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1.
The biological characteristics of eels from the Asi River, Turkey, were assessed between December 2017 and November 2018. Eels were sampled monthly using fyke nets (operated by professional fishermen), yielding a total of 509 specimens. Total length and weight were measured, sex, age and maturity stages (silver or yellow eel) were determined. Catch Per Unit Effort (CPUE) was calculated for both biomass per unit effort and numbers caught on a monthly and annual basis. The length-weight relationships (LWRs) of silver and yellow eel was W = 0.009*TL3.22 (n = 262) and W = 0.0106*L3.09 (n = 247), respectively. The age of the sampled fish ranged from year class II to VI. Von Bertalanffy growth parameters were estimated as L=69.25 cm, K = 0.43 1/year, t0= −0.41 and phi prime index ( ǿ )= 3.31 for all samples. The overall eel fishing mortality rate (F) was 0.31 year-1, and the exploitation and survival rates of silver stage eels were estimated with 30% and 39%, respectively.  相似文献   

2.
The European eel Anguilla anguilla has declined significantly over recent decades, exceeding its safe biological limits and probably reaching a historical minimum (1% of the 1960 recruitment level). Twenty‐three migrating females at the IV stage of silvering were collected from the Comacchio lagoon (Northern Italy); they showed high levels of cholesterol and triglycerides, probably due to lipid accumulation before migration across the Atlantic Ocean. Electrophoretic patterns showed high amounts of putative α and β globulins, probably due to high concentrations of lipoproteins. Genetic results obtained from a total of 49 specimens (20 from the Comacchio lagoon and 29 from Val Dogà) agree with the panmixia hypothesis, lacking any genetic differentiation over time and space at neutral loci. In conclusion, despite the small sampling and the population decline in recent years, high levels of variability were recorded.  相似文献   

3.
The Working Group on Eel of the International Council for the Exploration of the Sea (ICES) regularly reports that a significant amount of stocked eels in Europe was pre‐grown in aquaculture farms prior to stocking—so called “farmed eels.” The ICES advices chemical marking of stocked recruits to ensure their traceability throughout all life stages. To date, however, there was a lack of knowledge concerning the most suitable chemical substance and its application on farmed eels. The aim of this study was to fill this gap by presenting successful attempts of marking those eels with alizarin red S (ARS). An ARS concentration of 150 mg L?1 buffered with 150 mg L?1 Tris(hydroxymethyl)aminomethane applied as an immersion bath over 9 h was sufficient to mark a total of 3572 kg of farmed eels (6.5–8.0 g mean body weight). The marking success was 100% on otoliths and highest stocking density of up to 67.1 kg m?3 (corresponding 54.0 kg m?2) turned out to have no effect on mortality which was consistently below 1%.  相似文献   

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5.
《Zoology (Jena, Germany)》2015,118(6):413-423
The life cycle of the European eel (Anguilla anguilla) remained a mystery until the 20th century, when Schmidt discovered that the Sargasso Sea was its spawning area. However, many aspects of the eel's life cycle remain poorly understood. Among these is the bimodal distribution in head shape, with broad- and narrowheaded phenotypes reported in the yellow eel stage. Although this has been linked to dietary preferences of the yellow eels, very little is known about why, how and when this dimorphism arises during their ontogeny. To determine whether this dimorphism indeed appears in relation to trophic niche segregation, we examined head shape variation at an earlier ontogenetic stage, the glass eel stage, as at this stage eels are considered to be non-feeding. Head shape was studied in a large dataset, containing glass eels captured from the Yser river mouth, the Leopold Canal (Belgium) and from the rivers Severn, Trent and Parret (UK), by both taking measurements (head width/head length) and using an outline analysis. Our results show that there is already considerable variation in broadness and bluntness of the head at the glass eel stage. In most cases, equal support for a unimodal and bimodal head shape distribution is found, whereas some cases support head shape bimodality in glass eels, suggesting that glass eel head shape might be shifting from a unimodal to a bimodal distribution. This, in combination with the observation that variation in head width/head length ratios in non-feeding glass eels shows a similar range as in feeding yellow eels, indicates that head shape in European eel might be at least partially determined through other mechanisms than trophic segregation.  相似文献   

6.
Magnetic orientation of European silver eels(Anguilla anguilla) was tested in an octagonal tank. Orientation was determined from photo-registrations of eel positions in tests performed alternately in the natural magnetic field and a field with the horizontal component rotated 180°. Tests were performed in LD 11 : 13. At a daytime light intensity of 100 lux the fish were diurnally active, while at 0.10 lux crepuscular or nocturnal activity dominated. The eels probably differed in preferred orientation, largely depending on the clockwise or anti-clockwise swimming of some of the animals. Therefore there was no preferred direction common to all eels. The orientation of single eels differed, however, significantly between the two magnetic fields, suggesting that the eels responded to the geomagnetic field.  相似文献   

7.
Otolith Sr:Ca ratios from 32 of 34 European eel Anguilla anguilla collected from three freshwater sites in the River Asi, southern Turkey, indicated that they were resident in fresh water without apparent exposure to salt water since the elver stage. The Sr:Ca ratio criterion indicative of residence in fresh water was more than twice that of values from other European countries. Otolith Sr:Ca ratios of A. anguilla from fresh waters can vary among regions, possibly reflecting regional-specific water chemistry. Hence, the use of Sr:Ca ratios determined in one region to interpret results from a different region might lead to misclassification of migratory life-history types.  相似文献   

8.
The European eel (Anguilla anguilla) is a catadromous fish that spawns in the Sargasso Sea. As larvae, eels cross the Atlantic Ocean and reach the continental slope of Europe, where they metamorphose into post‐larval glass eels. These reach the continent, where some enter fresh water, some remain in marine waters, and others move between fresh and marine waters. After 5–25 years, as adult silver eels, they migrate back from fresh water to the Sargasso Sea to spawn and die. The glass eel stage is a critical step during which the eels cross the continental shelf and recruit to estuaries, where they facultatively transition to fresh water. Extensive research has been conducted to understand the behavioural mechanisms and environmental cues that aid and guide glass eels' migration. Glass eels follow odours and salinity gradients, they avoid light, and they change orientation and depth according to the tides. Recent work revealed that European glass eels also use Earth's magnetic field and lunar cues to orient. However, while we understand many aspects of their orientation behaviour, a unifying theory describing how glass eels migrate from the continental slope to fresh water is lacking. The goal of this review is to develop a comprehensive hypothesis on the migration of European glass eels, integrating previous knowledge on their orientation behaviour with recent findings on magnetic and celestial orientation. This review follows the journey of a hypothetical glass eel, describing the nature and the role of orientation cues involved at each step. I propose that, although glass eels have the sensory capacity to use multiple cues at any given time, their migration is based on a hierarchical succession of orientation mechanisms dictated by the physical properties of the environments that they occupy: (i) lunar and magnetic cues in pelagic water; (ii) chemical and magnetic cues in coastal areas; and (iii) odours, salinity, water current and magnetic cues in estuaries.  相似文献   

9.
The up-river migration of eels in the R. Thames has been studied at a number of locations since 1985. The run occurs between April and October, with the majority of eels moving over a distinct period averaging 47 days in May–June. The timing of the run varies and its commencement appears to be mainly dependent on water temperature. The number of unpigmented elvers migrating from the estuary to fresh water is variable and most appear to spend at least one year in the estuary. The average size and age of migrants increases with distance upstream. Migrants leaving the estuary were mainly 1–3 river-year (<14 cm) pigmented juveniles, though individuals of up to 45 cm were encountered. At a site 15 km from the estuary the majority of migrants were between 4 and 8 river-year eels (20–30 cm). Mark–recapture studies showed variable tendencies to display migratory behaviour among individuals, with some taking up to 2 years to reappear in a trap when released a few hundred metres below it. The implications of the results with respect to the commercial fishery in the estuary, populations in the catchment, and the contribution of Thames eels to international stocks and recruitment are discussed.  相似文献   

10.
A study was conducted on the parasitofauna of Anguilla anguilla caught in the section of the Tiber river south of Rome (Italy). The sample examined consisted of 417 specimens that were classified by stage (elver, young yellow eel, yellow eel and silver eel) and season of sampling. The following species were identified: Trypanosoma granulosum, Eimeria anguillae, Myxidium giardi, Sphaerospora reichenowi, Myxobolus sp., Trichodina anguilli, Ichthyohodo sp., Gyrodactylus anguillae, Bothriocephalus claviceps, Proteocephalus macrocephalus, Raphidascaris acus, Acanthocephalus clavula. Each parasite species was considered in its prevalence according to stage and season. For some parasite the pathogenic effect was also examined in a histological study of serial sections. The results of the statistical analysis of the single parasitic infection as related to different seasons pointed to marked seasonal trends only in some species (Trypanosoma granulosum and Gyrodactylus anguillae). The intensity of infection assessed for acanthocephalan infection only was found to affect neither weight nor condition index (weight/length3). No difference in weight or condition index was found to be ascribable to the presence or absence of a species. An attempt to find a rule-of-thumb algorithm to gauge the overall effect to parasitic diseases failed to give sufficiently significant results.  相似文献   

11.
Length–weight relationships for European silver eel (Anguilla anguilla) sub‐populations in six countries (13 catchments) along the latitudinal gradient in the area of distribution in this study use three important sampling programs; these results concur with previous studies on European eel with the equation: W = 0.0010 L3.148 for the entire data set (sexes combined). Significant differences were observed among the 13 catchments, possibly implying differences in body condition. Male and female LWRs varied significantly between and among catchments, suggesting that the capacity of silver eels to migrate and reproduce successfully varies according to the sites.  相似文献   

12.
The introduced parasite Anguillicoloides crassus is thought to play an important role in the decline of freshwater eel (Anguilla spp.) populations. These nematodes are known to negatively affect many fitness-related traits in eels. We used experimental infections to study the effect of A. crassus on the relative size or mass of organs, and the expression of functionally relevant genes (total of 12 parameters) that are involved in the silvering process of Anguilla anguilla. Our results showed that the liver mass, the hemoglobin α-chain, and androgen receptors α expression levels were significantly higher in infected eels, whereas the freshwater rod opsin expression level and the gut mass were significantly lower in infected eels. Our results suggested that infected eels were at a more advanced stage in the silvering process than uninfected counterparts of similar size. These results may be explained by 2 hypotheses. First, A. crassus could trigger physiological mechanisms involved in the silvering process as a side-effect of infection. Second, eels may adjust their life history traits in response to infection. The implications for eel migration and reproductive success may be either negative or positive, depending on whether the response to A. crassus infection results in an additional cost of the parasite or is due to the phenotypic plasticity of the host.  相似文献   

13.
The global population of European eel (Anguilla anguilla) is rapidly declining, and migration barriers in rivers are believed to be one of several key causes. While progress has been made in the development of bypass solutions, they are often constructed based on a limited knowledge of swimming behaviour. A bypass close to the stream bed is often recommended at fish passage facilities to accommodate downstream eel migration. The results of this recommendation are poorly studied, and the few studies that exist show varying bypass efficiencies. The current study used acoustic telemetry with depth sensors to explore the three-dimensional migratory behaviour of downstream-migrating silver eels. The eels were tracked as they approached a hydropower plant with a state-of-the-art angled bar rack and full-depth bypass. Downstream and upstream swimming differed in preferred vertical and lateral positions. During periods of local downstream movement, the density of observations was largest in the upper middle section, away from the river boundaries and in higher velocities. Conversely, when moving upstream, eels tended to avoid the upper layers of the middle part of the river, swimming closer to the riverbed and using the bank areas to a greater extent. Downstream-moving fish swam higher in the water column during night and in turbid conditions (high discharge). When approaching the impassable bar rack and the full-depth bypass, the eels searched most intensely but not exclusively along the bottom third of the rack, often exploring at new depths after changing direction. The impediment passage efficiency was 100% when both bypass solutions were considered. The study provides knowledge of the swimming behaviour of silver eels, which is relevant for the design of bypass solutions for eels at migration barriers.  相似文献   

14.
15.
Yellow eels(Anguilla anguilla) taken during summer from random commercial trapnet samples in the littoral area of Helgoland (n=116) and from a freshwater area of the River Elbe near Hamburg (n=109) were examined with regard to their karyological (i.e. existence of female sex chromosomes) and gonadal sex. In 47 % and 21 % of the two samples, respectively, chromosomes were unidentifiable because of insufficient numbers of mitotic plates. All eels from Helgoland, except one phenotypically undetermined fish, exhibited female gonads: 48 had female sex chromosomes and 13 were karyologically males. As found previously in the River Elbe, eels with male gonads predominated (n=55); 25 were undifferentiated. Of the gonadal males 26 were karyological males and 16 karyological females; the rest could not be identified by chromosome patterns. In contrast, all but one of the Elbe eels with female gonads (n=28) had female sex chromosomes. Some aspects of the sex reversal documented in the eel are considered.  相似文献   

16.
A herpesvirus has been isolated for the first time from a population of European eels Anguilla anguilla cultured in a recirculated system in Taiwan. Syncytia formation was detected in EP-1 (eel epidermis) cell cultures inoculated with cell-free homogenates prepared from both integument and visceral organs of moribund fish. Inoculation of homogenates onto EK (eel kidney) cell cultures induced giant cell formation. Subsequent passages produced a consistent and progressive cytopathic effect (CPE) in cell cultures. In this study, EP-1 cell cultures infected with EEHV (European eel herpesvirus) were examined using an electron microscope. Numerous nucleocapsids of about 100 nm in diameter were found within the nucleus of infected cells, whereas enveloped particles were observed within the cytoplasm. The mature viral particle, about 235 nm in diameter, had an electron-dense core with a hexagonal nucleocapsid surrounded by a coarse capsule. Histopathological examination of moribund fish showed epithelial hyperplasia with intracytoplasmic metabolic inclusions in the skin. Macrophage aggregates were found in liver, spleen, and kidney. A pair of primers designed from channel catfish virus and salmonid herpesvirus 1 was used in a polymerase chain reaction. A 402 bp fragment was amplified and cloned from genomic DNA of EEHV. The nucleotide homology was 99% (298 of 300) with DNA polymerase of eel herpesvirus (anguillid herpesvirus). EEHV nucleic acids were detected within melanomacrophages in the skin, liver, spleen and kidney by in situ hybridization (ISH).  相似文献   

17.
Measurements taken from a series of photographs of otoliths of 37 eels caught in a stretch of river of 150m length were used to prepare a growth curve based on back-calculated measurements. Values of L ∞= 1045 and K = 0.046 were determined. Mean incremental growth calculated as 33 mm per year agreed closely with increments determined from growth observed up to 4 years after tagging eels from the same location. Inconsistencies in results of age determinations based on burned otoliths are described but it is concluded that the technique yields results of sufficient reliability to be used in practical management situations.  相似文献   

18.
Ichthyological Research - This study was carried out to determine the relationships between various morphological parameters, body condition data, and digestive and stress enzyme activities due to...  相似文献   

19.
Seaward migration of silver eels occurred at nights during autumn at decreasing water temperature. Maximum migration-rate occurred mid-October at medium and decreasing water discharge. Most eels descended in the first quarter of the lunar cycle, few eels descended at full moon. Most males and small females migrated earlier in the season than the larger females. The migration speed of silver eels in fresh water was independent of body length.  相似文献   

20.
To investigate the extent to which European silver eels Anguilla anguilla, originating from stocking programmes in the Baltic Sea tributaries, effectively contribute to the spawning stock, two hundred and seventy‐four formerly stocked A. anguilla. emigrating from the Schwentine River near Kiel, Germany, were tagged with T‐Bar anchor tags. A total of 29 Anguilla spp. were recaptured (c. 11%) up to 14 months after release. Stocking history of recaptured A. anguilla. was confirmed by otolith microchemistry. Recapture locations were concentrated around the outlet of the Baltic Sea (Danish Belt Sea) with 62% of all recaptures reported here or in the Kattegat. Recaptured Anguilla spp. showed a reduction in both LT and mass (mean ± s.d . = ?1·5 ± 0·9 cm and ?125·3 ± 50·1 g) while average total fat content remained in the order of values previously reported as high enough to provide energy resources to allow successful completion of the spawning migration (mean ± s.d . = 28·4 ± 4·4%). The documented mean rate of travel (0·8 km day?1), however, indicated a delay in the target‐oriented migration that might be interpreted as a delayed initial migration phase of orientation towards the exit of the Baltic Sea.  相似文献   

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