Assessment of the environmental drivers of European glass eel (Anguilla anguilla) recruitment in transitional waters

The European eel (Anguilla anguilla) has undergone an unprecedented population decline since the 1980s, with current recruitment levels fluctuating from 3 to 15% of historical levels for the last 20 years. Monitoring of glass eels and elvers as 0?+?recruitment is an essential step in helping to understand the trend in recruitment and to better quantify the current recruitment time series. Two locations within the Shannon estuary on the west coast of Ireland were monitored for glass eel recruitment from January to April in 2017 and 2018. This study used a generalised linear mixed model to examine a range of environmental variables impacting on glass eel abundance in transitional waters. Results found that water temperature and moon phase were the most important variables. Tidal height and cloud cover also influenced the abundance of glass eels but to a lesser extent. This study found that focussing survey efforts on nights around the full moon when water temperatures exceed 5℃ will allow a catch which is representative of the population in an estuary. Glass eel monitoring needs a long-term sampling plan in order to account for annual fluctuations apparent in glass eel recruitment.

     

Long-term numerical changes and regulation in a river stock of European eel Anguilla anguilla

1. The objective was to explore the regulatory mechanisms underlying the numerical dynamics of a river stock of European eel Anguilla anguilla based on a purely observational study. We used a 21-year data set on the Rio Esva (Asturias, northwestern Spain) stock of eels, encompassing eel density at an estuary site and at nine sites distributed among three tributaries selected to differ in the distance from the tidal limit and fishery data on glass eels from the nearby Rio Nalón estuary.
2. The abundance of glass eels and estuarine and in-stream densities declined simultaneously over a 15-year period. However, following a historical minimum in the abundance of glass eels around the year 2000, the density continued to decline in the estuary whereas density in the streams began an increase that has continued to the present.
3. In-stream cohort mortality rate ( Z ) varied from 0.0016 day⁻¹ to only 0.0003 day⁻¹ (on an annual basis from 0.57 to 0.099 year⁻¹). The negative effects of intra-cohort and inter-cohort densities on mortality rate emphasized the responses of the riverine stock to compensatory density-dependence. As a consequence, the number of eels surviving to commence the silvering stage was similar in magnitude in the strongest and weakest cohorts.
4. These results suggest that, even if the low abundance of glass eels persists, in-stream density may increase towards a stable value regulated by in-stream density-dependence.     

Inter and intra-estuary variability in ingress, condition and settlement of the American eel Anguilla rostrata: implications for estimating and understanding recruitment

The objective of this study was to quantify spatial and temporal variability of anguillid glass eel ingress within and between adjacent watersheds in order to help illuminate the mechanisms moderating annual recruitment. Because single fixed locations are often used to assess annual recruitment, the intra-annual dynamics of ingress across multiple sites often remains unresolved. To address this question, plankton nets and eel collectors were deployed weekly to synoptically quantify early stage Anguilla rostrata abundance at 12 sites across two New Jersey estuaries over an ingress season. Numbers of early-stage glass eels collected at the inlet mouths were moderately variable within and between estuaries over time and showed evidence for weak lunar phase and water temperature correlations. The relative condition of glass eels, although highly variable, declined significantly over the ingress season and indicated a tendency for lower condition A. rostrata to colonize sites in the lower estuary. Accumulations of glass eels and early-stage elvers retrieved from collectors (one to >1500 A. rostrata per collector) at lower estuary sites were highly variable over time, producing only weak correlations between estuaries. By way of contrast, development into late-stage elvers, coupled with the large-scale colonization of up-river sites, was highly synchronized between and within estuaries and contingent on water temperatures reaching c. 10−12° C. Averaged over the ingress season, abundance estimates were remarkably consistent between paired sites across estuaries, indicating a low degree of interestuary variability. Within an estuary, however, abundance estimates varied considerably depending on location. These results and methodology have important implications for the planning and interpretation of early-stage anguillid eel surveys as well as the understanding of the dynamic nature of ingress and the spatial scales over which recruitment varies.     

Role of Eel Odour on the Efficiency of an eel, Anguilla anguilla, Ladder and Trap

The influence of eel odour on the efficiency of a freshwater eel ladder and trap was assessed during two glass eel and yellow eel springtime upstream migration seasons in the Vilaine estuary, France. This test consisted of alternatively directing the outflow from the trap holding bin, away from, or onto the trap access ladder and comparing catches. Catches were 1.4 higher for both glass eels and juvenile eels when the trap water was directed towards the pass. The experiment had little influence on predicting ladder catches when compared to the environmental parameters. The distance of detection of the scent was calculated to range between 0 and 5m from the ladder.     

Long-term effects of hydropower installations and associated river regulation on River Shannon eel populations: mitigation and management

The Shannon, Ireland’s largest river, is used for hydroelectricity generation since 1929. Subsequently, the Electricity Supply Board assumed responsibility for management of its eel stocks, due to the impact of the hydro-dam on recruitment to the commercial fishery. In order to negate a decline in juvenile recruitment resulting from the installation of hydroelectric facilities, management was focused on stocking lakes with elvers and fingerling eels. These were trapped at the hydropower facilities and in estuarine tributaries during their up-stream migrations. Due to the decline of natural recruitment in more recent times, attempts have also been made to develop an estuarine glass eel fishery. Stock levels are then monitored through annual surveys of the population trends of juvenile (glass eel, elver), growing phase (yellow eel) and downstream migrating pre-spawners (silver eels). Survey results and fishery management programmes are reviewed in this article. In addition to the long-term effects the hydroelectric facilities have had on the stock levels, there is also an annual effect on the migratory patterns of downstream migratory silver eels. In the lower reaches of the river system flow rates are regulated by the hydroelectric stations. We review previous work that had highlighted the importance of flow in determining the timing of the silver eels migrations, and assess the relationship between flow and migration in more detail through the use of hydroacoustic and telemetric studies. Current research on seaward migrating silver eel populations, suggests that spawner escapement rates can most effectively be increased by trapping migrating eels at fishing weirs located up-stream of the power station and transporting them towards the estuary. Guest editors: R. L. Welcomme & G. Marmulla Hydropower, Flood Control and Water Abstraction: Implications for Fish and Fisheries     

Assessment of The Annual European eel (Anguilla Anguilla Linnaeus, 1758) Catches In Three Suggested Major Monitoring areas To Support The development of eel fishery management system In Turkey

Thirteen river basins draining into the Marmara, Aegean and Mediterranean Seas were selected for evaluation of European eel (Anguilla Anguilla L., 1758) catch in Turkey. These basins were grouped into three so-called “eel monitoring area” (EMA). The official province-based annual landing data between 1996 and 2018 were assessed for each of the monitoring areas. Statistically significant decreases over the years were observed in the landing data in all of the monitoring areas. The adequate protection of the European eel standing stocks, particularly in some critical eel habitats such as Köyceğiz lagoon showed over time a small decrease in eel catches in EMA-2. However, a substantial decrease was detected in the province of Adana, which includes the Seyhan and Ceyhan basins, and these are located in the EMA-3. The catch data mainly based on the commercial fisheries and the restriction on export seems to be the main reason for the decline in catches in the three EMAs. Thus the suggested monitoring areas (EMAs) are needed to structure research programmes on basic knowledge on escapement ratio of migrating silver eels, arrival and upstream migration of glass eels in the rivers of each of the EMAs in order to build area specific management strategies. These monitoring areas may provide practical units that management can use in future fisheries regulations.     

Seasonal and long-term changes in biometrics of eel larvae: a possible relationship between recruitment variation and North Atlantic ecosystem productivity

The European eel larval life has two stages: the oceanic leptocephalus, in which growth is dependent on food supply, and the metamorphosis glass eel stage, in which feeding and growth are stopped until immigration in estuarine waters. Data from fisheries and experimental surveys in European estuaries at different periods between the 1930s and the 1990s showed a simultaneous decrease of mean length and abundance level. A monthly survey at the Vilaine estuary (Bay of Biscay) during 2 years provided a periodical model of seasonal variation in length and weight of newly recruited glass eels. Otolith microstructure was used to back-calculate larval stage durations and the timing of transatlantic migration. Glass eels entering the estuary in autumn are longer and more numerous than those arriving in summer. They migrated across the ocean during spring and summer, seasons when plankton production is likely to reach maximum levels. The proposed hypothesis regarding the determinism of recruitment in the eel, on the seasonal and on the yearly range, is that leptocephalus growth and glass eel survival are partly dependent on the plankton production at the oceanic scale. Eel recruitment dynamics could provide an integrated figure of large-scale environment fluctuations.     

Head shape dimorphism in European glass eels (Anguilla anguilla)

The life cycle of the European eel (Anguilla anguilla) remained a mystery until the 20th century, when Schmidt discovered that the Sargasso Sea was its spawning area. However, many aspects of the eel's life cycle remain poorly understood. Among these is the bimodal distribution in head shape, with broad- and narrowheaded phenotypes reported in the yellow eel stage. Although this has been linked to dietary preferences of the yellow eels, very little is known about why, how and when this dimorphism arises during their ontogeny. To determine whether this dimorphism indeed appears in relation to trophic niche segregation, we examined head shape variation at an earlier ontogenetic stage, the glass eel stage, as at this stage eels are considered to be non-feeding. Head shape was studied in a large dataset, containing glass eels captured from the Yser river mouth, the Leopold Canal (Belgium) and from the rivers Severn, Trent and Parret (UK), by both taking measurements (head width/head length) and using an outline analysis. Our results show that there is already considerable variation in broadness and bluntness of the head at the glass eel stage. In most cases, equal support for a unimodal and bimodal head shape distribution is found, whereas some cases support head shape bimodality in glass eels, suggesting that glass eel head shape might be shifting from a unimodal to a bimodal distribution. This, in combination with the observation that variation in head width/head length ratios in non-feeding glass eels shows a similar range as in feeding yellow eels, indicates that head shape in European eel might be at least partially determined through other mechanisms than trophic segregation.     

Determination of the state of emaciation of eel (Anguilla Anguilla) in migration in coastal and estuary zones

The objective of this work is to characterise the variations of the quality of the glass eels recruitment during a season of migration, from November till March and before and after the transition from marine to continental environment. The emaciation state measured from the ratio DNA/Dry weight and percentage of body water is analysed from a sampling of glass-eels collected at sea and in estuary, in the South of the Bay of Biscay during the migration period 1999/2000. The length and the mass are taken into account, they decrease during the season while the individuals are more and more pigmented. However, our work shows that groups of glass eels arriving on the coasts of the Bay of Biscay are homogeneous from the point of view of their energetic and hydride reserves with a high individual variability within these groups. Also no significant difference in losses of water and energetic reserve were observed among the individuals migrating at the beginning and during tide stream. Fluctuations of the biochemical indication and the percentage of water between the glass eels caught in sea and in estuary show that the transition between maritime and continental environment is not a mobilising event of a lot of energy for glass eels. Differences were only observed in December.     

Structure and Innervation of Inner Ear Sensory Epithelia in the European Eel (Anguilla anguilla L.)

Hair cell orientations of all inner ear sensory epithelia in glass eel, yellow eel and silver eel are presented. The patterns of hair cell orientation do not change with age. All sensory epithelia increase in area during growth of the eel. Examination of the hair cell population in macula utriculi show constant hair cell densities and increased hair cell population during development. Further, regional differences in hair cell densities and hair cell types are observed. The hair cells/axons ratio increases 3-fold from glass eel to silver eel stadium. Nerve stainings in silver eel reveal complex innervation patterns with large stubby fibres confined to restricted regions. Histograms of nerve fiber diameters show marked differences from glass eel to silver eel. Growth of sensory epithelia is discussed.     

Impact of glass eel fishery on by-catch fish species: a quantitative assessment

The present study evaluated the effect of the glass eel fishery on by-catch ichthyofauna in the Ebro River delta by investigating the primary responsible factors of discard mortality, in order to assess the adequacy of fishing and sorting procedures, and assess the ecological impact of this traditional activity. Results revealed that glass eel fishery had a negative impact on by-catch ichthyofauna mainly composed of mugilid fry and small-size estuarine species. Data showed that between 10 and 69% of incidental species died as a consequence of glass eel capture and sorting procedures. Differences in by-catch fish mortality rates depended on the glass eel sorting time, the total weight of catch, the discard ratio, by-catch fish size, and by-catch fish species composition. The total weight of the catch and the discard ratio were two of the main parameters affecting discarded fish mortality, since both had a direct effect on the period of time required for glass eel sorting during which by-catch fish were exposed to air and suffered from very aggressive sorting procedures that caused severe external and internal injuries. By-catch fish measuring less than 40 mm L _T were more susceptible to glass eel fishery procedures than those larger (>60 mm L _T), independent of the species considered or their stage of development. This fishery, similar to other ones, should strive to minimization the amount of discards and the mortality of discarded fish, by improving the selectivity of the fish trap and developing new sorting and selection procedures. E. Gisbert and M. A. López have contributed equally to this study. Guest editors: S. Dufour, E. Prévost, E. Rochard & P. Williot Fish and diadromy in Europe (ecology, management, conservation)     

Recruitment abundance estimation: Role of glass eel (Anguilla anguilla L.) response to light

Most fish populations are declining worldwide and their management would benefit from a better estimation of recruitment. In glass eels, field studies suggest that estuarine migratory glass eels are sensitive enough to light to change their vertical location according to factors such as water turbidity and/or moon brightness. The response of glass eel (Anguilla anguilla L.) to light was tested in the laboratory using boxes where fish could choose between a lit and an unlit side. Responses were quantified as the proportion of glass eels remaining in the unlit chamber. Decreasing light levels were used and tested on different “age” glass eels (“age” in days since capture). In addition, measures of light at different depths of the water column were carried out in the Adour estuary (43°30′ N, 1°30′ W). The glass eel light avoidance level was lower in non-pigmented glass eel (less than 10 ^− 10 W cm ^− 2), than in pigmented ones (10 ⁻⁹-10 ^− 8 W cm ^− 2). These results and field data on the measurement of light energy in the water column of Adour estuary are compared with previously published data on the estuarine migration of glass eel.     

Environmental cues and extended estuarine residence in seaward migrating eels (Anguilla australis)

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The changing times of Europe's largest remaining commercially harvested population of eel Anguilla anguilla L.

This study quantifies the processes involved in regulating the European eel population of Lough Neagh, a lake in Northern Ireland. The relationship between glass eel input and silver eel output for the 1923–1997 cohorts was best described by a Beverton–Holt stock recruitment model. Glass eel input time series was not complete and was thus derived from the relationship between catches elsewhere in Europe and Lough Neagh, together with the addition of stocked glass eel. Silver eel output was the sum of silver eel escapement, catch and yellow eel catch converted to silver eel equivalents. Natural mortality increased with glass eel density, ranging from 0.017 to 0.142 year⁻¹. The mean carrying capacity increased from ≈3.25 M silver eels (≈26 kg ha⁻¹) for the 1923–1943 cohorts to ≈5.0 M (≈40 kg ha⁻¹) for the 1948–1971 cohorts before regressing back to ≈3.25 M. The total silver eel output was highest during the late 1970s/early 1980s at 35–45 kg ha⁻¹ year⁻¹ and lowest during the early years of the 20th century and is currently at 10–15 kg ha⁻¹ year⁻¹. The findings are discussed in relation to (a) the ecological changes that have occurred within the lough, associated with eutrophication and the introduction of roach (Rutilus rutilus L.), and (b) the decline of the wider European eel stock across its distribution range. The findings from this study have relevance for the wider management of the European eel stock.     

Anguilla japonica is already magnetosensitive at the glass eel phase

Magnetosensitivity of the Japanese eel Anguilla japonica at the glass eel phase (newly metamorphosed juveniles) was examined by conditioning and electrocardiography. The glass eels were conditioned to an imposed magnetic field of 192 473 nT parallel to the fish body placed along the earth's west‐east axis. After 10 to 40 conditioning runs, all the glass eels exhibited a significant conditioned response ( i.e . slowing of the heart beat) to a 192 473 nT magnetic field and even to a 12 663 nT magnetic field that combined with the geomagnetic field (32 524 nT) at the laboratory and produced a resultant magnetic field of 21° easterly. These results indicate that glass eels have high magnetosensitivity and probably acquire geomagnetic information early in life. It is hypothesized that silver‐phase adult eels find their way back to the oceanic spawning ground by reversing the geomagnetic direction that had been detected and 'memorized' during the glass eel phase when migrating from the open ocean towards the continental shelf and coastal waters.     

Compilation of Japanese fisheries statistics for the Japanese eel, <Emphasis Type="Italic">Anguilla japonica</Emphasis>, since 1894: a historical dataset for stock assessment

Fishery sustainability and the extinction risk of the Japanese eel, Anguilla japonica, are of global concern. The landings of the Japanese eel in Japan comprise a large part of the landings in East Asia. This study provides a compiled dataset of the annual fisheries statistics of the Japanese eel in Japan for stock assessment. The Japanese government has been recording Japanese eel statistics annually since 1984 in five series of annual reports by conducting systematic questionnaire surveys of fisheries managers and associations; however, most of these data are stored in analog format. The key variables in the dataset include the harvest weight of eels, the harvest weight and number of seeds for aquaculture, the number of eels stocked, and the number of management entities engaged in the eel fishery. The levels of spatial aggregation of the variables include the site (river and lake), prefecture, inland and coastal waters, and total in Japan. We also incorporated location data (latitude and longitude) of the site and prefecture into the dataset. Eel harvest includes primarily yellow eels (late juvenile stage) and silver eels (mature stage). Seed harvest in inland waters includes glass eels (intermediate stage between leptocephalus and elver) and elvers (early juvenile stage). Seed harvest from coastal waters comprises glass eels. This dataset provides information to assess long-term trends in the Japanese eel population.     

A comprehensive hypothesis on the migration of European glass eels (Anguilla anguilla)

The European eel (Anguilla anguilla) is a catadromous fish that spawns in the Sargasso Sea. As larvae, eels cross the Atlantic Ocean and reach the continental slope of Europe, where they metamorphose into post‐larval glass eels. These reach the continent, where some enter fresh water, some remain in marine waters, and others move between fresh and marine waters. After 5–25 years, as adult silver eels, they migrate back from fresh water to the Sargasso Sea to spawn and die. The glass eel stage is a critical step during which the eels cross the continental shelf and recruit to estuaries, where they facultatively transition to fresh water. Extensive research has been conducted to understand the behavioural mechanisms and environmental cues that aid and guide glass eels' migration. Glass eels follow odours and salinity gradients, they avoid light, and they change orientation and depth according to the tides. Recent work revealed that European glass eels also use Earth's magnetic field and lunar cues to orient. However, while we understand many aspects of their orientation behaviour, a unifying theory describing how glass eels migrate from the continental slope to fresh water is lacking. The goal of this review is to develop a comprehensive hypothesis on the migration of European glass eels, integrating previous knowledge on their orientation behaviour with recent findings on magnetic and celestial orientation. This review follows the journey of a hypothetical glass eel, describing the nature and the role of orientation cues involved at each step. I propose that, although glass eels have the sensory capacity to use multiple cues at any given time, their migration is based on a hierarchical succession of orientation mechanisms dictated by the physical properties of the environments that they occupy: (i) lunar and magnetic cues in pelagic water; (ii) chemical and magnetic cues in coastal areas; and (iii) odours, salinity, water current and magnetic cues in estuaries.     

Application of otolith microchemistry to estimate the migratory history of Japanese eel Anguilla japonica on the Sanriku Coast of Japan

The age and migratory history of the Japanese eel, Anguilla japonica Temminck & Schlegel, collected in Miyako Bay along the Sanriku coast of Japan, was examined using the otolith microstructure and analysis of strontium (Sr) and calcium (Ca) concentrations conducted with wavelength dispersive X‐ray spectrometry by an electron microprobe. The line analysis of Sr : Ca ratios along the life history transect of each otolith showed a peak (ca. 15–17 × 10^?3) which corresponded with the period of their leptocephalus and early glass eel stages in the ocean. The mean Sr : Ca ratios from the elver mark to the otolith edge indicated that there were eels with several general categories of migratory history, including sea eels that never entered freshwater (average Sr : Ca ratios, ≥6.0 × 10^?3), and others that entered freshwater for brief periods but returned to the estuary or bay. This evidence of the occurrence of sea eels in this northern area indicates that Japanese eels of the Sanriku coast do not necessarily migrate into freshwater rivers during recruitment as do glass eels at the beginning of their growth phase; even those that do enter freshwater may later return to the marine environment. Thus, anguillid eel migrations into freshwater are clearly not an obligatory migratory pathway, but rather a facultative catadromy with seawater or estuarine residents as an ecophenotype.     

Donor life stage influences juvenile American eel Anguilla rostrata attraction to conspecific chemical cues

The present study investigated the potential role of conspecific chemical cues in inland juvenile American eel Anguilla rostrata migrations by assessing glass eel and 1 year old elver affinities to elver washings, and elver affinity to adult yellow eel washings. In two‐choice maze assays, glass eels were attracted to elver washings, but elvers were neither attracted to nor repulsed by multiple concentrations of elver washings or to yellow eel washings. These results suggest that A. rostrata responses to chemical cues may be life‐stage dependent and that glass eels moving inland may use the odour of the previous year class as information to guide migration. The role of chemical cues and olfaction in eel migrations warrants further investigation as a potential restoration tool.     

Influence of oceanic factors on Anguilla anguilla (L.) over the twentieth century in coastal habitats of the Skagerrak,southern Norway

The European eel (Anguilla anguilla L.) is distributed in coastal and inland habitats all over Europe, but spawns in the Sargasso Sea and is thus affected by both continental and oceanic factors. Since the 1980s a steady decline has been observed in the recruitment of glass eels to freshwater and in total eel landings. The eel is considered as critically endangered on the International Union for the Conservation of Nature and Natural Resources Red List of species. The Skagerrak beach seine survey from Norway constitutes the longest fishery-independent dataset on yellow/silver eels (starting in 1904). The Skagerrak coastal region receives larvae born in the Sargasso Sea spawning areas that have followed the Gulf Stream/North Atlantic Drift before they penetrate far into the North Sea. The Skagerrak coastal time series is therefore particularly valuable for exploring the impacts of oceanic factors on fluctuations in eel recruitment abundance. Analyses showed that Sargasso Sea surface temperature was negatively correlated with eel abundance, with a lag of 12 years revealing a cyclic and detrimental effect of high temperatures on the newly hatched larvae. The North Atlantic Oscillation index and inflow of North Atlantic water into the North Sea were negatively correlated with eel abundance, with a lag of 11 years. Increased currents towards the North Atlantic during high North Atlantic Oscillation years may send larvae into the subpolar gyre before they are ready to metamorphose and settle, resulting in low recruitment in the northern part of the distribution area for these years. The Skagerrak time series was compared with glass eel recruitment to freshwater in the Netherlands (Den Oever glass eel time series), and similar patterns were found revealing a cycle linked to changes in oceanic factors affecting glass eel recruitment. The recent decline of eels in the Skagerrak also coincided with previously documented shifts in environmental conditions of the North Sea ecosystem.