Search Results (15)

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 × L^2.732, with silver-stage eels showing a higher growth rate compared to yellow-stage eels, which had a relationship of W = 0.0184 × L^2.397. 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. Full article

The southern coast of Central Java Province is one of Indonesia’s largest eels (Anguilla spp.) fishing grounds. The annual eel catches in this region showed an increasing trend in 2011–2014 but then plummeted in 2019–2021. However, studies on stock status are lacking to provide an effective management plan to prevent the collapse of eel fisheries. Therefore, this study assessed the state of freshwater eels in Southern Central Java using a data-limited method, catch-based maximum sustainable yield (CMSY). The analysis revealed a swift stock depletion, shifting from a healthy state in 2011 to a concerning red zone in 2015–2018. Subsequently, the stock began a recovery phase in 2019, but the recent trajectory raised concerns as it approached the red zone again, signaling the urgency of implementing a rebuilding plan. The simulation of several rebuilding scenarios suggests that reducing fishing mortality to 80% of the F_MSY (maximum sustainable fishing mortality) can balance production and conservation objectives, achieving the quickest stock rebuilding with minimal catch loss. This study showed that eel stock could be rebuilt by reducing fishing pressure to preserve a surplus of individuals who can complete their migration and spawning cycles. Full article

The European eel, Anguilla anguilla (Linnaeus, 1758), is a species of high conservation and commercial value. Also, with a high aquacultural value, it was one of the most farmed species in Europe before its decline. Conservation measures for this species are currently being implemented, some of which are the release of adults to allow them to migrate to spawning grounds and artificial reproduction to minimise the impact of overfishing. Much progress has been made regarding the closing of the life cycle, but several problems remain to be solved, such as the weaning and growth of larvae. In this regard, the study of local populations and the identification of the best spawners could be a good way forward as it would help to identify spawners with high reproductive potential and good offspring quality. In this study, we compared the reproductive performance of female eels from migratory areas of the Northern Adriatic Sea at different maturation stages (10 pre-migrant stage; 10 migrant stage), treating both groups with the same hormonal protocol (weekly carp pituitary extract, injection of 17α,20β-Dihydroxy-4-pregnen-3-one at complete oocyte maturation). The research showed that eels at the beginning of metamorphosis (SI III), i.e., eels not yet ready to undertake migration, achieve reproductive performance equal to that of eels ready for migration (SI IV). Their performance was on par in both qualitative and quantitative terms. The optimal results in hatching (with values of 65.8 ± 3.2% for pre-migrant and 68.2 ± 4.1% for migrant) and survival rates (with value of 25.7 ± 5.4% pre-migrants; 27.2 ± 3.7% migrants), as well as the results about the time to reach full gonadal maturation, may have positive implications when considering release and restocking measures. Full article

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

Fish poisoning (FP) affects human health, trade and livelihood in Fiji, where management has depended mainly on traditional ecological knowledge (TEK). This paper investigated and documented this TEK through a 2-day stakeholder workshop, group consultation, in-depth interviews, field observations, and analyses of survey data from the Ministry of Fisheries, Fiji. Six TEK topics were identified and classified as preventative and treatment options. The preventive approach involves identifying toxic reef fishes, the spawning season of edible seaworms, hotspot areas of toxic fishes, folk tests, and locating and removing toxic organs. For example, 34 reef fish species were identified as toxic. The FP season was associated with the spawning of balolo (edible seaworm) and the warmer months of October to April (cyclone seasons). Two well-known toxic hotspots associated with an abundance of bulewa (soft coral) were identified. Folk tests and locating and removing toxic fish organs are also practised for moray eels and pufferfish. At the same time, various locally available herbal plants are used to treat FP as the second line of defence. The TEK collated in this work can help local authorities better identify the sources of toxicity, and applying TEK preventive measures could stem the tide of fish poisoning in Fiji. Full article

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

European eel is thought to be a symbol of the effects of global change on aquatic biodiversity. The species has persisted for millions of years and faced drastic environmental fluctuations thanks to its phenotypic plasticity. However, the species has recently declined to historically low levels under synergistic human pressures. Sublethal chemical contamination has been shown to alter reproductive capacity, but the impacts and required actions are not fully addressed by conservation plans. This paper proposes a modelling approach to quantify the effects of sublethal contamination by anthropogenic pollutants on the expression of life history traits and related fitness of the critically endangered European eel. Material and Methods: We sampled female silver eels from eight different catchments across Europe previously shown to be representative of the spectrum of environmental variability and contamination. We measured 11 fitness-related life history traits within four main categories: fecundity, adaptability and plasticity, migratory readiness, and spawning potential. We used machine learning in models to explore the phenotypic reaction (expression of these life history traits) according to geographical parameters, parasite burdens (the introduced nematode Anguillicoloides crassus) and anthropogenic contaminants (persistent organic pollutants (POPs) in muscular tissue and trace elements (TEs) in gonads, livers and muscles). Finally, we simulated, the effects of two management scenarios—contamination reduction and contamination increase—on the fecundity and recruitment. Results: Contamination in our sampling was shown to have a stronger control on life history traits than do geographic and environmental factors that are currently described in the literature. We modelled different contamination scenarios to assess the benefit of mitigation: these scenarios suggest that reducing pollutants concentrations to the lowest values that occurred in our sampling design would double the fecundity of eels compared to the current situation. Discussion: Remediation of contamination could represent a viable management option for increasing the resilience of eel populations, with much more effects than solely reducing fishing mortality. More broadly, our work provides an innovative way for quantitative assessment of the reaction norms of species’ biological traits and related fecundity to contamination by organic and inorganic pollutions thus opening new management and conservation pathways to revert the erosion of biodiversity. Full article

The aim of this project is to promote the recovery of ecological connectivity within 10–20 years in the lower Ebre River and Delta, and a healthy and sustainable population of diadromous fish, including European sturgeon (Acipenser sturio), European eel (Anguilla anguilla), twaite shad (Alosa fallax) and sea lamprey (Petromyzon marinus), among other aquatic native species. It is focused on long-term sustainable investments, adapting all present river obstacles to allow fish migration, increasing the river spawning habitat availability and the distribution and growth areas of migrating fish. The main activities of the LIFE MIGRATOEBRE Project (LIFE13 NAT/ES/000237) are: (1) Build a fish ramp at Xerta’s weir (located at 58 km from the Mediterranean sea; in 2023), and a fish ramp at Ascó’s weir (located at 104 km from the sea; built in 2017), and monitor them regularly. (2) Apply ship locks fish-friendly improved management at Xerta’s weir (located at 56 km from the sea), and monitor it regularly. (3) Undertake a pilot project of European sturgeon restocking through an experimental release in the lower Ebre (in 2023). (4) Develop a communication campaign and a community involvement plan for students, the general public, farmers, fishermen, anglers, electric companies, tourism stakeholders and regional and local authorities. This included the production of a great temporary exhibition and a network of volunteers. The potential spawning and feeding areas for European sturgeon, twaite shad and sea lamprey were identified and cartographed. Extensive monitoring of the target fish population of the lower Ebre river was carried out (2000 fish were marquet with PIT tags). Between 2017 and 2020, 150 fish (Chelon labrosus, Mugil cephalus, Alosa fallax and Anguilla anguilla) were monitored by acoustic telemetry (using 47 Vemco receivers located along the river and at in the lagoons of the Ebre delta). Obtained results indicate that the Ebre is optimal for the recovery of the target species of this project, but at the same time it would be very important to carry out a control of invasive species. This LIFE project started on 1 July 2014 and, after several extensions, it will finish on 30 June 2024. More information can be found at: www.migratoebre.eu. Full article

Anguillid eels are the iconic example of catadromous fishes, because of their long-distance offshore spawning migrations. They are also a good model for research on the onset mechanisms of migrations to breeding areas, because the migrations begin in inland waters. When eels transform from yellow eels to silver eels, it is called silvering. Silver eels show various synchronous external and internal changes during silvering, that include coloration changes, eye-size increases, and gonadal development, which appear to be pre-adaptations to the oceanic environment and for reproductive maturation. A strong gonadotropic axis activation occurs during silvering, whereas somatotropic and thyrotropic axes are not activated. Among various hormones, 11-ketotestosterone (11-KT) drastically increases during spawning migration onset. Gradual water temperature decreases simulating the autumn migratory season, inducing 11-KT increases. Administration of 11-KT appeared to cause changes related to silvering, such as early-stage oocyte growth and eye enlargement. Moreover, 11-KT may be an endogenous factor that elevates the migratory drive needed for the spawning migration onset. These findings suggested that water temperature decreases cause 11-KT to increase in autumn and this induces silvering and increases migratory drive. In addition, we newly report that 11-KT is associated with a corticotropin-releasing hormone that influences migratory behavior of salmonids. This evidence that 11-KT might be among the most important factors in the spawning migration onset of anguillid eels can help provide useful knowledge for understanding endocrinological mechanisms of the initiation of spawning migrations. Full article

Anguillid eels grow in freshwater but spawn in the open ocean. Almost all of them undertake long migrations, consisting of several thousands of kilometers, between their feeding and their distant oceanic spawning areas. The cues that guide eels over long distances to the spawning area are unknown. The Earth’s magnetic field is one, if not the only, reliable cue that can guide them between these areas. To test whether the use of magnetic cues is compatible with what we know about the life-history and migration of eels, the patterns of magnetic inclination and intensity along the migratory routes of five anguillid species were investigated. Regardless of the species and the differing routes between life stages, larvae of those species always drift along paths of increasing magnetic inclination and intensity, while adults follow reverse gradients. This is consistent with an imprinting/retracing hypothesis. The proposed navigation mechanism suggests that larvae imprint the target magnetic intensity or inclination isoline value upon hatching, and then years later retrace the magnetic gradient until they reach the target isoline value which they can follow to find their conspecifics for reproduction. Such a mechanism does not require a high level of precision to find a specific area but does require imprinting of the magnetic gradient experienced during the early life of the eel. There is already evidence for the imprinting of a magnetic compass direction in glass eels as well as yellow and silver eels. Knowledge about the orientation cues and biological mechanisms used by marine organisms to navigate and orient are important for taking appropriate management steps that are likely to help the conservation of vulnerable or endangered species. Full article

Spatio-temporal variability of marine soundscapes reflects environmental dynamics and local habitat health. This study characterizes the coastal soundscape of the Cres-Lošinj Natura 2000 Site of Community Importance, encompassing the non-tourist (11–15 March 2020) and the tourist (26–30 July 2020) season. A total of 240 h of continuous recordings was manually analyzed and the abundance of animal vocalizations and boat noise was obtained; sound pressure levels were calculated for the low (63–2000 Hz) and high (2000–20,000 Hz) frequency range. Two fish sound types were drivers of both seasonal and diel variability of the low-frequency soundscape. The first is emitted by the cryptic Roche’s snake blenny (Ophidion rochei), while the second, whose emitter remains unknown, was previously only described in canyons and coralligenous habitats of the Western Mediterranean Sea. The high-frequency bands were characterized by bottlenose dolphin (Tursiops truncatus) vocalizations, indicating dolphins’ use of area for various purposes. Boat noise, however, dominated the local soundscape along the whole considered periods and higher sound pressure levels were found during the Tourist season. Human-generated noise pollution, which has been previously found 10 years ago, is still present in the area and this urges management actions. Full article

Adult American eels (Anguilla rostrata) are vulnerable to hydropower turbine mortality during outmigration from growth habitat in inland waters to the ocean where they spawn. Imaging sonar is a reliable and proven technology for monitoring of fish passage and migration; however, there is no efficient automated method for eel detection. We designed a deep learning model for automated detection of adult American eels from sonar data. The method employs convolution neural network (CNN) to distinguish between 14 images of eels and non-eel objects. Prior to image classification with CNN, background subtraction and wavelet denoising were applied to enhance sonar images. The CNN model was first trained and tested on data obtained from a laboratory experiment, which yielded overall accuracies of >98% for image-based classification. Then, the model was trained and tested on field data that were obtained near the Iroquois Dam located on the St. Lawrence River; the accuracy achieved was commensurate with that of human experts. Full article

Multiple lines of evidence substantiate the existence of a very large aggregation of the basketwork eel, Diastobranchus capensis, on the small (3 km²) Patience Seamount off southeast Australia. The aggregation appears to be present year-round, but largest in the austral autumn when composed of spawning eels. Twenty eels caught in April 2015 (14 female, 6 male) were all in advanced stages of spawning condition. The eel’s abundance in the aggregation was very high as measured at seamount, local and regional scales. Hydroacoustic measurement of the spawning aggregation’s dimensions (~100 × 1000 m) and conservative counts of 100 s of eels along camera transects of ~1000–2000 m length indicate 10,000 s individual eels may have been present. The absence of other known spawning locations indicates the Patience Seamount is a regional-scale spatial anchor for spawning. The aggregation was protected in a marine park in 2007 following a decades-long impact from bottom trawling, indicating that the population can be expected to stabilise and recover. Monitoring the aggregation’s status, and validating seasonal spawning, provide important opportunities to examine conservation-led recovery in the deep sea as part of Australia’s new national strategy of Monitoring, Evaluation, Reporting and Improvement (MERI) for conservation values within marine parks. Full article

During the long migration from river habitats to the spawning ground, the Japanese eel undergoes sexual maturation. This spawning migration occurs concurrently with morphological changes, such as increases in eye size; however, the mechanisms by which sex steroids and their receptors influence these changes in peripheral tissues remain unclear. The aim of this study was to investigate changes in the eyes of female Japanese eels during sexual maturation, and our research focused on estrogen receptor (ER)α and ERβ transcripts. During ovarian development, the gonadosomatic index increased and yolk-laden oocytes developed rapidly. These changes occurred in conjunction with a steady increase in plasma levels of estradiol-17β (E₂). Concomitant increases in transcript levels of ERα and ERβ in eye, brain, pituitary, and ovary were also observed. Fluorescence in-situ hybridization analyses revealed that ERα and ERβ transcripts were present in the choriocapillary layer and photoreceptor layer of the eyes, and the analysis also revealed that their signals in these layers became stronger in mature females compared to those observed in immature females, suggesting that under the influence of gonadotropins, morphological changes in the eyes are regulated by E₂ through the activation of its receptors. In conclusion, E₂ plays a crucial role in physiological adaptations that occur in peripheral tissues during the spawning migration. Full article

The Japanese eel (Anguilla japonica), European eel (Anguilla anguilla), and American eel (Anguilla rostrata) are migratory, catadromous, temperate zone fish sharing several common life cycle features. The population genetics of panmixia in these eel species has already been investigated. Our extensive population genetics analysis was based on 1400 Gb of whole-genome sequence (WGS) data from 84 eels. It demonstrated that a Japanese eel group from the Kuma River differed from other populations of the same species. Even after removing the potential adapted/selected single nucleotide polymorphism (SNP) data, and with very small differences (fixation index [Fst] = 0.01), we obtained results consistently indicating that panmixia does not occur in Japanese eels. The life cycle of the Japanese eel is well-established and the Kuma River is in the center of its habitat. Nevertheless, simple reproductive isolation is not the probable cause of non-panmixia in this species. We propose that the combination of spawning area subdivision, philopatry, and habitat preference/avoidance accounts for the non-panmixia in the Japanese eel population. We named this hypothesis the “reproductive isolation like subset mapping” (RISM) model. This finding may be indicative of the initial stages of sympatric speciation in these eels. Full article