Eel Population Research Articles

Migratory life cycle of Anguilla anguilla: a mirror symmetry with A. japonica.

The European and Japanese eel populations have declined significantly in recent decades. To effectively manage and conserve them, gaining a better understanding of their migratory life cycles is important. Previous research on the spawning ecology and larval dispersal of European and Japanese eels has led to many significant discoveries and advancements for their migratory life cycles. However, different scholars hold varying views on their migratory life cycles, especially concerning the European eel, therefore this article aims to provide a comprehensive review of research from multiple disciplines concerning the spawning ecology and larval dispersal of European and Japanese eels and to propose migratory life cycles of these two species. The migratory life cycle of the European eel is as follows: European silver eels undertake a year-long spawning migration from September to January to reach the Sargasso Sea for spawning before the next spawning season, typically between December and May. After hatching, European eel leptocephali are transported by the Gulf Stream, Frontal Countercurrents, North Atlantic Current, and Azores Current and return to Europe and North Africa for growth. Recruitment of European glass eels mainly occurs between October and June of the following year, and the recruiting season is more concentrated in countries closer to the spawning area and more dispersed in countries farther away. The consistent recruitment pattern and the growth rate of leptocephali suggest a larval transport period, also called larval duration, of around 1 year. Understanding the migratory life cycle of European eels can facilitate the evaluation or development of their conservation measures.

The origin and evolution of European eel rhabdovirus dominant genotype.

The Eel Virus European X (EVEX) is a significant pathogen contributing to the decline of eel populations. As an important evolutionary driving force, it is crucial to understand whether homologous recombination (HR)occurs between EVEXs for revealing the evolutionary patterns of the virus. This study indicates that HR may enhance genetic diversity and accelerate the evolution and spread of EVEX. Phylogenetic analysis reveals that the current popular EVEX is primarily composed of a dominant recombinant genotype. Further investigation suggests that recombination events, which likely occurred approximately 54 years ago, may alter codon preferences, highlighting the adaptive advantages this provides and enhancing the virus's ability to infect its eel host. The emergence of this advantageous genotype may be driven by environmental selection pressures, consistent with natural selection principles. In summary, our findings suggest that HR might plays an important role in EVEX evolution, facilitating its adaptation to changing environmental conditions.

Trophic niche of a nonnative invader and environmental drivers of its increasing populations in the coastal Everglades

AbstractThe Florida Everglades is a critically important, but highly threatened ecosystem that is becoming increasingly susceptible to the invasion of non-native species. This study investigated the ecological role of the invasive peacock eel (Macrognathus siamensis) within this ecosystem using 15 years of electrofishing data and stable isotope analysis. We investigated the population trends of peacock eels at the marsh-mangrove ecotone of the Shark River Estuary, the environmental factors contributing to their abundance, and the potential interactions they may have with native fish assemblages and coastal food webs. We used stable isotope analysis to provide insights into the basal resource contribution to peacock eels and hypervolume analysis to determine peacock eel trophic niche size and overlap with native species. Results of this study found that peacock eel abundance has rapidly increased, and their populations are strongly related to hydroclimatic regimes. Peacock eel abundance was positively associated with warmer water temperatures and greater marsh inundation periods. The trophic niche of peacock eels was significantly smaller in volume than that of native sunfishes (Lepomis spp.) indicating lower intraspecific resource use variability and suggesting a limited potential for inter-specific competition with these taxa. However, in recent years, the catch of peacock eels has outnumbered the catch of all native sunfishes combined. The feeding habits and pervasiveness of peacock eels in the coastal Everglades could lead to a decrease in abundance of benthic prey items targeted by peacock eels and alter food web dynamics in the system. Based on these data, peacock eel populations are predicted to continue to increase, highlighting the importance of continued monitoring of their potential impact on native fish assemblages and food webs.

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.

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).

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

Glass eels at the continental edge of Europe: revisiting catchment recruitment at the international River Minho/Miño

The glass eel catch from the international River Minho, western Iberian Peninsula, forms the southernmost Atlantic series in the composite index outside the North Sea assessing European eel recruitment. Here, new experimental yield data from the lower estuary (1981–2022) and fishers´ daily records (1990–2022) were modelled to describe and compare seasonal and interannual trends. Seasonality matched river discharge climatology, possibly a more general feature in the southern range of the species distribution, with the difference between high and low season becoming less marked in recent periods of lower abundance. Glass eel yield showed a sharp decline during the 1980s, in line with the recruitment index outside the North Sea and with total Minho catch, but not with recent local estimates of catch per unit effort. This decline is corroborated by trends in fishers´ daily records, suggesting that the reduction in effort in the Minho was the consequence of a gradual adjustment to recruitment in the catchment mediated by progressively tighter local management measures. The inefficiency of fisheries regulatory measures to guarantee stock recovery alone calls for more comprehensive measures to reverse anthropogenic impacts on continental eel populations, but the task seems Herculean.

Recruitment Patterns and Environmental Sensitivity of Glass Eels of Anguilla japonica in the Yangtze Estuary, China.

The decline of Japanese eel (Anguilla japonica) populations in the Yangtze River estuary represents a critical conservation concern. Eleven-years of daily catch data during recruitment periods (i.e., January-April, 2012-2022) indicate that annual catch averaged from 153 to 1108 eels, and show a bimodal pattern in glass eel arrivals. Utilizing seasonal-trend decomposition and generalized additive models, we demonstrated a strong correlation between catch abundance, optimal water temperatures, and lunar cycles. An auto-regressive integrated moving average (ARIMA) model predicts an increase in glass eel numbers for 2023-2024 but also points to a concerning trend of delayed recruitment timing since 2016, attributable to the 0.48 °C per decade rise in sea surface temperatures. This delay correlates with a significant decrease in the average body weight of glass eels, suggesting potential energy deficits that may hinder successful upstream migration. This study not only furthers our understanding of glass eel recruitment dynamics but also underscores the urgent need for targeted conservation measures. Additionally, it highlights the importance of sustained, detailed monitoring to mitigate the detrimental effects of climate change on these eels, vital for preserving the Yangtze River's ecological integrity.

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.

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.

An app as a tool for detecting migrant females of Anguilla anguilla to support the wild population

The eel (Anguilla anguilla) population in Europe has declined dramatically since the 1980s and shows no signs of recovery. There are several threats to this species: migratory barriers, loss of habitats, hydroelectric dams, overfishing, and illegal trade. As a result, the eel has become part of the ICUN endangered species and is protected by various institutions. Information data on the maturation stage is needed to monitor silver eel escapement and assess population trends. In the sampling activities spanning from 2012 to 2022 silver index was calculated on 1852 eels in the northern Adriatic Sea. With the help of machine learning technology, we trained an algorithm in pupil recognition and the calculation of horizontal eye diameter in eels. This study allowed us: a) to identify a single parameter to discriminate the sexual maturity of the eel and thus to know the female with a migratory instinct; b) to use this parameter as a proxy to develop an easy and user-friendly app for all management operators.

Diet and trophic interactions between catadromous eels and sympatric fish in Kenyan east flowing river systems

Biotic interactions such as competition and predation are important ecological drivers of population structure. Interactions among higher trophic level fish can contribute to further population declines in species, such as eels, made vulnerable by overexploitation or environmental change. Furthermore, trophic interactions may further predispose eel populations to collapse, but this is poorly understood, particularly along the Western Indian Ocean (WIO) rivers. This study evaluated stomach contents of fish captured with glass and commercial fyke nets in the Athi and Ramisi Rivers, which discharge into the WIO. Stomach contents were examined using dissecting microscope to establish diet composition. Eels primarily consumed assorted fish (43 %), and crustaceans (36 %); such as penaeid shrimp (14%) and prawns (13%) and crab (9%), thus belonged to a higher trophic level (TL) of 3.47 than native (2.98) or introduced (2.8) sympatric fish species. Diet breadth of eels was significantly lower (0.20) than for sympatric fish species (0.27), attributed to higher diet specialization. The TL of carnivorous fish (3.19) and their diet compared well with those of eels, even though diet preference differed significantly among fish types. Consequently, eels ranked as vulnerable by the IUCN are further threatened by previously undescribed competition from carnivorous fish.

The evolving story of catadromy in the European eel (Anguilla anguilla)

Abstract Anguillid eels were once considered to be the classic example of catadromy. However, alternative life cycles have been reported, including skipping the freshwater phase and habitat shifting between fresh, brackish, and saltwater throughout the growth phase. There is a lack of knowledge regarding these alternate life strategies, for example, the proportion of individuals in the population that adopt them compared to classic catadromy. We provide a description of these alternate life cycle strategies in temperate anguillids, their possible drivers, and the methods available to investigate them. These methods (lethal and non-lethal), include otolith microchemistry, fatty acid and stable isotope analyses, parasite identification, blood transcriptomics, and electronic tags. We argue that since the current management framework for the European eel and other temperate eels is based mainly on the freshwater component of the population, it ignores eels growing in saline waters. Many of the factors that are thought to be responsible for the precipitous decline of the eel population are more prevalent in freshwater systems. Therefore, the contribution of saline eels may be more important than currently estimated. The habitat-shifting ability of eels may be all the more crucial for the persistence and recovery of those species that are endangered.

Overestimating management progress—modelled vs. monitored silver eel escapement in a North Sea draining river

Abstract The success of European efforts towards the recovery of the European eel (Anguilla anguilla) population will rely on accurate assessments of local stock status for the implementation of conservation measures. Yet, direct and continuous monitoring of the escapement of potential spawners (“silver eels”) is unfeasible in most habitats. Therefore, population models are widely used to estimate local silver eel escapement, but require input information on recruitment, demographic characteristics, and mortalities that are often estimated with great uncertainties. We conducted a combined mark–recapture and acoustic telemetry study across two migration seasons to quantify the actual silver eel escapement in a sub-catchment of the German river Ems. Results were compared with predictions from the demographic model used to provide stock parameters in Germany according to the EU eel-regulation. Mark–recapture results suggested an annual female silver eel escapement of ∼15–17 tons, while the demographic model predicted 90–98 tons, indicating a considerable overestimation. Our results suggest that realistic prediction of silver eel escapement is hardly feasible without high-quality input information and highlight the need for site-specific model calibrations against monitoring data. Overestimations of local stock sizes are problematic if they obscure the necessity for adequate conservation measures, hindering their implementation.

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.

Patterns in longitudinal distribution of American Eel (Anguilla rostrata) population characteristics in rivers of Puerto Rico

AbstractAmerican Eel (Anguilla rostrata) population characteristics and distribution remain drastically understudied in the Caribbean region. We conducted the first island‐wide study to evaluate the distribution of density, length, and sex in relation to distance from the mouth of 23 stream reaches in five rivers in Puerto Rico. We found 287 American Eel at a mean density of 156.7 fish/ha. Sex ratios favored females (52:3) but most of the eel sampled were undifferentiated or undetermined. Generalized linear modeling indicated that upstream habitats with low eel density were optimal environments for production of large and female eel. Our findings indicate that Puerto Rico could significantly contribute to the spawning population and strengthen knowledge of American Eel in the Caribbean. Our findings could be used to guide local and range‐wide conservation efforts to ensure stream connectivity and access to optimal upstream habitat for survival of the population.

Climate Change Vulnerability Assessment of the Eel Fishery in Aparri, Cagayan, Philippines

The global glass eel population is continuously declining over the years. Apart from the anthropogenic factors, the global production decline suggests that the changing climatic condition may have influenced its downtrend. This study assessed the climate change vulnerability of the eel fishery in Aparri, Cagayan, being one of the major collection areas in the Philippines. Vulnerability assessment (VA) was conducted using the Fisheries Vulnerability Assessment Tool (FishVool) through focus group discussion (FGD) with 25 participants and key informant interview (KII) with 50 respondents whose experiences are up to 10 years. Results showed that the overall climate change vulnerability of the eel fishery in Aparri, Cagayan, Philippines is high due to high exposure (4.76) to extreme weather events but medium in sensitivity (3.35) and adaptive capacity (3.2). The results of this study provided valuable information to government fisheries planners, policymakers, and other fisheries stakeholders in developing policies and management plans that will reduce vulnerability and build climate-resilient eel fishery.

GENETIC DIVERSITY AND POPULATION STRUCTURE OF THE EUROPEAN EEL (ANGUILLA ANGUILLA) IN BALTIC LAKELAND

Anguilla anguilla (Linnaeus, 1758) is the unique catadromous fish species in the Latvian fauna. The Baltic Lakeland area representing naturally recruited and introduced eels. In recent decades, its population has decreased not only in Latvia, but also in Europe. European eel critically endangered due to overfishing of glass eels, blocking of migratory paths, deaths in HPS turbines, water pollution and diseases and parasites. Only some bodies of water are freely accessible to natural migration of eels in Latvia.Currently, its position has been recognized as being critical, and a range of normative acts have been adopted for its restoration, such as the Regulation EC 1100/2007. The restocking programs of the European eel Anguilla anguilla have been conducted for nearly one century in Latvia.This study provides the first data on population structure of freshwater eels in Baltic Lakeland, for use in eel conservation and management of aquaculture on a regional and/or global scale. By analysing the sequences of the Cyt b gene of mtDNA for individuals caught in locations throughout in Baltic Lakeland, we determined the population genetic structure of A. anguilla in the area. The diversity of haplotypes was studied in ten waterbodies from part of Baltic Lakeland, namely Lake Sīvers, Lake Usmas, Lake Ķišezers, Lake Liepājas, Lake Alūksnes, Lake Rāznas, Lake Vialikija Švakšty, Lake Svir, Lake Myadzyel, Myadzelka river. Additionally, this study investigated the affinity of the Latvian populations to other A. anguilla populations around the world.This is the first report about eel’s population genetic diversity in Baltic Lakeland. Haplotype variation was different in all investigated waterbodies. In current studies seven new unique haplotypes were detected. Eel population in Baltic Lakeland shows quite high genetic diversity and rapid population expansions, which possibly is results of intensive restocking program. Sequences characterized Anguilla rostrata in Lake Alūksnes were detected.

К вопросу обоснования величины коэффициента промыслового возврата европейского угря (Anguilla anguilla) в заливах Калининградской области

The cessation of stocking of the Vistula Lagoon with eels by Poland in the period from 1995 to 2004, and the ongoing depression of the eel population, led to a decrease in eel catches in both parts of the bay to 4–5 tons / year by the end of the first, most of the second decade of the 21st century. The tendency of eel population decrease was even more pronounced in the Curonian Lagoon, which has never been stocked with eel in historical retrospective. Eel catches in the Russian part of the bay fell from 150 tons in 1970 to 3-4 tons by the 1990s. and less than 1 ton in the present century. Stocking pasture reservoirs with young eels is a way not only to preserve eel stocks, but also to reasonably increase catches. In solving this problem, the question of the value of the commercial return coefficient becomes.

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

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.