Anguillid Eels Research Articles

Effectiveness of a Strobing Light System for Deflecting Downstream Migrating European Silver Eels (Anguilla anguilla)

ABSTRACTAnthropogenic structures, such as dams, delay, injure and kill during downstream migrating silver‐phase Anguillid eels. To enhance spawner escapement from rivers, efforts are being made to develop devices that guide eels towards safe passage routes. We tested the ability of a strobing light array to deflect downstream migrating European silver eels (Anguilla anguilla) by comparing the distribution of eel catches in a series of nets when lights were on (treatment periods) and off (control periods). Deflection efficiency of the light array was 80.6% (95% CL = 74.3%–85.3%), estimated by comparing the proportion of the total biomass captured in nets located immediately downstream of the light array during control and treatment periods. Eels caught during treatment periods (Mean ± S.E.; 452 ± 11 mm) were significantly smaller than those caught during control periods (538 ± 8 mm), which suggested that smaller eels were less capable of avoiding the light array. Future research should determine specifications for operation of strobe light arrays, including effects of light intensity and strobe rate, angle of array deployment, and effects of environmental factors on deflection efficiency.

Reconstructing the experienced temperature during the larval oceanic migration of anguillid eels from otolith stable oxygen isotopes

Migration routes and the depth patterns of anguillid eel larvae migrating long distances from spawning grounds in the ocean remain poorly understood. We used otolith stable isotope analysis to study the oceanic migrations of anguillid eels by reconstructing experienced water temperature histories of larvae. The otolith stable oxygen isotopes (δ18Ootolith) of recruited Anguilla japonica glass eels were analyzed to assess the relationship with the experienced water temperature of the early larval stage in laboratory experiments. A negative linear relationship between rearing water temperature and δ18Ootolith values was observed for eel leptocephali reared at five different temperatures between 19 °C and 27 °C. Subsequently, the obtained equation between water temperature and δ18Ootolith was applied to estimate the water temperature experienced by wild-caught glass eels during their larval oceanic migration according to recruitment latitude, season, and species. The δ18Ootolith values of A. japonica glass eels were significantly higher at higher recruitment latitudes from Taiwan to Japan and later recruitment periods of 6 months, from November to March, indicating that the individuals were exposed to lower water temperatures along the Kuroshio flowing northward as they reached the higher latitudes in the later recruitment seasons. Furthermore, the δ18Ootolith values of the temperate A. japonica were higher than those of the tropical Anguilla marmorata, recruited at the same locations in southern Japan and Taiwan. This suggests that larval migratory behavior in the ocean may differ between the two eel species, although they have sympatric spawning areas in the western North Pacific. Collectively, these results suggest that δ18Ootolith provides a reasonable estimation of the experienced water temperature and may prove useful for reconstructing the early migratory history of anguillid eels in the ocean.

Interannual early life history variation and larval dispersal of three tropical anguillid eel species recruiting in French Polynesia

Interannual early life history variation and larval dispersal of three tropical anguillid eel species recruiting in French Polynesia

Environmental DNA Metabarcoding Details the Spatial Structure of a Diverse Tropical Fish Assemblage in a Major East African River System

ABSTRACTManagement and conservation of species‐rich tropical freshwater systems require reliable information on the diversity and distribution of species present. Here, we used environmental DNA metabarcoding to reveal the diversity of the fishes in the Rufiji River catchment of central Tanzania. Across 174 samples from 49 sites, and using a newly developed reference library, we mapped the presence of 66 fish species from an estimated 91 that we are confident are present in the system. We found clear evidence of community structuring of the assemblage linked to key environmental gradients—elevation, temperature, and turbidity. We also identified core distributions of rare or threatened taxa, including migratory species such as the anguillid eels. With a focused analysis of 50 samples collected over a small spatial scale (<2 km) from the Kilombero River, we showed that each single sample can capture an average of 23.1 species, while three samples can capture 39.4 species, from a total of 56 species encountered in the 50 samples. Collectively the results help to identify species vulnerable to ongoing change in the catchment, including dam construction and agricultural intensification. The results clearly demonstrate how eDNA‐based metabarcoding can reliably describe the diversity and distributions of riverine fish species across a catchment, providing standardized information that will be valuable for environmental management.

Genetic Diversity and Distribution of Haplotypes of Freshwater Eel in Baltic Lakeland Based on Mitochondrial DNA D-Loop and Cytochrome b Sequence Variation

The European eel (Anguilla anguilla (L.)) is a unique catadromous euryhaline fish and the only anguillid eel species classified as ‘critically endangered’. The Baltic Lakeland area contains naturally recruited and introduced eels. There is great uncertainty regarding a baseline for the location and number of naturally dispersed eels in the region. Little is known about the genetic structure of the European eel populations in Baltic countries. The estimation of population genetic structure is important for the efficient management of naturally recruited and introduced eels. Two mitochondrial regions were used to investigate the genetic structure within and between eel samples from 11 waterbodies. In this study, new, unique, and widely distributed haplotypes were revealed. The studied eel population in the Baltic Lakeland shows high genetic diversity, which is possibly a result of intensive restocking programs. Sequences characterized for Anguilla rostrata were revealed in both mitochondrial regions. Understanding the genetic structure of eel populations worldwide is crucial for conservation efforts. Eel restocking in waterbodies where natural migration is restricted contributes to diversity loss for the world gene pool of eels.

Highly contiguous genome assembly and gene annotation of the short-finned eel (Anguilla bicolor pacifica)

In East Asia, anguillid eels are commercially important. However, unlike other species, they have not been successfully cultivated throughout their lifecycle. Facing population decline due to overharvesting and environmental pressures, the industry is turning to alternatives, such as Anguilla bicolor pacifica (short-finned eel). However, genomic data for short-finned eels are unavailable. Here, we present in-depth whole-genome sequencing results for short-finned eel obtained using two sequencing platforms (PacBio Revio, and Illumina). In this study, we achieved a highly contiguous genome assembly of the short-finned eel, comprising 19 pseudochromosomes encompassing 99.76% of the 1.087 Gb genome sequence with an N50 of 16.88 and 61.07 Mb from contig and scaffold, respectively. Transcripts from four different tissues led to the annotation of 23,095 protein-coding genes in the eel genome, 98.66% of which were functionally annotated. This high-quality genome assembly, along with the annotation data, provides a foundation for future functional genomic studies of short-finned eels.

Rethinking fish-friendliness of pumps by shifting focus to both safe and timely fish passage for effective conservation

Globally, catadromous freshwater eels of the genus Anguilla are of conservation concern, including critically endangered European eel (Anguilla anguilla). Pumping stations that move river water to a higher elevation severely impact eels during their seaward spawning migration. Fish-friendly pumps can mitigate fish injury and mortality but here we uniquely rethink a fish-friendly pump as a fish passage solution. In this pluriannual study, the seasonal timing of pump operation was misaligned with the typical silver eel migration period. Eels were almost exclusively nocturnal but night-time pumping represented as little as 5.6% a year. Night-time eel approaches were primarily influenced by pump duration and temperature, but did not align with lunar phase, unlike in unregulated rivers. After reaching the pumping station, eel passage was influenced by weedscreen aperture and increased when the aperture was increased. Passive sensor collision suggested non-pump infrastructure could cause injury and mortality to eels. It is therefore recommended pump operation should align with the timing of silver eel migration, weedscreen and pump entrance efficiencies should be maximised, and non-pump infrastructure must have low fish injury risk. Ultimately, considering the entire structure a fish passage solution will help ensure fish-friendly pumps have high conservation value for anguillid eels globally.

Ecology and conservation of growth-phase Anguillid eels

Ecology and conservation of growth-phase Anguillid eels

Stable Production of a Recombinant Single-Chain Eel Follicle-Stimulating Hormone Analog in CHO DG44 Cells.

This study aimed to produce single-chain recombinant Anguillid eel follicle-stimulating hormone (rec-eel FSH) analogs with high activity in Cricetulus griseus ovary DG44 (CHO DG44) cells. We recently reported that an O-linked glycosylated carboxyl-terminal peptide (CTP) of the equine chorionic gonadotropin (eCG) β-subunit contributes to high activity and time-dependent secretion in mammalian cells. We constructed a mutant (FSH-M), in which a linker including the eCG β-subunit CTP region (amino acids 115-149) was inserted between the β-subunit and α-subunit of wild-type single-chain eel FSH (FSH-wt). Plasmids containing eel FSH-wt and eel FSH-M were transfected into CHO DG44 cells, and single cells expressing each protein were isolated from 10 and 7 clones. Secretion increased gradually during the cultivation period and peaked at 4000-5000 ng/mL on day 9. The molecular weight of eel FSH-wt was 34-40 kDa, whereas that of eel FSH-M increased substantially, with two bands at 39-46 kDa. Treatment with PNGase F to remove the N glycosylation sites decreased the molecular weight remarkably to approximately 8 kDa. The EC50 value and maximal responsiveness of eel FSH-M were approximately 1.23- and 1.06-fold higher than those of eel FSH-wt, indicating that the mutant showed slightly higher biological activity. Phosphorylated extracellular-regulated kinase (pERK1/2) activation exhibited a sharp peak at 5 min, followed by a rapid decline. These findings indicate that the new rec-eel FSH molecule with the eCG β-subunit CTP linker shows potent activity and could be produced in massive quantities using the stable CHO DG44 cell system.

High genetic diversity in anguillid glass eels across three Southeast Asian countries revealed through mitochondrial DNA D-loop sequences, with notes on population structure

Understanding the genetic diversity and population structure of anguillid eels is crucial for effective management. In this study, we used the mitochondrial DNA (mtDNA) D-loop marker to identify anguillid glass eels and assessed their genetic diversity across six sampling sites spanning three Southeast Asian countries. We also examined the population structures of Anguilla marmorata and A. celebesensis. We validated nine distinct species and subspecies of anguillid eels from 465 specimens collected in Indonesia (n = 246), the Philippines (n = 119), and Vietnam (n = 100). High gene diversities were observed across the species, ranging from 0.979 (A. celebesensis) to 1.000 (A. bicolor bicolor, A. marmorata, and A. interioris). Correspondingly, nucleotide diversity values were relatively low, varying from 0.036 (A. marmorata) to 0.057 (A. interioris). A. marmorata (n = 162) displayed a broad distribution, being observed across all six sites, with a significant yet low average fixation index of 0.05755. Pairwise comparisons of FST values between Cimandiri and other locations (except for Cagayan) were relatively high and statistically significant (p < 0.01), indicating significant genetic differentiation of Cimandiri samples compared to most locations in this study. On the other hand, A. celebesensis (n = 110), exhibited a non-significant and low fixation index (0.007), indicating no genetic differentiation in samples from Poso and Poigar Rivers, Indonesia. Our findings highlight the essential need for coordinated management strategies among regions that share common stocks. Additionally, this study underscores the efficacy of the mtDNA D-loop in precisely identifying glass eel species/subspecies and elucidating the genetic diversity and population structure of tropical anguillid eels.

Transcriptome analysis of the Japanese eel (Anguilla japonica) during larval metamorphosis

BackgroundAnguillid eels spend their larval period as leptocephalus larvae that have a unique and specialized body form with leaf-like and transparent features, and they undergo drastic metamorphosis to juvenile glass eels. Less is known about the transition of leptocephali to the glass eel stage, because it is difficult to catch the metamorphosing larvae in the open ocean. However, recent advances in rearing techniques for the Japanese eel have made it possible to study the larval metamorphosis of anguillid eels. In the present study, we investigated the dynamics of gene expression during the metamorphosis of Japanese eel leptocephali using RNA sequencing.ResultsDuring metamorphosis, Japanese eels were classified into 7 developmental stages according to their morphological characteristics, and RNA sequencing was used to collect gene expression data from each stage. A total of 354.8 million clean reads were generated from the body and 365.5 million from the head, after the processing of raw reads. For filtering of genes that characterize developmental stages, a classification model created by a Random Forest algorithm was built. Using the importance of explanatory variables feature obtained from the created model, we identified 46 genes selected in the body and 169 genes selected in the head that were defined as the “most characteristic genes” during eel metamorphosis. Next, network analysis and subsequently gene clustering were conducted using the most characteristic genes and their correlated genes, and then 6 clusters in the body and 5 clusters in the head were constructed. Then, the characteristics of the clusters were revealed by Gene Ontology (GO) enrichment analysis. The expression patterns and GO terms of each stage were consistent with previous observations and experiments during the larval metamorphosis of the Japanese eel.ConclusionGenome and transcriptome resources have been generated for metamorphosing Japanese eels. Genes that characterized metamorphosis of the Japanese eel were identified through statistical modeling by a Random Forest algorithm. The functions of these genes were consistent with previous observations and experiments during the metamorphosis of anguillid eels.

Larger yellow-phase Japanese eels show short-range homing in a freshwater river.

The homing behavior and site fidelity to habitats in various fishes, including anguillid eels (genus Anguilla), are fascinating. However, little is known about how yellow-phase eels exhibit homing behavior and the sensory mechanisms involved. Using acoustic telemetry, we investigated the homing behavior of 18 Japanese eels, A. japonica, with total lengths ranging from 204 to 570 mm, in a narrow freshwater river in inland central Japan, where salinity gradient, tidal current, and magnetic sense cannot be used for their homing, but where olfaction could play a role. The tagged eels captured upstream and downstream were released downstream and upstream, respectively. The results showed that large eels, over approximately 400 mm in total length, exhibited homing behavior to their original sampling locations (likely to shelters and foraging sites, where they probably spent a longer time than in other locations and grew successfully) from outside their home ranges, predominantly during the dark period. Homing success was not affected by the two capture locations, indicating that eels did not use olfactory cues for short-range homing in freshwater rivers.

Environmental DNA Reveals Geographic Distributions of Two eel Species, Anguilla japonica and A. marmorata, in Western Kyushu, Japan.

Some anguillid eels migrate thousands of kilometers from their spawning grounds, dispersing across vast geographic areas to fresh and brackish water habitats, where they settle and grow. Japanese eels (Anguilla japonica) and giant mottled eels (A. marmorata) are both found in Japan, although their distributions differ, and their exact distributions are poorly known. We assumed that topographic distribution patterns of Japanese and giant mottled eels must differ among and within rivers along the northwest coast of Kyushu, Japan. Environmental DNA (eDNA) analysis was conducted at 87 sites in 23 rivers. Japanese eel eDNA was detected in 19 rivers (82.6%) and that of giant mottled eels was detected in eight (34.8%). We detected giant mottled eel eDNA in five rivers where they were previously unknown. eDNA for Japanese eels was detected at six of nine sites in the north (66.7%), 13 of 23 sites in Omura (56.5%), and 37 of 55 sites in the south (67.3%). In contrast, giant mottled eel eDNA was detected at one of nine sites in the north (11.1%), no sites in Omura, and 15 of 55 sites in the south (27.3%). There was no correlation between eDNA concentrations of the two species at 10 sites in the five rivers where eDNA of both species was detected. These findings suggest differences in the distribution of the two eel species and the northern distributional limit of giant mottled eels in the area facing the East China Sea.

Factors influencing migration of short-finned eels (Anguilla australis) over 3 years from a wetland system, Lake Condah, south-east Australia, downstream to the sea.

Anguillid eel populations are under threat globally. A particularly vulnerable life-cycle stage is the migration of mature adult eels downstream from freshwater habitats through estuaries into the sea to spawn. This study investigated the factors associated with downstream migration of the short-finned eel Anguilla australis (Richardson 1841) from a coastal wetland (Lake Condah) in south-east Australia, using acoustic telemetry. Migration was associated with time of the year, higher water level and river flows, decreasing water temperature, and darker moon phases. Larger individuals and those in better condition were more likely to migrate from the wetland. Downstream migration peaked in spring, in contrast to the typical autumn migration period for other temperate anguillids. Variable responses, in comparison to other studies, highlight how migration cues may not be universal. In south-east Australia, short-finned eels may have evolved to migrate in multiple phases by first migrating to the estuary during typical seasonal spring flow pulses (e.g., to avoid being stranded in upland reaches during dry summer periods) and then migrating into the ocean in autumn. More research is needed to unravel these processes and causes, especially considering that the relationship between migration and hydrology may be complex and confounded (e.g., by human-induced disruptions to migratory pathways).

Social network analysis as a tool to inform anguillid eel conservation and management

Abstract Eel Management Plans demand European silver eel (Anguilla anguilla) escapement to the sea of at least 40% of that expected historically in the absence of human impacts. Landlocked lentic waterbodies, such as drinking water reservoirs, host substantial numbers of eel, which could represent a significant contribution to catchment-based conservation targets. To optimize netting strategies for eel management policies, information on their aggregation behaviour is currently needed but lacking. We performed a fine-scale acoustic tracking study to monitor the movements of 86 European eel in a UK reservoir. Social network sampling and sensitivity analyses were used to assess whether eel aggregate at scales relevant for current capture techniques. European eel were found to aggregate at spatial and temporal scales of 50 m and 2 days, respectively, which complements current capture methodologies and recommendations. Aggregations were not driven by fixed resources, indicating that other factors, such as sociality, may drive aggregation behaviour. Results also show that current netting practices could be optimized by increasing netting lengths from 50 to 80 m. In addition to aiding conservation and management protocols, these results provide an ecological foundation for exploring the role of social behaviour in this Critically Endangered species.

Future tasks for the conservation and sustainable use of Japanese eel: A review of the stocking/restocking of Anguillid eel species

Future tasks for the conservation and sustainable use of Japanese eel: A review of the stocking/restocking of Anguillid eel species

An Anguillid lens: how Eels reconnect people and waterways

The population decline of American Eels (Anguilla rostrata) in North America represents multiple crises: ecosystem destruction, industrial impact, forced relocation, cultural disenfranchisement, water degradation, and biodiversity loss. Once a key component of Indigenous societies, trade routes, economies, and ceremonial cycles, today Eels experience migration barriers and habitat degradation stemming from waterway abuse and land use alteration. The development of agricultural and industrial social norms has led to dramatic reductions in Eel populations across North America. Fishery agencies familiar with Eel life cycles often list species as depleted, and disappearance of Eels is well-accepted in all sectors. Related species of Anguillid Eels are in decline globally. Yet, in some areas, American Eels continue to migrate, grow, and even thrive. This article proposes Eel continuity as powerful survivorship in troubled times, a counter-narrative to overwhelming stories of environmental pessimism. Although Eels are often discredited, reviled, and disrespected by Americans and Canadians alike, their contributions to marine and aquatic zones are underestimated and poorly understood. This article employs a reflexive method of dialogue and commentary from two Eel advocates with interdisciplinary training and backgrounds, who envision relational, wisdom-based practices that meaningfully integrate Indigenous, community, academic, and other forms of knowledge about Eels and about water.

Barcoding a glass eel migration: a snapshot of taxa migrating upstream in the Palu River, Central Sulawesi, Indonesia

Eels of the genus Anguilla are high value food fish with a catadromous life cycle. As tropical anguillid glass eels migrate to freshwater in multi-species schools, effective management of these and other diadromous resources requires reliable data. Previous research using two morphological methods concluded that between three and five anguillid eels recruit to the Palu River in Central Sulawesi, Indonesia with interannual and seasonal variations in species composition, size structure, and co-migrating taxa. Some glass eels and other taxa could not be identified based on external morphology. To clarify the taxonomic uncertainty regarding species recruiting to the Palu River this study applied DNA barcoding using the Cytochrome Oxidase I (COI) mitochondrial DNA (mtDNA) molecular marker. Taxa identified from the first sampling of glass eels and other taxa recruiting in these mixed schools included three anguillid eels, five other fishes, two crustacea and one mollusc, presenting a snapshot of taxa migrating upstream in the Palu River. Current GenBank accessions did not enable species level identification of two fish larvae (families Eleotridae and Tetraroginae). The sequences will add to Indonesian accessions in the GenBank repository and inform resource management, including the use of environmental DNA (eDNA) methods for monitoring Indonesian anguillid eels.

Tidal and circadian patterns of European eel during their spawning migration in the North Sea and the English Channel

Technological advances in tracking methods enable the mapping of anguillid eel migration routes from continental habitats to their spawning sites in the ocean. However, the behaviour and orientation abilities of anguillids are still poorly understood, and have only rarely been studied on the continental shelf. Here we present the results of a study into the vertical and horizontal movement behaviour of 42 European eels (Anguilla anguilla L.) tagged with electronic tags that migrated through the North Sea and English Channel towards and into the Atlantic Ocean during their spawning migration. We used actograms, periodograms and linear mixed effects models to determine the periodicity and significance of the timing and pattern of vertical movement and activity. Overall, eels had a complex behavioural repertoire that included classical diel vertical migration (DVM), reverse DVM and vertical movement behaviours that synchronized with tidal patterns. All of the eels that were tracked showed one or more of these behaviours during their time at liberty, and many exhibited all of them. We also observed that the eels had a higher horizontal migration speed when the current in the favourable direction was stronger. This, together with the vertical movement synchronized with the tides, suggests the eels adopt selective tidal stream transport. Finally, tracked eels had a higher vertical movement range at night compared to daytime. We hypothesize that these behaviours are driven by bio-energetic efficient movement, navigation and predator avoidance.

Comparison of Reproductive Aspects of the Tropical Eel Anguilla bicolor (McClelland 1884) in Freshwater and Estuarine Habitats

The tropical anguillid eel, Anguilla bicolor McClelland, experiences significant growth up to the adult size in both freshwater and estuarine ecosystems, encountering salinity gradients that may impact their reproduction. Therefore, this study aimed to investigate the reproductive aspects of A. bicolor collected from Serayu River (freshwater) and Segara Anakan (estuary), Cilacap, Central Java, during July 2020 field surveys. The examined parameters included eye index (EI), fin index (FI), gonadosomatic index (GSI), hepatosomatic index (HSI), and sex ratio of eel specimens in freshwater and estuary. Furthermore, the total length (TL) and body weight (BW) ranged from 260 - 630 mm and 18 - 419 g in freshwater, and 260 - 630 mm and 18 - 447 g in estuary. Significant differences (P&lt;0.01) were observed in the reproductive characteristics between the two habitats (P&lt;0.01), with higher eye index, fin index, gonadosomatic index, and hepatosomatic index in freshwater-caught eels than the values obtained in estuary inhabitants. The mean of these four parameters were 6.99 ± 2.98%, 4.08 ± 0.71%, 1.70 ± 1.13%, and 1.92% ± 1.07 in freshwater, while 3.48 ± 1.60%, 3.45 ± 0.479%, 1.12 ± 0.23%, and 0.28 ± 0.19% were found in estuary. The sex ratio was significantly affected by habitat, with female eels dominating in freshwater (62.5%) compared to estuary (32%). These results suggested that eels in freshwater habitats might exhibit early gonadal maturation compared to those in estuary.