Anthropogenic spreading of anguillid herpesvirus 1 by stocking of infected farmed European eels,Anguilla anguilla (L.), in the Schlei fjord in northern Germany

The Schlei fjord in northern Germany is the recipient water of a comprehensive eel, Anguilla anguilla (L.), stocking programme. Since 2015, stocked eels become alizarin red S marked, but to date no control mechanism is implemented in this stock enhancement measure to prevent anthropogenic spreading of diseases. Consequentially, it was possible that farmed stocking cohorts of 2015 and 2016 (in total ca. 1040 kg) were subsequently tested positive for anguillid herpesvirus 1 (AngHV 1). For this study, 100 eels [total length (TL) 24.3–72.9 cm, age ca. 1–6 years] were caught in 2016 and investigated with regard to AngHV 1 infection, parasite load (Anguillicoloides crassus) and body conditions. 68% of the eels were found to be virus positive while larger specimens were more often infected. In addition, a fitted generalized linear model (area under the curve = 0.741) demonstrated that an increase in individual TL is accompanied with an increased risk of clinically relevant virus loads. Anguillicoloides crassus turned out to be an important stressor for eels, because parasite and virus load revealed a significant positive correlation. The results of this study evidently show the urgent need of a disease containment strategy for eel stocking programmes.     

Effects of feeding stimulants on the feed consumption,growth and survival at glass eel and elver stages in the European eel (Anguilla anguilla)

Elvers and glass eels of Anguilla anguilla were fed diets containing two types of feeding stimulants (FS) that were based on processed marine (MBFS) and yeast proteins (YBFS). Elvers (1.5 ± 0.3 g) were fed seven diets (MBFS and YBFS diets at 20 g kg^?1, 40 g kg^?1 and 60 g kg^?1 plus control) for 60 days. Glass eels (250 ± 100 mg) were weaned to 60 g kg^?1 MBFS, 60 g kg^?1 YBFS and control diets for 30 days. Diets containing 60 g kg^?1 FS had a beneficial effect in terms of growth, homogenous size distribution and feed intake in elvers. Elvers fed 60 g kg^?1 MBFS and YBFS diets grew 11.9% and 5.6% faster than the control group. No differences in growth and size distribution were detected in glass eels fed 60 g kg^?1 MBFS and YBFS diets. However, FS affected the digestive system maturation; fish fed the 60 g kg^?1 MBFS and YBFS diets showed higher and intermediate values in the specific enzyme activities in comparison with the control group. This study revealed that the incorporation of FS into a pelleted diet was beneficial on the overall performance of European glass eels and elvers. However, the observed results were different depending on the eel’s stage of development, as well as the type and inclusion level of the FS.     

Detection of Herpesvirus anguillae during two mortality investigations of wild European eel in England: implications for fishery management

Herpesvirus anguillae (HVA) was detected during disease investigations of European eel, Anguilla anguilla L. at two stillwater fisheries in central England. These represent the first records of HVA from UK eels. Both mortalities were eel‐specific and took place during August 2009 and July 2010 at water temperatures between 17 and 19.4 °C. Pathological changes consistent with HVA infection included haemorrhaging in the fins, skin lesions and necrosis within the gills and liver. Transmission electron microscopy revealed active virion replication within the gill tissue. An initial assessment of risk is presented, indicating that HVA represents a high disease risk to UK eel stocks. However, further studies are required to establish the distribution of HVA before a reliable assessment of impact may be obtained. Until then, the detection of HVA holds important implications for eel conservation and management, in particular eel stocking activity.     

Recovery from transportation by road of farmed European eel (Anguilla anguilla)

The objective of this study was to assess the effects of transportation of marketable eel (0.15 kg) in the Netherlands with respect to welfare. Eels (Anguilla anguilla) were obtained from a commercial farm and acclimatized for 7 weeks at the laboratory. Fish were transported according to regular commercial procedures. The animals were placed in water‐filled transport tanks on the trailer. Fish density increased from 72 kg m^?3 (husbandry) to 206 kg m^?3 (fasting) and was further increased to 270–290 kg m^?3 during transport. Fish transport lasted 3 h after which the eels were returned to laboratory recirculation systems to measure parameters indicative of stress load, i.e. mortality, plasma cortisol, lactate and non‐esterified fatty acids (NEFA) as well as gill morphology. Samples were taken at 0, 6, 24, 48 and 72 h after transport in transported fish and non‐transported counterparts (controls). Transportation affected water quality within known tolerable limits. No mortality during or after transport was observed. After 6 h, plasma cortisol levels had returned to baseline. However, energy metabolism had increased suggesting that transportation of eels resulted in an increased energy demand that lasted for at least 72 h in the fasted animals. Thus, it is conceivable that exposure to adverse conditions, prior to stunning/killing, in a slaughterhouse may result in allostatic overload in eel.     

Co‐Infection with Pseudomonas anguilliseptica and Delftia acidovorans in the European eel,Anguilla anguilla (L.): a case history of an illegally trafficked protected species

Inspections by customs agents at Barcelona airport discovered 420 kg of contraband glass eels prepared for shipment to Hong Kong. After confiscation of these animals by police, they were transported to holding facilities to be maintained until after a judicial hearing. Upon arrival, they were separated into two groups and held under ambient flow‐through conditions in fresh water. During their captivity period, several peaks in mortality occurred and multiple bacterial strains were isolated from moribund animals. Sequencing of 16S rDNA was used to determine specific identity of the isolates. An initial isolation of Pseudomonas anguilliseptica was treated with oxytetracycline. A subsequent isolation of Delftia acidovorans proved resistant to oxytetracycline and was treated with gentamicin in combination with sulphadiazine–trimethoprim. Once the health condition of the animals was stabilized, they were partitioned into groups and subsequently released as part of a restocking effort for the species following the guidelines of Regulation (EC) 1100/2007 (Anon 2007). This represents the first record for both bacterial species in the host Anguilla anguilla in the Spanish Mediterranean.     

Detection of Herpesvirus anguillae (AngHV‐1) in European eel Anguilla anguilla (L.) originating from northern Poland—assessment of suitability of selected diagnostic methods

The Community Action Plan requests EU member states to implement measures that ensure the recovery of the severely depleted European eel stocks. One of the main threats is posed by Anguillid herpesvirus 1 (AngHV‐1) leading to increased mortality in both wild and farmed eels. Following recommendations of the OIE to minimize the risk of obtaining false‐negative results, the main aim of the study was to optimize diagnostic methods for AngHV‐1 detection using conventional PCR, nested PCR and in situ hybridization assay. While 53.3% of the individual organ samples were tested positive for AngHV‐1 by PCR, the additional virus analysis via nested PCR revealed that the actual prevalence was 93.3%. In the cell cultivation passages, a cytopathic effect was hardly found in the first two rounds. In the third passage onto cell cultures, a lytic CPE was detected. The identification and confirmation of the viruses obtained from cell cultures as well as directly from the organ tissues were proceeded by PCR, nested PCR and sequencing of the PCR products. While no positive signal was detectable in the first round by PCR using samples from the third cell culture passages, the nested PCR provided weak but visible positive signals.     

Presence of viruses in wild eels Anguilla anguilla L,from the Albufera Lake (Spain)

A virological analysis was conducted on wild eels from the Albufera Lake (Spain). A total of 179 individuals at different growth stages were collected in two different surveys (2004 and 2008). Presence of anguillid herpesvirus (AngHV‐1), aquabirnavirus and betanodavirus was confirmed by PCR procedures in both surveys, although the number of detections was clearly higher in 2008 (83% of the eels analysed resulted positive for virus presence). AngHV‐1 was the viral agent most frequently detected, followed by aquabirnaviruses. Betanodaviruses were detected by the first time in wild eels, and although the detections were only made by nested PCR, high percentage of positives were achieved. In addition, in 2008, seven aquabirnaviruses were isolated. Phylogenetic analysis performed using partial sequences of both genomic segments of aquabirnaviruses indicated that the seven isolates could be typed as WB (genogroup I) on the basis of segment A sequences, but when segment B was used six of them clustered with C1 strain (genogroup V) and one was typed as Ab (genogroup II). These results indicate natural reassortment between different strains of aquabirnaviruses in the eels. Although betanodaviruses were not isolated in cell culture, the analysis of the sequence of the nested PCR product indicated that they clustered with SJNNV genotype. The diversity of viral agents and the high level of viral detections suggest that viral infections may play a more prominent role in the decline of the European eel than initially thought.     

Comparative imaging of European eels (Anguilla anguilla) for the evaluation of swimbladder nematode (Anguillicoloides crassus) infestation

This study compares diagnostic imaging tools in detecting the parasitic swimbladder nematode Anguillicoloides crassus in Anguilla anguilla (L.) and focuses on ultrasound in an attempt to develop a non‐destructive, field diagnostic test. Ultrasound use could allow the parasite to be diagnosed without decreasing the number of critically endangered European eels through post‐mortem. In the preliminary study, eels were examined with computed radiography, computed tomography, magnetic resonance imaging, 14 MHz high‐end ultrasound and 5 MHz low‐end portable ultrasound, and the results were compared with post‐mortem findings. This ultrasound scanning technique did not produce any promising results. A second batch of eels was examined using the same high‐end and low‐end ultrasounds, but employing a different scanning technique and comparing the results with post‐mortem. This second study, scanning along the midline from below, allowed for the detection of anomalies associated with moderately infected animals. None of the eels used in this study were severely infected; thus, no conclusions can be made regarding the use of ultrasound in those animals. Overall, it was found that none of the techniques were useful in diagnosing mildly infected individuals; therefore, no single diagnostic imaging tool is sensitive enough to replace post‐mortem for definite diagnosis.     

Migration of eels tagged in the Baltic Sea and Lake Mälaren—in the context of the stocking question

Eels (Anguilla spp.) are in decline worldwide, and the signs of reduced recruitment have been observed in continental Europe since the early 1970s. To aid recovery of the European eel, stocking is used by many European countries as a management option. In this study, tagging experiments were conducted to follow eel migration from Lake Mälaren and four sites along the Swedish east coast in the Baltic Sea. The recaptured tagged eels were retrieved from fishermen, allowing for the opportunity to investigate their origin (brackish water, stocked in freshwater or a mix in between) by otolith microchemistry and to assess for morphological differences after tagging. Several changes took place; for example, eye index increased while weight and condition decreased with migrated distance and time until recapture. In Lake Mälaren, the majority of tagged eels did not migrate out of the outlets, irrespective of their origin. Most of them were caught in the opposite direction and continued to be caught in the lake 1–3 years after tagging, with significant weight losses. Overall, overwintering is suggested to be an inferior option, but it is uncertain whether this is a natural behaviour or a result of translocation and restocking. For coastal eels, origin had no effect on migratory behaviour; a majority of the tagged eels migrated towards the outlet of the Baltic Sea. Interestingly, a minority of the recaptured eels originated from stocked fish. Instead, recaptures were dominated by natural immigrants that had spent most of their lives in brackish waters.     

Demographic assessment of a stocking experiment in European Eels

Since the 1980s, the European eels’ stocks have dramatically decreased with no sign of recovery, resulting in their classification as Critically endangered on the IUCN red list of threatened species. The European Council Regulation 1100/2007 requires that 35% of glass eels caught annually by fishing be released in European waters for restocking. However, the efficiency of this measure on population viability has never been evaluated. Here, we estimated demographic parameters of a stocked population of French eels using a multistate capture–recapture model. Using these estimates, we then estimated population size and predicted the number of future genitors obtained by stocking. We found that the stage in which eels were stocked did not influence their future survival and that the maximal number of silver eels was quickly reached, after 3 years following stocking. We concluded that stocking experiments in the Mediterranean region are efficient for fast production of genitors. We suggest that further studies should assess the quality of these genitors.     

Immune effects of bathing European eels in live pathogenic bacteria,Aeromonas hydrophila

The specific and non‐specific immune parameters and protection of European eels (Anguilla anguilla) were evaluated after bathing eels with Aeromonas hydrophila. Two hundred eels were distributed into two equal groups and bathed with Phosphate‐buffered saline (Control group) or 1.0 × 10⁷ cfu mL^?1 A. hydrophila (Test group) for 1 h respectively. Then, eels were bled aseptically from the caudal sinus on 1, 4, 7, 14 and 28 days post treatment. The blood cells were used to evaluate the cellular immunity and the serum was used to determine the titres of specific antibody as well as the activities of superoxide dismutase (SOD) and lysozyme. Eels from both groups were challenged by intraperitoneal injection of 1.0 × 10⁷ cfu of A. hydrophila on 28 and 42 days post bathing. The results show that eels bathed in A. hydrophila significantly (P < 0.05) enhanced the proliferation of different types of blood cells and the serum titres of anti‐A. hydrophila antibody. The Relative Percent Survival (RPS) after challenge on 28 and 42 days post bathing in Test group vs. Control group was 40% and 50% respectively. These results suggest that bathing European eels in A. hydrophila would positively affect specific as well as non‐specific immune parameters and protect against A. hydrophila infection in freshwater farming.     

A size‐age model based on bootstrapping and Bayesian approaches to assess population dynamics of Anguilla anguilla L. in semi‐closed lagoons

A size‐age modelling technique is presented for assessing the vital rates, stock and recruitment of eel populations in semi‐closed lagoons with fully monitored migration of silver eels. Data for yellow and silver European eels (Anguilla anguilla L.) were obtained in 2011 from the Comacchio lagoon (Italy). The analysis was performed in three steps: (i) correction of yellow eel data, which are affected by the fyke nets selectivity during samplings, (ii) estimation of survival curve, stock, recruitment and metamorphosis rates of the population (calibration using data from 2011) and (iii) validation of the model using the observed amount of silver eel migrating population of the next year. A bootstrap procedure was used to assess the level of uncertainty for each parameter using the 95% intervals of the highest posterior density distribution HPDD (Bayesian approach). The measured abundance of silver eels was 0.56 ind·ha^?1, while the yellow eel abundance and recruitment were estimated by the model for 2011 at 8.77 ind·ha^?1 and 5.99 ind. ha^?1 respectively. The model performance during validation was satisfactory as the observed total mass of migrating population of 2012 (3777 kg) was inside the 95% HPDD intervals (3197–3839 kg) of model's predictions. The estimated stocks and recruitment were at least ten times lower from the respective estimations of previous studies of 1989 highlighting the crucial conditions of the population. The proposed modelling approach can provide significant information about eel population conditions, facilitating the evaluation of a range of management options in the context of eel conservation plans.     

Mortality of European eel after downstream migration through two types of pumping stations

Although numerous pumping stations (PS) have been used by water managers for numerous applications on rivers, canals and other water bodies, their impact on fish populations is poorly understood. This study investigates European eel, Anguilla anguilla (L.), mortality after natural downstream passage through a propeller pump and two Archimedes screw pumps at two PSs on two lowland canals in Belgium. Fyke nets were mounted permanently on the outflow of the pumps during the silver eel migration periods. Based on the condition and injuries, maximum eel mortality rates were assessed. Mortality rates ranged from 97 ± 5% for the propeller pump to 17 ± 7% for the large Archimedes screw pump and 19 ± 11% for the small Archimedes screw pump. Most injuries were caused by striking or grinding. The results demonstrate that PSs may significantly threaten escapement targets set in eel management plans.     

Behaviour,escapement and mortality of female European silver eels within a regulated lowland river draining into the Baltic Sea

Migration patterns of 146 female silver eels on their way through fresh and brackish water stretches of the Warnow River into the Baltic Sea were studied using acoustic telemetry. The majority of eels (77%) regained their expected downstream migration behaviour after a median rehabilitation time of 2 days. Positive rheotactic behaviour was observed for 23% of tagged eels after release, probably influenced by experimental procedures. The observed silver eel escapement rate was 63%. Two migratory groups were classified by significantly different migratory speed, time of residence in brackish waters and biometric silvering characteristics. Although the classification of silvering stages is based on these measures, the proportions of these stages did not reflect the observed differences in migratory behaviour. In the context of a permanent silver eel descent in the freshwater part of the Warnow River, a permanent escapement was likewise occurring into the Baltic Sea.     

国内欧洲鳗鲡养殖中存在的主要问题

目前，国内采用饲养日本鳗鲡的传统方法来饲养欧洲鳗鲡，内中存在两个问题：一是鳗病，其中狂游病发病急，死亡率极高，而红头病虽然发病稍缓，但极难治疗，死亡也较严重；二是各阶段的生长速度明显低于日本鳗鲡。本文对这两大问题进行探讨。     

Protective immune responses of recombinant outer membrane proteins OmpF and OmpK of Aeromonas hydrophila in European eel (Anguilla anguilla)

Outer membrane proteins (Omps) of Gram‐negative bacteria have been proven to be efficient subunit vaccines against bacteriosis. In this study, OmpF and OmpK of Aeromonas hydrophila were expressed, and their immune protective effects in European eel (Anguilla anguilla) were evaluated. The genomic DNA of A. hydrophila 322A was used as a template, and two kinds of prokaryotic expression plasmids, pET‐32a‐OmpF and pET‐32a‐OmpK, were constructed. Recombinant OmpF protein (r‐OmpF) and r‐OmpK were purified and were proven to have antigenicity by Western‐blot analysis. r‐OmpF and r‐OmpK were used as immunogens to immunize European eel by intraperitoneal injection. The mRNA expression of 6 immune‐related genes (IgM, IL‐10, IRF3, IRF7, LysG4 and HexB) in the liver tissues of eels at 1 hr, 3 hr, 6 hr, 12 hr, 24 hr, 72 hr and 10 days postimmunization was analysed by real‐time PCR. At 30 dpi, the serum antibody response was measured by ELISA. Fish were attacked at 15 dpi by live 322A to assess the protective immunity of r‐OmpF and r‐OmpK. All the six tested genes responded to r‐OmpF or r‐OmpK vaccination at varying degrees. The serum antibody titre of r‐OmpF‐ and r‐OmpK‐immunized groups was 1:1,600 and 1:3,200 respectively. In addition, r‐OmpF gave 35.5% of the relative immune protection rate to European eels, while r‐OmpK gave 70.0%. By analysing the protective immunity and the regulatory role in the immune‐related gene expression of the two recombinant proteins that were studied, it was found that r‐OmpK was a potential vaccine candidate against A. hydrophila.     

微生物制剂在欧洲鳗海水土池养殖中的应用

在欧洲鳗海水土池养殖过程中使用微生物制剂——“绿珍QS-2”改良养殖水质,结果表明,后者能优化和稳定养殖水环境,减少病害,提高养殖产量,增加经济效益。养殖试验过程历时310d,试验池比对照池平均每亩增产314kg,产量提高20．4％,每亩增收1434元-1597元。     

Development and validation of a real‐time PCR assay for the detection of anguillid herpesvirus 1

Anguillid herpesvirus 1 (AngHV1) causes a haemorrhagic disease with increased mortality in wild and farmed European eel, Anguilla anguilla (L.) and Japanese eel Anguilla japonica, Temminck & Schlegel). Detection of AngHV1 is currently based on virus isolation in cell culture, antibody‐based typing assays or conventional PCR. We developed, optimized and concisely validated a diagnostic TaqMan probe based real‐time PCR assay for the detection of AngHV1. The primers and probe target AngHV1 open reading frame 57, encoding the capsid protease and scaffold protein. Compared to conventional PCR, the developed real‐time PCR is faster, less labour‐intensive and has a reduced risk of cross‐contamination. The real‐time PCR assay was shown to be analytically sensitive and specific and has a high repeatability, efficiency and r²‐value. The diagnostic performance of the assay was determined by testing 10% w/v organ suspensions and virus cultures from wild and farmed European eels from the Netherlands by conventional and real‐time PCR. The developed real‐time PCR assay is a useful tool for the rapid and sensitive detection of AngHV1 in 10% w/v organ suspensions from wild and farmed European eels.