measured in locations with a good ecological quality (MMIF≥0.7), was calculated for all compounds as an additional threshold value. Finally, fish concentrations were standardized for 5% lipid content (or 26% dry weight content for PFOS and Hg). Threshold values for PFOS and benzo(a)pyrene and the 95th percentiles for dioxins and fluoranthene were comparable to the existing standards. For all other compounds, the 95th percentile was higher than the current EQS_biota, while for HBCD, it was lower. These results strongly advise revising and fine-tuning the current EQS_biota, especially for ∑PBDE and HBCD.

Type of hormonal treatment administered to induce vitellogenesis in European eel influences biochemical composition of eggs and yolk-sac larvae

Egg biochemical composition is among the main factors affecting offspring quality and survival during the yolk-sac stage, when larvae depend exclusively on yolk nutrients. These nutrients are primarily embedded in the developing oocytes during vitellogenesis. In aquaculture, assisted reproduction procedures may be applied enabling gamete production. For the European eel (Anguilla anguilla), reproductive treatment involves administration of pituitary extracts from carp (CPE) or salmon (SPE) to induce and sustain vitellogenesis. In the present study, we compared the influence of CPE and SPE treatments on offspring quality and composition as well as nutrient utilization during the yolk-sac stage. Thus, dry weight, proximal composition (total lipid, total protein), free amino acids, and fatty acids were assessed in eggs and larvae throughout the yolk-sac stage, where body and oil-droplet area were measured to estimate growth rate, oil-droplet utilization, and oil-droplet utilization efficiency. The results showed that CPE females spawned eggs with higher lipid and free amino acid contents. However, SPE females produced more buoyant eggs with higher fertilization rate as well as larger larvae with more energy reserves (estimated as oil-droplet area). Overall, general patterns of nutrient utilization were detected, such as the amount of total lipid and monounsaturated fatty acids decreasing from the egg stage and throughout the yolk-sac larval stage. On the contrary, essential fatty acids and free amino acids were retained. Notably, towards the end of the yolk-sac stage, the proximal composition and biometry of surviving larvae, from both treatments, were similar.

Bioaccumulation and health risk assessment of heavy metals in European eels taken from Lakes Köyceğiz (Turkey) and Võrtsjärv (Estonia)

Monitoring heavy metal contaminants in fish is important for the assessment of environmental quality as well as food safety. In this study, European eel samples were collected from Lake Köyceğiz and Lake Võrtsjärv in 2017 and 2018. The concentrations of Mn, Cd, Zn, Pb, and Cu metals were measured by using GF-AAS in four selected tissues of eel, including liver, gill, skin, and muscle in both lakes. The pollution index (P_i, MPI) values were calculated for both lakes and the health risk for consumers was assessed for both adults and children in Turkey and Estonia. The estimated weekly intake (EWI), hazard index (HI), and lifetime cancer risk values (CRs) for the metals were calculated for both lakes. According to the results of this study, a significant difference was determined between the metal concentrations (especially Cu, Cd, and Pb) in the tissues of the eel samples taken from the two lakes. These results show that besides the pollution levels in the aquatic environment, physiological needs and metabolic activities in different habitats have a significant effect on metal accumulation in eels. In addition, HI was found to be < 1 for both adult and child consumers in both lakes, which indicates that consumers would not experience non-carcinogenic health effects. However, the values of CR for Pb and Cd were found negligible in Lake Köyceğiz, while the CR value for Pb was found to be very close to the danger limits in Lake Võrtsjärv.

Virus infections of the European Eel in North Rhine Westphalian rivers

Viral infections have been suggested to play a role in the decline of the panmictic population of the European eel (Anguilla anguilla). However, despite the importance of knowledge about pathogenic eel viruses, little is known about their spread in the wild European eel population and only a few eel pathogenic viruses have been described so far. In this study, we aimed to investigate the health status of the A. anguilla stock in North Rhine Westphalia (NRW) State of Germany. For this purpose, we examined tissue samples of 16 elvers, 100 yellow eels and 6 silver eels, sampled from the rivers Rhine, Lippe and Ems. Virus detection was performed via a combination of cell culture and PCR. Next to the detection of frequently encountered pathogenic eel viruses (anguillid herpesvirus 1 and eel virus European X (EVEX)), we isolated the eel picornavirus 1 (EPV-1) from tissue of yellow eels and elvers and demonstrate the distribution of EPV-1 in wild eel population in NRW.

Does tagging transparent fish increase predation risk? A laboratory study with glass eel (Anguilla anguilla) and sea bass (Dicentrarchus labrax)

Barriers in the estuaries of the rivers prevent the immigration of glass eels (Anguilla anguilla) arriving on the European coast every spring. This leads to an unnatural accumulation of migrating glass eels below the barriers, and this may lead to additional losses in glass eels by piscivorous fish. The proportion of predation losses can be estimated using mark-recapture techniques and abundance estimates in combination with stomach content analysis of piscivorous fish. Nonetheless, whether tagging transparent glass eels increases predation risk and what the digestion rate of glass eel is in piscivorous fish are unknown. This study aimed to determine whether there is an increased predation risk for tagged glass eel; it also studies glass eel digestion status in piscivorous fish after appointed time frames. A laboratory experiment with 48 trials was conducted. Tagged (visible implanted elastomer, VIE) and untagged glass eels were exposed to small (19.1-24.4 cm) and large (31.9-43.5 cm) sea bass (Dicentrarchus labrax) during a 2 h trial. In 48% of the trials, successful predation was present and 13% showed clear predation attempts in which bass did not capture glass eels. No significant difference was found in predation rate between tagged and untagged glass eels and between red and blue tagged glass eels. Large sea bass predated more, but all sizes consumed glass eel under laboratory conditions. Stomach content analysis showed intact glass eel bodies 4-6 h after ending the 2 h trial and parts of glass eel bodies up to 16-18 h. This study showed that tagging does not increase predation in mark-recapture studies using VIE-tags in transparent glass eel. It also shows that the proportion of predation in relation to local glass eel abundance can be estimated if stomach content analysis is conducted within 4-6 h after predation.

Impaired reproductive fitness despite high fecundity in European eel (Anguilla anguilla L.) from a Baltic Sea drainage area

The reproductive fitness of female European eels (Anguilla anguilla L.) from the Warnow River, Germany, Baltic Sea region, was evaluated by estimating fecundity and applying a spawning model. Individual fecundity ranged from 1,440,000 to 7,455,000 oocytes and mean relative fecundity equalled 5,956,015 oocytes kg^-1 . Modelling suggests that 43% of eels hold insufficient energy storages or are within 10% of complete depletion. The fraction of eels with insufficient energy supply increases from 19% to 35% if the effect of parasitic infection is considered.

Effect of abiotic factors and environmental concentrations on the bioaccumulation of persistent organic and inorganic compounds to freshwater fish and mussels

Many aquatic ecosystems are under persistent stress due to influxes of anthropogenic chemical pollutants. High concentrations can harm entire ecosystems and be toxic to humans. However, in case of highly hydrophobic compounds, their low water solubility precludes direct measurement in water, and thus alternative monitoring strategies are needed. In the present study, we investigated the extent to which bioaccumulated concentrations of persistent compounds can be predicted by concentrations in environmental compartments (water and sediment). Due to their high biomagnification potential, Hg and PFOS were included in this analysis as well. At 44 field locations in Flanders (Belgium), we monitored the concentrations of 11 priority compounds and their derivatives, included in the Water Framework Directive, in both sediment and water (where feasible) and biota (European perch, European eel and freshwater mussels). Besides, some sediment (i.e. total organic carbon (TOC) and clay content) and water characteristics were measured (i.e. pH, oxygen level, conductivity, nitrate, nitrite and dissolved organic carbon (DOC)). Measurements of HCB, HCBD, cis-heptachlorepoxide, HBCD and PFOS in sediment and ∑PCB in water showed a lower detection frequency than in fish samples. While PCB profiles were comparable between all matrices, for PBDE clear differences were detected between sediment and fish profiles, with BDE99 contributing the most for sediment (34%) and BDE47 for fish (≥44%), followed by BDE99 for perch (28%) and BDE100 for eel (25%). Water concentrations for PFOS and benzo(a)pyrene were predictive of respective bioaccumulated concentrations. HCB, ∑PCB and ∑PBDE, concentrations in fish were dependent on sediment concentrations and negatively related to organic compound levels (p < 0.05). Furthermore, pH and nitrite were negatively associated with accumulated concentrations in eel for HCB and PFOS, respectively (p < 0.05). Strong relationships between bioaccumulation and sediment and/or water concentrations strengthened the basis for surrogate monitoring methods. Finally, the extrapolation potential of Hg, ∑PBDE, PFOS, HBCD and ∑PCB between both fish species offered new opportunities in extrapolating different European monitoring frameworks.

The changing times of Europe's largest remaining commercially harvested population of eel Anguilla anguilla L

This study quantifies the processes involved in regulating the European eel population of Lough Neagh, a lake in Northern Ireland. The relationship between glass eel input and silver eel output for the 1923-1997 cohorts was best described by a Beverton-Holt stock recruitment model. Glass eel input time series was not complete and was thus derived from the relationship between catches elsewhere in Europe and Lough Neagh, together with the addition of stocked glass eel. Silver eel output was the sum of silver eel escapement, catch and yellow eel catch converted to silver eel equivalents. Natural mortality increased with glass eel density, ranging from 0.017 to 0.142 year^-1 . The mean carrying capacity increased from ≈3.25 M silver eels (≈26 kg ha^-1 ) for the 1923-1943 cohorts to ≈5.0 M (≈40 kg ha^-1 ) for the 1948-1971 cohorts before regressing back to ≈3.25 M. The total silver eel output was highest during the late 1970s/early 1980s at 35-45 kg ha^-1  year^-1 and lowest during the early years of the 20th century and is currently at 10-15 kg ha^-1  year^-1 . The findings are discussed in relation to (a) the ecological changes that have occurred within the lough, associated with eutrophication and the introduction of roach (Rutilus rutilus L.), and (b) the decline of the wider European eel stock across its distribution range. The findings from this study have relevance for the wider management of the European eel stock.

Migrating silver eels return from the sea to the river of origin after a false start

The European eel's singular spawning migration from European waters towards the Sargasso Sea remains elusive, including the early phase of migration at sea. During spawning migration, the movement of freshwater resident eels from river to sea has been thought to be irreversible. We report the first recorded incidents of eels returning to the river of origin after spending up to a year in the marine environment. After migrating to the Baltic Sea, 21% of the silver eels, tagged with acoustic transmitters, returned to the Narva River. Half returned 11-12 months after moving to the sea, with 15 km being the longest upstream movement. The returned eels spent up to 33 days in the river and migrated to the sea again. The fastest specimen migrated to the outlet of the Baltic Sea in 68 days after the second start-roughly 1300 km. The surprising occurrence of returning migrants has implications for sustainable management and protection of this critically endangered species.

What are the best upland river characteristics for glass eel restocking practice?

The fitness of restocked European eel (Anguilla anguilla), an endangered fish species, was studied in relation to the environmental variables of habitats in six upland rivers that are typologically different in terms of their hydromorphological and physicochemical characteristics, food resources and fish communities. These rivers received a total of 76,370 imported glass eels in 2017. During a three-year period, we monitored eels with respect to total length, annual growth rate, condition factor and density using capture-mark-recapture experiments to understand the effects of the characteristics of receiving rivers on restocking success levels. Our results showed the survival of the restocked eels in the six rivers and revealed significant differences between them in terms of total length, condition factor and density. Better performance in eel yield variables was observed in a eutrophic alkaline river with greater roughness of riverbed substrates, dominant pool- and riffle-type flow facies and lower brown trout density. The variables conductivity and total hardness had higher explanatory power and were strongly associated with increased eel density. This study suggests that a well-selected habitat/river in a restocking programme can be beneficial for the species and recommends restocking practice as a management tool to achieve eel conservation goals.

Role of mucosal immune response and histopathological study in European eel (Anguilla anguilla L.) intraperitoneal challenged by Vibrio anguillarum or Tenacibaculum soleae

The external mucus layer that covers fish skin contains numerous immune substances scarcely studied that act as the first line of defence against a broad spectrum of pathogens. This study aimed to characterize and describe for the first time several humoral immune defence parameters in the skin mucus of the European eel (Anguilla anguilla) after intraperitoneal injection with Vibrio anguillarum or Tenacibaculum soleae. This study evaluated several immune-related enzymes and bactericidal activity against fish pathogenic bacteria in the skin mucus of European eels at 24, 48, and 72 h post-challenge. The results demonstrated that European eel skin mucus showed significant increments in peroxidase and lysozyme activity at 48 and 72 h after V. anguillarum challenge, compared to other experimental groups. In the case of antiprotease activity, an increase was observed at 24 h in the skin mucus of fish challenged with V. anguillarum compared to unchallenged fish, while this activity was undetected at 48 and 72 h. In contrast, protease activity had decreased at 48 and 72 h in the skin mucus of fish challenged with V. anguillarum compared to the unchallenged group. Regarding bactericidal activity, a high growth capacity of T. soleae was observed in the skin mucus of all experimental groups. Interestingly, the skin mucus from fish challenged with V. anguillarum exhibited increased bactericidal activity against this bacterium at 48 h, compared to unchallenged fish. Finally, severe histopathological alterations were observed in the gills and liver at the end of the trial (72 h), whereas the skin showed only an overspread presence of goblet cells in the challenged fish compared to unchallenged fish. The present results may give new insights into the mucosal immune system of this primitive species with potential applications in aquaculture.