European Silver Eels Research Articles

Impact of a short‐term exposure to tributyl phosphate on morphology, physiology and migratory behaviour of European eels during the transition from freshwater to the marine environment

AbstractMigrating silver European eels were exposed for 5 days in a laboratory to an environmental level of tributyl phosphate (TBP), tagged with acoustic transmitters and released below the Tange hydropower station, on the River Gudenaa, Denmark. The subsequent movements of the eels were monitored as they migrated through Randers Fjord and into the Kattegat Sea using an array of acoustic receivers. In laboratory‐based studies, exposure of eels for 5 days to a nominal concentration of 0.5 μg·l−1 TBP significantly affected plasma glucose concentration and reduced plasma levels of sodium and chloride both in freshwater and 3 days after transfer to saltwater. However, there were no mortalities when the eels were transferred to full strength sea water. Exposure to TBP did not appear to have a significant effect on the subsequent movements of the eels in the river or fjord. The eels moved rapidly through the fjord predominantly at night. The results of the study are discussed in relation to the impact of diffuse pollution on the behaviour and physiology of European eels.

Tracking the migratory success of stocked European eels Anguilla anguilla in the Baltic Sea

To investigate the extent to which European silver eels Anguilla anguilla, originating from stocking programmes in the Baltic Sea tributaries, effectively contribute to the spawning stock, two hundred and seventy-four formerly stocked A. anguilla. emigrating from the Schwentine River near Kiel, Germany, were tagged with T-Bar anchor tags. A total of 29 Anguilla spp. were recaptured (c. 11%) up to 14 months after release. Stocking history of recaptured A. anguilla. was confirmed by otolith microchemistry. Recapture locations were concentrated around the outlet of the Baltic Sea (Danish Belt Sea) with 62% of all recaptures reported here or in the Kattegat. Recaptured Anguilla spp. showed a reduction in both L(T) and mass (mean ± s.d. = -1.5 ± 0.9 cm and -125.3 ± 50.1 g) while average total fat content remained in the order of values previously reported as high enough to provide energy resources to allow successful completion of the spawning migration (mean ± s.d. = 28.4 ± 4.4%). The documented mean rate of travel (0.8 km day(-1)), however, indicated a delay in the target-oriented migration that might be interpreted as a delayed initial migration phase of orientation towards the exit of the Baltic Sea.

Model prediction vs. reality—testing the predictions of a European eel (Anguilla anguilla) stock dynamics model against the in situ observation of silver eel escapement in compliance with the European eel regulation

Abstract Prigge, E., Marohn, L., Oeberst R., and Hanel, R. 2013. Model prediction vs. reality—testing the predictions of a European eel (Anguilla anguilla) stock dynamics model against the in situ observation of silver eel escapement in compliance with the European eel regulation – ICES Journal of Marine Science, 70: 309–318. A direct monitoring of European silver eel (Anguilla anguilla, L) escapement from rivers and estuaries has been proven to be challenging, and a Europe-wide documentation of escaping silver eel numbers therefore hardly seems realistic. To reinforce management decisions, policy-makers are thus widely reliant on the accuracy of escapement models. A 3-year programme of silver eel escapement monitoring was undertaken to compile model input data and revise an eel population model (German Eel Model II; GEM II) already used in the decision-making process of management authorities. By compiling necessary input data and analysing vital system-specific population characteristics, it was possible to compare the documented silver eel escapement with the modelled potential silver eel escapement. Resulting model predictions were close to actually monitored escapement numbers, which were distinctly lower than reference escapement values for the same freshwater system given in the implementation report of the German Eel Management Plans. Applying different commercial and recreational catch scenarios revealed the sensitivity of the model. The results show the potential of the GEM II and highlight the importance of high-quality input data to use model predictions as the basis for management measures.

Recommendations on size and position of surgically and gastrically implanted electronic tags in European silver eel

Information on European silver eel Anguilla anguilla anatomy was collected to gain information on limitations on size and placement of electronic tags. To reduce the eel’s ability to bite its own sutures, it may be an advantage to make surgical incisions close to the head, but this increases the risk of cutting the liver. Recommended placement of an incision was slightly further from the head than one-fourth of the fish’s body length (L T) to avoid damaging the liver. Long, flexible tags comprising various components can be adjusted to the narrow body cavity and undulating movements of eels. There was space for surgically implanting a 100 mm long tag (11 mm in diameter) in the body cavity of eels with L T ≥380 mm. During gastric tagging, tag length is limited by stomach length. Silver eels with L T 380 to 998 mm had stomach lengths of 47 to 185 mm, indicating that there was space for short gastric tags in the smallest eels, but that there was space for relatively long tags in larger eels. The distance from the snout to the start of the stomach constituted 15 to 23% of L T, indicating how far the transmitter should be inserted during tagging. This information aids the development of tags and tagging methods that consider the unique morphological and behavioral features of eels.

Evaluation of surgical implantation of electronic tags in European eel and effects of different suture materials

Effects of implanting data-storage tags in European eel, Anguilla anguilla, and the suitability of different suture materials (braided permanent silk, permanent monofilament, absorbable and absorbable antibacterial) were examined. The tags consisted of an electronic unit and three floats on a wire, making them flexible and able to follow the swimming movements of the eel. No mortality occurred, and tagged fish did not differ from the control fish in growth. Sutures were shed or dissolved slowly. After 4 weeks, there was no difference among the groups in the proportion of sutures left. After 6 months, fish with braided silk had largely shed their sutures, fish with monofilament sutures had the majority of sutures left, whereas the fish with absorbable sutures were intermediate in between. Fish with monofilament sutures showed the least-extensive inflammation reactions and fastest wound healing. Antibacterial treatment had no effect on inflammation or healing rates. After 6 months, the tag started to become expelled through the incision in five fish (12%). The internal reaction appeared stronger around the floats, suggesting that the coating material of the floats created a tissue reaction, which should be further investigated. Intraperitoneal implantation appears to be a suitable tagging method for European silver eel, and it is recommended to close incisions using permanent monofilament sutures.

Adrenergic regulation of hepatic glycogenolysis in yellow and silver European eel

Adrenergic regulation of hepatic glycogenolysis in yellow and silver European eel

Trace elements in sediments and bioaccumulation in European silver eels (Anguilla anguilla L.) from a Mediterranean lagoon (SE Italy)

Samples of surface sediments and tissues (liver and muscle) of commercially available European silver eels (Anguilla anguilla L.) collected from Varano lagoon (Italy) were analysed to determine trace element contents. Univariate and multivariate analyses were performed to highlight both the differences between sampling sites and the influence of channel discharges. Atomic ratios indices for sediment data and biological enrichment factors (BEF) for eel tissues were calculated in order to evaluate the enrichment factor due to human activities. The highest levels of As (11.9 µg g−1) and Zn (14.1 µg g−1) were observed in the south-eastern zone of the lagoon, which is influenced by urban and agricultural discharges. The low levels of Hg observed in this study (0.04 µg g−1) led us to exclude both natural and human local sources of this element. Trace element concentrations of all elements were lower in muscle than in liver tissue. Significant enrichment of Cu and Zn was found in livers.

The efficacy of Aqui-S vet. (iso-eugenol) and metomidate as anaesthetics in European eel (Anguilla anguillaL.), and their effects on animal welfare and primary and secondary stress responses

The effects of 40 mg/L metomidate and 300 mg/L iso-eugenol (i.e. 600 mg/L Aqui-S vet.) anaesthesia on the stress response in European silver eel during a process of complete anaesthesia, recovery and long-term survival were studied. Metomidate is a hypnotic agent with no analgesia, whereas Aqui-S vet. is a true anaesthetic. There was no difference between fish exposed to Aqui-S vet. and metomidate, with respect to time to anaesthesia (mean 3.8 and 2.6 min respectively) and recovery (mean 7.6 and 6.5 min respectively). For the metomidate group, the plasma cortisol concentration increased to a peak during the recovery phase, and was significantly higher than for the Aqui-S vet. group, which showed no such increase. Fish were kept in tanks with seawater for monitoring of long-term survival (4 months) after anaesthesia treatment, and no mortality was observed in either group. The results indicate that metomidate is a potential stressor to European eels during exposure, and we do not recommend using metomidate in this species. Aqui-S vet., however, seems promising as a stress-reducing anaesthetic for European eel, and if used properly could improve animal welfare and survivability during and after common ecology-related procedures, as capture, tagging and size measuring.

Loss of European silver eel passing a hydropower station

Summary The aim of this study was to assess escapement success of silver eels, Anguilla anguilla (L.), in a lowland river while passing a reservoir and a hydropower station. It was hypothesized that passage success would be lowest at the hydropower station and that survival and migration speed would be highest in the free-flowing river section upstream the reservoir. Forty-five female silver eels 56–86 cm in length were tagged with acoustic transmitters and released in November 2006. Their migration was monitored via automatic listening stations (ALS) in various sections of the river, covering a total migration distance of 64 km. Survival and progression rate of downstream migration was highest in the upstream river section and significantly lower in the reservoir. The eels apparently had trouble finding their way past the turbines and spent between 1.5 and 35 h in the forebay. The results show that within the study period, only 23% of the tagged eels reached the tidal limit, mainly due to difficulties in passing the hydropower dam. With such high loss-rates, the escapement goals set in the management plan cannot be achieved.

Influence of temperature regime on endocrine parameters and vitellogenesis during experimental maturation of European eel ( Anguilla anguilla) females

We examined the effect of temperature in European silver eels during their maturation induced by injections of carp pituitary extract on endocrine parameters: pituitary fshβ and lhβ expression, plasma 17β-estradiol (E2) and vitellogenin, estrogen receptor 1 ( esr1), and vitellogenin 2 ( vtg2) expression in liver. A variable thermal regime (T10) that increased from 10° to 14° and 17 °C was compared with a constant 20 °C regime (T20) during 12 weeks. T10 caused a faster development until week 8, higher fshβ, lhβ, esr1 expression, and higher E2 levels. The results strongly suggest that T10 is inducing a higher endogenous FSH level which increases the E2 circulating level during vitellogenesis. A variable thermal regime induced an fshβ expression and E2 profile in vitellogenic hormonally matured eel females that were more similar to the profile observed in other naturally maturing fish.

Cholinesterase activities and sensitivity to pesticides in different tissues of silver European eel, Anguilla anguilla

Cholinesterase (ChE) activities were characterized in silver European eel, Anguilla anguilla, grown in the brackish lagoon of Comacchio (Italy). All specimens were harvested at the “lavoriero”, a traditional eel trapping weir that captures eels while leaving internal waters at the onset of reproductive migration. To our knowledge, no investigation on ChE was reported in silver eels. Therefore a first characterization of enzyme activity in muscle, brain, liver and plasma of silver eel was carried out, in the presence of different substrates, selective inhibitors, and four pesticides representative of the carbamate and organophosphate classes. Brain and white skeletal muscle showed similar ChE activities, 5- and 10-fold higher than those detected in liver and plasma, respectively. Km values of 0.31 and 0.30mM, and Vmax values of 40.28 and 35.47nmol min–1 mg protein–1 were obtained in brain and muscle ChE, respectively. Acetycholinesterase was the predominant ChE form in all tissues, as concluded by comparing the effects of BW 284c51, iso-OMPA and eserine. ChE activities in brain and muscle were significantly inhibited by in vitro treatment with pesticides, with the following order of potency: carbofuran>carbaryl>chlorpyrifos≥diazinon.

Temporal progression in migratory status and sexual maturation in European silver eels during downstream migration

The onset of downstream migration of European eels is accompanied by a cessation of feeding and the start of sexual maturation which stresses the link between metabolism and sexual maturation, also suggesting an important role for exercise. Exercise has been tested with eels in swim tunnels and was found to stimulate the onset of sexual maturation. In this study, we have investigated the interplay between migration and maturation in the field during the downstream migration of female silver eels. Temporal changes in migratory status and sexual maturation among silver eels of the upstream Rhine River system over 3 months of the migration season (August, September and October) were determined in biometrical parameters, plasma 17β-estradiol and calcium levels, oocyte histology and gonadal fat levels. Furthermore, the ecological relevant parameters age as determined by otolithometry and health aspects indicated by haematocrit, haemoglobin and swim-bladder parasite load were measured. Silver eels were estimated to be 14 years old. A strong temporal progression in migratory stage was shown over the months of downstream migration. Catches probably represented a mix of reproductive migrants and feeding migrants of which the ratio increased over time. Furthermore, this study confirmed our hypothesis linking the migratory stage to early maturation as indicated by enlargement of the eyes, oocyte growth and fat deposition in the oocytes, exactly the same changes as found induced by exercise but not ruling out environmental influences. Migrants show extensive fat uptake by the oocytes, probably stimulated by the swimming exercise. In addition, at least 83% of the silver eels in this spawning run may have suffered from negative effects of swim-bladder parasites on their swimming performance.

Early marine migration of European silver eel Anguilla anguilla in northern Norway

To study the migratory behaviour in wild northern European silver eel Anguilla anguilla during sea entry and early marine migration, 32 individuals were tagged with acoustic transmitters and registered at four automatic listening station arrays from the mouth of the north Norwegian River Alta and throughout the Alta Fjord. The A. anguilla entered the fjord during all parts of the tidal cycle and did not seem to utilize the outgoing tidal currents. They migrated mainly during the night, in both the river mouth and the fjord. On average, they spent 2·7 days travelling from the river mouth to the outermost array, 31 km from the river mouth, corresponding to an average migratory speed of 0·5 km h(-1) . The A. anguilla generally migrated in the central part of the fjord and in the uppermost 10-25% of the water column, but with frequent dives to greater depths. Already 4 km after sea entry, A. anguilla were observed diving deeper than 130 m within 20-30 min periods. Hence, this study demonstrated that A. anguilla may perform an active diving behaviour during the early marine migration. The study took place in a pristine area with a minimum of anthropogenic interventions and by individuals from a population still uninfected by the introduced parasite Anguillicoloides crassus. The results may therefore be used as a baseline for future studies of the A. anguilla early marine migration.

The health status of European silver eels, Anguilla anguilla, in the Dutch River Rhine Watershed and Lake IJsselmeer

The worldwide decline of the eel population is thought to be caused by several factors, among which eel diseases. To investigate diseases of European silver eels Anguilla anguilla in the Netherlands, in Nov–Dec 2004 12 silver eels, and in Aug–Dec 2005 80 eels were caught in downstream parts (rivers) of the River Rhine and in Lake IJsselmeer. The eels were measured and weighed, necrotized, and individually checked for presence of external and internal parasites, bacterial infections and viruses, and blood smears were made for haematology. This is the first multidisciplinary study of the health of Dutch silver eels. In the small (2 × 6 eels) pilot study of 2004 in the River Rhine and Lake IJsselmeer respectively, most eels showed aspecific fin haemorrhages, some had ectoparasites, nearly none had parasites in the intestine, half of the groups had Anguillicola crassus in their swimbladder, only few had Trypanosoma's in their blood, and no primary virus or bacterial infections were found, although one eel from Lake IJsselmeer was positive for anguillid herpesvirus 1 (AngHV-1, former Herpesvirus anguillae, HVA) in the PCR test only. The blood of all 6 and 2/6 of the eels respectively was considered abnormal, and the eels had a proper condition. In 2005, in 50 eels from the River Rhine and 30 from Lake IJsselmeer respectively, again aspecific fin haemorrhages were often seen, some of the eels had ectoparasites and parasites in the intestine, most eels had A. crassus in their swimbladder, 32% and 53% had Trypanosoma's in their blood, from 44% and 13% of the eels AngHV-1 was isolated, and 44% and 27% were tested positive by PCR, with a peak in August, 10% of both groups of eels had an internal bacterial infection, mostly due to Aeromonas spp. The blood of about half of the eels was considered abnormal, but the eels had a proper condition. It was concluded, that the silver eels of this study had a proper Fulton condition factor (values 2.00–2.26), with aspecific fin haemorrhages, often were Trypanosoma-, A. crassus- and AngHV-1-infected, dependent on the season, and often showed an abnormal haematology. A. crassus causes injuries and is a chronic stress factor, more than the other parasites, which were mostly found less prevalent. In fact, lymphocytosis was directly related to A. crassus infection. Stress and injuries by A. crassus might induce disease through the presence of virus (AngHV-1), relevant in the health status of the silver eels during their spawning migration if ambient water temperatures would enhance a clinical infection of AngHV-1 disease. Moreover this virus might potentially decrease the survival of the silver eels by itself, because spawning migration to the Sargasso Sea takes wild eels to temperate/tropical areas in which the clinical infection by AngHV-1 is surely enhanced.

Sex-related differences in aerobic capacities and reactive oxygen species metabolism in the silver eel

Silver European eels (Anguilla anguilla L.) need to develop important aerobic capacities to cope with their long fasting spawning migration at depth, particularly males which are about half the size of females. Moreover, they have to face potential oxidative stress because reactive oxygen species (ROS) production is linked to the increase in metabolic rate. Thus, aerobic metabolism was globally evaluated in male and female silver eels exposed to a 10.1 MPa hydrostatic pressure (1,000 m depth). Oxygen consumption (MO(2)), ROS production and antioxidant enzyme activities were measured in the muscle fibres. Males showed a trend in both higher rate of aerobic metabolism and ROS production than females. After pressure exposure, ROS production was inversely correlated to metabolic rate only in males. By facilitating MO(2) rise with no harmful effects by ROS, the supposed enhanced aerobic capacities of males could speed up the sustained swimming. In females, the tendency to lower metabolic rate and higher catalase activity would make them less vulnerable to ROS effects. These results are in agreement with the hypothesis for different migration depths between genders.

Swimming physiology of European silver eels (Anguilla anguilla L.): energetic costs and effects on sexual maturation and reproduction

The European eel migrates 5,000–6,000 km to the Sargasso Sea to reproduce. Because they venture into the ocean in a pre-pubertal state and reproduce after swimming for months, a strong interaction between swimming and sexual maturation is expected. Many swimming trials have been performed in 22 swim tunnels to elucidate their performance and the impact on maturation. European eels are able to swim long distances at a cost of 10–12 mg fat/km which is 4–6 times more efficient than salmonids. The total energy costs of reproduction correspond to 67% of the fat stores. During long distance swimming, the body composition stays the same showing that energy consumption calculations cannot be based on fat alone but need to be compensated for protein oxidation. The optimal swimming speed is 0.61–0.67 m s−1, which is ~60% higher than the generally assumed cruise speed of 0.4 m s−1 and implies that female eels may reach the Sargasso Sea within 3.5 months instead of the assumed 6 months. Swimming trials showed lipid deposition and oocyte growth, which are the first steps of sexual maturation. To investigate effects of oceanic migration on maturation, we simulated group-wise migration in a large swim-gutter with seawater. These trials showed suppressed gonadotropin expression and vitellogenesis in females, while in contrast continued sexual maturation was observed in silver males. The induction of lipid deposition in the oocytes and the inhibition of vitellogenesis by swimming in females suggest a natural sequence of events quite different from artificial maturation protocols.

Survival and progression rates of large European silver eel Anguilla anguilla in late freshwater and early marine phases

The population of European silver eel Anguilla anguilla has declined tremendously in the last decades. The cause of this decline is unknown, and it is necessary to investigate the migra- tory behaviour and survival rates of silver eels during the reproductive migration in order to under- stand if the decline is related to factors acting during that migration. We estimated survival and pro- gression rates of European silver eel migrating in the lower part of the River Gudenaa and during the first phase of the marine migration in the Randers Fjord in Denmark. Fifty migrating silver eel (total body length: 56 to 84 cm) were captured, and each was equipped with an acoustic transmitter. Their migration was subsequently monitored using an array of automatic listening stations, and progres- sion rate and mortality in the river, inner part of the fjord and outer part of the fjord were estimated. Survival was high in fresh water. However, 60% of eels were lost in the inner and outer fjord, sup- porting the hypothesis that mortality is large in the early phase of the marine migration and that fish- ing may be a major cause of mortality of silver eels. There was no indication that the slowest-migrat- ing individuals were more prone to fishing mortality than the faster-migrating individuals. Progression rate increased as the eels proceeded downriver and out of the fjord. The migration was predominantly nocturnal, both in the river and fjord. Based on the available evidence, a considerable increase in eel survival in the river-fjord system will be needed in order to fulfil the goals in the Euro- pean Union recovery plan for eels.

Swimming suppresses hepatic vitellogenesis in European female silver eels as shown by expression of the estrogen receptor 1, vitellogenin1 and vitellogenin2 in the liver

BackgroundWhen European silver eels (Anguilla anguilla) venture into the Atlantic Ocean for their 6,000 km semelparous spawning run to the Sargasso Sea, they are still in a prepubertal stage. Further sexual development appears to be blocked by dopaminergic inhibition of hypothalamus and pituitary activity. Recently, we found that swimming for several weeks in freshwater stimulated the incorporation of fat droplets in the oocytes. So, it was hypothesized that long term swimming in seawater would release the inhibition further and would also stimulate the production of vitellogenin by the liver.MethodsFor this study a swim-flume was constructed to allow simulated migration of migratory female silver eels for 3 months (1,420 km) in natural seawater at 20 degrees C. Primers were designed for polymerase chain reactions to measure the mRNA expression of estrogen receptor 1 (esr1), vitellogenin1 (vtg1) and vitellogenin2 (vtg2) genes in the liver of European female silver eels.ResultsIn comparison to resting eels, swimming eels showed a diminished expression of esr1, vtg1 and vtg2 in the liver. They also had lower plasma calcium (Ca; indicative of vitellogenin) levels in their blood. This showed that vitellogenesis is more strongly suppressed in swimming than in resting eels. However, when eels were subsequently stimulated by 3 weekly carp pituitary extract injections, the expression of the same genes and plasma levels of Ca strongly increased in both groups to similar levels, thus equalizing the initial differences between resting and swimming.ConclusionsIt is concluded that vitellogenesis remains suppressed during resting and even more during swimming. The fact that swimming stimulates fat deposition in the oocytes but suppresses vitellogenesis indicates that these events are separated in nature and occur sequentially. Swimming-suppressed vitellogenesis may imply that in nature eels undergo vitellogenesis and final maturation near or at the spawning grounds.

Behaviour and passage of European silver eels (Anguilla anguilla) at a small hydropower plant during their downstream migration

Between 2004 and 2007, 116 downstream migrant silver eels (Anguilla anguilla) were monitored at a hydropower plant on the Gave de Pau river in South-West France using radio and PIT telemetry. The objectives of the study were: (i) to determine the environmental conditions when eels arrived and passed the facility; (ii) to determine the rate of eel escapement (passage other than via the turbines); (iii) to describe the behaviour of eels faced with the intake structure and the permeability of the intake trashracks for the different sizes of eel; and (iv) to determine whether surface bypasses originally designed for salmon could be effective for eels. Five types of behaviour of silver eels in the forebay and at the plant intakes were identified. The study showed the key factor influencing both eel behaviour and the route taken through the plant was variation in river discharge. Escapement rate was related to eel length and the spill flow to river flow ratio, which could be described by a logistic regression model. The surface bypasses originally designed for salmon were found to aid downstream eel migration significantly. At velocities < 0.40 m*s−1, no eels, even the largest, for which the racks are a physical barrier, were found impinged on the trashracks.

Developmental dynamics of cone photoreceptors in the eel

BackgroundMany fish alter their expressed visual pigments during development. The number of retinal opsins expressed and their type is normally related to the environment in which they live. Eels are known to change the expression of their rod opsins as they mature, but might they also change the expression of their cone opsins?ResultsThe Rh2 and Sws2 opsin sequences from the European Eel were isolated, sequenced and expressed in vitro for an accurate measurement of their λmax values. In situ hybridisation revealed that glass eels express only rh2 opsin in their cone photoreceptors, while larger yellow eels continue to express rh2 opsin in the majority of their cones, but also have <5% of cones which express sws2 opsin. Silver eels showed the same expression pattern as the larger yellow eels. This observation was confirmed by qPCR (quantitative polymerase chain reaction).ConclusionsLarger yellow and silver European eels express two different cone opsins, rh2 and sws2. This work demonstrates that only the Rh2 cone opsin is present in younger fish (smaller yellow and glass), the sws2 opsin being expressed additionally only by older fish and only in <5% of cone cells.