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Spatio-temporal variations in glass eel recruitment at the entrance pathways of a Mediterranean delta
Abstract
Understanding spatio-temporal dynamics of glass eel recruitment is necessary to characterize eel population status and the stock of future elvers. Despite numerous studies that have characterized recruitment across Europe, multiple systems along the Mediterranean coasts need a deeper look. We built a Bayesian State-Space Model to investigate the temporal variations in glass eel recruitment in the Rhône delta (France). The model was suited to address the spatial heterogeneity due to the complexity of this system constituted by numerous entrance pathways. Over 13 years, we found inter-annual variations without a particular trend. Here, an overall migration peak occurred in February–March for the system, but substantial spatial variations in the resulting recruitment were visible. Spatial variations in the inter-annual dynamics and migration peak pointed out the necessity to account for spatial heterogeneity in the dynamics of glass eel recruitment. The highlighted inter-annual variations are consistent with analysis of coincident time-series in other sites in Europe (e.g. WGEEL), and the migration peak’s timing is similar to peaks observed in other estuaries. Spatial variations in the migration peak suggest a potential effect of local environment conditions on the recruitment. Our model provided a suitable approach to investigate temporal variations while accounting for spatial heterogeneity.
... Recent studies have shown tide gate management could improve the migration of glass eels and the number of individuals migrating upstream (Mouton et al., 2011(Mouton et al., , 2012, although success is not guaranteed (Van Wichelen et al., 2021). Tide gates, by impacting glass eel migration could be an additional source of variability in glass eel recruitment, which is known to be variable temporally and spatially (ICES, 2020), even within a hydro-system (Bouchard et al., 2022). Investigating how tide gates impact glass eel migration and how managers can mitigate these impacts is, thus, necessary. ...
... We found temporal variations in the three levels of information analyzed here. In overall, the temporal dynamics obtained displays a peak in 2014, which agrees with previous results for glass eels in Europe (Bornarel et al., 2018;Drouineau et al., 2016;ICES, 2020;Bouchard et al., 2022). Concerning the intra-seasonal variability, we found a higher probability of capture in February, March, and April associated with higher capture levels during these months. ...
... Concerning the intra-seasonal variability, we found a higher probability of capture in February, March, and April associated with higher capture levels during these months. In particular, that indicates a migration peak occurs in February-April, which is supported by previous findings for the Mediterranean coasts (Zompola et al., 2008;Briand et al., 2019;Lefebvre et al., 2003;Podda et al., 2020;Kara and Quignard, 2019;Bouchard et al., 2022;Lanzoni et al., 2022). A migration peak in glass eel recruitment has also been found across Europe, the timing varying mostly according to a latitude gradient (Gascuel et al., 1995;Desaunay and Guerault, 1997). ...
Anthropogenic barriers such as tidal gates impair animal migration and ecological continuity. For migratory fishes, barriers may alter or impeach both upstream and downstream migrations. Juveniles of European eels, namely glass eels, have to pass those barriers when migrating upstream, whereas they have limited swimming capacities and are dependent on several environmental variables. Hydrological conditions, which can be modified by barrier management, are critical for glass eels. Here, we investigated the links between hydrological conditions and barrier management and the recruitment of glass eels at a connection between the Mediterranean Sea and a lagoon. To do that, we modeled the recruitment of glass eels over ten seasons accounting for temporal variations and co-variable effects. We modeled three aspects of recruitment: presence, level, and composition. We found that monthly and inter-annual effects explained the main part of the variations in glass eel recruitment but accounting for environmental effects improved our models, with a positive effect of temperature, for instance. We associated low levels of catches and recruitment of rather old individuals to high water flow rates when water flows out of the lagoon. Those results call for further studies on how sluice gate management may improve glass eel recruitment and seem to indicate that local stakeholders should adjust the number of open gates depending on the water level on both sides of the barrier.
The Vaccarès Lagoon System, located in the central part of the Rhône Delta (France), is a complex shallow coastal lagoon, exposed to a typical Mediterranean climate and a specific hydrological regime affected by man-controlled exchanges with the sea and agricultural drainage channels. In this article, we report the results obtained by a series of monitoring programs, with different spatial and temporal resolutions. Long-term datasets from 1999 to 2019 with data collected on a monthly basis and a high spatial resolution highlighted the significant spatial heterogeneity in salinity regimes, and helped to determine the long-term evolution of the total mass of dissolved salt. High-frequency surveys allowed to characterize the water levels and salinity dynamics seasonal response to (i) the exchanges with the Mediterranean Sea, (ii) the exchanges with agricultural drainage channels, and (iii) the rain and evaporation. In addition, wind effects on salinity variations are also explored. This work shows how different spatial and temporal monitoring strategies provide complementary information on the dynamic of such a complex system. Results will be useful and provide insight for the management of similar lagoon systems, accommodating for both human activities and ecological stakes in the context of global change.
State–space models (SSMs) are an important modeling framework for analyzing ecological time series. These hierarchical models are commonly used to model population dynamics, animal movement, and capture‐recapture data, and are now increasingly being used to model other ecological processes. SSMs are popular because they are flexible and they model the natural variation in ecological processes separately from observation error. Their flexibility allows ecologists to model continuous, count, binary, and categorical data with linear or nonlinear processes that evolve in discrete or continuous time. Modeling the two sources of stochasticity separately allows researchers to differentiate between biological variation and imprecision in the sampling methodology, and generally provides better estimates of the ecological quantities of interest than if only one source of stochasticity is directly modeled. Since the introduction of SSMs, a broad range of fitting procedures have been proposed. However, the variety and complexity of these procedures can limit the ability of ecologists to formulate and fit their own SSMs. We provide the knowledge for ecologists to create SSMs that are robust to common, and often hidden, estimation problems, and the model selection and validation tools that can help them assess how well their models fit their data. We present a review of SSMs that will provide a strong foundation to ecologists interested in learning about SSMs, introduce new tools to veteran SSM users, and highlight promising research directions for statisticians interested in ecological applications. The review is accompanied by an in‐depth tutorial that demonstrates how SSMs can be fitted and validated in R. Together, the review and tutorial present an introduction to SSMs that will help ecologists to formulate, fit, and validate their models.
Transitional ecosystems and, particularly, Mediterranean lagoons represent important habitats for the European eel (Anguilla anguilla) population. In these habitats many anthropogenic pressures can disturb eel movements and, in turn, negatively affect the population. Despite the importance of movements during the non-migrant growing stage in eels, this topic is understudied in Mediterranean lagoons. We thus aim to describe the diel and seasonal phenology and the effect of environmental drivers on non-migrant eel movements. Videos obtained from an Adaptive Resolution Imaging Sonar (ARIS) acoustic camera that continuously recorded from October 2018 to April 2020 were processed to evaluate the daily number of eels swimming toward the lagoon. More than 60% of the 7207 eels observed were females with a size >45 cm. Movements were year-round and predominantly during the night. A Boosted Regression Tree analysis demonstrated that, among the 10 environmental drivers studied, flow velocity, water temperature, discharge of the main tributary, wind velocity and atmospheric pressure, had the strongest influence on eel movement activity. Non-migrant eel movements should be better incorporated into lagoon management plans through actions such as limiting dredging activities from 18:00 to midnight, especially when the water flows toward the lagoon and when the water temperature is higher than 12 °C.
Silver eel samples, collected from the lagoons of Fogliano and Caprolace (Italy), were investigated for a broad range of contaminants (29 polychlorinated biphenyls, 9 polybrominated diphenyl ethers, 5 dichlorodiphenyltrichloroethane, 5 chlordanes, hexachlorobenzene, 3 hexachlorocyclohexane, and 5 metals). Concentrations of targeted compounds stand for a general low contamination pattern. Infestation by Anguillicola crassus and virus infections were also examined. No parasite infestation was found, while infected silver eels had a low prevalence for EVEX, and, for the first time in the Mediterranean area, for AngHV-1. Overall, a good quality status of escaping silver eels, for both lagoons, was highlighted by the use of integrative Indexes. A quality assessment of the ecological status of the two lagoons was carried out developing an expert judgment approach, in order to characterize the habitat of eel stocks. A Final Pressure Index was derived, whose values showed an overall limited global anthropogenic impact acting on both lagoons. Results stand for the suitability of an integrated approach to assess lagoon habitats and eel local stocks quality. This could be proposed as a tool to identify sites yielding high quality eel spawners in the Mediterranean region, in order to set up suitable management frameworks, providing elements to appraise and discuss the potential of coastal lagoons in the Mediterranean region towards the recovery of the eel global stock.
1. The European eel, Anguilla anguilla, is a catadromous and migratory species of commercial importance. Its complex life cycle results in its exposure to many risk factors, which have resulted in stock declines across all life stages since the 1970s.
2. The temporal recruitment dynamics of juvenile eels (glass eels and elvers) were investigated in a small Mediterranean estuary (Sardinia, Italy). The composition of the population and the monthly and seasonal variations in the abundances of juvenile eels was assessed over 78 sampling events (from February 2017 to February 2018). Furthermore, the effects of abiotic variables on the abundances of glass eels and elvers were investigated using generalized additive models (GAMs).
3. Glass eels had the greatest abundance during the winter months, whereas elvers had the greatest abundance during spring. Modelling revealed that the abundance of glass eels was mostly explained by the combined effects of water temperature (12.3-14.5 C), tidal coefficient (40-110 cm), moon phase, season, and river mouth condition, whereas the abundance of elvers was associated with water temperature (14-21 C), dissolved oxygen content (>7 mg/L), and season. These results suggest that the annual recruitment of juvenile eels occurs throughout the year, with clear seasonal migration dynamics.
4. The use of multiple statistical approaches allowed us to identify the importance of several environmental variables in regulating the recruitment dynamics, providing useful information for conserving eel stocks through the restoration of the natural flow regime and the connectivity between freshwater habitats and the sea.
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.
Most studies on glass eel (Anguilla anguilla) migration are performed in natural estuaries, where they enter freshwater systems to live there for a period of years before they swim back again to the sea to reproduce. In these natural systems, river flows play a major role in attracting and directing migrating eels. However, coastal areas get urbanized more and more and characterized by anthropogenic barriers and hampered or artificial water flows. The effects of these flows on glass eel migration are poorly understood. Therefore, in this study glass eel were sampled at water pumping stations in a constructed part of the Rhine delta in the Netherlands. A mixed linear-effect model was used to determine effects of freshwater flows from water pumping stations on glass eel catch. We found that freshwater flows from water pumping stations had a significant but small effect on glass eel catch. Pumping activity had no significant effect on glass eel catch at sample locations with a continuous freshwater flow from fish passages. However, a low predictive value of the model and low numbers of individuals per sample prohibited strong conclusions on effects of anthropogenic freshwater flows on glass eel migration. More individual tracking techniques should be used to improve understanding migratory behavior of glass eel.
We present a general size- and age-structured demographic model of the continental phase of catadromous eels’ life cycle, allowing for the incorporation of (i) fluctuations of juvenile recruitment, (ii) density-dependent settlement and sex determination, (iii) plastic body growth and sexual dimorphism, and (iv) vital rates driven by body size rather than age. The model can be used to assess (i) standing stock, (ii) fishing yield, and (iii) spawner production under different recruitment and management scenarios. We demonstrate the model by applying it to a European eel stock (Camargue lagoons, southern France) that was continuously monitored for 17 years (1993–2009). Results indicate that the upper limit to settlement in the Camargue water system is ∼419 eels ha−1 per year. A balanced sex ratio occurs for a settlement of ∼94 eels ha−1, shifting towards males at higher densities. Catchability is higher in adult silver eels than in sub-adult yellow eels, possibly due to behavioural differences. Estimated standing stock ranged between 7 and 19 kg ha−1. Fishing yield was 5–7 kg ha−1 per year, while spawner escapement ranged between 0 and 13 kg ha−1 per year (<30% of what would occur in the absence of fishing and obstacles to migration).
Threats to aquatic biodiversity are expressed at broad spatial scales, but identifying regional trends in abundance is challenging owing to variable sampling designs, and temporal and spatial variation in abundance. We compiled a regional dataset of brook trout Salvelinus fontinalis counts across their southern range representing 326 sites from eight states between 1982-2014, and conducted a statistical power analysis using Bayesian state-space models to evaluate the ability to detect temporal trends by characterizing posterior distributions with three approaches. A combination of monitoring periods, number of sites and electrofishing passes, decline magnitude and different revisit patterns were tested. Power increased with monitoring periods and decline magnitude. Trends in adults were better detected than young-of-the-year fish, which showed greater inter-annual variation in abundance. The addition of weather covariates to account for the temporal variation increased power only slightly. Single- and three-pass electrofishing methods were similar in power. Finally, power was higher for sampling designs with more frequent revisits over the duration of the monitoring program. Our results provide guidance for broad-scale monitoring designs for temporal trend detection.
Temperate eels Anguilla anguilla (European eel), A. rostrata (American eel) and A. japonica
(Japanese eel) are three catadromous species which have been declining since
the 1970s/1980s despite their remarkable adaptive capacity. Because of their specific
life cycles, which share distant oceanic spawning grounds and continental
growth stage, eels are affected by five components of the global change: (a) climate
change affecting larval survival and drift, (b) an increase in pollution leading to high
levels of contamination exacerbated by their high lipid levels, (c) increasing fragmentation
and habitat loss that reduce dramatically the amount of available habitats and
induce increased spawner mortality, (d) the appearance of Anguillicola crassus a parasitic
alien nematode that impairs spawning success, and (e) the impact of commercial
and recreational fisheries for all life stages of eel. In this context, the rapid increases
of pressures during the “Great Acceleration” have surpassed the adaptive capacity of
eels. This illustrates that cumulative effects of global change can lead to the collapse
of species, even in species that have amazingly high adaptive capacities
European eel (Anguilla anguilla) recruitment has been declining at least since the early 1980s at the scale of its distribution area. Since the population is panmictic, its stock assessment should be carried out on a range-wide basis. However, assessing the overall stock during the continental phase remains difficult given its widespread distribution among heterogeneous and separate river catchments. Hence, it is currently considered by the International Council for the Exploration of the Sea (ICES) more feasible to use glass eel recruitment data to assess the status of the overall population. In this study, we used Glass Eel Recruitment Estimation Model (GEREM) to estimate annual recruitment (i) at the river catchment level, a scale for which data are available, (ii) at an intermediate scale (6 European regions), and (iii) at a larger scale (Europe). This study provides an estimate of the glass eel recruitment trend through a single index, which gathers all recruitment time-series available at the European scale. Results confirmed an overall recruitment decline to dramatically low levels in 2009 (3.5% of the 1960-1979 recruitment average) and highlighted a more pronounced decline in the North Sea area compared to elsewhere in Europe. © International Council for the Exploration of the Sea 2017. All rights reserved.
Obstacles in rivers are considered to be one of the main threats to diadromous fish. As a result of the recent collapse of the European eel, the European Commission introduced a Regulation, requiring to reduce all sources of anthropogenic mortality, including those caused by passing through hydropower turbines. Improving knowledge about migration triggers and processes is crucial to assess and mitigate the impact of obstacles. In our study, we tracked 97 tagged silver eels in a fragmented river situated in the Western France (the River Dronne). Using the movement ecology framework, and implementing a Bayesian state-space model, we confirmed the influence of river discharge on migration triggering and the distance travelled by fish. We also demonstrated that, in our studied area, there is a small window of opportunity for migration. Moreover, we found that obstacles have a significant impact on distance travelled. Combined with the small window, this suggests that assessment of obstacles impact on downstream migration should not be limited to quantifying mortality at hydroelectric facilities, but should also consider the delay induced by obstacles, and its effects on escapement. The study also suggests that temporary turbines shutdown may mitigate the impacts of hydropower facilities in rivers with migration process similar to those observed here.
The size of the European eel population is below safe biological limits. To assess the status of the stock properly, it is
necessary to increase the number of recruitment series based on scientific monitoring. In this study, the spatio-temporal
variation in glass eel density in a South European Atlantic estuary, the Oria, has been examined using experimental fisheries
and fishery data. Glass eel density was predicted using a mixed generalized additive model. Current and depth were selected
as covariates and date as a random variable then extrapolated to the whole sampling point volume to obtain the daily recruitment
(mean, 12.76 kg; range, 0–72.8 kg). The average seasonal daily recruitment and fishery data were combined to obtain the seasonal
recruitment (mean, 1144 kg; range, 682–1593 kg) and exploitation rate (mean, 31.1%; range, 6.2–48.7%). The information of
spatio-temporal dynamics in glass eel density gathered in this study will help to improve the design of a recruitment-monitoring
scheme at the European level. The integration of glass eel fishery data and scientific estimates is crucial to obtain a recruitment
index in the Bay of Biscay, the area with the largest glass eel recruitment in Europe.
The European eel Anguilla anguilla is thought to be in a multi-decadal decline across its range. Although its northern Atlantic sub-populations are well-studied,
little is known about the historical trend and current status of eel stock in the Mediterranean Sea. To fill this gap, we
gathered catch data for 86 lagoon fisheries in nine countries across the Mediterranean basin and analysed historical trends
and geographical and environmental patterns. We found a region-wide decline in eel catch, beginning in the mid-1970s and exceeding
the simultaneous decline in non-eel fisheries, as well as lower productivity in larger lagoons and those in the southern Mediterranean.
Additionally, we developed a population dynamics model to provide a preliminary estimate of pristine, potential, and actual
escapement of spawning adults (silver eels) across the Mediterranean basin under historical and current conditions. Model
results suggest that current escapement is 35% of escapement at pristine biomass levels, <40% target set by EC regulation
1100/2007. Furthermore, we estimate that a complete closure of lagoon fisheries would achieve 57% of pristine escapement under
current recruitment levels. Though preliminary, this analysis represents a first step towards a full assessment of the role
of the Mediterranean sub-population in overall eel stock recovery.
Recruitment of the three northern hemisphere eel species (European eel Anguilla anguilla, American eel Anguilla rostrata and Japanese eel Anguilla japonica) has reduced significantly over the past thirty-five years. The stock of the European eel is described as being outside safe biological limits, with urgent action required by European Union Member States to assist recovery of the panmictic stock. Stock recruitment models and estimates of silver eel output from a river catchment are strongly influenced by the degree of certainty in estimating key population parameters of each life history stage. Therefore, management decisions aimed at enhancing eel populations rely on sound scientific evidence, based upon a fundamental understanding of the complex anguillid eel life cycle. This review paper focuses on the estuarine entry phase of the eel life cycle and synthesises the current scientific knowledge with regard to glass eel migratory behaviour, sampling methods and abundance estimates within estuaries. Although the behavioural and environmental processes modulating glass eel migration patterns are reasonably well understood, site specific factors play a significant role in determining fine scale distribution patterns at an individual estuary level. Given the large resource commitment required to adequately sample this key life history stage, behavioural studies of migration patterns on a local scale are crucially important to aid the design of robust sampling programmes aimed at quantifying seasonal abundance and annual recruitment.
Oceanic dispersal and connectivity have been identified as crucial factors for structuring marine populations and designing Marine Protected Areas (MPAs). Focusing on larval dispersal by ocean currents, we propose an approach coupling Lagrangian transport and new tools from Network Theory to characterize marine connectivity in the Mediterranean basin. Larvae of different pelagic durations and seasons are modeled as passive tracers advected in a simulated oceanic surface flow from which a network of connected areas is constructed. Hydrodynamical provinces extracted from this network are delimited by frontiers which match multi-scale oceanographic features. By examining the repeated occurrence of such boundaries, we identify the spatial scales and geographic structures that would control larval dispersal across the entire seascape. Based on these hydrodynamical units, we study novel connectivity metrics for existing reserves. Our results are discussed in the context of ocean biogeography and MPAs design, having ecological and managerial implications.
Data collected to inform time variations in natural population size are tainted by sampling error. Ignoring sampling error in population dynamics models induces bias in parameter estimators, e.g., density-dependence. In particular, when sampling errors are independent among populations, the classical estimator of the synchrony strength (zero-lag correlation) is biased downward. However, this bias is rarely taken into account in synchrony studies although it may lead to overemphasizing the role of intrinsic factors (e.g., dispersal) with respect to extrinsic factors (the Moran effect) in generating population synchrony as well as to underestimating the extinction risk of a metapopulation.
The aim of this paper was first to illustrate the extent of the bias that can be encountered in empirical studies when sampling error is neglected. Second, we presented a space-state modelling approach that explicitly accounts for sampling error when quantifying population synchrony. Third, we exemplify our approach with datasets for which sampling variance (i) has been previously estimated, and (ii) has to be jointly estimated with population synchrony. Finally, we compared our results to those of a standard approach neglecting sampling variance. We showed that ignoring sampling variance can mask a synchrony pattern whatever its true value and that the common practice of averaging few replicates of population size estimates poorly performed at decreasing the bias of the classical estimator of the synchrony strength.
The state-space model used in this study provides a flexible way of accurately quantifying the strength of synchrony patterns from most population size data encountered in field studies, including over-dispersed count data. We provided a user-friendly R-program and a tutorial example to encourage further studies aiming at quantifying the strength of population synchrony to account for uncertainty in population size estimates.
The main oceanic and local environmental traits forcing the glass eel Anguilla anguilla (L., 1758) recruitment to the southernmost European estuary, the Guadalquivir (SW Spain), were studied during nine successive migration seasons (June 1997–December 2006) using a fishery-independent experimental survey at three sampling sites in the estuary. A clear seasonal pattern was observed: density was highest between late autumn and spring with two migration peaks, and lowest during summer. Short-term (inter-month) changes in glass eel density were partially driven by local environmental variables, such as estuarine turbidity, local rainfall and water temperature. Long-term (inter-annual) changes were positively correlated with oceanic factors related to recruitment success (NAO index and primary production at the spawning area) as well as local environmental factors (westerly and southerly wind mixing indices and rainfall). Spatial changes in glass eel density within the estuary depended on tidal and light situations although maximum densities were mainly observed at diurnal and/or nocturnal flood tides. Although the decrease in the abundance of European glass eels has been widely known since the 1980s, during this study there was no evidence of a declining trend, probably because of an insufficiently long time series.
Les pompes d'irrigation constituent la principale connexion aquatique entre l'île de Camargue et d'autres milieux d'eau douce. Nous avons suivi le passage de poissons dans 2 sites parmi les 15 stations de pompage du bassin du Fumemorte (68 km2) pendant toute la période rizicole, c'est-à-dire de fin avril à début octobre 1997. Les poissons peuvent survivre au passage par la pompe. En grande majorité, ce sont des individus de petites tailles (2-5 cm). Cependant, des individus de grandes tailles (jusqu'à 46 cm) sont passés vivants. On estime à 14 700 le nombre d'individus entrés en Camargue par la pompe du Sambuc, et à 98 000 pour l'ensemble du bassin du Fumemorte. Vingt-quatre espèces ont été capturées par pompage. Les espèces dominantes numériquement sont le gardon, l'anguille, le goujon et la brème bordelière. Ces résultats sont comparés aux données sur les peuplements d'autres milieux de Camargue, afin de discuter l'importance de l'immigration par pompage dans le fonctionnement des peuplements de poissons de Camargue. Ils suggèrent que (1) le pompage permet l'introduction d'espèces nouvelles en Camargue, (2) les populations de certaines espèces passant par les pompes ne sont maintenues en Camargue que par cette immigration (e.g., hotu, chevaine, barbeau), (3) les poissons utilisant les marais temporaires viennent davantage du réseau de drainage que du réseau d'irrigation (e.g., épinoche, gobie commun, athérine), et (4) certaines espèces pompées ont des populations abondantes en Camargue (e.g., anguille, Pseudorasbora parva, ablette, brèmes), pour celles-ci, l'importance de l'immigration par pompage dans le fonctionnement de leurs populations en Camargue reste difficile à évaluer.
Une campagne d’échantillonnage des civelles Anguilla anguilla a été menée de novembre 2000 à mai 2001 au grau de la Fourcade (Saintes-Maries-de-la-Mer, côte méditerranéenne française). Des civelles ont été capturées sur toute la période d’échantillonnage, mais les maxima d’abondance se situent en janvier-février et, dans une moindre mesure, en avril. Afin d’optimiser le recrutement dans les lagunes littorales intérieures, la mesure de gestion proposée est donc de favoriser l’ouverture des martelières au milieu de l’hiver.
Le suivi des proportions mensuelles des différents stades pigmentaires (de V A à VI A4) permet de conclure à un vieillissement généralisé des civelles en recrutement de novembre à mars, puis à l’arrivée d’un deuxième flux de civelles jeunes en avril. Parallèlement, il est observé une diminution très nette des masses et des longueurs moyennes mensuelles, et ce, même en ne considérant qu’un stade pigmentaire donné (en l’occurrence V B). Ces résultats sur l’évolution des caractéristiques biométriques et pigmentaires sont en accord avec les deux séries de travaux déjà menés sur la façade méditerranéenne française (années 30 et 70), ainsi qu’avec les données publiées en différents points de la façade atlantique.
L’analyse comparée de ces données avec celles issues de la dernière étude en date sur la côte méditerranéenne française (Bages-Sigean, Languedoc-Roussillon, campagne 1974-75 ; LECOMTE-FINIGER, 1976) montre une composition pigmentaire mensuelle totalement différente, et révèle une diminution significative de la longueur des civelles, de l’ordre de 5 % en 25 ans.
Glass eel migration of the European eel Anguilla anguilla (L., 1758) in the eastern Mediterranean is poorly known despite the increasing state of anxiety for the future of the stock. In the present study, glass eel migration was investigated from October 1999 to April 2000, using experimental fishery with fyke nets, in two coastal systems along the western Greece (Ionian Sea): at the Sagiada marsh in the delta area of Kalamas River and at the mouth of Alfios River. The main period of the glass eel entrance was from December to March and the migration pattern was similar to those observed along the Atlantic coast of southwestern Europe. The variance of daily glass eel catches between the two studied sites was significantly different. The univariate and bivariate time series spectral analysis showed that glass eel short-term freshwater migration: (1) consisted of waves with periods from 5 to 40 days and (2) was correlated with environmental factors such as water temperature, atmospheric pressure, rainfall and moonlight. The observed differences between the two sites reveal the importance of the inland ecosystem characteristics.
Habitat use patterns of 3 species of temperate eels, Anguilla anguilla, A. japonica and A. rostrata, were investigated using otolith strontium:calcium ratio life history transects. Published and unpublished data from 6 sites (Canada, United States, Sweden, France, Taiwan and Japan) sam- pled across the geographical range of each eel species were compiled. Sr:Ca patterns indicated that the 3 species displayed similar patterns of habitat use. In all sites, patterns of habitat use consisted of either residency in one habitat (fresh, brackish, or marine) or movements between habitats. One movement pattern consisted of either a single change or 2 changes of habitat from fresh to brackish waters, or from brackish water to freshwater. Seasonal movements between fresh and brackish waters were observed for all 3 species. When only a single habitat switch event was detected, it occurred between 3 and 5 yr of age. Occurrence of eels with no freshwater experience was demon- strated, but such eels accounted for a smaller proportion of the overall sample than eels with some (even brief) freshwater experience. Contrary to the common convention that these are obligate catadromous species, we must now consider them as facultative catadromous, with far more flexibil- ity in habitat use. The most variable parameter among study sites was the relative proportion, rather than the diversity, of lifetime spent in the various habitat use patterns. Eels found at higher latitudes exhibited a greater probability of remaining in the lower reaches of watersheds in brackish water. Diversity of habitat use appears to be a common strategy of temperate eel species, and, as a life his- tory tactic, is under environmental control.
Age and total length (LT) data from a 11 year monitoring of the Anguilla anguilla eel population of the Camargue lagoons (Rhone delta, southern France) were collected for glass, yellow and silver eels. Three distinct models were calibrated to describe the growth process of undifferentiated eels, females and males, respectively. Uncertainty of parameter estimates was evaluated by bootstrap- ping. Females were characterized by larger asymptotic body size (LT) than males (580 � 50 v. 388 � 13 mm) and faster growth, whilst the Brody growth coefficient was larger for males than for females (meansS.D.3 � 00 10� 3 � 1� 68 10� 3 v .1 � 73 10� 3 � 0� 50 10� 3). Sexual differentiation was estimated to begin at 204 � 38 mm meanS.D., i.e. at the end of the second year in the lagoons, well before the LT at which macroscopic differentiation became possible (c. 300 mm). Males probably leave the lagoon or die (due to either natural or fishing mortality) within the first 3 years, whilst females can remain up to 5 years. Sexual differentiation and maturation have a major role in shaping the LT structure of the population. The LT and mass (M )d ata were fitted by allometric curves M ¼ aLTb . The calibration of distinct curves for data from different years indicated that the allometric coefficient a was subject to wider interannual fluctuations than the allometric exponent b. A negative correlation linked the average LT and the allometric exponent (r ¼� 0� 58, P < 0� 01).
Remotely sensed tracking data collected on animal movement is vastly un-derutilized due to a lack of statistical tools for appropriate analysis. Features of such data that make analysis particularly challenging include the presence of estimation errors that are non-Gaussian and vary in time, observations that occur irregularly in time, and com-plexity in the underlying behavioral processes. We develop a state–space framework that simultaneously deals with these features and demonstrate our method by analyzing three seal pathway data sets. We show how known information regarding error distributions can be used to improve inference of the underlying process(es) and demonstrate that our frame-work provides a powerful and flexible method for fitting different behavioral models to tracking data.
Traditional estimation of migration speed is based on telemetry or marking of individual fish. Only a few numbers of fishes (or batches) are followed, often over short periods of time. We propose a method based on capture data from the professional fishery. It is applied to glass eel (Anguilla anguilla) in the Gironde basin (France).
Migration speed is estimated using 2 methods: a rough method based on the analysis of seasonal abundance peaks for two métiers and a more precise method of cross-correlation, which compares speeds between several fishing areas. The methods are coherent and lead to mean migration speed ranging from 3 to 4 km/day.
We define an index of efficiency of migration using selective tidal stream transport (STST efficiency index) as the ratio of the observed migration speed to the potential speed (flood tide current speed and swimming speed). For the glass eel in the Gironde basin this index ranges between 0.15 and 0.19. The glass eel behavior, which can explain this low value, is reviewed.
Our STST efficiency index can be successfully applied for many fish or crustaceans using selective tidal stream transport. It can also be used for one species to compare results obtained on different basins.
Bayesian statistics is an approach to data analysis based on Bayes’ theorem, where available knowledge about parameters in a statistical model is updated with the information in observed data. The background knowledge is expressed as a prior distribution and combined with observational data in the form of a likelihood function to determine the posterior distribution. The posterior can also be used for making predictions about future events. This Primer describes the stages involved in Bayesian analysis, from specifying the prior and data models to deriving inference, model checking and refinement. We discuss the importance of prior and posterior predictive checking, selecting a proper technique for sampling from a posterior distribution, variational inference and variable selection. Examples of successful applications of Bayesian analysis across various research fields are provided, including in social sciences, ecology, genetics, medicine and more. We propose strategies for reproducibility and reporting standards, outlining an updated WAMBS (when to Worry and how to Avoid the Misuse of Bayesian Statistics) checklist. Finally, we outline the impact of Bayesian analysis on artificial intelligence, a major goal in the next decade.
Diadromy in eel is facultative and the diverse propensities in glass eel estuarine migration may lead to a large spatial dispersion, which may has profound influences on the species sex determination. In this study, we sought to clarify the relationship between European glass eel energetics and their pattern of migration behavior, in the framework of the conditional strategy. Marine glass eels were sampled in autumn and spring, stratifying high and low energetic status, respectively. Their migration behavior was determined in experimental installations that mimic tides using a change in water current direction every 6.2 h. When synchronized to the current reversal, glass eels were called active while individuals hiding in the substratum were considered as non-active. Then, for each active fish, a level of swimming activity was determined and both migration behavior and level of swimming activity were correlated to the individual wet weight (used as a proxy of energy stores), standard metabolic rate (relative SMR) and transcriptomic profile of metabolism related genes. Results showed that spring glass eels presented a lower probability to migrate, a lower wet weight and a higher expression of genes involved in energy stress resistance than autumn ones, supporting a conditional strategy based on individual's energy status. However, within each season, no wet weight difference was observed between active and non-active fish. In autumn glass eels, migration behavior was weakly related to relative SMR while in spring, none of the parameters analyzed allows discriminating active and non-active glass eels. The level of swimming activity in active fish was related to their relative SMR in autumn and to their wet weight in spring. Altogether, our results could not validate a conditional strategy but spring glass eels displayed some signs of energy distress and a lower level of swimming activity than autumn ones
1.Mathematical modelling is playing an increasing role in studies of ecological systems. This requires the communication of the details of a mathematical model, including the use of mathematical notation to represent ecological variables, parameters and processes. 2.In our experience, the clarity of mathematical notation varies between papers and can often be inconsistent with general conventions. Poor notation can impede communication and understanding of ideas, and make models appear more complicated than necessary. 3.Here we present some guidelines, including: (i) define every term in an equation, (ii) use fonts appropriately (italicise mathematical symbols, use bold lower case for vectors and bold upper case for matrices),(iii) use subscripts appropriately (to index quantities, for example by year),(iv) use superscripts appropriately (to indicate a power, the transpose of a matrix or the steadystate value of a quantity), (v) avoid multi‐letter variable names, and (vi) revisit notation early on in a project to see if it should be refined. 4.Although we focus mainly on ecology, our guidelines should be of interest to researchers applying models in evolutionary biology and broader areas of biology. This article is protected by copyright. All rights reserved.
In the past three decades the European eel Anguilla anguilla experienced up to 99% decline in recruitment in some parts of its distribution range. In addition to this long-term trend, glass eel recruitment exhibits significant inter-annual variability partially explained by large-scale oceanographic processes as well as local-scale meteorological and hydrological variables. A 40-year long time series of glass eel fishery catch in the Minho estuary, Portugal, was used to assess (a) whether local recruitment followed the general pattern of decline of A. anguilla observed elsewhere, and (b) whether environmental variables may explain inter-annual fluctuations in glass eel recruitment in the Minho estuary. The analysis shows that, in contrast to the majority of coastal systems in northern Europe and the Mediterranean, CPUE of glass eel in the Minho estuary did not exhibit a marked decline between 1974–2015 and never dropped below 20% of the 10-year mean (1974–1983), taken as baseline. The difference between the recruitment trend observed in the Minho river and that reported by WGEEL for wider European geographical scales highlights the need to calculate recruitment indices with a higher geographic resolution to better support the assessment of the status of the European eel population.
Mediterranean wetlands are increasingly managed to maintain their functions and services following modification in water allocation, embankment and climate change, calling for proactive and adaptive water management. In a first step, we used long-term monitoring of water levels in 37 adjacent embanked marshes in the Camargue as a repeated non-controlled experiment to build a hydrological model. Without information on water input/output by marsh users, we could nevertheless estimate evapotranspiration under flooding and dry conditions, and soil water coefficient. The model provided a high predictive accuracy (adjusted R2 = 0.73-0.83) of monthly water levels when applied to an independent sample of 12 marshes. In a second step we developed an interactive decision-aid tool that allows users to visualize the impact of their management strategy (desired water level at a specific month) on subsequent water levels, and their consequence on different components of the ecosystem over a 10-yr period.
Given the importance of reliable recruitment estimates when assessing temperate eel stocks and enforcing appropriate management measures, surprisingly few analytical tools have been developed to estimate yearly glass eel recruitment. Of the models that do exist, large-scale models generally rely on strong assumptions relating to fishing activity, while other models generally estimate recruitment at the river basin scale. With the aim of filling this gap, we developed the GEREM (Glass Eel Recruitment Estimation Model) to estimate glass eel recruitment at different nested spatial scales. Our model simultaneously estimates annual recruitment at the river catchment level, at an intermediate spatial scale such as Eel Management Units (EMUs), and at a larger scale (e.g., a country). Provided sufficient data become available in the future, the analysis could be extended to the scale of the distribution area, which would be consistent with the population scale. In this study, the model was applied to France, using various recruitment indices obtained from 1960 to 2013. This provided trends and absolute recruitment estimates consistent with current expert knowledge. A sensitivity analysis was carried out to assess the robustness of results to sources of uncertainty. This type of model fills an important gap in the range of quantitative tools presently available to estimate recruitment. It could be used in the future to establish total allowable catches in countries such as France where glass eels are fished commercially.
Abstract – Social interactions impact growth rate of animals in different ways such as direct or indirect competition for food and/or territories. In this study, behaviour of glass eels and elvers was described throughout three trials of 9 days spread out over 3 months. The impact of recruitment season (spring and autumn runs) and glass eels’ propensity to migrate (assessed thanks to an experimental sorting) on individual growth was studied. Overall, spring elvers were more aggressive and active than autumn elvers while autumn fish were more gregarious and reached a higher specific growth rate than spring fish. At the individual level, gregariousness and swimming, respectively, promote and impair growth rate. In addition, skinny individuals and individuals with high motivation to get food achieved better growth rate suggesting catch-up growth processes. These findings may help in clarifying the complex relations between behaviour, growth, river basin colonisation and future sex (which is environmentally determined in eel) and then contribute to the urgent need of scientific knowledge to guide the stocking programme of this endangered species.
Estuarine barriers may significantly reduce the upstream migration of diadromous fish species like the European eel (Anguilla anguilla L.). Previous research showed that limited barrier opening during tidal rise was a cost-efficient and effective mitigation option to improve upstream glass eel migration, without significant intrusion of sea water. This paper aims to optimise this adjusted barrier management to improve eel passage at a tidal barrier complex at the mouth of the River Yser, Flanders, Belgium, one of the most important migration routes for glass eel in Flanders. Specifically, three hypotheses were tested. The first hypothesis analysed the impact of the number of barriers opened on the upstream glass eel migration. The second hypothesis evaluated the relation between the size of the barrier opening and glass eel migration. Finally, we tested whether the suggested adjusted barrier management may lead to a significant increase in conductivity in the River Yser. Increased opening of one barrier appeared more efficient than opening several barriers slightly. Conductivity increased during periods of extreme drought and at base flow, but decreased within 24 h after the first peak flow. This indicates that adjusted barrier management does not entail salt intrusion in the Yser basin, as long as this management is not applied in extremely dry periods. Since the adjusted barrier management is easily implemented and could be applied on numerous tidal barriers, the presented results may contribute to restoration of eel populations worldwide and be of interest to a wide range of river managers and stakeholders.
EACH spring large numbers of elvers enter the Dutch Wadden Sea through the Marsdiep tidal inlet and migrate to the places where freshwater is discharged, such as at the sluices in the enclosing dam of the Ijssel Lake at Kornwerderzand and Den Oever (Fig. 1).
A Lagrangian model is presented of the current-carried migration of the leptocephali (larvae) of the European eel (Anguilla anguilla) across the North Atlantic Ocean from the spawning area in the Sargasso Sea to the adult range in Europe and North Africa. The success of larvae in crossing the Atlantic Ocean and reaching particular latitude bins on the eastern side depended strongly on starting location in the Sargasso Sea and migration depth. In the model domain, silver eel spawners can develop strategies for spawning location and migration depth to preferentially target particular regions in the adult range. This observation may help to explain the presence of gradients in molecular markers in eel samples collected across Europe. Spawning in the period of late winter spring maximizes the average food availability along the 2-year larval trajectory. The fastest transatlantic larval migration in the model is about 2 years, and the route to Europe takes most of the larvae past the east coast of North America in the first year. These model results are consistent with the hypothesis that the European and American eel (Anguilla rostrata) could separate themselves on different sides of the Atlantic Ocean on the basis of the different durations of their larval stages.
For the distribution of the European eel (Anguilla anguilla), only Schmidt (1909) has conducted substantial investigations, yielding a qualitative description (Atlantic and Mediterranean coasts of Europe and Northern Africa). In this article, a meta-analysis of reported fishing yields is presented, showing a major concentration of glass eel yield in the Bay of Biscay (and possibly farther south) and of yellow and (or) silver eel yield in the western Mediterranean. Fisheries target the glass eel stage at highest stock density and shift to the silver eel stage at low density. Because there is no suitable habitat in the Sahara, the southern limit is, contrary to Schmidt's belief, primarily determined by continental conditions. From the centre of the distribution to the north, a long and slow decline in density occurs. The mismatch between northern temperatures and the species' preference, in combination with the very low abundance, indicates that the European eel is best seen as a warm-water species, like most other eel species (Anguilla spp.), showing a considerable northern diaspora.
Glass eel (Anguilla anguilla) upstream migration was studied in the River Tiber estuary to obtain a better understanding of spatial and temporal migration dynamics within the season of ascent. Using data from glass eel fisheries, time series analysis of daily catches per unit of effort revealed a fortnightly cycle that can be related to invasion waves possibly corresponding to tidal currents. The amplitude of these waves appeared to correspond to the tidal area of the estuary. Furthermore, glass eels apparently had a delay in this area before resuming upstream migration.
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.
The freshwater eels of the genus Anguilla, which are catadromous, migrate between freshwater growth habitats and offshore spawning areas. A number of recent studies, however, found examples of the temperate species Anguilla anguilla, Anguilla rostrata, Anguilla japonica, Anguilla australis and Anguilla dieffenbachii that have never migrated into fresh water, spending their entire life history in the ocean. Furthermore, those studies found an intermediate type between marine and freshwater residents, which appear to frequently move between different environments during their growth phase. The discovery of marine and brackish-water residents Anguilla spp. suggests that they do not all have to be catadromous, and it calls into question the generalized classification of diadromous fishes. There has been little available information, however, concerning migration in tropical Anguilla spp. Anguilla marmorata, shows three fluctuation patterns: (1) continuous residence in fresh water, (2) continuous residence in brackish water and (3) residence in fresh water after recruitment, while returning to brackish water. Such migratory patterns were found in other tropical species, Anguilla bicolor bicolor and Anguilla bicolor pacifica. In A. b. bicolor collected in a coastal lagoon of Indonesia, two further patterns of habitat use were found: (1) constantly living in either brackish water or sea water with no freshwater life and (2) habitat shift from fresh water to brackish water or sea water. The wide range of environmental habitat use indicates that migratory behaviour of tropical Anguilla spp. is facultative among fresh, brackish and marine waters during their growth phases after recruitment to the coastal areas. Further, the migratory behaviours of tropical Anguilla spp. appear to differ in each habitat in response to inter and intra-specific competition. The results suggest that tropical Anguilla spp. have a flexible pattern of migration, with an ability to adapt to various habitats and salinities. The ability of anguillids to reside in environments of various salinities would be a common feature between tropical and temperate species without a latitudinal cline. Thus, the migration of Anguilla spp. into fresh water is clearly not an obligatory behaviour. This evidence of geographical variability among Anguilla spp. suggests that habitat use is determined by environmental conditions in each site.
A glass eel fishery exists downstream of the Arzal estuarine dam in the Vilaine (Brittany, France). Catch statistics were collected between 1996 and 2000, and processed using a subsampling technique which allowed data from a reliable subsample to be extrapolated to the whole fishery. During the same period, glass eel migration into fresh water was monitored using a glass eel trap located on the dam – the upstream limit of the fishery. The analysis of glass eel biology and exploitation shows that the glass eel fishery is very intensive and that there was more or less no escapement during the fishing seasons studied. The proportion of the stock successfully migrating towards fresh water, as compared with the total catch is estimated to range between 0.3 and 3.9%. In such a fishery, the fishing effort affects the abundance. Consequently, the total catch has to be used instead of catch-per-unit-effort to estimate abundance.