European eel restocking programs based on wild-caught glass eels: Feasibility of quarantine stage compatible with implementation of prophylactic measures prior to scheduled reintroduction to the wild

Abstract European eel Anguilla anguilla recruitment into the rivers of the northeastern Atlantic has declined substantially since the 1980s. Monitoring of recruiting juveniles, or glass eels, is usually undertaken in small estuaries and rivers. Sampling of large-scale estuaries is rare, due to the size of the sampling area and the resources needed to provide adequate sampling levels. Here we describe surveys for glass eels in the UK’s largest estuarine system, the Severn Estuary/Bristol Channel. We sampled across a 20 km-wide stretch of the estuary in 2012 and 2013, using a small-meshed net deployed from a commercial fishing trawler, and the surveys yielded over 2500 glass eels. Eels were more abundant in the surface layer (0–1.4 m depth) than at depth (down to 8.4 m depth), were more abundant close to the south shore than along the north shore or middle of the estuary, and were more abundant in lower salinity water. Numbers were higher in the second year than in the first and eels were more abundant in February than April. The difficulties and logistics of sampling in such a large estuary are discussed, along with the level of resources required to provide robust estimates of glass eel abundance.

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Glass eels (Anguilla anguilla) imprint the magnetic direction of tidal currents from their juvenile estuaries

Communications Biology ◽

10.1038/s42003-019-0619-8 ◽

2019 ◽

Vol 2 (1) ◽

Cited By ~ 5

Author(s):

Alessandro Cresci ◽

Caroline M. Durif ◽

Claire B. Paris ◽

Steven D. Shema ◽

Anne Berit Skiftesvik ◽

...

Keyword(s):

Anguilla Anguilla ◽

Magnetic Compass ◽

European Eel ◽

Sargasso Sea ◽

North African ◽

Compass Direction ◽

Tidal Flows ◽

Glass Eels ◽

Cardinal Directions ◽

Magnetic North

Abstract The European eel (Anguilla anguilla) hatches in the Sargasso Sea and migrates to European and North African freshwater. As glass eels, they reach estuaries where they become pigmented. Glass eels use a tidal phase-dependent magnetic compass for orientation, but whether their magnetic direction is innate or imprinted during migration is unknown. We tested the hypothesis that glass eels imprint their tidal-dependent magnetic compass direction at the estuaries where they recruit. We collected 222 glass eels from estuaries flowing in different cardinal directions in Austevoll, Norway. We observed the orientation of the glass eels in a magnetic laboratory where the magnetic North was rotated. Glass eels oriented towards the magnetic direction of the prevailing tidal current occurring at their recruitment estuary. Glass eels use their magnetic compass to memorize the magnetic direction of tidal flows. This mechanism could help them to maintain their position in an estuary and to migrate upstream.

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Age, growth, and condition of European eel (Anguilla anguilla) from six lakes in the River Havel system (Germany)

ICES Journal of Marine Science ◽

10.1093/icesjms/fsm093 ◽

2007 ◽

Vol 64 (7) ◽

pp. 1414-1422 ◽

Cited By ~ 24

Author(s):

Janek Simon

Keyword(s):

Growth Rate ◽

Condition Factor ◽

Energy Content ◽

Anguilla Anguilla ◽

European Eel ◽

Annual Increment ◽

Otolith Increments ◽

Gross Energy ◽

Glass Eels ◽

Mean Annual Increment

Abstract Simon, J. 2007. Age, growth, and condition of European eel (Anguilla anguilla) from six lakes in the River Havel system (Germany). – ICES Journal of Marine Science, 64: 1414–1422. A total of 199 female yellow European eels (Anguilla anguilla), 21.6–66.2 cm long and 3–14 years old, was collected by electro-fishing from six lakes in the River Havel system (Germany) in spring 2001. The condition and the growth rate, estimated by otolith increments, varied between eels within single lakes and between lakes. Fulton's condition factor ranged from 0.10 to 0.24 and the gross energy content varied between 4.3 and 15.3 MJ kg−1. There were no significant differences in mean condition factor (0.16–0.18) or gross energy content (6.5–9.3 MJ kg−1) between lakes. Fastest growth was in Lake Blankensee (mean 5.3 cm year−1), and the slowest in Lake Sacrow (mean 4.0 cm year−1). For all lakes combined, the overall mean annual increment was estimated to be 4.5 cm year−1. The biggest annual increment on the otoliths was generally laid down during the first and second years in fresh water, when the growth rate was 6.1–8.5 cm year−1. Then, in the subsequent 12 years, the annual increment remained almost constant or decreased slightly (with lake-dependent values of between 1.6 and 6.8 cm year−1). In the River Havel system, the time between stocking of the lakes with glass eels and the recapture of eels at 45 cm body length was 7–10 years. The physiologically possible maximum length (L∞ values) of eels lay in the range 50–130 cm. In comparison with previous investigations (between the 1950s and the 1970s), the only difference observed was a trend towards slower growth.

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Bacterial and parasitic factors diagnosed in eel farmed in Poland in the years 2010-2014

Medycyna Weterynaryjna ◽

10.21521/mw.5568 ◽

2016 ◽

Vol 72 (10) ◽

pp. 647-651

Author(s):

Elżbieta Terech-Majewska ◽

Alicja Bernad ◽

Stanisław Robak ◽

Joanna Pajdak ◽

Patrycja Schulz ◽

...

Keyword(s):

Health Assessment ◽

Anguilla Anguilla ◽

Good Health ◽

European Eel ◽

Test Results ◽

Aeromonas Sobria ◽

Controlled Conditions ◽

Etiologic Agents ◽

Controlled Rearing ◽

Glass Eels

European eel Anguilla anguilla L. is a fish species highly valued in European fisheries, currently reared under controlled conditions (Aquatic Recirculation System - RAS). In order to protect the health of fish, regular check-ups are carried out in specialized veterinary laboratories. Health hazards are recognized on the basis of clinical, pathological, microbiological, and parasitological surveys. The aim of the study was to analyze the results of health assessment of eels reared under controlled conditions, which was performed in the Diagnostics Laboratory of Fish and Crayfish Diseases, Department of Veterinary Hygiene, Voivodeship Veterinary Inspectorate in Olsztyn, in the period from 2010 to 2014. The results are presented according to the etiologic agents, divided into bacterial and parasitic diseases. In this period, 73 samples (100%) were examined, including 5 samples (6.85%) from glass eels and 68 (93.15%) from elvers. Microorganisms isolated from the collected material were mainly conditionally pathogenic, such as Pseudomonas fluorescens, Aeromonas hydrophila, Shewanella putrefaciens, Aeromonas sobria, Flavobacterium spp., and Chryseobacterium indologenes. The parasitological examination revealed infection with Trichodina spp., Ichthyophtirius multiphilliis, and Pseudodactylogyrus spp. The most frequently diagnosed were gill monogeneans, detected in 34 cases (75.55% of all positive parasitological test results). In 21 cases (61.8%) the infestation manifested itself as a disease. The remaining 13 cases were asymptomatic carriers (38.2%). The analysis shows that the main problem in the controlled rearing of eel are parasitic infestations, which may be endemic and pose a constant threat. Systematic monitoring and preventive measures are necessary throughout the rearing and fattening of fry to maintain a good health status of eels.

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The relationship between the moon cycle and the orientation of glass eels ( Anguilla anguilla ) at sea

Royal Society Open Science ◽

10.1098/rsos.190812 ◽

2019 ◽

Vol 6 (10) ◽

pp. 190812 ◽

Cited By ~ 5

Author(s):

Alessandro Cresci ◽

Caroline M. Durif ◽

Claire B. Paris ◽

Cameron R. S. Thompson ◽

Steven Shema ◽

...

Keyword(s):

Anguilla Anguilla ◽

Lunar Cycle ◽

Global Scale ◽

European Eel ◽

Sargasso Sea ◽

The Moon ◽

Detection Mechanism ◽

Freshwater Habitats ◽

Glass Eels ◽

Open Question

Links between the lunar cycle and the life cycle (migration patterns, locomotor activity, pulses in recruitment) of the European eel ( Anguilla anguilla ) are well documented. In this study, we hypothesized that the orientation of glass eels at sea is related to the lunar cycle. The European eel hatches in the Sargasso Sea and migrates across the Atlantic Ocean towards Europe. Upon reaching the continental shelf, the larvae metamorphose into glass eels and migrate up the estuaries, where some individuals colonize freshwater habitats. How glass eels navigate pelagic waters is still an open question. We tested the orientation of 203 glass eels in a transparent circular arena that was drifting in situ during the daytime, in the coastal Norwegian North Sea, during different lunar phases. The glass eels swimming at sea oriented towards the azimuth of the moon at new moon, when the moon rose above the horizon and was invisible but not during the other moon phases. These results suggest that glass eels could use the moon position for orientation at sea and that the detection mechanism involved is not visual. We hypothesize a possible detection mechanism based on global-scale lunar disturbances in electrical fields and discuss the implications of lunar-related orientation for the recruitment of glass eels to estuaries. This behaviour could help glass eels to reach the European coasts during their marine migration.

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Effect of management measures on glass eel escapement

ICES Journal of Marine Science ◽

10.1093/icesjms/fsm071 ◽

2007 ◽

Vol 64 (7) ◽

pp. 1402-1413 ◽

Cited By ~ 14

Author(s):

Laurent Beaulaton ◽

Cédric Briand

Keyword(s):

Anthropogenic Impacts ◽

European Eel ◽

Anthropogenic Pressure ◽

Stock Management ◽

Management Scenarios ◽

Anthropogenic Pressures ◽

Glass Eel ◽

Management Measures ◽

Glass Eels ◽

Different Levels

Abstract Stocks of European eel (Anguilla anguilla) have declined continuously and steadily, since 1980. A model, GEMAC, namely Glass Eel Model to Assess Compliance, has been developed with the objective of assessing anthropogenic impacts on glass eels in estuaries and evaluating the effects of management measures, to support initiatives aimed at helping the eel stocks recover. The model is described and applied to two estuaries with contrasting anthropogenic pressures: the Vilaine and the Garonne. It assesses the proportion of settled glass eels relative to a non-impacted situation with current (%S/R) or pristine recruitment (%S/R0). The estimated %S/R (%S/R0) is 5.5% (1.1%) for the Vilaine and 78% (19%) for the Garonne, in accord with the different levels of anthropogenic pressure in these two estuaries. A sensitivity analysis shows that the assessment of %S/R is accurate, and that in a data-poor context, the %S/R is under-assessed, as required by the precautionary approach. Seven management scenarios are explored all aiming to halve the anthropogenic pressure, but in fact leading to different levels of glass eel escapement, from almost zero to a 13-fold increase. This variation emphasizes the need for the estuarine context of eel stock management to be carefully evaluated for effectiveness when implementing management measures.

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Pre-release processes influencing short-term mortality of glass eels in the French eel (Anguilla anguilla, Linnaeus 1758) stocking programme

ICES Journal of Marine Science ◽

10.1093/icesjms/fsv074 ◽

2015 ◽

Vol 73 (1) ◽

pp. 150-157 ◽

Cited By ~ 8

Author(s):

Q. Josset ◽

T. Trancart ◽

V. Mazel ◽

F. Charrier ◽

L. Frotté ◽

...

Keyword(s):

Body Condition ◽

Anguilla Anguilla ◽

Management Plan ◽

The Body ◽

European Eel ◽

The European Union ◽

Short Term ◽

Good Body Condition ◽

Short Term Mortality ◽

Glass Eels

Abstract The decline in the European eel has led the European Union to require that its Member States establish an Eel Management Plan, which includes a set of measures to help preserve and restore the wild eel populations. Stocking has been conducted in France since 2011 as part of this Management Plan. This stocking programme is based on a protocol designed by the French National Museum of Natural History and includes an assessment of the short-term (15 d after release) mortality and three recapture surveys at 6, 12, and 36 months after release. The present study aims to assess the different processes influencing the short-term mortality to provide recommendations to reduce mortality and increase the efficiency of the releases. A linear model was built to examine the different variables selected modelling. The resulting model explained 56.4% of the total deviance; the main factor in terms of explained deviance was the operation (31.2%), which actually includes a number of handling parameters. The other factors influencing the short-term mortality were the release year, the fishmonger that provided the glass eels, the duration of stalling (period of captivity) before release, the body condition, the marking, and the type of mortality test. It appears that some of these variables could be adjusted to reduce the short-term mortality. For instance, selecting the most suitable release sites to stock or selecting glass eels with good body condition, adjusting the duration of stalling, or limiting the marking should help reduce short-term mortality.

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Ocean warming and acidification may challenge the riverward migration of glass eels

Biology Letters ◽

10.1098/rsbl.2018.0627 ◽

2019 ◽

Vol 15 (1) ◽

pp. 20180627 ◽

Cited By ~ 1

Author(s):

Francisco O. Borges ◽

Catarina P. Santos ◽

Eduardo Sampaio ◽

Cátia Figueiredo ◽

José Ricardo Paula ◽

...

Keyword(s):

Anguilla Anguilla ◽

Ocean Warming ◽

Choice Test ◽

European Eel ◽

Upstream Migration ◽

Migratory Activity ◽

Critically Endangered Species ◽

Dramatic Decline ◽

Glass Eels ◽

And Shifts

The dramatic decline of European eel ( Anguilla anguilla ) populations over recent decades has attracted considerable attention and concern. Furthermore, little is known about the sensitivity of the early stages of eels to projected future environmental change. Here, we investigated, for the first time, the potential combined effects of ocean warming (OW; Δ + 4°C; 18°C) and acidification (OA; Δ − 0.4 pH units) on the survival and migratory behaviour of A. anguilla glass eels, namely their preference towards riverine cues (freshwater and geosmin). Recently arrived individuals were exposed to isolated and combined OW and OA conditions for 100 days, adjusting for the salinity gradients associated with upstream migration. A two-choice test was used to investigate migratory activity and shifts in preference towards freshwater environments. While OW decreased survival and increased migratory activity, OA appears to hinder migratory response, reducing the preference for riverine cues. Our results suggest that future conditions could potentially favour an early settlement of glass eels, reducing the proportion of fully migratory individuals. Further research into the effects of climate change on eel migration and habitat selection is needed to implement efficient conservation plans for this critically endangered species.

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To boldly climb: behavioural and cognitive differences in migrating European glass eels

Royal Society Open Science ◽

10.1098/rsos.150665 ◽

2016 ◽

Vol 3 (1) ◽

pp. 150665 ◽

Cited By ~ 15

Author(s):

T. Podgorniak ◽

S. Blanchet ◽

E. De Oliveira ◽

F. Daverat ◽

F. Pierron

Keyword(s):

Cognitive Abilities ◽

Coping Styles ◽

Anguilla Anguilla ◽

European Eel ◽

Transcription Level ◽

Risky Behaviour ◽

Cognitive Differences ◽

Climbing Behaviour ◽

Glass Eels ◽

The Brain

European eel ( Anguilla anguilla ) is a catadromous fish species that received substantial attention as its population has markedly declined in the last three decades. The possible causes of this decline include habitat fragmentation factors such as dams and weirs. In some cases, these obstacles are equipped with fish friendly passage devices that may select young eels according to their climbing behaviour. We tested how individual climbing tendency was related to the event of fishway passage experienced in the field and classified fish climbing profiles as climbing ‘leaders’, ‘followers’, ‘finishers’ and ‘no climbers’. Moreover, we analysed the brain transcription level of genes related to neurogenesis and synaptic plasticity and compared it to climbing profiles. We found that fish from the upstream segments of an impounded river had a higher climbing propensity. Their behaviour was also more repeatable throughout the whole test than the obstacle-naive fish from the downstream segment. Moreover, we found that boldly climbing ‘leaders’ had lower levels of transcription of synapse-related genes than the climbing ‘followers’. These differences could be related to coping styles of fish, where proactive ‘leaders’ express a routine and risky behaviour, whereas reactive fish need an environmental assessment before exploratory behaviour. Our study showed that differences in climbing propensity exist in glass eels separated by water obstacles. Moreover, eels could adopt climbing different strategies according to the way they deal with environmental stress and to the cognitive abilities they possess.

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Oceanic fronts in the Sargasso Sea control the early life and drift of Atlantic eels

Proceedings of The Royal Society B Biological Sciences ◽

10.1098/rspb.2010.0900 ◽

2010 ◽

Vol 277 (1700) ◽

pp. 3593-3599 ◽

Cited By ~ 63

Author(s):

Peter Munk ◽

Michael M. Hansen ◽

Gregory E. Maes ◽

Torkel G. Nielsen ◽

Martin Castonguay ◽

...

Keyword(s):

Life Cycle ◽

Life Histories ◽

Anguilla Anguilla ◽

European Eel ◽

Sargasso Sea ◽

American Eel ◽

Wide Range ◽

Frontal Processes ◽

Glass Eels ◽

Atlantic Eels

Anguillid freshwater eels show remarkable life histories. In the Atlantic, the European eel ( Anguilla anguilla ) and American eel ( Anguilla rostrata ) undertake extensive migrations to spawn in the oceanic Sargasso Sea, and subsequently the offspring drift to foraging areas in Europe and North America, first as leaf-like leptocephali larvae that later metamorphose into glass eels. Since recruitment of European and American glass eels has declined drastically during past decades, there is a strong demand for further understanding of the early, oceanic phase of their life cycle. Consequently, during a field expedition to the eel spawning sites in the Sargasso Sea, we carried out a wide range of dedicated bio-physical studies across areas of eel larval distribution. Our findings suggest a key role of oceanic frontal processes, retaining eel larvae within a zone of enhanced feeding conditions and steering their drift. The majority of the more westerly distributed American eel larvae are likely to follow a westerly/northerly drift route entrained in the Antilles/Florida Currents. European eel larvae are generally believed to initially follow the same route, but their more easterly distribution close to the eastward flowing Subtropical Counter Current indicates that these larvae could follow a shorter, eastward route towards the Azores and Europe. The findings emphasize the significance of oceanic physical–biological linkages in the life-cycle completion of Atlantic eels.

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