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.     

Increased abundance of European eel (Anguilla anguilla) in acidified Norwegian rivers after liming

The prevalence and abundance (density 100 m^?2) of European eel (Anguilla anguilla) were studied by means of electrofishing in 13 acidified rivers in Norway that had been limed to restore acceptable water quality for Atlantic salmon (Salmo salar). Status of Atlantic salmon in these rivers varied prior to liming, from being entirely lost in six highly acidified rivers and in different stages of decline in seven less acidified rivers. Four of the rivers are heavily affected by hydropower development. The abundance and prevalence of European eel increased significantly during the study period. The best model for predicting eel abundance was that with four explanatory variables: time after liming, time after liming squared, status of salmon stocks and hydropower regulation. The eel density was expected to increase by a factor of almost 5 after 10 years of liming. The model also predicts that a river with a formerly reduced Atlantic salmon stock has a 2.8 times higher density of eel than rivers with formerly lost salmon stocks. Before liming, European eel were on average recorded at 15 and 41% of the sampling stations in rivers with formerly lost and reduced Atlantic salmon populations, respectively, increasing to 49 and 68% in individual rivers, respectively, after 10 years of liming. The recovery of European eel in these formerly acidified rivers by means of liming took place during the same period as their abundance declined in other parts of their distribution area in Norway and elsewhere in Europe.     

Movements of the European eel (Anguilla anguilla) in a chalk stream

Abstract – The movements of wild European eels (Anguilla anguilla L.) were monitored continuously over a 2‐year period in a tributary of the River Itchen, Hampshire, UK, using a passive integrated transponder (PIT) antenna system. The time of these movements was then related to a number of monitored and calculated environmental parameters. No eel movements were recorded in the winter months when mean stream temperature was below 10 °C. The numbers of movements peaked in May, when mean water temperatures and day length were increasing. The movement of eels was significantly (P < 0.001, R² = 0.45, N = 72) correlated with the time of sunset, with the majority (72%) recorded during the hours of darkness. The results of this investigation have implications for the design/timing of surveys of yellow eel populations and their habitat associations and suggest that successfully determining the microhabitat preferences of active resident eels will require remote monitoring throughout the 24‐h period.     

Did lack of spawners cause the collapse of the European eel,Anguilla anguilla?

Since the 1980s, a 90% decline in recruitment of European eel, Anguilla anguilla (L.), has occurred across most of Europe. Whether the continental stock has equally declined is uncertain. This study compiles available landings statistics since the beginning of the 20th century and identifies trends over time and space, using a statistical model that takes varying levels of reporting into account. Landings in the pre‐1940s reached over 40 000 tonnes yr^?1, declined during World War II, rose to a peak of 40 000 tonnes yr^?1 in the 1960s (coincident with a peak in re‐stocking) and dropped to an all time low of <20 000 tonnes yr^?1 in the 1990s. The decline in recruitment since the early 1980s was preceded by a decline in landings two or more decades earlier, indicating a decline of the continental stock. Considering the continental stock and the spawning stock must have declined in parallel, insufficient spawning stock biomass might have caused the recruitment collapse currently observed.     

First isolation of Flavobacterium psychrophilum associated with reports of moribund wild European eel (Anguilla anguilla) in Scotland

In late April 2015, the River Dee Trust informed Marine Scotland Science, Fish Health Inspectorate (FHI), that there had been observations of dead and moribund European eels on the River Dee. Later in May, the Spey Fishery Board also reported a number of moribund European eels in a rotary screw smolt trap on the River Spey. In total, 10 cases involving moribund eels were investigated in 2015 and one case in 2016. In addition, a health screen was conducted to investigate the potential presence of Flavobacterium psychrophilum in healthy eels and Atlantic salmon from the River Dee in 2015. Externally, the diseased eels demonstrated white patches in different locations of the body. In all cases, F. psychrophilum was detected by bacterial isolation and/or molecular methods. Three isolates were further characterized by whole‐genome sequencing (WGS) as belonging to sequence type 15 (ST15). Histological examination of diseased European eels revealed lesions at the level of the integument. The pathogen screen for F. psychrophilum in wild healthy fish tested negative by PCR. Further investigation is required to understand the pathogenicity of this bacterium on the health of eels and the potential impact on the wild salmonid population.     

Patterns of silver eel (Anguilla anguilla L.) sex ratio in a catchment

Abstract –  Changes in the numbers and size-class structure of European silver eels, Anguilla anguilla , in the River Frémur (France) were examined over a 9-year period after installation of downstream eel passes. The number of silver eels migrating downstream peaked in 1999, then decreased strongly and steadily after 2000, reaching relatively low levels. At the same time, a gradual shift in the silver eel sex ratio from a dominance of males (size from 270 to 442 mm, age from 3 to 6 years) to females (size from 366 to 1112 mm, age from 4 to 9 years) was recorded. Possible explanations for the escapement patterns observed are environmental sex determination and the installation of eel passes on the main hydraulic engineering structures in 1992 and 1996.     

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.     

中草药对欧洲鳗病原菌的抑制作用研究

本文测试了中草药对五株欧洲鳗致病菌的单独、联合抑菌效果，并测试了中草药、化学治疗剂联合抑菌效果。黄连、地锦草、乌梅、黄芩对病原菌抑制效果显著，中草药联合无显著提高药效作用，其作用效果与抑菌效果最强药物的含量相关，中草药化学治疗剂联合对病原菌作用效果显著。     

Plasma lipoproteins in European eels (Anguilla anguilla): effects of estradiol

A dose-response effect was observed on the plasma concentration of vitellogenin (ALPP, alkali-labile protein P) over a 33 day time-course with repetitive estradiol-17 treatment of European eel (Anguilla anguilla). The plasma estradiol-17 concentration in the treated groups was not proportional to the doses injected. In the groups treated with a medium (0.5 mg kg BW^-1) or high dose (5 mg kg BW^-1) of estradiol there was a significant increase in the plasma total cholesterol concentration during the 33 day time course. The most marked dose-response effect of estradiol was that of plasma total lipids. The distribution of cholesterol between the different lipoprotein classes was dose-related: high density lipoproteins (HDL) predominated in the controls and in the low and medium dose groups, while low density lipoproteins (LDL) predominated over HDL in the group given the high estradiol dose. Plasma HDL concentration was markedly decreased in the high dose treated group; this finding is interesting since HDL plays an important role in the uptake of cholesterol from the extrahepatic tissue.     

Movement behaviour of large female yellow European eel (Anguilla anguilla L.) in a freshwater polder area

Due to a recruitment decline of more than 90% in 30 years, the European eel (Anguilla anguilla L.) has been classified by IUCN as critically endangered. Although the species has been studied intensively to obtain knowledge to improve management, studies about the resident yellow stage are relatively scarce. In this study, 52 large female yellow eels were tagged with acoustic transmitters in a Belgian polder system and tracked by a network of 23 automatic listening stations. We studied both circadian and seasonal movement patterns and the effect of environmental variables on these patterns. Large female yellow eels were most active at night in late summer and early autumn. A generalised linear mixed model showed that their movement is only slightly influenced by environmental variables. Moreover, as yellow eels show high site fidelity (i.e., the majority was detected only in the habitat type of their catch‐release location), they do not encounter many human‐induced connectivity problems in polder systems, which makes these systems highly suitable as eel growth habitat. These results can contribute to an effective eel management regarding habitat protection and restoration.     

Histological and hormonal changes in the European eel (Anguilla anguilla) after exposure to environmental cocaine concentration

The aim of this study was the assessment of histological and hormonal changes induced in the European eel from environmental concentrations of cocaine. Silver eels were exposed to 20 ng L^?1 of cocaine during 50 days; at the same time, control, vehicle control and two post‐exposure recovery groups (3 and 10 days) were made. The general morphology of the skin and the intestine, and the plasma levels of prolactin, cortisol and dopamine were evaluated. In the skin, cocaine decreased the number and size of mucous cells, increased the thickness of the epidermis and altered the club cells and the basal lamina. In the intestine, cocaine increased the thickness of the epithelium and the number of mucous cells and reactivated the structure of the intestine and of the intestinal musculature. Moreover, cocaine increased plasma prolactin, cortisol and dopamine levels. These results suggest that cocaine induced histological changes, directly and/or through the hormonal changes observed. Considering the complex life cycle of the eel, the changes induced by cocaine in the skin, the intestine and the endocrine system could threaten the ability of the eel to successfully migrate and reproduce.     

Modification of essential fatty acid composition in broodstock of cultured European eel Anguilla anguilla L

Farmed eels had lower levels of arachidonic acid (20:4 n‐6) (ARA) and higher ratios of eicosapentaenoic acid (20:5 n‐3) (EPA):ARA compared to wild European eels collected from the Baltic Sea and southern Norwegian coast. Eels fed a formulated feed (JD) with a distribution of essential fatty acids (EFA) resembling wild European eel were sampled after 0, 5, 10, 14 and 44 weeks of feeding to examine changes in fatty acid composition (FAC) in ovaries, visceral fat and muscle. The results showed a slow but steady incorporation of EFA. Lipids are incorporated in the oocytes early in oogenesis, and the leading cohort of oocytes is rich in lipid droplets before the onset of vitellogenesis. This indicates that feeding with optimized broodstock feeds should start early to allow the incorporation of EFA in the first cohort of oocytes. At least 14 weeks of feeding is required to change lipid EFA in broodstock eel to resemble EFA in the diet or in wild fish. After 44 weeks of feeding, ARA was significantly higher in the neutral lipids of ovaries (1.9%) compared to visceral fat (1.2%) or muscle (1.0%). EPA:ARA ratios decreased two‐ to threefold in all tissues examined during that time. ARA and docosahexaenoic acid (22:6 n‐3) (DHA) had accumulated in ovarian polar lipids.     

Differences in head shape of the European eel, Anguilla anguilla (L.)

Differences in eel, Anguilla anguilla (L.), head shape were the result of greater increases in head width, rather than changes in head length. The ratio of head width:total length (HW:TL) increased significantly ( t -test, P < 0.05) from the glass-yellow eel stage. Cultured yellow eels were exclusively narrow headed (mean HW:TL = 0.027, range = 0.023–0.032), while wild yellow eels displayed an array of head shapes (mean HW:TL = 0.033, range = 0.023–0.046). Therefore, broad heads (HW:TL 0.033) occurred only among wild yellow eels sampled and may have resulted from diet. Cultured yellow eels consumed only small pellet material. Of wild yellow eels stomachs containing food, 78% of broad-headed eels consumed large and/or hard-bodied organisms (e.g. beetles, fish, molluscs and Notonecta sp.), while 83% of narrow-headed eels consumed exclusively small/soft-bodied prey (e.g. amphipods and chironomids).     

Modelling daily catches of silver-phase European eel (Anguilla anguilla) in two hydropower-regulated rivers

Estimation of silver eel production, Anguilla anguilla L., is fundamental for the management of eel stocks. In the hydropower-regulated rivers Shannon and Erne, Ireland, production is calculated using catch data from a conservation trap and transport programme. However, in both rivers gaps in silver eel catch datasets tend to occur, which can lead to biases in production estimates. Generalised additive models (GAMs) were used to model daily catch in these rivers based on a variety of environmental variables. Final models for the Shannon and Erne explained 83.7% and 78.8% of deviance in daily catch, respectively. A second model on the Erne included catch from a nearby fishing site in an attempt to increase explanatory power and explained 91.7% of deviance. Although model accuracy was increased, reliance on catch from another site may limit the applicability of the model. Model predictions were combined with estimates of fishing efficiency to predict production for the Shannon (36,210 kg; 0.85 kg/ha) and Erne (66,899–67,047 kg; 2.55–2.56 kg/ha). These values represented a 9.3% and 2.8%–3.0% increase on production estimated from incomplete catch records alone for the Shannon and Erne, respectively.     

Fluctuations in European eel (Anguilla anguilla) recruitment resulting from environmental changes in the Sargasso Sea

European eel decline is now widely observed and involves a large number of factors such as overfishing, pollution, habitat loss, dam construction, river obstruction, parasitism and environmental changes. In the present study, we analyzed the influence of environmental conditions in the Sargasso Sea and Atlantic ocean circulation on European glass eel recruitment success. Over a recent 11‐yr period, we showed a strong positive correlation between an original index of glass eel recruitment and primary production (PP) in eel spawning area. Moreover, PP was negatively correlated with temperature in the Sargasso Sea. Therefore, we used sea temperature as an inverse proxy of marine production. A close negative relationship has been found over the last four decades between long‐term fluctuations in recruitment and in sea temperature. These findings were reinforced by the detection of a regime shift in sea temperature that preceded the start of the decline in glass eel recruitment in the early 1980s. By contrast, variations in integrative indices measuring ocean circulation, i.e. latitude and strength of the Gulf Stream, did not seem to explain variations in glass eel recruitment. Our results support the hypothesis of a strong bottom‐up control of leptocephali survival and growth by PP in the Sargasso Sea on short and long time scales. We argue that sea warming in the eel spawning area since the early 1980s has modified marine production and eventually affected the survival rate of European eels at early life stages.     

On the actual recruitment of European eel (Anguilla anguilla) in the River Ems,Germany

Data from the historical River Ems glass eel fisheries in the 20th Century have been used for the ICES recruitment series. Commercial catches ceased from 1980 and dropped to zero; thus, eel recruitment data are since lacking. The present study assessed the actual eel recruitment to the River Ems with regard to the local tidal conditions. Standardised measurements were performed on relative glass eel occurrence at the tidal weir from 2014 and quantitative sampling at the next weir upstream from 2013. Eel migration at the tidal weir was dominated by glass eels and increased 2–3 month earlier than at the upstream weir, where migration was dominated by pigmented eels. In 2016, glass eels were marked at the tidal weir and pigmented elvers were sampled at the upstream weir. The 2016 eel recruitment was estimated at about 1% of the mean historical recruitment. Eel recruitment at the two weirs was not directly linked. Many eels seem not to pass the upstream weir. Further investigations are needed to reveal whether the results represent a regular pattern. In the long run, the perpetuation of the recruitment series is also foreseen.     

Two forms of cytochrome P450 cDNA from 3-methylcholanthrene-treated European eel Anguilla anguilla

ABSTRACT:     The cytochrome P450 (CYP) represents a large group of microsomal monooxygenases that catalyze drugs as well as a host of lethal environmental contaminants such as dioxins, leading to either detoxification and excretion from the animal or generation of carcinogenic intermediates. In the present study two forms of cDNA were cloned (Eu MC1 and Eu MC2) for European eel CYP1A genes by polymerase chain reaction (PCR) techniques. The cDNA of Eu MC1 was 3368 bp long coding 521 amino acid residues, and that of Eu MC2 was 2464 bp long coding 517 amino acid residues. Identities of deduced amino acid sequences between Eu MC1 and Japanese eel CYP1A1 and that between Eu MC2 and the second form of Japanese eel CYP1A were 98% and 97%, respectively, showing decisively that Eu MC1 and Eu MC2 are orthologous to Japanese eel CYP1A1 and the second form of CYP1A, respectively. A striking difference between the two eel species was that the Eu MC1 peptide was two amino acid residues longer than that of the Japanese eel CYP1A1. Existence of two loci of CYP1A in Japanese and European eels may suggest that the two forms of CYP1A exist widely among the eel species, because the divergence between the two eel species has been shown to be close to the basal divergence among eels. The identities in CYP1A may help to estimate genetic distance between European and Japanese eels.     

Experimental validation of geosmin uptake in rainbow trout,Oncorhynchus mykiss (Waldbaum) suggests biotransformation

The bioconcentration of waterborne geosmin in rainbow trout, Oncorhynchus mykiss (Waldbaum) was assessed. Fifty rainbow trout with a mean (SD) weight of 226.6 (29.0) g and lipid content of 6.2 (0.6) % (w/w) were exposed to geosmin in static water for 0, 2, 4, 6, 8, 12, 24, 36, 48 and 120 hr, with one tank containing five fish for each exposure period. Geosmin concentrations were measured in fish tissue and water samples collected over time. With time the geosmin concentration in the fish increased and decreased in the water. However, the total absolute amount of geosmin in the system declined over time which could be explained by induction of biotransformation. This is in accordance with the decreasing lipid normalized geosmin levels in the liver compared with the liver‐free carcass. Geosmin distribution within rainbow trout clearly is not exclusively governed by the lipid content of tissues. In vivo geosmin bioconcentration in rainbow trout is slower and the body burden reached is lower than the generally accepted theoretical model predicts.     

The influence of phytoestrogens and oestradiol-17β on growth and sex determination in the European eel (Anguilla anguilla)

Under aquaculture conditions, European eels (Anguilla anguilla) produce a high percentage of males (80–95%) that normally stop growing at 100–200 g. Females continue to grow to 500–750 g and obtain higher market value. Therefore, increasing the percentage of females in a population would be beneficial to the culture of eels. The present study was carried out in order to examine the effect of oestradiol and phytooestrogens on sex differentiation and growth rate of eels. Juvenile European eels with undifferentiated gonads were fed pellets containing oestradiol‐17β (E2) or phytooestrogens for 100–150 days. Feeding E2 resulted in 50–61% increase in body weight compared with the control. Oestradiol‐17β and phytooestrogens both elevated significantly the percentage of females in the population. Feeding E2 at 20 mg kg^?1 feed resulted in 70% females, while lower concentration of E2 (2 mg kg^?1) resulted in only 30% after 100 days (Experiment 1). The same dose given for 150 days (Experiment 2) resulted in 88% females, indicating that both, the concentration and duration of E2 treatments had a significant effect on sex differentiation. Fish fed genistein at 2 mg kg^?1 for 100 days, resulted in 55% of females, but at a higher dose of 20 mg kg^?1 there were only 15% females. These results demonstrate that phytooestrogens can be used as alternatives to gonadal steroids for sex manipulation in eels, but the optimal concentrations and duration are still to be determined.