Research

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11 pages, 2186 KiB

Open AccessArticle

The Eel Ascending: The Influence of Lateral Slope, Climbing Substrate and Flow Rate on Eel Pass Performance

by Adam T. Piper, Paula J. Rosewarne, Charlotte Pike and Rosalind M. Wright

Fishes 2023, 8(12), 612; https://doi.org/10.3390/fishes8120612 - 18 Dec 2023

Cited by 1 | Viewed by 1461

Optimising the design of passage facilities to restore fluvial connectivity for juvenile European eel (Anguilla anguilla) is a key priority within conservation efforts for the species, across the majority of its freshwater range. Employing an experimental setup that simulated gravity-fed upstream [...] Read more.

Optimising the design of passage facilities to restore fluvial connectivity for juvenile European eel (Anguilla anguilla) is a key priority within conservation efforts for the species, across the majority of its freshwater range. Employing an experimental setup that simulated gravity-fed upstream eel passes, this study demonstrated that novel V-profile passes, which incorporate two lateral slopes (15°), performed better than laterally flat passes over the flow rates tested (0.2–0.6 L s⁻¹). For the small eel size used (60–80 mm length), the bristle substrate consistently outperformed studs, but the lateral slope had a greater effect on passage metrics than the substrate choice. Our findings strongly support the use of V-shaped channels for upstream migrating eel at fish passage facilities, particularly in scenarios where flow rates may be elevated and/or fluctuating, such as for gravity-fed passes. Full article

(This article belongs to the Special Issue Biology and Ecology of Eels)

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13 pages, 1818 KiB

Open AccessArticle

Detecting Japanese Eels (Anguilla japonica) and Revealing Their Distribution in Taiwanese Rivers by Environmental DNA Analysis

by Hsiang-Yi Hsu, Kai-Jen Wu and Yu-San Han

Fishes 2023, 8(10), 483; https://doi.org/10.3390/fishes8100483 - 27 Sep 2023

Viewed by 922

Abstract

The Japanese eel (Anguilla japonica) is the most prevalent freshwater eel species in Taiwan. However, its population has undergone a significant decline in recent decades due to factors such as overfishing, habitat destruction, and the effects of climate change. Urgent action [...] Read more.

The Japanese eel (Anguilla japonica) is the most prevalent freshwater eel species in Taiwan. However, its population has undergone a significant decline in recent decades due to factors such as overfishing, habitat destruction, and the effects of climate change. Urgent action is needed to conserve this species. Before implementing conservation measures, it is imperative to ascertain the distribution of Japanese eels in Taiwan’s rivers. This study’s primary objective was to assess the effectiveness of eDNA analysis as a method for detecting Japanese eels. To achieve this goal, we compared eDNA analysis data with results obtained from electrofishing, with the Fengshan and Shimen Rivers serving as our designated test sites. Additionally, we collected water samples from 34 other rivers across Taiwan to comprehensively assess the species’ wider distribution using eDNA analysis. Our findings demonstrated eDNA analysis’s viability for detecting Japanese eels. Of the 36 rivers tested, Japanese eel DNA was detected in samples from 21 rivers, scattered across northern, eastern, southern, and western Taiwan, with no specific concentration in any region. We also noted reduced detectability of Japanese eel DNA in highly polluted rivers, indicating that river pollution may have a potential impact on their population. In the future, expanding eDNA analysis to more rivers could identify additional rivers that Japanese eels inhabit. Subsequently, resource management and conservation efforts can be focused on these identified habitats. Furthermore, developing advanced eDNA-based methods for estimating the abundance or biomass of Japanese eels could enhance the flexibility of management and conservation measures. Full article

(This article belongs to the Special Issue Biology and Ecology of Eels)

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13 pages, 1861 KiB

Open AccessArticle

Estuarine-Specific Migration of Glass Eels in the Ems Estuary

by Jeroen B. J. Huisman, Henry J. Kuipers, Leopold A. J. Nagelkerke, Peter Paul Schollema and Inge van der Knaap

Fishes 2023, 8(8), 392; https://doi.org/10.3390/fishes8080392 - 27 Jul 2023

Cited by 1 | Viewed by 876

Abstract

Understanding recruitment of glass eels in estuaries is crucial for the conservation of the European eel (Anguilla anguilla). However, basic knowledge on estuarine-specific glass eel migration, including in estuarine harbours, is mostly lacking. Therefore, we studied glass eel migration in the [...] Read more.

Understanding recruitment of glass eels in estuaries is crucial for the conservation of the European eel (Anguilla anguilla). However, basic knowledge on estuarine-specific glass eel migration, including in estuarine harbours, is mostly lacking. Therefore, we studied glass eel migration in the Dutch–German Ems estuary and the harbour at Delfzijl (The Netherlands) and tagged glass eels with Visual Implant Elastomer tags (VIE tags). We released 2000 tagged glass eels into the Ems estuary itself and 1000 tagged glass eels into the tidal harbour at Delfzijl. At three estuarine locations, i.e., Delfzijl–Duurswold, Termunterzijl, and Nieuwe Statenzijl, glass eel collectors were strategically placed, each location being progressively situated further upstream in the Ems estuary. Most glass eels (n_untagged = 97,089, n_tagged = 74) were caught at Nieuwe Statenzijl, although this location is much further upstream. Lower numbers of glass eels (n_untagged = 1856, n _tagged = 31) were caught at Delfzijl–Duurswold and Termunterzijl (n_untagged = 1192, n_tagged = 7). Glass eels arrived approximately a week earlier at Nieuwe Statenzijl than at the other two locations, and the migration speed of tagged glass eels was highest at Nieuwe Statenzijl (>2 km/day) and lower (<1 km/day) at Delfzijl–Duurswold. Our study highlights that migration and the resulting potential recruitment of glass eels in estuaries and harbours may vary considerably both spatially and temporally. Further research on estuarine-specific factors that influence glass eel migration, such as the (anthropogenically altered) tidal action and flow, will provide valuable information on what influences glass eel migration in estuaries. Full article

(This article belongs to the Special Issue Biology and Ecology of Eels)

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18 pages, 3063 KiB

Open AccessFeature PaperArticle

Exploring European Eel Anguilla anguilla (L.) Habitat Differences Using Otolith Analysis in Central-Western Mediterranean Rivers and Coastal Lagoons from Sardinia

by Cinzia Podda, Jacopo Culurgioni, Riccardo Diciotti, Francesco Palmas, Elsa Amilhat, Elisabeth Faliex, Fabien Morat, Nicola Fois and Andrea Sabatini

Fishes 2023, 8(8), 386; https://doi.org/10.3390/fishes8080386 - 26 Jul 2023

Viewed by 1404

Abstract

An otolith shape and morphometric analysis was performed on European eel (Anguilla anguilla) subpopulations from five rivers and three coastal lagoons of Sardinia (central-western Mediterranean) to assess the role of different habitats on otolith development. Sagittal otolith shape was described by [...] Read more.

An otolith shape and morphometric analysis was performed on European eel (Anguilla anguilla) subpopulations from five rivers and three coastal lagoons of Sardinia (central-western Mediterranean) to assess the role of different habitats on otolith development. Sagittal otolith shape was described by 11 harmonics from elliptic Fourier descriptors. Comparisons among the harmonics were run through canonical discriminant analyses (CDAs). The CDA reclassification rate (75.7%) demonstrated a spatial environmental discrimination among local eel subpopulations of Sardinia. The Euclidean distance values demonstrated a dissimilarity between the river and lagoon groups. The form factor and roundness shape indices were significantly higher in the river group than in the lagoon group. The distances of the first three rings to the otolith core revealed site-specific otolith development. Moreover, the annual otolith growth rate was faster in the lagoon group than in the river group. The differences among the studied sites in terms of sagittal otolith shape could relate to changes in different local stocks potentially related to environmental peculiarities. Establishing a direct correlation between otolith morphology and environmental factors is challenging, and further studies are needed to investigate the relationship between habitat type/environmental variation and growth/body characteristics of eels. Nevertheless, the achieved results suggest that this method can be considered to be a valuable tool for studying the ontogeny of the European eel. Full article

(This article belongs to the Special Issue Biology and Ecology of Eels)

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16 pages, 5009 KiB

Open AccessFeature PaperArticle

Human Chorionic Gonadotropin Enhancement of Early Maturation and Consequences for Reproductive Success of Feminized European Eel (Anguilla anguilla)

by Arjan P. Palstra, Ida van de Ven, Pauline Jéhannet, Leo Kruijt, Henk Schipper, William Swinkels and Leon T. N. Heinsbroek

Fishes 2023, 8(6), 281; https://doi.org/10.3390/fishes8060281 - 25 May 2023

Cited by 1 | Viewed by 1136

Abstract

To induce oocyte development, eels are weekly injected with salmon or carp pituitary extract (CPE). The weekly handling and hormone peaks result in inferior oocyte quality; therefore, alternative treatments that improve oocyte quality and reproductive success require investigation. The enhancement of early sexual [...] Read more.

To induce oocyte development, eels are weekly injected with salmon or carp pituitary extract (CPE). The weekly handling and hormone peaks result in inferior oocyte quality; therefore, alternative treatments that improve oocyte quality and reproductive success require investigation. The enhancement of early sexual maturation by a single injection with human chorionic gonadotropin (hCG), administered prior to CPE treatment, was investigated. Fifty feminized eels were subjected to simulated migration, after which eels received either a hCG or a sham injection. After two months, the hCG-treated eels showed an increase in eye size, gonadosomatic index (GSI), and plasma 11-ketotestosterone concentration, when compared with the sham-injected controls. The hCG-treated eels showed increases in oocyte diameter and lipid area, and in ovarian expression of aromatase (cyp19), follicle stimulating hormone receptor (fshr) and lipoprotein lipase (lpl). Yolk was present in the oocytes of the hCG-treated eels, not yet in the oocytes of the controls. The hCG-induced deposition of yolk may relate to early-life treatment with 17β-estradiol during feminization. hCG-treated eels required four CPE injections less to mature than the controls. hCG treatment may benefit reproductive success in feminized eels by initiating vitellogenesis and reducing the hypophysation period, although larvae were obtained from most females in both groups. Full article

(This article belongs to the Special Issue Biology and Ecology of Eels)

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16 pages, 1620 KiB

Open AccessEditor’s ChoiceArticle

Biomass Quantification of the Critically Endangered European eel from Running Waters Using Environmental DNA

by Sara Fernandez, Álvaro Gutiérrez, Dumas Deconinck, Jose Luis Martinez, Almudena Alvarez, Isabel Marquez, Gonzalo Machado-Schiaffino and Eva Garcia-Vazquez

Fishes 2023, 8(6), 279; https://doi.org/10.3390/fishes8060279 - 24 May 2023

Cited by 1 | Viewed by 1761

Abstract

The European eel Anguilla anguilla is a critically endangered catadromous species. There is an urgent need for close surveillance of the populations that are still viable in European rivers. The species is difficult to observe in freshwater because of its bottom-dwelling behavior; the [...] Read more.

The European eel Anguilla anguilla is a critically endangered catadromous species. There is an urgent need for close surveillance of the populations that are still viable in European rivers. The species is difficult to observe in freshwater because of its bottom-dwelling behavior; the currently employed methods of eel monitoring in Europe based on the physical capture of individuals are stressful and may cause mortality. Here, we present a new highly sensitive method based on an A. anguilla-specific qPCR marker designed within the cytochrome oxidase I mitochondrial gene for application on environmental DNA (eDNA). Since the detectability of eDNA depends on the hydrographic conditions, we applied correction for altitude and a linear model and were able to predict the eel biomass from the eDNA in the different rivers of northern Spain still holding wild populations. The method was validated by electrofishing surveys. This novel eDNA-based marker allows for estimating the European eel biomass in running waters from small 1.5 L water samples and could complement, or replace in some cases, current eel surveys without disturbing wild populations. Full article

(This article belongs to the Special Issue Biology and Ecology of Eels)

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15 pages, 2826 KiB

Open AccessArticle

Temporal Pattern of the Occurrence of Japanese Glass Eels (Anguilla japonica) in the Pearl River Estuary

by Fangmin Shuai, Jie Li, Shunchao Yu and Jian Yang

Fishes 2023, 8(5), 256; https://doi.org/10.3390/fishes8050256 - 11 May 2023

Cited by 1 | Viewed by 2253

Abstract

Japanese eels (Anguilla japonica) are a typical migratory fish species with high commercial importance. The Pearl River estuary in southern China is an important natural growing ground for Japanese glass eels, but limited information on Japanese glass eel population characteristics is [...] Read more.

Japanese eels (Anguilla japonica) are a typical migratory fish species with high commercial importance. The Pearl River estuary in southern China is an important natural growing ground for Japanese glass eels, but limited information on Japanese glass eel population characteristics is available, despite their ecological importance. In this paper, we examined the annual patterns of the occurrence of Japanese glass eels in the Pearl River estuary from 2011 to 2022. The most frequently occurring Japanese glass eel’s total length is 5.3 cm. The collecting period extended from December to February, and the collection catch-per-unit-effort (CPUE) decreased significantly from 2011 to 2022. The generalized linear model (GLM) indicated that daily changes in Japanese glass eel collection were significantly affected by tidal range, water temperature, and lunar distance. The catch peak appeared when the tidal range rose to 1.7 m, and the water temperature dropped below 8 °C on the full moon days. Overall CPUE analysis showed no significant periodic and inter-annual variability in the period 2011–2022, with the ARIMA model suggesting that the CPUE is expected to remain stable but low in the coming years (2023–2026), although recruitment ultimately depends on the overall spawning stock. Full article

(This article belongs to the Special Issue Biology and Ecology of Eels)

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21 pages, 3097 KiB

Open AccessArticle

Recombinant Gonadotropins to Induce Oocyte Development In Vitro and In Vivo in the European Eel Anguilla anguilla

by Pauline Jéhannet, Arjan P. Palstra, Ignacio Giménez Nebot, Henk Schipper, William Swinkels, Leon T. N. Heinsbroek and Hans Komen

Fishes 2023, 8(3), 123; https://doi.org/10.3390/fishes8030123 - 22 Feb 2023

Cited by 4 | Viewed by 2207

Abstract

Commonly, female European eels are injected weekly with pituitary extract (PE) from carp (CPE) or salmon (SPE) to induce sexual maturation. However, a PE is a mixture of gonadotropins and other hormones that are not specific for eel and rapidly cleared from circulation. [...] Read more.

Commonly, female European eels are injected weekly with pituitary extract (PE) from carp (CPE) or salmon (SPE) to induce sexual maturation. However, a PE is a mixture of gonadotropins and other hormones that are not specific for eel and rapidly cleared from circulation. The aim of this study was therefore to test the effects of highly stable eel-specific recombinant gonadotropins (rGTHs) on oocyte development in vitro and in vivo in European eels. For the in vitro trial, the dose–effect responses of maturing eel oocytes on CPE and recombinant luteinizing hormone (rLH) were studied before and after 12 and 18 h of incubation. For the in vivo experiment, sexual maturation was stimulated by treatment with (i) CPE, (ii) recombinant follicle-stimulating hormone (rFSH) followed by CPE and (iii) rFSH followed by rLH. For the in vitro experiment, the expression of the nuclear progestin receptor 2 (pgr2) was induced by rLH, implying that rLH was preparing the oocyte for ovulation. For the in vivo experiment, the females treated with rGTHs had high gonadosomatic index (GSI) values (rFSH-CPE: 75, 77; rFSH-rLH: 80) in comparison with the females injected with CPE (50–60), suggesting that rFSH strongly induced vitellogenic growth. Larvae were produced for all treatment groups and for the first time by rGTH treatment alone but dose and timing still need optimization. Full article

(This article belongs to the Special Issue Biology and Ecology of Eels)

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Review

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18 pages, 4176 KiB

Open AccessReview

Side Effects of Human Drug Use: An Overview of the Consequences of Eels’ Exposure to Cocaine

by Luigi Rosati, Ivana Caputo, Lillà Lionetti, Mayana Karoline Fontes, Camilo Dias Seabra Pereira and Anna Capaldo

Fishes 2023, 8(3), 166; https://doi.org/10.3390/fishes8030166 - 17 Mar 2023

Viewed by 2035

Abstract

The widespread use of drugs is a global problem which affects not only humans but also the environment around them, as research is showing the presence of these substances in different environmental matrices, like air, water, and soil. Above all, due to the [...] Read more.

The widespread use of drugs is a global problem which affects not only humans but also the environment around them, as research is showing the presence of these substances in different environmental matrices, like air, water, and soil. Above all, due to the remarkable pharmacological properties of drugs, it is discovered that organisms accidentally exposed to them, as aquatic organisms, undergo behavioral and physiological changes that can compromise their health, survival, and reproduction ability. In addition to this, we must consider the ability of some drugs to accumulate within these organisms, thus entering the food chain, and the possible interactions that drugs in water can establish with each other and with other possible pollutants, making the final effects on exposed organisms unpredictable. This article is an overview of the effects of one of these drugs, cocaine, one of the drugs commonly found in the aquatic environment, on European eel, an endangered species and known biomonitor of aquatic contamination. Full article

(This article belongs to the Special Issue Biology and Ecology of Eels)

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13 pages, 2726 KiB

Open AccessFeature PaperReview

Swimbladder Function in the European Eel Anguilla anguilla

by Bernd Pelster

Fishes 2023, 8(3), 125; https://doi.org/10.3390/fishes8030125 - 22 Feb 2023

Cited by 2 | Viewed by 1876

Abstract

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 [...] Read more.

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. Full article

(This article belongs to the Special Issue Biology and Ecology of Eels)

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11 pages, 949 KiB

Open AccessReview

Footprints of Natural Selection in North Atlantic Eels: A Review

by José Martin Pujolar, Francesca Bertolini and Magnus W. Jacobsen

Fishes 2022, 7(6), 311; https://doi.org/10.3390/fishes7060311 - 28 Oct 2022

Cited by 1 | Viewed by 2010

Abstract

The study of natural selection and local adaptation is a thriving field of research. Local adaptation is driven by environment components and results in locally adapted phenotypes with higher fitness relative to other phenotypes from other locations in the species range. Tests of [...] Read more.

The study of natural selection and local adaptation is a thriving field of research. Local adaptation is driven by environment components and results in locally adapted phenotypes with higher fitness relative to other phenotypes from other locations in the species range. Tests of local adaptations have traditionally been done using transplant experiments, but the advent of next-generation sequencing methods have allowed the study of local adaptation to move from a phenotypic to a genomic approach. By using genome scans and state-of-the-art statistical tests, researchers can identify genes putatively under selection and study the genomic architecture of local adaptation, which often includes the observation of clustering of adaptive genes concentrated in fewer genomic regions known as “genomic islands of divergence”. The two species of North Atlantic eels, the European and the American eel, are excellent species for studying selection since they are panmictic and present large population sizes, show a wide distribution range across extremely heterogenous environments, and are subject to high mortalities. We reviewed studies of natural selection and local adaptation in American eel, European eel, between life cycle stages, between European and American eel. Finally, we discussed genome architecture in relation to local adaptation in eels and the role of both genetic (i.e., local adaptation) and non-genetic (i.e., phenotypic plasticity) in the survival of eels across their distribution range. Full article

(This article belongs to the Special Issue Biology and Ecology of Eels)

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