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.     

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.     

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.     

Population density, growth and diet of eels, Anguilla anguilla L., in two rivers in eastern England

Abstract. The population density, growth and diets of eels, Anguilla anguilla L., from four sites on two rivers in eastern England were studied over 2 years. Eel population density varied greatly between sites and seasons, few eels being caught during the winter. Mean population densities ranged from 50 to 518 eels/ha and mean biomass from 35 to 210kg/ha. Older eels were more numerous at upstream sites. Growth rates in the two rivers were generally similar, but eels 9–13 years old at upstream sites were significantly larger in terms of length and weight than their downstream counterparts.
Of 868 eels examined, only 425 stomachs contained food, the percentage of empty stomachs being negatively correlated with temperature. Asellus aquaticus was the predominant food item at all sites in all seasons and in the three size classes of eels studied. Megaloptera and Trichoptera were also important. Fish, largely bottom-dwelling species, were found mainly in the largest size-class of eels and in the winter, when few large eels were feeding. Niche breadth expands with increasing size of cel.     

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.     

The occurrence and distribution of Myxidium giardi Cépède, 1906 in wild and cultured European eels, Anguilla anguilla L., in England

Abstract. Myxidium giardi was found in wild and cultured eels. The prevalence rate in elvers was 77% in the River Torridge (Devon), 80% in the River Parrett (Somerset) and 90% in the River Ouse (Yorkshire). Adult eels from the River Ouse had a prevalence rate of 53%, Cultured fingerlings and adults had a prevalence rate of 60% and 1% , respectively. There was an apparent age-related pattern in the organ location of the histozoic trophozoite: an initially high prevalence of kidney trophozoites which gradually decreased with age; in the gills of elvers there was a low prevalence of histozoic trophozoites which increased in the adult stage. Histozoic trophozoites were also found in the stomach and intestine of elvers and fingerlings. A qualitative estimate of trophozoite numbers in each organ of elvers is given. The coelozoic trophozoite of M. giardi in the gall bladder of elvers and fingerlings is also described for the first time.     

Reading otoliths of eels, Anguilla anguilla (L.), of known age from Kolderveen, The Netherlands

Abstract. In this study we aged eels, Anguilla anguilla (L.), from a pond in the Netherlands stocked with elvers in 1966. Yellow eels were sampled from the pond in 1981. Burning and cracking the otoliths of these eels gave the expected age (15 years) in 27% of the cases, with a mean estimated age of 12·3 years. As some immigration of young eels into the pond must have occurred after stocking, the percentage of correctly determined specimens is even higher. Clearing the otoliths in ethanol was not a good method for these eels, giving a mean estimated age of 7·7 years and no age over 10 years. The use of the burning and cracking technique is recommended.     

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。     

Studies of feeding stimulation in young eels, Anguilla anguilla L., before and after first-feeding using a novel rapid-screening method

Olfactory attractiveness and gustatory palatability of different materials of potential importance in first-feeding of young eels, Anguilla anguilla (L.), were examined using a novel electromechanical device which was developed for rapid screening. Comparisons were made between non-feeding glass eels and elvers that had commenced feeding on a commercial formulated granule diet. Preparations of tubificid worms, earthworms (Eisenia foetida), cod roe, beef liver and potentially stimulatory chemicals were tested. Invertebrate-derived foodstuffs and combinations of amino acids, inosine 5′-monophosphate and glycine betaine were found to be more attractive than vertebrate foodstuffs pre-weaning. Attractiveness and palatability, especially of granules formulated for eels, were strongly influenced by learning after feeding had commenced. Implications for weaning of glass eels in aquaculture are discussed. Further uses of the rapid-screening device are discussed.     

Eel, Anguilla anguilla L., stock and yield in Polish lakes

Abstract. Details of mean annual slocking rate and catch of eels in a random sample of 68 Polish lakes over a period of 36 years are presented. Multiple correlation and curvilinear regressions are used to assess the effectiveness of stocking. Maximum stocking rate of 275 elvers/ha/year and maximum catch of 5.2 kg/ha were determined. Yields per recruit were highest in the lakes in which the greatest catches of other fish were made and stocking was most effective in lakes of depth less than 5 m.
The theoretical maximum catch of 5.2kg/ha was very much less than maxima observed elsewhere. Two possible contributory factors are the Polish practice of using relatively large-meshed seines for fishing, permitting the escape of eels of less than 50cm and the relatively small number of potential competing fish species in fisheries such as that of Lough Neagh in Ireland.     

The pathology of Myxidium giardi Cépède, 1906 infections in wild and cultured eels, Anguilla anguilla L.

Abstract. The histopathology of Myxidium giardi infections in wild and cultured elvers, fingerlings and adult eels is described. The lesions in the target organs of the kidney and gills were minimal. Marked granulomatous changes were evident in the spleen, peritoneal fat and, to a lesser extent, the liver and gas gland. The granulomatous reaction consisted of a mononuclear infiltration into the spore mass and proliferative fibrosis of the surrounding connective tissue, Histological evidence of an immunological response is described. The types of M. giardi cysts are described-a primary interstitial cyst of the kidney and gill, a secondary cyst of the gills and a glomerular spore mass. Primary interstitial cysts in the kidney were observed to rupture and formed spore masses in the interstitial tissue. Evidence of vascular spread and the presence of spore emboli is presented. It is suggested that primary multiplication occurs in the primary interstitial cyst of the kidney and gills, and secondary multiplication in the secondary gill cyst and glomerular spore mass from which mature spores are released to the environment.     

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.     

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.     

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.