Dermocystidium from cultured eels, Anguilla anguilla L., in Scotland

Abstract. An outbreak of Dermocystidium infection in cultured eels, Anguilla anguilla L., from Scotland is described. The disease appeared in the winter months but did not cause significant mortalities or adversely affect the growth of the eels. Infected eels had large, elongate white cysts on the gills. The cysts consisted of a homogeneous layer of parasite origin surrounded by a fibrous capsule and hyper-plastic epithelium. Within the cysts were small spherical parasite cells with a densely staining central area and peripheral granular, vacuolated cytoplasm. The cell wall appeared to be broken down in many cases and the cells appeared degenerate. The cysts eventually collapsed and the cavity was filled with epidermal tissue.     

Riverine migration of young eels Anguilla anguilla (L.)

Eels of length 6–50 cm, trapped in the course of up-stream migration in the River Shannon, were sampled during 10 seasons from 1973 to 1983. Migration normally began at the end of May at water temperatures of 13 or 14°C, but the extreme dates ranged from 17 May to 24 June. In five out of seven years, migration ended between 10 and 19 September; in the other two years it was on 29 July and 6 October. Positive correlations were observed between date of first migration and temperature between 15 and 21 May. No connection between temperature and date of end of migration was apparent.Length measurements of 5008 eels showed that large individuals (> 15 cm) migrated throughout the season, but that the majority of smaller eels (< 10 cm) had a shorter migration period from mid-June to mid-August.The ages of 158 specimens were determined. The maximum age was 10 +; the majority ranged from 1 + to 3 +. Elvers of 0+ were scarce or absent early in the season and were never plentiful. Numbers of eels caught ranged from 131000 to 417000 per year.It was concluded that a number of factors influenced migration of the eels, the time of year perhaps being the strongest, with water temperature in May being secondary. Migration of the greatest numbers occurred in the year following the greatest immigration of elvers to the river. This suggests that migration may be influenced by population pressure in the lower reaches of the river.     

Sexual dimorphism in yellow European eels, Anguilla anguilla (L.)

Sexual dimorphism in yellow eels, Anguilla anguilla (L.). was studied to clarify the differences between sexes. The focus was on the relationship between length/weight and sex. Three samples of eels, grown in captivity in different places in Israel, were studied. Females were significantly lighter than undifferentiated eels in the length range 33-47 cm. The regression coefficient did not differ significantly between females and undifferentiated eels. Sex induction, by a single injection of human chorionic gonadotropin, resulted in a higher loss of body weight during differentiation into the female than into the male.     

European eels, Anguilla anguilla (L.), infected with Anguillicola crassus exhibit a more pronounced stress response to severe hypoxia than uninfected eels

The parasite, Anguillicola crassus is a non-native species that infects naive European eels, Anguilla anguilla, and causes pathological damage to the swimbladder, potentially compromising their ability to cope with hypoxic conditions. This study aimed to elucidate whether anguillicolosis exacerbates the stress responses to exposure to hypoxic water, conditions that have been implicated in mass mortalities of wild infected European eels. Blood parameters in infected and uninfected eels were measured during exposure to severe hypoxia over an 8-h period. Infected fish showed significantly higher levels of plasma cortisol compared with uninfected eels after 4 h of hypoxia. Uninfected fish showed an almost twofold increase in plasma glucose after 8-h exposure to hypoxia but infected fish showed no significant change, so that the plasma glucose concentration was significantly higher in uninfected eels than in infected eels. Both groups showed similar elevations in blood haematocrit, suggesting a similar catecholamine response in infected and uninfected eels. The lack of a hyperglycaemic response in infected eels, despite indirect evidence of a catecholamine response to hypoxia, may reflect an increase in glucose turnover. The data suggest that anguillicolosis results in a significantly greater corticosteroid stress response to hypoxia accompanied by a higher metabolic cost.     

Temperature-dependent activity of brackish water yellow eels, Anguilla anguilla L.

Abstract. Female yellow eels, Anguilla anguilla L., were caught with fyke nets in the brackish water bight Hallangspollen, Norway during 1983 and 1984. Fishing was performed continuously for a variable number of days (defined as the fishing period), and the nets were monitored at uneven intervals during the fishing period. The eel activity, expressed as catch per unit effort (CPUE), was positively correlated (R²= 0·74; P < 0·001) with water temperature (defined as a regression of temperature variation during the fishing period). The multiple regression model of CPUE on water temperature was improved by including the number of fishing days in each fishing period in the regression model (R²= 0·91; P < 0·001). With high fishing efforts the predicted CPUE decreased below that which was predicted by the temperature model alone.     

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.     

溴氰菊酯在欧洲鳗鲡体内药代动力学研究

在试验水温(25±1)℃时,按100μg·L-1的剂量给欧洲鳗鲡药浴溴氰菊酯36h后,采用气相色谱法测定血浆和肌肉中的药物浓度,研究了溴氰菊酯在欧洲鳗鲡体内的代谢及消除规律.采用DAS自动化药动学分析程序对数据进行分析.结果表明:欧洲鳗鲡血浆和肌肉中溴氰菊酯经时过程均符合一级吸收一室开放模型,血浆中主要药动学参数为:T1/2k为148.296h,Cmax为18.446μg·L-1,Tmax为14.7h.肌肉中主要药动学参数为:T1/2k为386.912h,Cmax为44.291μg·kg-1,Tmax为49.6h.肌肉作为可食性组织,且消除较慢,因此选取肌肉组织作为残留检测的靶组织,以10μg·kg-1为最高残留限量,在本试验条件下,建议休药期不低于9d.     

鳗鲡疱疹病毒对欧洲鳗鲡的致病性研究

为了明确鳗鲡疱疹病毒(Anguillid herpesvirus, AngHV)的致病性,本实验采用一株从鳗鲡“脱黏败血综合征”病料中分离的AngHV(NA16108),研究了其对欧洲鳗鲡幼鳗的致病性。结果显示,注射AngHV的鳗鲡体表出现黏液脱落、鳍条出血、红头等症状;鳃部出现黏液增多、出血,肝脏褪色、肿大,脾脏和肾脏肿大等病变;进一步的组织病理学观察发现,鳗鲡的体表黏液及黏膜上皮细胞脱落,次级鳃瓣增生、呼吸细胞肿胀坏死,脾脏细胞坏死、黑色素细胞聚集,肾小管管壁上皮细胞坏死、管腔变窄等病理症状;这与鳗鲡“脱黏败血综合征”的发病特征一致。致死率分析显示,从第4天开始攻毒组鳗鲡开始出现死亡,第7天和第14天的累计死亡率分别达到26.7%和56.7%;荧光定量PCR检测显示,在攻毒鳗鲡肝脏、脾脏、肾脏、肠道、鳃和皮肤黏液中均可检测到AngHV;另外,用鳗鲡卵巢细胞系(eel ovary cell line, EO)从攻毒鳗鲡主要内脏器官中重新分离出AngHV,表明鳗鲡发生了AngHV的系统侵染。研究表明,AngHV是鳗鲡“脱黏败血综合征”的致病病原,这为深入开展该病的发病机制及防控研究奠定了基础。     

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.     

Effect of insulin in the diet on the growth of European eels (Anguilla anguilla L.)

The effect of oral administration of insulin, in various concentrations, on the growth of European eels (Anguilla anguilla L.) was studied. In order to determine whether the insulin penetrated through the stomach or gills to the blood system, 5 ml insulin, suspended in an 0.6% solution of NaCl, was inserted via the mouth of eels, and the insulin content in the blood measured by radioimmunoassay immediately, and at one and two hours after administration. A control group was given 0.6% NaCl alone. Significantly increased levels of insulin in the blood plasma were found in eels which received high insulin concentrations compared to the control group. Eels administered 20 ppm and 40 ppm insulin in the diet grew significantly faster than a control group fed a diet without insulin, and a group fed 5 ppm insulin.     

Age and growth of yellow eel, Anguilla anguilla (L.), determined by two different methods

Abstract– Ageing and calculation of growth rate of eels has always been controversial, and remarkably little validation has been reported. This article describes the growth of yellow eels from three lakes in the Burrishoole system, Ireland, as determined by back-calculation from otoliths prepared by burning and cracking. Eels captured by fyke net ranged in length from 25.0 cm to 98.5 cm and the ages ranged from 6 to 49 years. Back-calculated growth was slow, with mean annual increments of 2.08 cm, 1.40 cm and 1.97 cm for the three lakes. Yellow eels captured in the Burrishoole system in 1987 and 1988 were marked by Pan Jet inoculated alcian blue dye and released back into the three lakes. Recaptures made over the following three years yielded considerable growth data. Mean annual growth rates recorded were 2.30 cm, 1.07 cm and 0.97 cm. These data compare favorably with the calculated growth rates, thereby validating the otolith analysis for the eels in the Burrishoole system.     

Studies of sampling methods and of techniques for estimating populations of eels, Anguilla anguilla L.

Abstract. The performance of fyke netting was compared with that of electrofishing in an artificially stocked closed pond and in open waters with natural populations in the Thames catchment, England. Seine netting, trapping and fyke nets of different mesh size were also compared using wild populations. Studies of mark-recapture and catch-depletion techniques for estimating population sizes and structures were included. It was concluded that no single technique was ideal because of the difficulties inherent in adequately sampling eels in all aquatic environments. Recommendations are made and the implications of fyke net efficiency for commercial fishing are discussed.     

Dietary accumulation of polychlorinated biphenyls, chlorinated pesticides and mercury by cultivated eels, Anguilla anguilla L.

Abstract. Polychlorinated biphenyls (PCBs), chlorinated pesticides and mercury were analysed in cultivated eels, Anguilla anguilla L., and their food. In one experiment the eels were mainly fed with a low-fat diet. In another experiment cultivated eels were fed a high-fat diet. Both experiments showed that the major part of the organochlorine compounds and mercury present in the diet was retained by the eel. The retention of the organochlorine compounds and mercury was estimated at 50–70% after an exposure period of about 1 year.
Although the mechanisms of accumulation of mercury and organochlorine compounds clearly differ, no differences in retention were found. The remaining mercury, PCBs and pesticides which were not retained possibly pass through the alimentary canal rather than being accumulated first and metabolized or excreted during the feeding period.
Due to these high levels of accumulation monitoring levels of PCBs, chlorinated pesticides and mercury in the diet is essential for protection of the quality of cultivated eels.     

The protein sparing effect of carbohydrates in the diet of eels (Anguilla anguilla)

The effect of partially replacing protein by carbohydrates in eel (Anguilla anguilla) diets was studied. The growth of eels fed 40% protein and 38% wheat meal was greater than that of eels fed 50% protein and 20% wheat meal or 30% protein and 56% wheat meal.The body fat of eels fed a high carbohydrate diet was greater (weight/weight) than the body fat of eels fed a low carbohydrate diet. The food conversion value was lower in eels fed 40% protein than eels fed 50% or 30% protein. However, the protein efficiency ratio was lower in eels fed a low percentage of protein (30%) than that of eels fed a high percentage of protein (40% or 50%).     

Preliminary investigations on husbandry, nutrition and growth of glass eels and elvers, Anguilla anguilla L.

Abstract. Some effects of husbandry and feeding on growth of glass eels and elvers of the European eel. Anguilla anguilla L., were investigated. In the first 7 months after arrival the recorded mortality amounted to 34% of the initial population. due to escapes and possibly cannibalism the observed survival after 7 months was only 21% of the initial population.
Mean growth rate of the whole population was 7.1 g day⁻¹ kg^−0.8. Growth proved to be highly variable. Growth rate was clearly affected by feeding level but not by initial stocking density.
Maximum growth rate was approximately 8g day, ¹ kg^−0.8 with feed conversions of 1.7–2.0. An optimal feeding level of 7.5g day¹ kg^−0.8 resulted in feed conversions of 1.3–1.6.