Glass Eels Research Articles

Evaluation of the pigmentation stages and body proportions from the glass eel to yellow eel in Anguilla japonica

The pigmentation stages of Anguilla japonica were classified from the glass eel to yellow eel stage using 412 wild-caught eels from Hamana Lake and detailed developmental observations of 10 laboratory-reared eels. The sequential appearance of pigment was similar to that in A. anguilla, except for delayed pigment formation on the nerve cord of A. japonica. The general classification of pigmentation stages (VA–VIB) in A. anguilla was applicable to A. japonica, but the VB stage could be separated into two stages (VB1, VB2) in A. japonica. The completion of guanine deposition on the intra-abdominal membrane may be a trait to discriminate the yellow eel stage (VII) from the VIB stage. In natural conditions, wild-caught 0-age eels (n = 3,298) did not increase in size and retained a slender body form until VIA4, and then became thicker during VIB. The transition between body forms and growth may correspond to their ecological change from using passive transport in glass eels with tidal flow into rivers, to their settlement and further movements of elvers upstream. Considering this behavioral transition and pigmentation progression, it is proposed that the terms "glass eel" and "elver" could be defined as stages VA–VIA4 and VIB, respectively.

Survival and growth of European eels stocked as glass‐ and farm‐sourced eels in five lakes in the first years after stocking

AbstractEuropean eels Anguilla anguilla stocked as wild‐sourced glass eels showed a better overall performance of growth and survival compared with farm‐sourced eels after stocking in five isolated lakes within a 7‐year study period. Eels stocked as farm eels lost their initial size advantage over eels stocked as glass eels within 3–5 years after stocking. Population sizes estimated for consecutive stocking batches indicated that 8–17% of eels stocked as farm eels survived 3–6 years after stocking compared with 5–45% of eels stocked as glass eels. This study coupled with results of previous studies suggests that stocking of farm eels may have no advantage in growth and survival compared with stocking of glass eels if stocking occurs at an optimal time in spring. In addition, the use of relatively expensive farm eels may provide no general advantage over stocking of glass eels. However, if glass eels are only available for stocking purposes very early in the year, lower survival rates than obtained in the present study can be assumed and stocking with relatively more expensive farm eels could possibly be a better option.

Demographic assessment of a stocking experiment in European Eels

AbstractSince the 1980s, the European eels’ stocks have dramatically decreased with no sign of recovery, resulting in their classification as Critically endangered on the IUCN red list of threatened species. The European Council Regulation 1100/2007 requires that 35% of glass eels caught annually by fishing be released in European waters for restocking. However, the efficiency of this measure on population viability has never been evaluated. Here, we estimated demographic parameters of a stocked population of French eels using a multistate capture–recapture model. Using these estimates, we then estimated population size and predicted the number of future genitors obtained by stocking. We found that the stage in which eels were stocked did not influence their future survival and that the maximal number of silver eels was quickly reached, after 3 years following stocking. We concluded that stocking experiments in the Mediterranean region are efficient for fast production of genitors. We suggest that further studies should assess the quality of these genitors.

New insights regarding gonad development in European eel: evidence for a direct ovarian differentiation

In European eel, it has been proposed that the undifferentiated gonad would develop into either an intersexual stage (Syrski organ) or directly into an ovary. The Syrski organ could then develop into either an ovary or a testis. In the present study, glass eels were raised until they reached a minimum size of 29 cm for histological sex assessment. In addition, some undifferentiated individuals with size encompassing 15-28 cm were sampled in a female-biased population (Oir River). We also investigated aromatase gene expression, which is known to be involved in the process of fish sex differentiation. Gonad histology revealed that intersexual eels were characterized by a small number of oocytes within a predominant testis-like structure. Males were significantly smaller than intersexual eels, which suggests that all males do not necessarily pass through an intermediate intersexual stage. Aromatase transcript levels in intersexual eels gonads and testes were similar but significantly lower than in ovaries and were comparable between ovaries and undifferentiated gonads from the females-biased population. In addition, condition factor was lower in female than in intersexual individuals. Together, these results provide evidence that ovaries would not develop from the Syrski organ.

Co‐Infection with Pseudomonas anguilliseptica and Delftia acidovorans in the European eel, Anguilla anguilla (L.): a case history of an illegally trafficked protected species

Inspections by customs agents at Barcelona airport discovered 420 kg of contraband glass eels prepared for shipment to Hong Kong. After confiscation of these animals by police, they were transported to holding facilities to be maintained until after a judicial hearing. Upon arrival, they were separated into two groups and held under ambient flow-through conditions in fresh water. During their captivity period, several peaks in mortality occurred and multiple bacterial strains were isolated from moribund animals. Sequencing of 16S rDNA was used to determine specific identity of the isolates. An initial isolation of Pseudomonas anguilliseptica was treated with oxytetracycline. A subsequent isolation of Delftia acidovorans proved resistant to oxytetracycline and was treated with gentamicin in combination with sulphadiazine-trimethoprim. Once the health condition of the animals was stabilized, they were partitioned into groups and subsequently released as part of a restocking effort for the species following the guidelines of Regulation (EC) 1100/2007 (Anon 2007). This represents the first record for both bacterial species in the host Anguilla anguilla in the Spanish Mediterranean.

DISTRIBUTION OF TROPICAL EEL GENUS Anguilla IN INDONESIA WATER BASED ON SEMI-MULTIPLEX PCR

Tropical eels living in Indonesian waters are known to be composed of several species, but their real listing together with their distribution ranges need to be established. The main difficulties are the very high number of islands with perennial rivers where these species are living during the growth phase of their life cycle. It is difficult, sometimes impossible, to determine the species using morphological characters, moreover on glass eels. In order to establish the geographic distribution of tropical eels of the genus Anguilla in Indonesian waters, a total 1,115 specimens were collected between 2008 and 2012. Sample collection was done in the growth habitats that are rivers and estuaries by commercial nets of different categories according to the fish size. All samples were identified genetically using the recently developed semimultiplex PCR method. We recognized four species and subspecies with wide distribution: Anguilla bicolor bicolor, Anguilla bicolor pacifica, Anguilla marmorata, and Anguilla interioris; two species with limited distribution, close to endemism: Anguilla celebesensis and Anguilla borneensis and one subspecies Anguilla nebulosa nebulosa that is only spread in river flowing into Indian Ocean.

Optimisation of adjusted barrier management to improve glass eel migration at an estuarine barrier

Estuarine barriers may significantly reduce the upstream migration of diadromous fish species like the European eel (Anguilla anguilla L.). Previous research showed that limited barrier opening during tidal rise was a cost-efficient and effective mitigation option to improve upstream glass eel migration, without significant intrusion of sea water. This paper aims to optimise this adjusted barrier management to improve eel passage at a tidal barrier complex at the mouth of the River Yser, Flanders, Belgium, one of the most important migration routes for glass eel in Flanders. Specifically, three hypotheses were tested. The first hypothesis analysed the impact of the number of barriers opened on the upstream glass eel migration. The second hypothesis evaluated the relation between the size of the barrier opening and glass eel migration. Finally, we tested whether the suggested adjusted barrier management may lead to a significant increase in conductivity in the River Yser. Increased opening of one barrier appeared more efficient than opening several barriers slightly. Conductivity increased during periods of extreme drought and at base flow, but decreased within 24 h after the first peak flow. This indicates that adjusted barrier management does not entail salt intrusion in the Yser basin, as long as this management is not applied in extremely dry periods. Since the adjusted barrier management is easily implemented and could be applied on numerous tidal barriers, the presented results may contribute to restoration of eel populations worldwide and be of interest to a wide range of river managers and stakeholders.

Population genetics of the American eel (Anguilla rostrata): FST = 0 and North Atlantic Oscillation effects on demographic fluctuations of a panmictic species

We performed population genetic analyses on the American eel (Anguilla rostrata) with three main objectives. First, we conducted the most comprehensive analysis of neutral genetic population structure to date to revisit the null hypothesis of panmixia in this species. Second, we used this data to provide the first estimates of contemporary effective population size (Ne ) and to document temporal variation in effective number of breeders (Nb ) in American eel. Third, we tested for statistical associations between temporal variation in the North Atlantic Oscillation (NAO), the effective number of breeders and two indices of recruit abundance. A total of 2142 eels from 32 sampling locations were genotyped with 18 microsatellite loci. All measures of differentiation were essentially zero, and no evidence for significant spatial or temporal genetic differentiation was found. The panmixia hypothesis should thus be accepted for this species. Nb estimates varied by a factor of 23 among 12 cohorts, from 473 to 10,999. The effective population size Ne was estimated at 10,532 (95% CI, 9312-11,752). This study also showed that genetically based demographic indices, namely Nb and allelic richness (Ar), can be used as surrogates for the abundance of breeders and recruits, which were both shown to be positively influenced by variation during high (positive) NAO phases. Thus, long-term genetic monitoring of American glass eels at several sites along the North American Atlantic coast would represent a powerful and efficient complement to census monitoring to track demographic fluctuations and better understand their causes.

Effect of starvation, body size, and temperature on the onset of metamorphosis in Japanese eel (Anguilla japonica)

We assessed the effects of starvation, body size, and water temperature on the onset of metamorphosis in leptocephali of Japanese eel ( Anguilla japonica Temminck and Schlegel, 1846) as determined by the morphological criteria of proportion, preanal length, and body depth to total length. Leptocephali of mean total length 55.6 mm that had been reared in captivity for 241 days from hatching were divided into unfed (n = 28) and fed (n = 30) groups in triplicate and reared for an additional 2 weeks. The mean percentage of larvae starting metamorphosis within 2 weeks was significantly higher in the unfed than in the fed groups (70% vs. 28.6%), suggesting that food deprivation acted as a cue for metamorphosis. The critical size for metamorphosis was a total length of 50–55 mm; smaller larvae did not start metamorphosis even in the absence of food, whereas larvae reaching that critical size were induced to undergo metamorphosis by starvation. The start of metamorphosis under unfed conditions was independent of diel-varying water temperature (day 23 °C; night 21–29 °C), suggesting a high plasticity in response to a wide range of environmental temperatures. These findings suggest methods for the efficient production of glass eels, as well as new insights into the mechanism of eel metamorphosis.

Comparison of growth and condition of European eels stocked as glass and farm sourced eels in lakes in the first 4 years after stocking

European glass eels Anguilla anguilla showed a better overall performance of growth and condition compared to farm sourced eels after stocking in six isolated lakes within a 4-year study period. It can be concluded that the stocked farm sourced eels needed a longer period to switch from artificial food to natural prey and to adapt to new foraging strategies.

Unusual Cases of Adult Intersexuality in the European Eel Anguilla anguilla

Two cases of intersexuality are reported for the first time in European eel, Anguilla anguilla (at the beginning of the silver eel stage), within 140 fish caught as glass eels in the south-west of France and reared in tanks at 17°C. Cysts containing spermatozoa were observed in ovaries with pre-vitellogenic oocytes. This feature is very uncommon, especially owing to the fact that male cells do not normally reach this stage in captivity, and an environmentally controlled transdifferentation process may not be excluded. Besides, the expression of the gonadal aromatase gene (cyp19a1a) was found to be higher in these 2 intersexual fish compared to normal females, although these results must be considered with caution since only 2 intersexual fish were available. A possible feminizing effect of this ‘abnormal’ upregulation of aromatase is discussed.

Viral diseases of wild and farmed European eel Anguilla anguilla with particular reference to the Netherlands

Diseases are an important cause of losses and decreased production rates in freshwater eel farming, and have been suggested to play a contributory role in the worldwide decline in wild freshwater eel stocks. Three commonly detected pathogenic viruses of European eel Anguilla anguilla are the aquabirnavirus eel virus European (EVE), the rhabdovirus eel virus European X (EVEX), and the alloherpesvirus anguillid herpesvirus 1 (AngHV1). In general, all 3 viruses cause a nonspecific haemorrhagic disease with increased mortality rates. This review provides an overview of the current knowledge on the aetiology, prevalence, clinical signs and gross pathology of these 3 viruses. Reported experimental infections showed the temperature dependency and potential pathogenicity of these viruses for eels and other fish species. In addition to the published literature, an overview of the isolation of pathogenic viruses from wild and farmed A. anguilla in the Netherlands during the past 2 decades is given. A total of 249 wild A. anguilla, 39 batches of glass eels intended for farming purposes, and 239 batches of farmed European eels were necropsied and examined virologically. AngHV1 was isolated from wild yellow and silver A. anguilla from the Netherlands from 1998 until the present, while EVEX was only found sporadically, and EVE was never isolated. In farmed A. anguilla AngHV1 was also the most commonly isolated virus, followed by EVE and EVEX.

Offshore Spawning for the Newly Discovered Anguillid Species Anguilla luzonensis (Teleostei: Anguillidae) in the Western North Pacific

Abstract: A new anguillid eel species, Anguilla luzonensis, was recently discovered on Luzon Island of the northern Philippines, but little is known about its life history. DNA identification was used to determine that five leptocephali of this species (29.2–51.2 mm) were collected offshore in the western North Pacific (13°–17.5° N, 125°–141° E) in April, June, and July between 2002 and 2009. One leptocephalus was caught west of Luzon and four were close to the spawning areas of Anguilla japonica and Anguilla marmorata. Otolith microstructure showed that the leptocephali were up to 103–138 days old. Glass eels from northern Luzon in previous studies that were thought to be Anguilla celebesensis now appear likely to have been A. luzonensis, and they were estimated by otolith analysis to have long larval durations similar to those of A. marmorata. Estimated hatching dates of these glass eels and the A. luzonensis leptocephali were both in the February to May season. Offshore presence of leptocephali of A. l...

Late arrival of Anguilla japonica glass eels at the Sagami River estuary in two recent consecutive year classes: ecology and socio-economic impacts

To determine the actual status of the recruiting glass eel stock of Anguilla japonica, we conducted a continuous monthly monitoring program for 2 years between November 2009 and October 2011 at the Sagami River estuary, Japan. A total of 114 and 372 A. japonica glass eels were observed in the 2009–2010 and 2010–2011 seasons, respectively. Recruitment patterns were the same in both years, starting in early winter (December in 2009 and November in 2010), increasing slightly until March, decreasing in April, but attaining the largest peak in June, after which no glass eels were observed during August–October. The Japanese eel has been known to spawn mainly in the summer, and glass eels recruit to their freshwater growth habitats during the winter to early spring. Our results clearly demonstrate an unexpected late arrival of glass eels in the early summer for two recent consecutive year classes. The summer recruitment found in our study indicates the unusual phenology of the Japanese eel, which may be a possible response to recent climate change.

Sympatric spawning but allopatric distribution of Anguilla japonica and Anguilla marmorata: temperature- and oceanic current-dependent sieving.

Anguilla japonica and Anguilla marmorata share overlapping spawning sites, similar drifting routes, and comparable larval durations. However, they exhibit allopatric geographical distributions in East Asia. To clarify this ecological discrepancy, glass eels from estuaries in Taiwan, the Philippines, Indonesia, and China were collected monthly, and the survival rate of A. marmorata under varying water salinities and temperatures was examined. The composition ratio of these 2 eel species showed a significant latitude cline, matching the 24°C sea surface temperature isotherm in winter. Both species had opposing temperature preferences for recruitment. A. marmorata prefer high water temperatures and die at low water temperatures. In contrast, A. japonica can endure low water temperatures, but their recruitment is inhibited by high water temperatures. Thus, A. japonica glass eels, which mainly spawn in summer, are preferably recruited to Taiwan, China, Korea, and Japan by the Kuroshio and its branch waters in winter. Meanwhile, A. marmorata glass eels, which spawn throughout the year, are mostly screened out in East Asia in areas with low-temperature coastal waters in winter. During summer, the strong northward currents from the South China Sea and Changjiang River discharge markedly block the Kuroshio invasion and thus restrict the approach of A. marmorata glass eels to the coasts of China and Korea. The differences in the preferences of the recruitment temperature for glass eels combined with the availability of oceanic currents shape the real geographic distribution of Anguilla japonica and Anguilla marmorata, making them “temperate” and “tropical” eels, respectively.

Oceanic and local environmental conditions as forcing mechanisms of the glass eel recruitment to the southernmost European estuary

The main oceanic and local environmental traits forcing the glass eel Anguilla anguilla (L., 1758) recruitment to the southernmost European estuary, the Guadalquivir (SW Spain), were studied during nine successive migration seasons (June 1997–December 2006) using a fishery-independent experimental survey at three sampling sites in the estuary. A clear seasonal pattern was observed: density was highest between late autumn and spring with two migration peaks, and lowest during summer. Short-term (inter-month) changes in glass eel density were partially driven by local environmental variables, such as estuarine turbidity, local rainfall and water temperature. Long-term (inter-annual) changes were positively correlated with oceanic factors related to recruitment success (NAO index and primary production at the spawning area) as well as local environmental factors (westerly and southerly wind mixing indices and rainfall). Spatial changes in glass eel density within the estuary depended on tidal and light situations although maximum densities were mainly observed at diurnal and/or nocturnal flood tides. Although the decrease in the abundance of European glass eels has been widely known since the 1980s, during this study there was no evidence of a declining trend, probably because of an insufficiently long time series.

Genomics in Eels — Towards Aquaculture and Biology

Freshwater eels (genus Anguilla), especially the species inhabiting the temperate areas such as the European, American and Japanese eels, are important aquaculture species. Although artificial reproduction has been attempted since the 1930s and large numbers of studies have been conducted, it has not yet fully succeeded. Problems in eel artificial breeding are highly diverse, for instance, lack of basic information about reproduction in nature, no appropriate food for larvae, high mortality, and high individual variation in adults in response to maturation induction. Over the last decade, genomic data have been obtained for a variety of aquatic organisms. Recent technological advances in sequencing and computation now enable the accumulation of genomic information even for non-model species. The draft genome of the European eel Anguilla anguilla has been recently determined using Illumina technology and transcriptomic data based on next generation sequencing have been emerging. Extensive genomic information will facilitate many aspects of the artificial reproduction of eels. Here, we review the progress in genome-wide studies of eels, including additional analysis of the European eel genome data, and discuss future directions and implications of genomic data for aquaculture.

Lipid Class and Fatty Acid Content of the Leptocephalus Larva of Tropical Eels

The leptocephalus larva of eels distinguishes the elopomorph fishes from all other bony fishes. The leptocephalus is long lived and increases in size primarily through the synthesis and deposition of glycosaminoglycans. Energy stored during the larval stage, in the form of glycosaminoglycan and lipids, is required to fuel migration, metamorphosis and metabolism of the subsequent glass eel stage. Despite the importance of energy storage by leptocephali for survival and recruitment, their diet, condition and lipid content and composition is essentially unknown. To gain further insight into energy storage and condition of leptocephali, we determined the lipid class and fatty acid concentration of larvae collected on a cross-shelf transect off Broome, northwestern Australia. The total lipid concentration of two families and four sub-families of leptocephali ranged from 2.7 to 7.0 mg g wet weight(-1), at the low end of the few published values. Phospholipid and triacylglycerol made up ca. 63 % of the total lipid pool. The triacylglycerol:sterol ratio, an index of nutritional condition, ranged from 0.9 to 3.7, indicating that the leptocephali were in good condition. The predominant fatty acids were 16:0 (23 mol%), 22:6n-3 (docosahexaenoic acid, DHA, 16 mol%), 18:0 (8.2 mol%), 20:5n-3 (eicosapentaenoic acid, EPA, 6.7 mol%), 18:1n-9 (6.4 mol%) and 16:1n-7 (6.3 mol%). The DHA:EPA ratio ranged from 2.4 to 2.9, sufficient for normal growth and development of fish larvae generally. The leptocephali had proportions of bacterial markers >4.4 %, consistent with the possibility that they consume appendicularian houses or other marine snow that is bacteria rich.

Differential effects of behaviour, propensity to migrate and recruitment season on glass eels and elvers’ growing performance

Abstract – Social interactions impact growth rate of animals in different ways such as direct or indirect competition for food and/or territories. In this study, behaviour of glass eels and elvers was described throughout three trials of 9 days spread out over 3 months. The impact of recruitment season (spring and autumn runs) and glass eels’ propensity to migrate (assessed thanks to an experimental sorting) on individual growth was studied. Overall, spring elvers were more aggressive and active than autumn elvers while autumn fish were more gregarious and reached a higher specific growth rate than spring fish. At the individual level, gregariousness and swimming, respectively, promote and impair growth rate. In addition, skinny individuals and individuals with high motivation to get food achieved better growth rate suggesting catch‐up growth processes. These findings may help in clarifying the complex relations between behaviour, growth, river basin colonisation and future sex (which is environmentally determined in eel) and then contribute to the urgent need of scientific knowledge to guide the stocking programme of this endangered species.

Larval Japanese eel (Anguilla japonica) as sub‐surface current bio‐tracers on the East Asia continental shelf

AbstractLarval Japanese eel (leptocephali) are passively transported from their spawning sites of the North Equatorial Current to the Kuroshio and its branch waters for 4–6 months before reaching the East Asian coasts. The larvae mainly stay within water depths between 50–150 m. The dispersal dynamics of larvae thus should reflect the sub‐surface oceanic currents on the East Asia continental shelf. An analysis of Japanese glass eel catch data in East Asian countries during 1985 to 2009, and for Taiwan from 1968 to 2008, indicates that the overall annual catch is generally correlated across countries of East Asia, and between north and west areas of Taiwan. The Kuroshio and its branch waters disperse glass eels throughout East Asian habitats, and the glass eel distribution matches the flow directionality of oceanic currents. Recruitment in western Taiwan occurs with a sequential southwestern to northwestern direction, suggesting that the Taiwan Strait Current penetrates the western coast of Taiwan in the sub‐surface layer in winter. The monthly averaged sub‐surface 50 m circulation pattern in the vicinity of Taiwan and modeled tracer experiments also support the northward winter sub‐surface current in Taiwan Strait. These results suggest that the larval Japanese eel could serve as a valuable bio‐tracer of sub‐surface currents, and the earlier recruitment dynamics of Japanese glass eels in Taiwan could be a good predictor for the subsequent catch in other East Asia areas.