Behavioural responses of European silver eels(Anguilla anguilla) to the geomagnetic field

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.     

A comparison of highly unsaturated fatty acid levels in wild and farmed eels (Anguilla anguilla).

1. Absolute and relative amounts of eicosapentaneoic acid (EPA) and docohexaenoic acid (DHA) in muscle of eels from four different fish farms were compared with samples from wild eels from two different areas of northern Italy. 2. Farmed eels were richer in DHA and EPA than wild animals. 3. The addition of cod liver oil to the diet of farmed eels led to a significant accumulation of EPA and DHA, but no change in total lipid content. 4. The calculation of two indices related to highly unsaturated fatty acid (HUFA) content (FLQ = fish lipid quality; AI = aterogenic index), indicated the higher nutritional value of farmed vs wild fish. 5. We conclude that farmed eels are an adequate source of fish products for human nutrition and propose use of the above-mentioned indices as an effective means for assessing fish nutritional quality for populations at high risk of chronic degenerative disease.     

Head shape dimorphism in European glass eels (Anguilla anguilla)

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.     

Descent of European silver eels, Anguilla anguilla L., in a Norwegian watercourse

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.     

Swim bladder nematodes (Anguillicoloides crassus) disturb silvering in European eels (Anguilla anguilla)

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.     

Isolation and molecular characterization of herpesvirus from cultured European eels Anguilla anguilla in Taiwan

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).     

Critical swimming speed of yellow‐ and silver‐phase European eel (Anguilla anguilla,L.)

The prime objective of this study was to evaluate differences between the swimming performance of two distinct life stages of European eels. The critical swimming speed (U_crit) of 29 yellow‐ and 33 silver‐phase eels was evaluated in a swim tunnel. Silver‐phase eels showed a better swimming performance (U_crit = 0.66 ms^?1) than yellow individuals (U_crit = 0.43 ms^?1). Male and female silver eels reached an identical U_crit despite their different sizes, which may be a strategy to increase the synchronization of arrival at the spawning grounds.     

Behavioural responses of glass eels (Anguilla anguilla) towards amino acids

The behavioural responses of glass eels of Anguilla anguilla towards amino acids were investigated by binary choice experiments testing different concentrations of 14 L-amino acids in fresh water and salt water. Glass eels responded to solutions of individual amino acids down to a concentration of 10⁻⁸M, 10⁻⁹m. Media of different salinities influenced the responses. In freshwater, gln and thr were strongly attractive; asn, ala, met, glu and ile induced significant avoidance; gly elicited multimodal reactions depending on concentrations; his, lys, phe, val, leu and asp hardly influenced behaviour. In salt water, only gly, asn and lys significantly influenced the behaviour of the glass eels.     

Age determination and growth rate of eels, Anguilla anguilla (L)

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.     

Vaccination of eels (Anguilla japonica and Anguilla anguilla) against Anguillicola crassus with irradiated L3

The original host of the swimbladder nematode Anguillicola crassus, the Japanese eel (Anguilla japonica) and the recently colonized European eel (Anguilla anguilla) were immunized with 40 irradiated (500 Gy) 3rd-stage larvae (L3) of this parasite and challenged with an infection of 40 normal L3. The immunization induced a significant reduction of the number of adult worms developing from the challenge infection in A. japonica, but not in A. anguilla. The induced resistance (calculated using the relation of the number of adult worms in immunized eels and in non-immunized control eels) in A. japonica was 87.3%+/-30.4%. Following a single infection, the percentage of adult worms found in A. japonica was lower as compared to A. anguilla, and the few adult worms were much smaller, revealing a lower susceptibility of A. japonica to A. crassus in comparison to A. anguilla. Both eel species developed an antibody response against A. crassus, but the level of antibody responses was not positively correlated with the protection against infection, suggesting that the antibody response is not a key element in resistance of eels against A. crassus. This study suggests that the original host of A. crassus is able to mount efficient protective immune responses against its parasite, whereas the newly acquired host seems to lack this ability.     

Pop up satellite tags impair swimming performance and energetics of the European eel (Anguilla anguilla)

Pop-up satellite archival tags (PSATs) have recently been applied in attempts to follow the oceanic spawning migration of the European eel. PSATs are quite large, and in all likelihood their hydraulic drag constitutes an additional cost during swimming, which remains to be quantified, as does the potential implication for successful migration. Silver eels (L_T = 598.6±29 mm SD, N = 9) were subjected to swimming trials in a Steffensen-type swim tunnel at increasing speeds of 0.3–0.9 body lengths s⁻¹, first without and subsequently with, a scaled down PSAT dummy attached. The tag significantly increased oxygen consumption (MO₂) during swimming and elevated minimum cost of transport (COT_min) by 26%. Standard (SMR) and active metabolic rate (AMR) as well as metabolic scope remained unaffected, suggesting that the observed effects were caused by increased drag. Optimal swimming speed (U _opt) was unchanged, whereas critical swimming speed (U _crit) decreased significantly. Swimming with a PSAT altered swimming kinematics as verified by significant changes to tail beat frequency (f), body wave speed (v) and Strouhal number (St). The results demonstrate that energy expenditure, swimming performance and efficiency all are significantly affected in migrating eels with external tags.     

Persistence of herpesvirus of eel Herpesvirus anguillae in farmed European eel Anguilla anguilla.

Herpesvirus of eel Herpesvirus anguillae (HVA) was isolated repeatedly from farmed eel of an outwardly healthy stock, but virus isolation was much greater in an experimental group of fish that were injected with dexamethasone. The results suggest that HVA can establish a latent infection in eel. Previous exposure of these eels to HVA virus was shown by detection of HVA-specific antibodies. These eels did not show clinical signs after a secondary infection with HVA. Tracing of seropositive eel stocks, which had previous contact with HVA, and of HVA carrier fish can be useful to control disease outbreaks due to HVA infection.     

Parasitological study of a population of Tiber River eels (Anguilla anguilla)

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.     

Migrations of elvers and juvenile European eels, Anguilla anguilla L., in the River Thames

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.     

Influence of Anguillicola crassus (Nematoda) and Ichthyophthirius multifiliis (Ciliophora) on swimming activity of European eel Anguilla anguilla

We investigated the swimming activity of 70 European eels Anguilla anguilla in relation to natural infection with 2 parasite species: the eel-specific swimbladder nematode Anguillicola crassus and the non-specific skin and gill protozoan Ichthyophthirius multifiliis. We measured how long individual eels exposed to a water current in a swimming channel with a steady-stream profile could withstand the water current. The parasites affected the swimming behaviour of eels in different ways. The maximum period of time the fish were able to swim against the current was not correlated with infection by A. crassus. In contrast, infection with I. multifiliis reduced the swimming time. The protozoan has a higher pathogenicity than the swimbladder nematode, at least in closed systems, where I. multifiliis is able to spread within a few days. Reduction in swimming capacity after infection with the ciliate averaged 47 % compared to capacity prior to infection. Thus, our results do not support the previously suggested strong negative relation between swimming activity of eels and intensity of A. crassus infection, at least in the short-term. However, there are indications in the literature that the pathological effects of A. crassus on the eel swimmbladder may involve a higher energy demand, possibly manifested in a prolonged spawning migration. As a result, eels heavily infected with this parasite may arrive too late at the spawning site to participate in mating. This could ensure a selection of 'good genes'.     

Physiological responses to acute temperature increase in European eels Anguilla anguilla infected with Anguillicola crassus

The swimbladder parasite, Anguillicola crassus has infected, and spread rapidly, through European eel Anguilla anguilla (L.) populations over the past 20 to 25 yr. Our aim in the present studies was to elucidate whether the presence of A. crassus in these eels alters their rapid physiological responses to an acute temperature increase, compared to the response of uninfected fish. Both infected and uninfected fish showed significant increases in plasma cortisol after 2 h at a raised environmental temperature with increased plasma glucose after 6 h. However, infected eels exhibited a slight lag in glucose mobilisation, which may be due to the metabolic cost of harbouring a sanguiverous parasite. Both infected and uninfected fish showed a significant increase in haematocrit after 6 h of temperature elevation, but only uninfected fish exhibited a significant increase in haemoglobin at this point. However, there were no significant changes in mean erythrocyte haemoglobin concentration in either group. Our results suggest that acute temperature increase alone is unlikely to cause significant mortality of A. crassus-infected European eels; however, the effects of chronic increases in temperature in combination with other factors such as toxicants and hypoxia requires examination.