Yellow Eels Research Articles

Evaluating Silvering Stages in European Eels: A Study on Biological and Morphometric Variations in the Asi River, Türkiye

The European eel (Anguilla anguilla) undergoes significant morphological and physiological changes during its transition from the yellow to the silver stage, which are critical for its long-distance spawning migration. This study aimed to investigate these changes in European eels from the Asi River, located in Hatay, Türkiye, during their silvering process. A total of 96 eels were sampled in February 2019, and various morphometric measurements, including total length, body weight, eye dimensions and height, and pectoral fin lengths, were taken. Liver and gonad weights were also measured to assess the hepatosomatic index (HSI). The length–weight relationship for silver eels was described by the equation, W = 0.0072 × L2.732, with silver-stage eels showing a higher growth rate compared to yellow-stage eels, which had a relationship of W = 0.0184 × L2.397. The average total length of silver eels (431.2 ± 16.7 mm) was significantly greater than that of yellow eels (382.4 ± 11.9 mm). Additionally, pectoral fin length was significantly longer in silver eels (20.8 ± 1.1 mm) compared to yellow eels (14.8 ± 0.9 mm). The hepatosomatic index (HSI) for silver eels was also found to be higher than for yellow eels, indicating increased liver size as an adaptation for energy storage during migration. Eye height, a key indicator of silvering, showed a substantial increase during the transition, with silver-stage eels having an average eye height of 5.3 ± 0.2 mm compared to 4.2 ± 0.1 mm in yellow-stage eels.

European eel (Anguilla anguilla) survival modeling based on a 22-year capture-mark-recapture survey of a Mediterranean subpopulation.

Since the 1980s, the European eel (Anguilla anguilla) has declined by over 90% in recruitment across its European and North African distribution area. This diadromous fish spawns at sea and migrates into continental waters, where it grows for three to more than 30 years, depending on habitat conditions and location. During their growth, different habitat use tactics can locally influence the life-history traits of eels, including their survival rates. Thus, the spatio-temporal dimension of this species is crucial for management. Based on a rare Mediterranean long-term survey of more than 20 years (2001-2022) in an artificial drainage canal connected to a vast brackish lagoon (the Vaccarès lagoon), we aimed to study the dynamics of one subpopulation's life-history traits. We used Bayesian multistate capture-mark-recapture (CMR) models to assess the temporal variability in survival and abundance at both seasonal and inter-annual scales, considering life-stage structure. High survival rates and low detection probabilities were found for the undifferentiated and female yellow stages. In contrast, female silver eels exhibited lower survival rates and higher capture probabilities. Estimating detection probabilities and survival rates enabled accurate assessment of relative abundance across different life stages and time periods. Our findings indicated a substantial decrease in the abundance of undifferentiated and female yellow eels in the early 2000s, whereas the abundance of female silver eels remained consistently low yet stable throughout the study period. Considering the life stage seemed essential to study the dynamics of the eel during its continental growing period. The present results will provide key elements to propose and implement suitable sustainable environmental management strategies for eel conservation.

The expression profiles of cyp19a1, sf-1, esrs and gths in the brain-pituitary during gonadal sex differentiation in juvenile Japanese eels

Eels are gonochoristic species whose gonadal differentiation initiates at the yellow eel stage and is influenced by environmental factors. We revealed some sex-related genes were sex dimorphically expressed in gonads during gonadal sex differentiation of Japanese eel (Anguilla japonica); however, the expression of sex-related genes in the brain-pituitary during gonadal sex differentiation in eels is still unclear. This study aimed to investigate the sex-related gene expressions in the brain-pituitary and tried to clarify their roles in the brain and gonads during gonadal sex differentiation. Based on our previous histological study, the control eels developed as males, and estradiol-17β (E2) was used for feminization. Our results showed that during testicular differentiation, the brain cyp19a1 transcripts and aromatase proteins were increased significantly; moreover, the cyp19a1, sf-1, foxl2s, and esrs (except gperb) transcripts in the midbrain/pituitary also were increased significantly. Forebrain gnrh1 transcripts increased slightly during gonadal differentiation of both sexes, but the gnrhr1b and gnrhr2 transcripts in the midbrain/pituitary were stable during gonadal differentiation. The expression levels of gths and gh in the midbrain/pituitary were significantly increased during testicular differentiation and were much higher in males than in E2-feminized females. These results implied that endogenous estrogens might play essential roles in the brain/pituitary during testicular differentiation, sf-1, foxl2s, and esrs may have roles in cyp19a1 regulation in the midbrain/pituitary of Japanese eels. For the GnRH-GTH axis, gths, especially fshb, may be regulated by esrs and involved in regulating testicular differentiation and development in Japanese eels.

Studi kepadatan ikan sidat (Anguilla Spp) di Sungai Tarus Desa Mata Air Kabupaten Kupang pada bulan September sampai Oktober

The research conducted over approximately 60 days at Tarus River, Mata Air Village, Kupang Regency aimed to assess the variety and abundance of eel fish, a high-value commodity in the international market. The study focused on three stations along the river, where the density of eel fish was observed: station 1 (8 individuals), station 2 (5 individuals), and station 3 (2 individuals). Using digital scales, researchers measured the total weight of the caught fish, alongside noting the types of eel fish obtained. The findings, analyzed descriptively and presented through tables and figures, revealed the prevalence of eel fish type A. marmorata during the research period, primarily in the yellow eel and elver phases with varying sizes. Morphometric measurements indicated that Anguilla marmorata specimens typically ranged in total length (35-47 cm), dorsal length (20-30 cm), anal length (15-25 cm), head diameter (5-7 cm), body diameter (7-9 cm), and total weight (170-181 grams). Additionally, water quality assessments at the research site indicated temperatures ranging from 20-27oC, with a pH of 7.5.
 
 Keywords: Identification, eel fish (Anguilla marmorata).

Kajian keberadaan ikan sidat Anguilla marmorrata di Muara Sungai Wae Randang Desa Nanga Mbaur Kabupaten Manggarai Timur pada bulan November-Desember

Wae Randang is one of the rivers that flows in Ngana Mbaur Village and directly empties into the sea. This river has the potential for eel population, because its location is in accordance with the natural habitat of eel fish. Therefore, it is necessary to conduct research to assess the presence of eel Anguila marmorata in Wae Randang River, especially from November to December. The purpose of this study was to determine the length, weight, and diameter of A. marmorata eels found in certain periods in the river. The research was conducted from November to December 2022, with descriptive data analysis and presented in the form of figures and tables. In the study obtained as many as 11 eels A. marmorata caught during November to December. The fish had a total length of 30-52 cm, total weight of 48-323 g, anal length 17-30 g, dorsal length 19-42 g, body diameter 6-12 g, and head diameter 5-9 cm. Based on the length and weight of the recorded eels, the fish were in the elver and yellow eel stadia.
 
 Keywords: Eel, potential, study, Wae Randang

The inadequacies of estimating silver eel escapement by studying yellow eels caught in the coastal sea far away from the river mouth. A comment on Rohtla et al. (2021)

Abstract For the conservation and management of the critically endangered European eel, it is crucial to maintain an overview on the biomass of the eels moving from freshwater feeding areas to the sea for spawning. Rohtla et al. suggested an assessment of the escapement of eels from the Narva River to the Baltic Sea based on their study of origin of yellow eels from Estonian coastal areas and eels from Finnish coastal areas. Silver eels from the Estonian coast have not been included in the study. The number of descending eels in the Narva River has not been investigated. The sampling areas in the sea were located far from the river mouth, ranging from 100 to 500 km away. The article does not include an analysis of how to calculate the number of eels descending from the river based on the samples used. Thus, estimates can be significantly biased. We demonstrate that the approach used to estimate escapement is methodologically incorrect and needs further development before applying in eel management. This comment recommends methodological improvements for estimating eel escapement using the indirect method.

Reply to: Tuvikene et al. The inadequacies of estimating silver eel escapement by studying yellow eels caught in the coastal sea far away from the river mouth. A comment on Rohtla et al. (2021)

Abstract Restocking European eel (Anguilla anguilla) is generally considered as a conservation action, but its true conservation value remains unknown and is currently debated over. Estimation of the biomass of escaping silver eels is one way to partially measure the success of restocking. Tuvikene et al. in a comment to Rohtla et al. criticize the data that were used to reach the conclusion that eel conservation goals in a fully restocked eel management unit in Estonia are not being met. While acknowledging some potential and unavoidable sampling biases in our study that need to be addressed in the future, we stand by our general conclusions.

Functional Morphology as an Indicator of European Eel Population Status

In the area of the Neretva delta in the eastern Adriatic, where the European eel, Anguilla anguilla (Linnaeus, 1758) has been traditionally fished for centuries, a decline in its population has been observed, as in most of Europe. Despite several studies, systematic monitoring was not performed, and the causes of population decline are attributed to anthropogenic stressors, mainly overfishing and interventions that disrupt the migration. With the stock at a low level, there is a need for a detailed assessment of biological data and the determination of the “zero state” of the eel population in the areas where monitoring was not previously performed, such as the Neretva delta. This data would serve as a basis for the development of an appropriate monitoring and eel management plan. One of the under-researched aspects is still the eel’s morphology, which is closely related to all basic life functions. The aim of this work was to analyze in detail the morphological parameters of yellow and silver eels from the mouth of the Neretva River in different seasons and the relationships between the measured morphometric parameters and physiological indicators and to compare them with previously published results for different life stages across Europe. The samples were collected during spring, summer and autumn of 2021, and winter of 2022. Yellow eels were present in the catch throughout the sampling period, while silver eels were caught in the autumn and winter. Yellow and silver eels were significantly different regarding 22 morphometric measures that were analyzed. Isometric growth was recorded for yellow eels in the spring and autumn of 2021, and positive allometric growth was recorded for yellow eels in the summer and silver eels in the autumn of 2021 and winter of 2022. PCA showed that the main factor that separates the eels grouped by life stage in different seasons is the intestine length (IL), whereas the rest of the factors (weight—W; intestine weight—IW; liver weight—LW; and total length—TL) affect the groupings almost equally. Seasonal averages of the condition factor (CF) for yellow and silver eels did not differ statistically. Three indicators were used to describe intestine morphology: relative gut weight (RGW), relative gut length (RGL), and Zihler’s index (ZHI); and the only statistically significant difference between yellow and silver eels was recorded for the RGW. The hepatosomatic index (HSI) was significantly different between silver eels in winter and yellow eels in spring. In addition to supplementing the already known facts, this paper provides new information on the functional morphology of the European eel. Monitoring of these characteristics is crucial for management of the European eel fisheries as they are directly related to functional performance and affect the ability to maintain sustainable populations in anthropogenically altered environments.

Glass Eel Restocking Experiments in Typologically Different Upland Rivers: How Much Have We Learned about the Importance of Recipient Habitats?

The efficiency of glass eel restocking as a conservation measure to restore the altered local eel stocks has never been evaluated by integrating the dimension of typological diversity of freshwater habitats in eel recruitment performance in terms of the abundance, density, growth, silvering, survival, catchability and eel yields. Here, we used the electrofishing method during a 6-year study to catch eels, and the most appropriate Jolly–Seber model was applied to estimate the demographic parameters in open populations. We found that most eels were yellow eels in the growth phase with a low abundance (eels 3+: 2.8% and eels 5+: 7.1%) of silver eels, which were only males at the MII migrating phase. Eel recruitment performance varied between sector/river habitats. Restocked eels showed annually positive allometric growth type with good length increments and better condition factors. They have survived in almost all sectors with a survival rate > 0.810. Eels were more abundant and denser (maximum 0.128 individuals m−2) in one sector with a high quality of habitats offering optimal living conditions in terms of the protection against predators and water flow, settlement and food availability, as revealed by it having the highest eel yields. In contrast, no eels were found in two sectors whose habitats offered a high threat of predation, poor burial properties and insufficient protection against water flow. Sector/river habitats play a key role in the success of yellow eel production and certainly, over time, future genitor production. This study provides recommendations for the management of eels and their habitats during restocking aimed at the conservation of this threatened species.

Infection with swim bladder nematode Anguillicola crassus in relation to European eel growth, age, and habitat along the German Baltic coast.

One possible reason for the European eel (Anguilla anguilla) population decline is the neozoan eel swim bladder nematode Anguillicola crassus. To investigate whether the prevalence of A. crassus and the associated swim bladder pathology is related to eel habitat, growth rate, and age, 728 yellow eels from 6 habitats differing in salinity and located along the German Baltic coast were examined between 2005 and 2009. The prevalence of A. crassus varied between habitats, ranging from 9 to 57%. Infection prevalence and the percentage of eels with a damaged swim bladder were significantly higher in inner coastal waters compared to more saline open coastal water. In infected eels, 1 to 32 adult and preadult individuals of A. crassus were observed. The mean infection intensity varied between habitats from 2 to 7 nematodes per eel but did not significantly differ between inner and open coastal waters. Infection prevalence and intensity decreased significantly with age when all open coastal waters and all habitats were combined. Both the lower prevalence of A. crassus and the swim bladder damage of older eels and of eels originating from open coastal water habitats suggest that these eels have a higher fitness for spawning migrations than eels from inner coastal waters. The present study underlines the importance of eel screening on a sufficiently small geographical scale for the accurate estimation of eel recruitment and the identification of priority areas that are likely to produce healthy silver eels.

Swimbladder Function in the European Eel Anguilla anguilla

Eels use the swimbladder for buoyancy control. The ductus pneumaticus connecting the esophagus with the swimbladder is closed soon after initial opening of the swimbladder in the glass eel stage, so that eels are functionally physoclist. Subsequent filling of the swimbladder is achieved by activity of gas gland cells in the swimbladder epithelium and countercurrent concentration in the rete mirabile. Gas gland cells produce and release lactic acid and CO2. In blood, acidification induces a release of oxygen from the hemoglobin (Root effect). The resulting increases in PO2 and PCO2 provide diffusion gradients for the diffusion of oxygen and CO2 into the swimbladder, the main gases secreted into the swimbladder. In addition, the partial pressure of these two gases remains elevated in venous blood leaving the swimbladder epithelium and returning to the rete mirabile. Back-diffusion from venous to arterial capillaries in the rete results in countercurrent concentration, allowing for the generation of high gas partial pressures, required for filling the swimbladder under elevated hydrostatic pressure. The transition of the yellow eel to the silver eel stage (silvering) is accompanied by a significant improvement in swimbladder function, but swimbladder volume cannot be kept constant during the daily vertical migrations silver eels perform during their spawning migration back to the spawning grounds in the Sargasso Sea. Infection of the swimbladder with the nematode Anguillicola crassus significantly impairs the function of the swimbladder as a buoyancy organ.

Seasonal activity and depth distribution of resident yellow-phase American eels (Anguilla rostrata) in a large fluvial ecosystem

Abstract The American eel (Anguilla rostrata) is a catadromous species occupying diverse habitats, but little is known about the specific activities of this elusive nocturnal fish. Mobile radio and acoustic telemetry were used to locate 33 transmitter-implanted resident yellow eels (729 ± 22 mm TL), acquiring 1613 locations from 2014 to 2017 in a 21-km2 slow-flowing section of the upper St. Lawrence River. Measurements of distance moved, water depth, and temperature were used to study activity, homing, and site fidelity. Movement was greatest in spring, late April (422 m, median semi-monthly), after emergence from winter dormancy, from their deepest habitat (3.71 ± 0.06 m) to their shallowest (1.74 ± 0.17 m). Activity was low in early summer and early fall (69 m). As temperature reached a maximum during midsummer (24.3 ± 0.29°C), most eels became more active (106 m) and moved deeper (2.46 ± 0.28 m), probably following prey fish. Activity increased in mid-to-late fall (277 m), decreased considerably at ≲8°C when settling into their winter habitat, and ceased at ≲4°C. Temperatures of ≃10°C (8.1–11.1°C) corresponded with the greatest seasonal activity and spring/fall commercial hoop-net catches. These spatial and temporal habitat insights can help focus sampling and assessment procedures, as well as habitat suitability modelling.

Spatial ecology of translocated American Eel (Anguilla rostrata) in a large freshwater lake

American Eel (Anguilla rostrata) undertake extensive migrations from their rearing grounds to spawn in the Sargasso Sea, and historically the upper St. Lawrence River and Lake Ontario provided an important source for large, fecund female eel. Following declines in the Lake Ontario population, glass eel were translocated from eastern Canada from 2006 to 2010. From 2016 to 2018, large, presumably translocated yellow eel (N = 230) with the potential to begin maturing and out-migrating within their year of capture were collected in spring and fall and tagged with acoustic transmitters. Eel were released into eastern Lake Ontario and tracked to better understand their movement patterns prior to and during migration, and the timing of migration. Most eels successfully migrated out of Lake Ontario (64%). Timing of migration was consistent regardless of year or tagging season and primarily occurred in late summer or fall, with cooling water temperatures and decreasing sky illumination associated with initiation for fall tagged eel. Eels were mostly detected in eastern Lake Ontario and those in western Lake Ontario were mostly detected in shallow waters (< 20 m) close to shore. Eels were detected on fewer receivers in the winter, suggesting reduced movements during this season. Finally, larger individuals spent less time in the system, particularly when tagged in the fall. These findings confirm that translocated eels can migrate out of Lake Ontario; however, the weeks when migration occurred were more aligned with timing in their natal range (i.e., eastern Canada) than with naturally recruited eels from Lake Ontario. This temporal mismatch requires further consideration, since it may influence arrival times of translocated eel to the spawning grounds and their recruitment potential. These results can be used to inform future assessments of eel translocation efficacy and can also aid in the design of future tracking studies to more completely explore the downstream migration success of eel translocated into the highly productive waters of Lake Ontario.

A Comment on: Migrating silver eels return from the sea to the river of origin after a false start (2021) by Tambets M et al.

A Comment on: Migrating silver eels return from the sea to the river of origin after a false start (2021) by Tambets M et al.

Habitat use and growth of yellow-stage European eel in coastal and freshwater ecosystems in Norway

Many anguillid eel species display facultative catadromy. Some eel spend their entire life cycle in marine coastal areas, but the geographical extent of this, especially at the extremes of their distributional ranges, is unknown. We analysed otolith Sr:Ca and Ba:Ca from yellow-stage European eel ( Anguilla anguilla) sampled along the coast of Norway and in several freshwater lakes (58°N–63°N), to infer their initial settlement and later life movement patterns with regards to habitat salinity. Most eel (80%) sampled in marine habitats ( n = 371) had settled and remained in marine water, but 20% had moved between marine and freshwater habitats and were hence classified as inter-habitat shifters. Among freshwater sampled eel ( n = 99), 80% had settled and remained in fresh water, but 20% were classified as inter-habitat shifters. The average growth rates for marine water residents, inter-habitat shifters, and freshwater residents were 35, 27, and 17 mm·year–1, respectively. Northern European shallow marine habitats may serve as important yellow eel growth habitats and may be critical to buffer the European eel population against the general decrease in continental recruitment.

Yellow eel (Anguilla anguilla) density trends along the German part of the southern Baltic Sea between 2009 and 2020

According to the recruitment indices provided by ICES for the years 2011–2021, the natural recruitment of the European eel remains at low level. However, fishery-dependent data series indicate an increasing trend in the southern Baltic Sea within recent years. In the light of the pan-European conservation efforts, the question is whether fishery-independent monitoring approaches can verify this positive trend. To monitor yellow eel density, a 1 ha enclosure system for non-tidal coastal waters was used in eight reference areas along the German part of the southern Baltic Sea (Federal State Mecklenburg-Western Pomerania) from 2009 to 2020. Changes in presence rate, mean yellow eel density and total length distribution were investigated. Additionally, the total stock size of yellow eels and the annual settlement rate of juvenile eels were estimated. Overall, the enclosure data indicate a positive trend in yellow eel stock size in recent years. An increasing mean yellow eel density was observed in five out of eight reference areas sampled. The density change was linked to an increased number of juvenile eels as evidenced by a significant change in the total length distribution. The mean annual settlement rate of juvenile eels was estimated as 13.2 eels of age class 0 + ha−1 year−1 in 2012–2017. Besides the increased settlement rate, other mechanisms of population dynamics or conservation measures might have contributed the observed positive trend in yellow eel densities.

Identification of Tropical Freshwater Eels, Anguilla spp., in Cagayan River, Philippines

Anguillid eels are high-value food fish with 16 known species worldwide. Due to its taxonomic ambiguity and insufficient data on species composition, this family of freshwater eels were inadvertently regarded as a single species. This study collected 63 freshwater eels at 3 life stages (glass eel, elver, yellow eel) from Cagayan River, Philippines, to evaluate the accuracy of morphological identification in comparison with molecular analysis. Morphological results displayed clear distinctiveness between Anguilla bicolor and Anguilla marmorata despite some overlapping physical characteristics between Anguilla marmorata and Anguilla luzonensis. Phylogenetic analysis revealed that 15 out of 23 glass eel specimens were molecularly identified as A. marmorata, while 8 were identified as A. bicolor pacifica. For elver specimens, there were A. bicolor pacifica (n = 10), A. marmorata (n = 8), and A. luzonensis (n = 2). For yellow eels, there were A. bicolor pacifica (n = 10) and A. marmorata (n = 10). The results showed the inaccuracy of morphological identification in determining the richness and composition of Anguillids, which otherwise proved that molecular analysis is a more reliable approach in identifying freshwater eel species.

Overwintering behaviour of yellow-stage European eel (Anguilla anguilla) in a natural marine fjord system

Like many animals, northern temperate eel can enter a hibernation-like state and become dormant during the winter. Knowledge of overwintering behaviour in eel is sparse and mainly based on anecdotal observations and a few experimental studies on thermal tolerance. We studied European eel (Anguilla anguilla) overwintering behaviour in a Skagerrak fjord in Southern Norway, during three consecutive years, using an array of acoustic receivers and acoustic tags with depth and temperature sensors. We obtained results from 55 yellow eel, of which 19 were studied for one winter, 35 for two winters and one for three winters. Dormancy was inferred to begin in September for the earliest individuals and lasted until May for the last, with the majority of eel dormant from at least late October–November until mid-April. The timing of dormancy was mainly related to photoperiod and less to temperature. More than 50% of eel became dormant when day length was <9 h and became active when day length was >14 h. Approximately 10% of eel remained active during the winter and 31% of eel changed their pattern between consecutive years. Some dormant individuals exhibited activity periods that interrupted their dormancy. Eel in the outer fjord nearer the open sea became dormant before eel in the inner more freshwater part of the fjord, and were dormant longer.

The study of European eel, Anguilla anguilla in the RiverNeretva estuary (Eastern Adriatic Sea, Croatia) using traditional fishery gear

Samples of the European eel, Anguilla anguilla from two different habitats in the River Neretva estuary region were analysed using two different traditional fishery gear, in the period from 2016 to 2019. The monthly eel catches in ten traditional fyke nets in Parila lagoon varied from 4.4 to 12.7 kg (±2.45 kg) in 2016 and 2.4 to 9.6 kg (±2.01 kg) in 2019. There were no statistical differences between monthly and total weights recorded in 2016 and 2019 (t=1.04; p=0.312). The b-coefficient of the length-weight (LW) relationship for the freshwater eel samples varied from 3,1036 to 3,3206 duringtwo-year period of the sampling. The LW relationship for the brackish population was significantly lower (b-coefficient 2,6513). The dominant eel stage in both sites was yellow eel, while silver stage was scarce. The fishery with traditional devices revealed that efficiency of these artisanal tools is low, pointing to poor abundance of eel in their common habitats. The recorded catch using bigger traditional eel trap gear during autumn spawning migrations was also dominated with yellow stage, while silver eel represented only 15,4% of the catch. The infestation of eels with the parasite,Anguillicoloides crassus was significantly higher in freshwater habitat (41% of eel specimens infested) than in brackish, Parila lagoon (7%). Poor efficiency of traditional gear, bad length and weight structure of the population and low b-coefficients of eels in their most important habitat, indicate poor status of European eel population and River Neretva estuarine ecosystems.

Morphological and gonadal histological characteristics of the silver-phase male Indo-Pacific eel Anguilla marmorata.

Eight silver-phase male Indo-Pacific eels (giant mottled eels) Anguilla marmorata were collected from three small rivers on subtropical Amami-Oshima Island, Japan, and their morphological and gonadal-histological characteristics were examined. The total length, body mass and age of the silver eels were 543.8 ± 49.3 (496-659)mm, 376.0 ± 116.0 (282-660)g and 14.6 ± 2.3 (11-19)years old, respectively. The silvering-related characteristics (eye index, pectoral-fin index, gut-somatic index) and reproductive characteristics (gonadosomatic index, histological stage) of the silver eels were significantly advanced compared to those of yellow eels, which is similar to other anguillid species.