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European Eel (european + eel)
Selected AbstractsEvaluation of relative distance as new descriptor of yellow European eel spatial distributionECOLOGY OF FRESHWATER FISH, Issue 4 2008H. Imbert Abstract,,, The spatial distribution of yellow European eel (Anguilla anguilla) smaller than 300 mm was analysed during the upstream colonisation process. A 9-year electric-fishing programme in the Gironde catchment (France) provided eel occurrence data in 256 sites and eel abundance data in 23 sites. Generalized linear models showed that small eel spatial distribution decreased with river slope, dam number and with downstream-upstream distance, estimated using either the distance from the tidal limit, called ,tidal distance', or the ,relative distance', calculated as the fish's position relative to the total distance between tidal limit and river source. This new descriptor should be considered in future eel distribution studies as it reveals fractal dimension in eel spatial distribution and may provide a standardised method to compare directly freshwater eel assessment between streams and catchments of different lengths. If the relevancy of this descriptor is subsequently confirmed, it may have important implications for the management of eel population conservation. [source] Winners and losers of conservation policies for European eel, Anguilla anguilla: an economic welfare analysis for differently specialised eel anglersFISHERIES MANAGEMENT & ECOLOGY, Issue 2 2010M. DOROW Abstract, Recreational specialisation theory was coupled with a discrete choice experiment to understand eel, Anguilla anguilla L., angler's heterogeneity in their reaction to regulatory changes and the associated welfare changes. Differently specialised eel anglers exhibited distinct preferences for catch variables and eel angling regulations. All anglers preferred slightly to moderately stricter regulations than are currently in place; however, such policies particularly benefited casual eel anglers. In contrast, advanced eel anglers would be most penalised by highly restrictive regulations as indicated by substantial reductions in economic welfare. Aversions to stricter regulations found for advanced anglers contradicted predictions from specialisation theory. From an eel management perspective, the implementation of some simple tools such as increased minimum-size limits will reduce angling mortality on eel and simultaneously increase the welfare of anglers. By contrast, highly restrictive eel angling regulations will result in considerable economic welfare losses of several million , per year for northern Germany alone. [source] Management issues relating to the European eel, Anguilla anguillaFISHERIES MANAGEMENT & ECOLOGY, Issue 6 2003A. Starkie Abstract In many European countries there is growing concern over reduced recruitment and catches of the European eel, Anguilla anguilla (L.). Evidence of the decline, together with possible reasons for the changes as detailed by various authors, are reviewed. It is suggested that the problems need to be addressed now and on various levels. Proposals for management actions, as outlined by the Environment Agency for England and Wales, are summarized by way of example. [source] Did lack of spawners cause the collapse of the European eel, Anguilla anguilla?FISHERIES MANAGEMENT & ECOLOGY, Issue 6 2003W. Dekker Abstract 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. [source] Fluctuations in European eel (Anguilla anguilla) recruitment resulting from environmental changes in the Sargasso SeaFISHERIES OCEANOGRAPHY, Issue 1 2008SYLVAIN BONHOMMEAU Abstract 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. [source] Protein profile study in European eel (Anguilla anguilla) seminal plasma and its correlation with sperm qualityJOURNAL OF APPLIED ICHTHYOLOGY, Issue 5 2010D. S. Peñaranda Summary Along with sperm quality parameters, the protein profile of European eel seminal plasma was analyzed during induced spermiation (n = 56 samples). Motility, Percentage of live cells, spermatozoa head morphometry and concentration showed low values during the initial weeks of spermiation and maintained high levels throughout the rest of the experiment. The protein profile gradient by SDS-PAGE (4,15%) registered four important electrophoretic bands around 80, 40, 26 and 12 KDa. Three of them showed significant differences in concentration during treatment (80, 40 and 12 KDa), and all of them showed the highest value on the 8th week. Both 80 and 12 KDa bands increased until the 8th week, followed by a progressive decline. One possible explanation for these profiles is that, in the first weeks of treatment, proteins originated from blood plasma are accumulated in the seminal plasma, and from the 8th week some of these proteins are incorporated into the spermatic membranes. The 40 KDa protein band also increased during the first 8 weeks, but maintained high concentrations in the seminal plasma for the rest of the experiment. One result confirms the theory that the presence of proteins in the seminal plasma having a molecular weight lower than 50 KDa increased spermatozoa motility, since the 40 KDa band displayed significantly higher values coinciding with the high percentages of spermatozoa motility. Seminal plasma proteins seem to have an important role in spermatogenesis and spermatozoa movement, but further studies are necessary to discover the identity of these proteins and their precise functions. [source] Anguillicoloides crassus infection of European eel, Anguilla anguilla (L.), in inland waters of Estonia: history of introduction, prevalence and intensityJOURNAL OF APPLIED ICHTHYOLOGY, Issue 2010A. Kangur Summary Eel fishery in Estonian inland waters depends entirely on the stocking of glass eels or pre-grown (farmed) eels. Via importation of live eels of 20,30 cm length the non-indigenous swimbladder nematode Anguillicoloides crassus was probably introduced via Germany into Lake Võrtsjärv in 1988, and has since spread to many inland waters of Estonia. In 1992, the parasite was found in eel caught from Lake Võrtsjärv. Between 1992 and 2002 and additionally in 2008, we examined in total 870 eels from Lake Võrtsjärv (270 km2) and in 2008, 63 eels from three small lakes for adult A. crassus. The aim of the study was to obtain information on the variation of A. crassus infection in eels in Estonian lakes, to determine the temporal dynamics of prevalence and intensity of infection, and to establish a relationship between the length of host and intensity of infection in the eels in Lake Võrtsjärv. There appeared to be a pronounced variation in prevalences of infected eels (from 3.7% to 100%) between the four investigated lakes. However, in Lake Võrtsjärv, the prevalence of adult A. crassus infection remained stable (mean about 65%) for many years. The average number of nematode per infected eel (mean intensity) ranged from 12.6 ± 2.5 in 1993 to 4.0 ± 0.6 in 1999 in Lake Võrtsjärv, while it was significantly higher (P < 0.0001) in the period 1992,1998 compared to 1999,2002 and 2008. The mean number of parasites per swimbladder was not related to eel length and no statistical difference was found in the condition factor of infected and non-infected eels. Although under normal environmental conditions A. crassus has not caused serious disease problems to eels in the study area, high intensity of parasite infection may contribute to eel kills due to oxygen deficiency in winter under the ice in Lake Võrtsjärv. [source] Growth and product quality of European eel (Anguilla anguilla) as affected by dietary protein and lipid sourcesJOURNAL OF APPLIED ICHTHYOLOGY, Issue 2 2003U. Luzzana Summary A 12-week feeding trial was undertaken to evaluate the effects of partial replacement of marine raw materials in the diet with soybean meal and beef tallow on growth and product quality of European eel (Anguilla anguilla). Based on growth, feed efficiency and chemical composition, soybean meal was found to be an unsuitable ingredient for eel diets as a substitute for fishmeal (25% on a protein basis), probably because of the low digestibility of its carbohydrate content. However, beef tallow can be used to replace 50% of fish oil without reduction in growth, provided that digestible carbohydrates are present in the diet. No major effects of protein and lipid sources in the diet were found on fillet chemical composition. Sensory analysis revealed no significant differences between fish fed the control and the experimental diets, with the exception of salty taste which was significantly higher in fish fed combined soybean meal and beef tallow compared with fish fed the control diet. [source] Timing and rate of sexual maturation of European eel in brackish and freshwater environmentsJOURNAL OF FISH BIOLOGY, Issue 2006D. Bevacqua Maturation rates in three European eel Anguilla anguilla populations increased from September to October and were slightly lower in fresh water than in brackish waters. Average and variance of total length at maturation were larger in females than males. [source] Distribution of genetic variation in farmed and natural stocks of european eelJOURNAL OF FISH BIOLOGY, Issue 2004J. M. Pujolar European eel (Anguilla anguilla; Teleostei) is a valuable commercial species. However, over the past 25 years, the population of European eel has been declining to such a degree that major concerns have been raised for its long-term conservation. Since little information is available on the life-cycle and genetic structure of European eel, it has been difficult to evaluate the existence of any population substructuring. Molecular genetic methods contribute to a better knowledge of the demography and population structure in marine fish. In addition, management strategies and conservation goals must consider information on genetic substructuring as well as on life history patterns. The aim of the study is to provide more detailed knowledge on the genetic variability, demography and population substructuring of European eel by analysing and comparing natural and farmed individuals. Natural eel samples have been obtained in two geographical sites (Netherlands, France) including temporal samples in a short-scale (within years) and a long-scale (between years). Simultaneously, farmed glass eels have been grown in two separate batches during one year. Batches have been monitored and genetic samples have been obtained during the year. A combination of selection-sensitive (allozymes) and selection-neutral markers (microsatellites) has been used in the study since selection seems to play an important role in the determination of the quality of future eel spawners. Results suggest a positive correlation between growth and genetic variability since individuals attaining a large length and mass present significant higher heterozygosities. [source] Endurance swimming of European eelJOURNAL OF FISH BIOLOGY, Issue 2 2004G. Van Den Thillart A long-term swim trial was performed with five female silver eels Anguilla anguilla of 0·8,1·0 kg (c. 80 cm total length, LT) swimming at 0·5 body lengths (BL) s,1, corresponding to the mean swimming speed during spawning migration. The design of the Blazka-type swim tunnel was significantly improved, and for the first time the flow pattern of a swim tunnel for fish was evaluated with the Laser-Doppler method. The velocity profile over three different cross-sections was determined. It was observed that 80% of the water velocity drop-off occurred over a boundary layer of 20 mm. Therefore, swim velocity errors were negligible as the eels always swam outside this layer. The fish were able to swim continuously day and night during a period of 3 months in the swim tunnel through which fresh water at 19° C was passed. The oxygen consumption rates remained stable at 36·9 ± 2·9 mg O2 kg,1 h,1 over the 3 months swimming period for all tested eels. The mean cost of transportation was 28·2 mg O2 kg,1 km,1. From the total energy consumption the calculated decline in fat content was 30%. When extrapolating to 6000 km this would have been 60%, leaving only 40% of the total energy reserves for reproduction after arriving at the spawning site. Therefore low cost of transport combined with high fat content are crucial for the capacity of the eel to cross the Atlantic Ocean and reproduce. [source] Regulation of post-larval development in the European eel: thyroid hormone level, progress of pigmentation and changes in behaviourJOURNAL OF FISH BIOLOGY, Issue 1 2003I. M. Jegstrup Based on thyroid hormone concentrations and histological criteria, glass eels of Anguilla anguilla caught in the French Atlantic coast in January and November were found to be late metamorphic. The pigmentation of the glass eels was demonstrated to be regulated by thyroid hormones, and evidence is given that together, the end of the progression of pigmentation, a fully developed gut and a change in behaviour marked the end of metamorphosis. [source] Effects on acid-base balance, methaemoglobinemia and nitrogen excretion of European eel after exposure to elevated ambient nitriteJOURNAL OF FISH BIOLOGY, Issue 3 2002C.-Y. Huang Haemoglobin, methaemoglobin, blood nitrite concentration and acid-base balance were measured in European eel Anguilla anguilla following exposure to 0 (control), 0·142, 0·356, 0·751 and l·549 mM nitrite in fresh water for 24 h. Blood GOT (glutamate oxaloacetate transaminase) and GPT (glutamate pyruvate transaminase) activities and whole animal ammonia-N and urea-N excretions were also measured. Blood nitrite, methaemoglobin, PO2 (oxygen partial pressure), GOT, and whole animal ammonia-N excretion and urea-N excretion increased directly with increasing ambient nitrite concentrations, whereas blood pH, PCO2, and HCO,3 were inversely related to ambient nitrite concentration. An accumulation of nitrite in the blood of A. anguilla following 24 h exposure to elevated ambient nitrite as low as 0·751 mM increased its blood methaemoglobin, PO2, GOT and nitrogen excretion, but decreased its PCO2 (carbon dioxide partial pressure), HCO,3 and functional haemoglobin. [source] Osmoregulation during the development of glass eels and elversJOURNAL OF FISH BIOLOGY, Issue 6 2000L. Birrell Drinking rates in glass eels and elvers of the European eel increased with environmental acclimation salinity from 0·07±0·02 (FW) to 0·70±0·09 ,l g- 1 h- 1 (SW) at month 1 and from 1·12±0·42 (FW) to 12·85±1·05 ± l g- 1 h- 1 (SW) at month 5. Drinking rates increased with time in both FW and SW groups. FW acclimated eels when challenged acutely with SW increased drinking rate rapidly immediately upon transfer (0,15 min) and the magnitude of this response increased with developmental time from month 1 to month 5. [source] Genetic variability is unrelated to growth and parasite infestation in natural populations of the European eel (Anguilla anguilla)MOLECULAR ECOLOGY, Issue 22 2009J. M. PUJOLAR Abstract Positive correlations between individual genetic heterozygosity and fitness-related traits (HFCs) have been observed in organisms as diverse as plants, marine bivalves, fish or mammals. HFCs are not universal and the strength and stability of HFCs seem to be variable across species, populations and ages. We analysed the relationship between individual genetic variability and two different estimators of fitness in natural samples of European eel, growth rate (using back-calculated length-at-age 1, 2 and 3) and parasite infestation by the swimbladder nematode Anguillicola crassus. Despite using a large data set of 22 expressed sequence tags-derived microsatellite loci and a large sample size of 346 individuals, no heterozygote advantage was observed in terms of growth rate or parasite load. The lack of association was evidenced by (i) nonsignificant global HFCs, (ii) a Multivariate General Linear Model showing no effect of heterozygosity on fitness components, (iii) single-locus analysis showing a lower number of significant tests than the expected false discovery rate, (iv) sign tests showing only a significant departure from expectations at one component, and, (v) a random distribution of significant single-locus HFCs that was not consistent across fitness components or sampling sites. This contrasts with the positive association observed in farmed eels in a previous study using allozymes, which can be explained by the nature of the markers used, with the allozyme study including many loci involved in metabolic energy pathways, while the expressed sequence tags-linked microsatellites might be located in genes or in the proximity of genes uncoupled with metabolism/growth. [source] Effect of different methods for the induction of spermiation on semen quality in European eelAQUACULTURE RESEARCH, Issue 15 2005Juan F Asturiano Abstract Five hormonal treatments with human chorionic gonadotropin (hCG) were tested for the induction of maturation and spermiation in male farmed eels. The main aim was to optimize previously used hormonal treatments to achieve shorter induction treatments, longer spermiation periods and/or higher sperm quality. Fish treated for just 3 weeks (treatment E) or until the onset of spermiation (treatment C) showed the worst results, while the treatment consisting of weekly administration of 1.5 IU hCG g,1 fish (treatment A) induced the highest percentage of spermiating males, the highest number of sperm samples and sperm volumes and densities similar to the rest of the treatments (B: half hormone dosage, or D: biweekly administration). Evaluation of the sperm quality was performed by computer-assisted sperm analysis (CASA), considering the percentage of total motile spermatozoa, the percentage of fast and medium-velocity spermatozoa, as well as different motility parameters. Sperm samples from A-D groups showed between 44% and 54% motile spermatozoa, and between 10% and 15% fast spermatozoa, while samples from E-treated males showed 0% motile cells. No significant differences were found in the spermatozoa straight line velocity (VSL), curvilinear velocity (VCL) or the angular velocity (VAP), neither spermatozoa beating cross frequency (BCF) between A,D groups. [source] Fluctuations in European eel (Anguilla anguilla) recruitment resulting from environmental changes in the Sargasso SeaFISHERIES OCEANOGRAPHY, Issue 1 2008SYLVAIN BONHOMMEAU Abstract 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. [source] Survival of juvenile European eels (Anguilla anguilla), transferred among salinities, and developmental shifts in their salinity preferenceJOURNAL OF ZOOLOGY, Issue 1 2005S. R. Crean Abstract After their oceanic migration, juvenile European eels Anguilla anguilla enter estuaries as glass eels, develop into pigmented elvers and migrate into fresh water. Fisheries often transfer such eels abruptly between salinities, principally glass eels and elvers from estuarine to fresh water. It is usually assumed that survival rates are high, but this required systematic investigation. Survival was found to be 100% over 21 days of glass eels and semi-pigmented elvers transferred abruptly from estuary conditions into fresh water, 50% sea water and full sea water. Fully pigmented elvers, however, showed significantly reduced survival when transferred into sea water. Salinity preference experiments with juvenile eels have historically been inconclusive. Here, in a choice chamber design, a clear developmental shift in salinity preference was found, with glass eels preferring 100% sea water, semi-pigmented elvers showing no clear preference and fully pigmented elvers preferring fresh water. We conclude that eel fisheries enhancement by abrupt transfer of juveniles among salinities is largely vindicated. In addition, developmental shifts in salinity preference have been clarified and this aids in the interpretation of eel migration patterns. [source] Distribution of genetic variation in farmed and natural stocks of european eelJOURNAL OF FISH BIOLOGY, Issue 2004J. M. Pujolar European eel (Anguilla anguilla; Teleostei) is a valuable commercial species. However, over the past 25 years, the population of European eel has been declining to such a degree that major concerns have been raised for its long-term conservation. Since little information is available on the life-cycle and genetic structure of European eel, it has been difficult to evaluate the existence of any population substructuring. Molecular genetic methods contribute to a better knowledge of the demography and population structure in marine fish. In addition, management strategies and conservation goals must consider information on genetic substructuring as well as on life history patterns. The aim of the study is to provide more detailed knowledge on the genetic variability, demography and population substructuring of European eel by analysing and comparing natural and farmed individuals. Natural eel samples have been obtained in two geographical sites (Netherlands, France) including temporal samples in a short-scale (within years) and a long-scale (between years). Simultaneously, farmed glass eels have been grown in two separate batches during one year. Batches have been monitored and genetic samples have been obtained during the year. A combination of selection-sensitive (allozymes) and selection-neutral markers (microsatellites) has been used in the study since selection seems to play an important role in the determination of the quality of future eel spawners. Results suggest a positive correlation between growth and genetic variability since individuals attaining a large length and mass present significant higher heterozygosities. [source] | |||||||||||||