Tracking the migratory success of stocked European eels Anguilla anguilla in the Baltic Sea

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 L_T 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.     

Are eels (Anguilla anguilla L.) planktonic feeders? Evidence from parasite communities

The metazoan parasite communities of eels Anguilla anguilla were investigated in three riverine localities and one lake in Devon. Altogether 21 species of parasite, the majority of the British eel parasite fauna, were recorded. Parasite communities in the three riverine localities were more similar to each other than to that of the lake. The four parasite component communities were typical and representative of those from eels in other parts of the U.K. They were characterized by low species diversity and domination by a single species: Pseudodacytlogyrus anguillae in the rivers and Diplostomum spathaceum in the lake. Both the dominant species and the majority of parasite species, and hence individuals, in any locality infected eels directly by free swimming planktonic larval stages or indirectly by the ingestion of infected planktonic copepod intermediate hosts. Examination of other species of fish in each locality showed that only Raphidascaris acus utilized a fish species as an intermediate or paratenic host to infect eels. Increases in prevalence and intensity of infection with size of eel by helminths such as Camallanus lacustris and Paraquimperia tenerrima were observed in localities in which transmission of these parasites could not have involved another fish species. Over 63% of the specialist, specific parasites of eels throughout Europe employ planktonic transmission to their hosts, as do several species of the commoner generalists. A study of eel parasite communities thus leads to the conclusion that eels are widely and closely associated in their habits with plankton and that eels of all sizes feed regularly upon planktonic copepods: the widely accepted view of eels as being more or less exclusively benthic in habit and feeding behaviour requires re-appraisal.     

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.     

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.     

Long-term stability in the richness and structure of helminth communities in eels,Anguilla anguilla,in Lough Derg,River Shannon,Ireland

A data set on intestinal helminth parasites was collected in the course of an 18 year investigation into the biology of eels in Meelick Bay, Lough Derg, River Shannon. This was used to test two hypotheses relating to the composition and structure of intestinal helminth communities, namely that eels in large rivers do not harbour richer and more diverse communities than those in small rivers but that community composition and structure are more stable over time than in small rivers. The helminth community was species poor, with only six species comprising the component community and a maximum infracommunity richness of three species. The community was overwhelmingly dominated by the acanthocephalan Acanthocephalus lucii, reflecting the importance of its intermediate host Asellus aquaticus in the eels' diet. The remaining helminth species contributed to species richness but made very little contribution to community diversity. Population levels of Acanthocephalus lucii fell and remained low between 1992 and 2000, probably reflecting increased movement of eels from other parts of the lough into Meelick Bay. Diversity values were low, but similar to those reported from other rivers in Britain and Europe. The results provided support for both hypotheses and indicated that in respect of richness, diversity and dominance, the helminth communities of eels in the River Shannon were typical of, and comparable to, those of other large rivers throughout Europe.     

Relation of water odour, salinity and temperature to ascent of glass-eels, Anguilla anguilla (L.): a laboratory study

The ascent of Anguilla anguilla (L.) which shows a variable pattern, has prompted much research that is not related to environmental conditions in a simple manner. Physico-chemical parameters of the water can be used by glass-eels as orienting cues during upstream migration. Temperature, salinity and odour linked with marine and river waters are three of the most important cues. Laboratory experiments were carried out by testing the responses of groups of 20 glass-eels, acclimated to either salt or fresh water at 11° C, towards water flows differing in temperature (8, 11 and 14° C), salinity (fresh water and 33‰ salt water) and odour (natural surface, marine or odourless tap water). Preferences were tested varying any one or two of the three factors at a time, or presenting the three factors simultaneously, in all possible combinations. The three factors tested singly reveal that thermal and salinity preferences recorded using odorous waters were analogous to those already observed with odourless water. Freshwater flows are preferred to salt-water. Temperatures below the acclimation temperature are preferred to higher temperatures. Natural odorous waters are preferred to odourless water at the same salinity. Data from 2900 choices recorded in 163 tests demonstrate that salinity is the most important factor guiding flow choice. Preference for fresh water is affected more by temperature than by odour. The latter, in turn, influences more thermal than salinity choices. Odour attractiveness mainly acts by reinforcing preferred stimuli or offsetting unpleasant ones. Preference differences were found between glasseels acclimated to fresh and salt water. Preference for fresh water strongly affects the choices of the former group, blunting the effects of thermal and odorous stimuli. Salt-water reared glass-eels, less conditioned by salinity, seem to be more sensitive to thermal and odorous attractions.     

A comprehensive hypothesis on the migration of European glass eels (Anguilla anguilla)

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.     

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.     

Cl- -HCO3- dependent ATPase in gills of freshwater and seawater adapted eels (Anguilla anguilla L.)

The gills of both seawater and freshwater adapted eels have an ATPase activity which is stimulated by anions in the presence of Mg2+. Plasma membranes were distinguished from mitochondrial membranes with specific enzyme markers, the membrane fractions separated on a discontinuous sucrose gradient, and the ATPase activity of the plasma membranes studied. Activation by the anions of Cl- or HCO3- followed Michaelis-Menten kinetics and was competitively inhibited by SCN-. The Cl- and HCO3- activation characteristics were determined: no differences between the plasma membrane ATPase activities of freshwater and seawater-adapted fishes were observed. Maximal activity measurements after solubilization of the enzymes by Triton X 100 confirmed these findings. The function of a membrane anion-dependent ATPase in the brachial epithelium of euryhaline fish is discussed.     

Modelling of unexploited and exploited populations of eels, Anguilla anguilla (L.), in the Thames Estuary

Eels were fyke-netted in different zones of the Thames Estuary at various times of year and subsamples sexed, aged and stomachcontentsexamined. Instantaneous mortality coefficients (Z) were calculated for each 3 cm length class for eels > 30 cm and used to construct a compartmental model to assess effects of natural and fishing mortality and migration on different life stages of males and females. Growth rates were good and fishing mortality effects low and masked by effects of emigration/immigration (and, possibly, local migrations). Density-dependent mortality appeared to be having minor effects. Males dominate freshwater sites, females the more saline zone and possible implications of food availability and density are discussed. It is concluded the stock is not being over-exploited but that elver recruitment is relatively low. Means of increasing exploitation are discussed, as are ways of increasing production of females to enhance elver recruitment to the Thames and other European rivers.     

Growth variability in individually confined elvers, Anguilla anguilla (L.)

Newly caught elvers Anguilla anguilla (L.) were individually confined, induced to feed and their survival and growth variability compared to those of similar elvers held communally in lowdensity populations. Similar trials were conducted with fast-growing and slow-growing older elvers that had been induced to feed while cultured communally. It was not difficult to induce feeding in newly caught elvers, nor did it appear necessary for elvers to be stimulated to feed by the presence of others. Isolation seemed to enhance growth variability in newly caught elvers.
In older elvers growth was generally depressed by isolation. Following a period of stress, e.g. weighing, small fish tended to grow more slowly than larger fish in the individual groups. This suggested some permanency of stunting; however, some markedly stunted fish taken from large populations showed remarkable recovery when isolated and exhibited very high instantaneous growth rates, over 6.5% day⁻¹.     

Factors determining length distribution and abundance of the European eel, Anguilla anguilla, in the River Mondego (Portugal)

1. The distribution and abundance of the European eel, Anguilla anguilla, were investigated in the Mondego River, a strongly modified river in central Portugal. Nine freshwater sites, located below the first impassable obstacle to eel migration, were surveyed for 2 years. The study was conducted monthly during the first year and seasonally during the second year. Fish were caught by electric fishing and, physical and biotic variables potentially influencing eel length distribution and abundance were determined at each sampling session. 2. Canonical correspondence analysis (CCA) was used to explain the relationships between abiotic and biotic habitat variables, and eel length distribution. Spatial variation was strongly associated with distance from the sea, number of obstacles, river width and percentage of instream cover. Depth, river flow, water temperature and cannibalism had weaker effects on the longitudinal distribution, but contributed as explanatory variables for the model. The influence of spatial variables outweighed the importance of temporal variables as predictors in this model. 3. Length distribution changed in space from a dominance of small eels (<100 mm) closer to the sea to larger eels (≥250 mm) in the upper reaches of the river. The smallest length class (eels <100 mm) was strongly associated with wider stretches where instream cover, mainly composed of aquatic submerged macrophytes, was abundant (>75% area coverage). On the contrary, larger eels (≥250 mm) preferred deeper stretches with stronger river flow and less instream cover. 4. The effect of biotic interactions with conspecifics as well as with the non‐native red swamp crayfish Procambarus clarkii, in sites with poor instream cover, modified this general pattern of distribution leading to fewer eels <100 mm at some sites closer to the sea. Therefore, despite the ubiquity and plasticity shown by this species, there were clear ontogenetic variations in habitat use such that the first two canonical axes of a CCA accounted for 58.4% of the spatial variability in size structure.     

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.     

Three-dimensional migratory behaviour of European silver eels (Anguilla anguilla) approaching a hydropower plant

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.     

Recruitment of Anguilla spp. glass eels in the Waikato River,New Zealand. Evidence of declining migrations?

The timing of Anguilla spp. glass eel recruitment into the Waikato River, North Island, New Zealand, was studied over a 2 year period (2004–2005). While glass eels of both the shortfin eel Anguilla australis and the endemic longfin eel Anguilla dieffenbachii were caught, the former comprised >97% of the species composition. There was a positive correlation of glass eel migrations with spring tides, with peak migration periods typically occurring within a few hours of the peak of high tide, and between 2 and 4 days after the day of spring tide. Both water temperature and discharge had significant inverse relationships with glass eel catches, with temperature explaining >30% of the variance in catch periodicity. Comparison of catch data 30 years apart showed that main migration periods appear to occur several weeks earlier today than previously. Reduced catch per unit effort and duration of runs from recent years' sampling (compared with the 1970s) indicate that a reduction in recruitment may also have occurred during this period, something recorded in other temperate species of Anguilla .     

Avoidance Responses to Infrasound in Downstream Migrating European Silver Eels, Anguilla anguilla

In an attempt to develop an efficient acoustic fish fence, we have designed an infrasound source able to generate large nearfield particle acceleration. The source generates water movements by means of two symmetrical pistons in an air-filled cylinder with 21cm bore. The pistons are driven by eccentric coupling to an electric motor, with 5cm p.p. amplitude. The piston movements are 180° out of phase. The piston reaction forces are thus opposed, leading to vibration free operation. The submergible infrasound source is operated freely suspended in the water mass. The emitted sound frequency is 11.8Hz. The particle acceleration is about 0.01ms^–2 at a distance of 3m, corresponding to the threshold intensity for deterring effects of infrasound on Atlantic salmon smolts. The sound source was employed to test the effect of intense infrasound on migrating European silver eels. Fish confined in a tank displayed startle behaviour and prolonged stress reactions, telemetrically monitored as tachycardia, in response to intense infrasound. The field tests were carried out in the River Imsa. A trap that catches all the descending eels is installed near the river mouth. The trap was separated in four equal sections. During the periods with infrasound exposure, the proportion of silver eels entering the section closest to the sound source was reduced to 43% of the control value. In the section closest to the opposite river bank, infrasound increased the proportion of trapped eels to 144% of the control values. This shift of the migrating eels away from the infrasound source was highly significant.