Glass Eels Research Articles

An overview of the Anguillid eel culture in Vietnam

The wild eel fishery and eel farming are playing an increasingly important role in aquaculture. Most of the Anguillid eel fries at their glass eel stage are caught in the two provinces of Vietnam, viz., Phu Yen and Binh Dinh, which accounts for over 80% and 10-15%, respectively. The stages of raising eels in Vietnam are very diverse with the survival rate of Anguilla Marmorata is approx. 60% through glass eel and elver stages. The major eel grow-out area of Vietnam is in Mekong delta. Ca Mau province is the highest in eel farming among the 14 provinces in the country. There are over 1,000 eel farms produced about 2,000–5,000 tons of eel in Vietnam at this moment. Around 95% of the total number of farms produced giant mottled eel (A. marmorata), and most of the production (70%) was used for domestic consumption, while30% was exported to the adjacent countries.

Effects of environmental factors within the spawning area and migration routes on the length of Anguilla japonica glass eels recruited to Taiwan

Growth extent varies considerably among recruiting cohorts of Japanese eels Anguilla japonica. However, the effects of oceanic variation on their growth, particularly during the larval stage, remain unclear. We sampled glass eels in northeast Taiwan from 2010 to 2019 and investigated the effects of sea surface temperature (SST), salinity, chlorophyll a (chl a) concentration, and El Niño-Southern Oscillation (ENSO) events on the total length (TL) of these eels by developing a generalized additive model. The results revealed that mean SST between 21 and 24.5°C in the spawning area was associated with TL. Larger TL values were observed from 0.03-0.07 mg m-3 in the spawning area, and along the migration routes TL was greatest when chl a was > 13 mg m-3. Other variables, including mean SST and salinity along the migration route, influenced TL but contributed less than 4.5%. Larger and smaller A. japonica glass eels were observed during El Niño and La Niña years, respectively. Specifically, El Niño years exerted the greatest influence (67.1%) on TL, and this was followed by mean SST (12%) and accumulated chl a concentration (11.4%) within the spawning area. Our results indicated that environmental factors within the spawning area influenced the TL of A. japonica glass eels to a greater extent than did those along the migration routes, and climatic ENSO events exerted an additional important effect regarding changes in TL. Taken together, our results provide fundamental ecological information and a basis for fisheries to more effectively manage A. japonica.

Perspective plan for sustainable eel management in Lake Poso, Central Sulawesi

Lake Poso has high eel potential and a strategic position in supporting the development of eel fisheries in Sulawesi. Lake Poso is out of 15 national priority lakes in natural resources and environmental management programs. The catch of eel from Lake Poso is reported to have been decreasing for the last twenty years. Several factors causing the decline in eel populations are fishing activities, disruption in migration routes, and other factors such as changes in habitat and environmental conditions. The sustainability of the life of the Poso eel is determined by the success of the eel migration process. The migration of adult eels to the sea and young eels from glass eels to freshwater habitats must continue. The perspective plans that can be done to manage Poso's eel resources include 1) Socializing the eco-biological characteristics of eels as catadromous fish; 2) Establishing conservation areas and regulation of eel fishing; 3) Increasing eel population through restocking 4) Integrating fisheries and aquaculture activities; 5) Developing eel culture technology through size segmentation production; 6) Developing an eel migration monitoring system; 7) Building a cross-sectoral forum and put a joint action program; 8) Proposing the Poso eel Research and Conservation Center.

Glass eel (Anguilla anguilla L.) behaviour after artificial intake by adjusted tidal barrage management

Tidal barrages on water ways constitute a major threat for diadromous fish species such as the critically endangered European eel (Anguilla anguilla L.). An unobstructed migration route between the spawning area in the Sargasso Sea and the freshwater growth habitats in Europe is crucial for the European eels’ long-term survival. Every spring however, millions of glass eels arrive at the European coast to find their inland migration route blocked. Adjusted tidal barrage management (ATBM), i.e. setting the tidal sluice doors ajar during tidal rise, is one of the mitigation measures to enhance glass eel colonisation. Although this management substantially improves the number of incoming glass eels, the fate of these migrants, confronted with abruptly shifting conditions upon sluice passage, remains un-investigated. Glass eel upstream dispersion after sluice passage by means of ATBM was investigated in a drainage canal connecting a small estuary with an adjacent polder area in Belgium. Large numbers of glass eel caught with eel ladders installed at a pumping station about 800 m upstream the tidal barrage indicate that glass eels were well able to switch to an active counter current swimming behaviour to continue upstream dispersion. On the other hand, some glass eels did not initiate active migration but settled in artificial substrates placed in the canal instead. These residents however appeared very successful in using the canal as feeding ground and in time substantially increased their overall fitness. The appearance of both migration strategies (switch to active migration or successful settlement) in this canal shows the potential of ATBM as a valuable tool for the management and restoration of the European eel population and calls for a wide application of this cost-efficient mitigation measure.

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

The anthropogenic spread of pathogens contributes to the decline of some species. However, restocking programs implemented to counteract the decline of such populations can also contribute to the spread of pathogens, in particular for endangered species that cannot be bred in captivity, where populations used in restocking must be sourced from the wild with no control over sanitary status. A prime example of this is the endangered species European eel (Anguilla anguilla). As part of eel restocking programs, glass eels are captured in estuaries for later release further inland. In cases where release occurs immediately after capture, pre-release quarantine is important for mitigation of sanitary risks. In this study, we explored the impact of different quarantine conditions on glass eel sanitary status, morphology, behaviour and quarantine/post-quarantine survival in order to evaluate the feasibility and optimization of quarantine stages. Replicate experiments were conducted in 2017 and 2018, using glass eels of different geographical origin. The results suggest that glass eels entering estuaries are free of pathogens and so that their capture at this early stage reduces the risk of the anthropogenic introduction of pathogens through restocking programs. They support the use of a 15-day pre-release quarantine period to allow easier management of release processes and the implementation of prophylactic and therapeutic measures as part of these useful conservation programs.

The lunar compass of European glass eels (Anguilla anguilla) increases the probability that they recruit to North Sea coasts

AbstractThe European eel hatches in the Sargasso Sea and migrates across the Atlantic Ocean toward Europe. At the continental shelf, larvae metamorphose into glass eels and then recruit to coastal habitats and estuaries. Among other cues, glass eels orient in situ using lunar cues, but what role this lunar compass plays in their recruitment to the coast is unknown. To assess this, we incorporated empirical in situ observations of glass eel swimming and lunar‐driven orientation into a biophysical advection model. We simulated dispersal of glass eels drifting with the North Atlantic Current to test the hypothesis that lunar‐driven swimming and orientation behavior affects recruitment to North Sea coasts. Particles were released from the continental slope north of Scotland, an obligate passage for migrating eel larvae. Four numerical experiments were conducted: one with passive drift and three including glass eel swimming speeds (ranging from 3 to 12 cm/s) and lunar‐driven orientation. With a speed of 3 cm/s, the lunar compass increased recruitment to the North Sea coasts of Southwestern Norway and Scotland by 34%–40%. Conversely, orientation behavior decreased recruitment to northern areas like Iceland (−46%), the Faroe Islands (−39%) and Northern Norway (−49%). Behavior affected the timing of recruitment to Southwestern Norway, causing peaks of abundance in May–June, but not to other regions. These results show that lunar‐driven orientation and swimming behavior observed in glass eels substantially increases their recruitment to North Sea coasts. Results agree with the distribution of eel in northern Europe, which decreases in abundance with increasing latitude.

Diadromous fish modified timing of upstream migration over the last 30 years in France

Abstract Numerous studies have documented a change in the phenology of migration of diadromous fish in response to climate change. Only few studies have, however, been conducted simultaneously for multiple species and at a large spatial scale. We investigated the change in the timing of upstream migration of diadromous fish species in France. We used an original dataset collected from 40 fish‐counting devices in 28 French rivers over 10–30 years for five diadromous taxa: Alosa spp., Anguilla anguilla (with a distinction between glass eel and yellow eel), Petromyzon marinus, Salmo salar, and Salmo trutta. Except for glass eel, we found that taxa shifted their migration for earlier arrival dates. This result is consistent with numerous studies reporting advancement of the phenology of species life‐events. On average, we highlighted a phenological change of −2.3 days per decade (min = −0.2, max = −3.7). Moreover, the North Atlantic Oscillation Index, sea surface temperature, air temperature, and river discharge were found to explain the timing of upstream migration of diadromous fish taxa, highlighting the importance of factors acting at different spatial scales. Given the important phenological changes observed in our study and more widely in the scientific literature, we recommend that managers incorporate these changes into the management rules; in particular, in the case of dams whose migratory transparency (i.e. the possibility for the fishes to cross the dam) is ensured by adaptive water management and gate operations. This study benefited from large‐scale monitoring of migratory phenology of multiple species and environmental variables. These monitoring data are valuable and could allow better predictive modelling of the response of species to climate change.

Early settlement and growth of stocked European glass eels in a fragmented watercourse

AbstractOne of the actions that has been implemented to support the recovery of the panmictic population of European eel is stocking of waterbodies where natural recruitment is low or null. However, growth conditions of the stocked eels can vary greatly. This circumstance emphasises the importance to determine ideal habitat conditions to contribute to the success of stocking actions and, consequently, to increase the production of silver eels. This study aims to evaluate the early settlement and growth of stocked glass eels in the upper reaches of a fragmented river. Stocking was carried out, in 2014, at three sites of an inland tributary of the Mondego river basin (Central Portugal), and its monitoring was conducted during the following two years, until 2016, along with the collection of environmental and hydromorphological parameters. The results showed a successful dispersion throughout the study area. Growth varied spatially, although environmental parameters have not clearly explained this variation, but overall with high growth rates and a positive allometric growth in this early stage of stocking, suggesting a good condition of the stocked individuals. This study showed that these upper reaches of fragmented watercourses, a shared feature amongst most European rivers that are currently inaccessible for natural recruitment, may be suitable habitats for eel stocking.

Estimation of glass eel (Anguilla anguilla) exploitation in the Severn Estuary, England

AbstractMark‐release‐recapture trials were conducted to determine the exploitation rate of glass eel Anguilla anguilla (L.) by handheld dip nets in the Severn Estuary in the spring of 2020. The glass eel were marked with Rhodamine B at a concentration of 0.1 g/L, 200 glass eel/L for 4.5 hr and then placed in fresh water for 36–48 hr to monitor mortality. In trial 1, 891 ± 100 of the 20,455 glass eel were recaptured while in trial 2,373 ± 172 of the 27,923 glass eel released were recaptured. The exploitation rate was estimated to be 4.36% (±0.49) in trial 1 and 1.33% (±0.62) in trial 2. The size of the glass eel population from trial 1 was estimated to be 24.69 t (22.46 – 28.81), and the overall exploitation rate of the fishery for the season was 7.8% (6.7% – 8.6%). Comparisons are made with studies in other estuaries and with conservation targets set by the EU Eel Regulation and the Eel Management Plan for the River Severn. The study suggests the fishery is not the main cause of the Severn RBD failing to meet escapement targets.

New clues on the Atlantic eels spawning behavior and area: the Mid-Atlantic Ridge hypothesis

The Sargasso Sea has long been considered as the only spawning area for Atlantic eels, despite the absence of direct observations. The present study raises a novel scenario, deviating from Schmidt’s dogma, begins with a review of historical and recent observations that were combined to build up a global theory on spawning ecology and migration behavior of Atlantic eels. From this, it is argued that a favorable spawning area could be located eastward of Sargasso Sea at the intersection between the Mid-Atlantic Ridge and the oceanic fronts. Ocean circulation models combined with 3D particle-tracking method confirmed that spawning at this specific area would result in larval distribution fitting the field observation. This study explores the hypothesis that leptocephali are able to swim and orientate to reach their specific growth areas. It proposes a novel framework about spawning ecology, based on orientation, navigation and meeting cues of silver eels to the spawning area. Together this framework may serve as a stepping-stone for solving the long-lasting mystery of eel reproduction which first came out 2,400 years ago and promotes the understanding of oceanic migration and reproduction of marine organisms.

Rethinking swimming performance tests for bottom-dwelling fish: the case of European glass eel (Anguilla anguilla)

Systematic experiments on European eel (Anguilla anguilla) in their juvenile, early life stage (glass eel), were conducted to provide new insights on the fish swimming performance and propose a framework of analysis to design swimming-performance experiments for bottom-dwelling fish. In particular, we coupled experimental and computational fluid dynamics techniques to: (i) accommodate glass eel burst-and-coast swimming mode and estimate the active swimming time (tac), not considering coast and drift periods, (ii) estimate near-bottom velocities (Ub) experienced by the fish, rather than using bulk averages (U), (iii) investigate water temperature (T) influence on swimming ability, and (iv) identify a functional relation between Ub, tac and T. Results showed that burst-and-coast swimming mode was increasingly adopted by glass eel, especially when U was higher than 0.3 ms-1. Using U rather than Ub led to an overestimation of the fish swimming performance from 18 to 32%, on average. Under the range of temperatures analyzed (from 8 to 18 °C), tac was strongly influenced and positively related to T. As a final result, we propose a general formula to link near-bottom velocity, water temperature and active swimming time which can be useful in ecological engineering applications and reads as {rm{U}}_{rm{b}}=0.174cdot left({{rm{t}}_{rm{ac}}}^{-0.36}cdot {rm{T}}^{0.77}right).

Burrowing behaviour of the European eel (Anguilla anguilla): Effects of life stage.

The European eel (Anguilla anguilla) is a fascinating species, exhibiting a complex life cycle. The species is, however, listed as critically endangered on the IUCN Red List due to an amalgam of factors, including habitat loss. This study investigated the burrowing behaviour and substrate preference of glass, elver and yellow stages of A. anguilla. Preference was determined by introducing eels in aquaria with different substrates and evaluating the chosen substrate for burrowing. In addition, burrowing was recorded using a camera in all substrate types and analysed for kinematics. The experiments showed that all of these life stages sought refuge in the sediments with particle sizes ranging from sand to coarse gravel. Starting from a resting position, they shook their head horizontally in combination with rapid body undulations until half of their body was within the substrate. High-speed X-ray videography revealed that once partly in the sediment, eels used only horizontal head sweeps to penetrate further, without the use of their tail. Of the substrates tested, burrowing performance was highest in fine gravel (diameter 1-2 mm; lower burrowing duration, less body movements and/or lower frequency of movements), and all eels readily selected this substrate for burrowing. However, glass eels and elvers were able to use coarse gravel (diameter >8 mm) because their smaller size allowed manoeuvring through the spaces between the grains. Further, burrowing performance increased with body size: glass eels required more body undulations compared to yellow eels. Interestingly, the urge to hide within the sediment was highest for glass eels and elvers. Documentation of substrate preference and burrowing behaviour of A. anguilla provides new information about their potential habitat use. Considering that habitat alterations and deteriorations are partly responsible for the decline of the eel, this information can contribute to the development of more effective conservation measures.

Rare earth element organotropism in European eel (Anguilla anguilla)

Rare earth elements (REEs) are metallic elements with electronic, magnetic, optical and catalytic properties which make them essential in many industrial and medical fields. REEs are therefore becoming emerging pollutants and it is important to understand their implications for ecosystem health. However, little knowledge of REE bioaccumulation in aquatic organisms is available and especially on their internal distribution in fish. In the present study, REE organotropism was determined in Anguilla anguilla from the Loire estuary (France) by determining burdens in a wide set of tissues, organs and biological fluids. Differences have been observed between life stages and genders. For yellow eels, the most accumulating organ was the gills (126.90 ± 50.78 μg/kg dw) and for silver eels, it was the liver (181.78 ± 62.04 μg/kg dw for males; 203.79 ± 111.86 μg/kg dw for females). The comparison between female silver and yellow eels shown that female silver individuals accumulated significantly more REEs in the urinary system (US), muscles, gonads, spleen and liver, while yellow individuals accumulated more in gills. The comparison between male and female silver eels also highlighted differences, indeed the females accumulated significantly more REEs in the US, gonads, skin and spleen, compared to males which accumulated significantly more in muscles and gills. REEs abundances are also different between organs, life stages and genders. The gonads of female silver eels exhibited a particular profile with the dominance of gadolinium (Gd) (up to 74.2% of ∑REEs). Moreover, the presence of Anguillicola crassus in the swim bladder of organisms seemed to have an impact on REE bioaccumulation: parasitized yellow eels present higher concentrations of REEs in muscles, gills, gonads and liver than non-parasitized individuals. Regarding glass eels, REE contribution profiles in the whole body were close to those of yellow and silver eel skin.

Modeling the Hydromechanical Solution for Maintaining Fish Migration Continuity at Coastal Structures

To promote water control in coastal areas developed for agriculture, tidal hydraulic structures (doors and flap gates) are used for the hydraulic management of irrigation and drainage networks. By closing when the water level increases due to the tide, they prevent sea water inflow. However, fish using the tidal wave to move forward (e.g., glass eels) are also blocked from accessing the hydrologic network. There are hydromechanical solutions to mitigate these problems, such as adding a wooden block to leave an opening at high tide or adding a spring to delay the closing of the structure. The latter solution allows the structure to remain open only during a part of high tide. Designed empirically, these solutions can either alter the hydraulic control performance or be insufficiently effective for fish passage. This study proposes an operating model approach for hydromechanical structures that takes these passage solutions into account. This approach makes it possible to evaluate the performance of the passage solutions for fish passage and hydraulic regulation. With the help of these models, it is possible to design solutions that ensure a compromise between the passability of fish species and agricultural uses.

Rhythmic activity of inshore migration of tropical freshwater glass eels of the genus Anguilla

Compared with temperate Anguilla, little is known about the early marine migration and recruitment mechanisms of these tropical eels. To understand the ecological and behavioural mechanisms of their inshore migration in tropical Anguilla, systematic research on their periodicity of inshore migration was conducted for 24 h over 9 to 13 consecutive days before and after the new moon dates four times in one year. Glass eel (juvenile) collections (151 to 4371 specimens during each survey period) in the Poigar River, north Sulawesi Island, Indonesia indicated a lunar periodicity, in which peak recruitment continued for 2 to 4 days during the new moon period; then, the recruitment sharply decreased before and after the new moon. Daily glass eel migration occurred in two peaks during the ebb (20:00) and flood (02:00) tides, while there was no significant different glass eel recruitment between the flood tide and ebb tides. Although the “selective tidal stream transport” that glass eels utilize during the flood tide for recruitment is widely accepted to be the main mechanism of inshore migration behaviour through estuaries to freshwater habitats for temperate glass eels, we did not find this mechanism during this study. These results suggest that the main recruitment mechanism for tropical glass eels may be associated with both endogenous rhythms (biological clock) and exogenous factors (tidal orientation) in the study site. The rhythmic activity of migrating tropical glass eels found in this study also contributes not only to understanding the biological and ecological implications but also provides clues for stock management and conservation of Anguilla, which have been critically declining in recruitment worldwide in recent decades.

New eDNA based tool applied to the specific detection and monitoring of the endangered European eel

European eel (Anguilla anguilla) populations have declined drastically over the last decades and it has been included in the IUCN red list of endangered species as critically endangered since 2007. Thus, continuous population monitoring is crucial in order to warrant the conservation of this emblematic species.Environmental DNA (eDNA) has been recently implemented as a powerful method for the detection and monitoring of freshwater species, particularly for endangered species, where eDNA methods have shown to be less invasive than other methods (e.g. electrofishing).In this study, we developed and validated, under controlled conditions, a new species-specific tool for detecting A. anguilla from water samples by means of eDNA. Furthermore, we applied a semi-quantitative approach for monitoring of glass eel at different depths (surface and bottom) during different seasons in two rivers of northern Spain (Nalón and Sella rivers). We detected a significantly higher proportion of positive DNA amplifications in bottom than surface samples. Moreover, the proportion of replicas with positive amplification varied along the estuaries sections examined, and especially in the different sampling months. The temporal detection trends found in this study were compatible with the known upstream migration pattern of this species.Altogether, this study contributes to the establishment of a simple, easy and cheap system based on eDNA, that could be routinely applied in conservation research and management programs to monitoring wild populations of endangered species.

Hydraulic management of coastal freshwater marsh to conciliate local water needs and fish passage

Water control structures, used to regulate water levels and flow exchange in coastal marshes, act as barriers during fish migration between the ocean and brackish or freshwater ecosystems. Usual fish pass solutions may be unsuitable for obstacles subject to significant water level variations such as tidal range. This study proposes new solutions that were developed, implemented and evaluated on a marsh controlled by a series of hydraulic structures. These solutions were based on soft physical modifications (passive management) of the control gates, and on adaptations of their operation rules (active management). To evaluate the impacts of these adaptations, a hydraulic model of the marsh was built. It solves the one-dimensional Saint-Venant equations and appropriate gate equations. The model was used to identify management rules of control structures in a way to improve fish migration without significantly affecting the initial hydraulic management of the marsh (i.e. targeted seasonal water levels). Fish passability of upstream structures could be improved by managing downstream ones. It was concluded that the combination of active and passive management of water control structures could largely increase the passability of these obstacles during glass eel migration, while limiting seawater intrusion in the marsh and maintaining water levels into a range compatible with marsh management needs.

Gene-associated microsatellite markers confirm panmixia and indicate a different pattern of spatially varying selection in the endangered Japanese eel Anguilla japonica

The Japanese eel (Anguilla japonica) is a commercially important fish species in East Asia and its recruitment has been rapidly declining since 1990s. Clarifying the genetic population structure of A. japonica is the basis of multinational cooperation on its management and protection, due to its large distribution range. Gene-associated markers have been proved powerful in delineating fine-scale population genetic structure and spatially varying selection. In the present study, we developed 24 polymorphic gene-associated microsatellite markers including 18 loci associated with the genes under selection in the two North Atlantic eel species (Anguilla anguilla and Anguilla rostrata) and 6 loci based on transcript sequences. A total of 13 geographic populations were sampled across its distribution range, including 11 samples from China (9 from China's mainland and 2 from Taiwan region), and 2 samples from Japan. A total of 416 individuals (mostly glass eels) were collected and genotyped at the 24 microsatellites. All measures of differentiation were accordant with a panmictic scenario (FST=-0.001) in A. japonica. No footprints of spatially varying selection were found, indicating that the selection pattern in A. japonica might be different from that in the two North Atlantic eel species. We suggest that A. japonica should be managed as a single unit and management and conservation efforts must be coordinated at the international level, as overexploitation in any region will decrease its recruitment across the whole distributional range.

Growth potential of Anguilla marmorata glass eels in different seasons in eastern Taiwan

Growth potential of Anguilla marmorata glass eels in different seasons in eastern Taiwan

Warming enhances lanthanum accumulation and toxicity promoting cellular damage in glass eels (Anguilla anguilla)

Cumulative and continuing human emissions of greenhouse gases to the atmosphere are causing ocean warming. Rising temperature is a major threat to aquatic organisms and may affect physiological responses, such as acid-base balance, often compromising species fitness and survival. It is also expected that warming may influence the availability and toxicological effects of pollutants, including Rare Earth Elements. These are contaminants of environmental emerging concern with great economic interest. This group comprises yttrium, scandium and lanthanides, being Lanthanum (La) one of the most common. The European eel (Anguilla anguilla) is critically endangered and constitutes a delicacy in South East Asia and Europe, being subject to an increasing demand on a global scale. Considering the vulnerability of early life stages to contaminants, we exposed glass eels to 1.5 μg L−1 of La for five days, plus five days of depuration, under a present-day temperature and warming scenarios (△T = +4 °C). The aim of this study was to assess the bioaccumulation, elimination and specific biochemical enzymatic endpoints in glass eels (Anguilla anguilla) tissues, under warming and La. Overall, our results showed that the accumulation and toxicity of La were enhanced with increasing temperature. The accumulation was higher in the viscera, followed by the head, and ultimately the body. Elimination was less effective under warming. Exposure to La did not impact acetylcholinesterase activity. Moreover, lipid peroxidation peaked after five days under the combined exposure of La and warming. The expression of heat shock proteins was majorly suppressed in glass eels exposed to La, at both tested temperatures. This result suggests that, when exposed to La, glass eels were unable to efficiently prevent cellular damage, with a particularly dramatic setup in a near-future scenario. Further studies are needed towards a better understanding of the effects of lanthanum in a changing world.