Search

International audience; Acoustic cameras, or imaging sonars, are high-potential devices for many applications in aquatic ecology, notably for fisheries management and population monitoring. However, how to extract such data into high-value information without a time-consuming entire data set reading by an operator is still a challenge. Moreover, the analysis of acoustic imaging, due to its low signal-to-noise ratio, is a perfect training ground for experimenting with new approaches, especially concerning deep learning techniques. We present hereby a novel approach that takes advantage of both convolutional neural network (CNN) and classical computer vision (CV) techniques, able to detect fish passages in acoustic video streams. The pipeline pre-treats the acoustic images to localise the signals of interest and to improve the detection performances. The YOLOv3-based model was trained with fish data from multiple species recorded by the two most frequently used models of acoustic cameras, the DIDSON and ARIS, including species of high ecological interest, as Atlantic salmon or European eels. The pre-treatment of images greatly improves the model performance, increasing its F1-score from 0.52 to 0.69. The model we developed provides satisfying results detecting almost 80% of fish passages and minimising the false-positive rate. On a validation data set, 40 h of videos and around 1 800 fish passages, the efficiency increases with the fish sizes, notably reaching a recall higher than 95% for Atlantic salmon. Conversely, the model appears much less efficient for eel detections on ARIS videos than on DIDSON data (31% recall vs 75%).

Anthropogenic pressures have resulted in declines in diadromous fish. Many diadromous fish which were commercially important are now threatened and protected. Little is known about their marine life history phases, and no observation-based Species Distribution Model exists for this group of species at sea. Yet, fisheries dependent and independent data could provide new insights into the distribution of diadromous fish at sea.We collated a database of 168 904 hauls from fisheries observer bycatch data and scientific fisheries surveys, from eastern Atlantic and Mediterranean waters. The distribution of eleven rare and data-poor diadromous fish (shads, lampreys, salmonids, the European eel, the thinlip mullet, smelt and the European flounder) were modelled. A Bayesian site occupancy model, that incorporates imperfect detection to account for repeat detections and non-detections, the non-random nature of fishing gear type and spatial autocorrelation was used. From the model outputs, we explored bycatch risk and the role of MPAs, required under the Marine Strategy Framework Directive and Habitat Directive and assessed.Diadromous fish were observed within relatively shallow coastal areas. Species specific gear bycatch trends were observed. Core distribution areas corresponded to their known water basin presence, indicating connectivity with their freshwater habitats. Numerous Habitat Directive Marine Protected Areas were found to be of relevance.Given the coastal distribution of these species, they are exposed to higher anthropogenic pressures from both terrestrial and marine environments. Risk of bycatch at sea for most species appears to be low. Nonetheless, for threatened individuals, even a small amount of bycatch may impact their populations, especially since misreporting is likely to be high. Differences in catchability between gears highlight potential benefits of limiting access of certain gears within protected areas to reduce bycatch.

International audience; The European eel Anguilla anguilla is listed as critically endangered by the IUCN. Among many threats, the introduced parasitic nematode Anguillicola crassus is suspected to alter the eels’ swim bladder and jeopardize their reproductive oceanic migration. To date, gaining knowledge about the distribution and prevalence of A. crassus requires individual sacrifice (over 50,000 eels were sacrificed for epidemiology studies since 2010). This paper describes a non-lethal molecular protocol for identifying prevalence of A. crassus in A. anguilla, based on searching for A. crassus DNA in the feces of eels. Tests using three DNA microsatellite markers specific to the nematode showed that molecular detection provided similar results to visual examination of the swim bladder in up to 80% of the cases, and allowed for comparison of prevalence among sites. Easy to implement, this non-lethal protocol for detecting A. crassus could be valuable for management plans of this endangered species.

Lampreys are ancestral jawless vertebrates with particularly complex life histories. Population declines resulting from increased anthropogenic pressure have been observed. For semelparous diadromous lampreys, the marine phase remains largely a black box, making targeted management and conservation measures difficult to implement. Here, we collated a database of 168904 hauls from both fisheries-dependent and fisheries-independent surveys between 1965 and 2019. Lampreys were observed in only 254 hauls (20 cm) and occurred in autumn, indicating inshore migration. Our observations provide insight into the ecology of lampreys at sea and highlight study locations and gear types, which may be more pertinent for future research. Greater awareness is needed during surveys to collate catch information on lampreys and improve understanding of their ecology and phenology at sea.

Estimation of abundance with wide spatiotemporal coverage is essential to the assessment and management of wild populations. But, in many cases, data available to estimate abundance time series have diverse forms, variable quality over space and time and they stem from multiple data collection procedures. We developed a hierarchical Bayesian modelling (HBM) approach that take full advantage of the diverse assemblage of data at hand to estimate homogeneous time series of abundances irrespective of the data collection procedure. We apply our approach to the estimation of adult abundances of 18 Atlantic salmon (Salmo salar) populations of Brittany (France) from 1987 to 2017 using catch statistics, environmental covariates, and fishing effort. Additional data of total or partial abundance collected in four closely monitored populations are also integrated into the analysis. The HBM framework allows the transfer of information from the closely monitored populations to the others. Our results reveal no clear trend in the abundance of adult returns in Brittany over the period studied. lations d'animaux sauvages. Dans de nombreux cas, toutefois, les donnees disponibles pour l'estimation de series chronologidifferentes procedures. Nous avons elabore une approche de modelisation bayesienne hierarchique (MBH) pour tirer pleinement parti de l'assemblage varie de donnees disponibles pour estimer des series chronologiques homogenes d'abondance, quelle que soit la procedure de collecte de donnees. Nous appliquons l'approche a l'estimation de l'abondance d'adultes dans 18 populations de saumon atlantique (Salmo salar) de Bretagne (France) de 1987 a 2017, en utilisant des statistiques sur les prises, des covariables faisant l'objet d'une surveillance etroite sont aussi integrees a l'analyse. Le cadre de MBH permet le transfert d'information l'abondance des adultes de retour en Bretagne au cours de la periode etudiee. [Traduit par la Redaction], L’estimation de l’abondance à vaste couverture spatiotemporelle est nécessaire pour l’évaluation et la gestion de populations d’animaux sauvages. Dans de nombreux cas, toutefois, les données disponibles pour l’estimation de séries chronologiques d’abondance prennent différentes formes, sont de qualité variable dans le temps et l’espace et ont été obtenues par différentes procédures. Nous avons élaboré une approche de modélisation bayésienne hiérarchique (MBH) pour tirer pleinement parti de l’assemblage varié de données disponibles pour estimer des séries chronologiques homogènes d’abondance, quelle que soit la procédure de collecte de données. Nous appliquons l’approche à l’estimation de l’abondance d’adultes dans 18 populations de saumon atlantique (Salmo salar) de Bretagne (France) de 1987 à 2017, en utilisant des statistiques sur les prises, des covariables environnementales et l’effort de pêche. D’autres données sur l’abondance totale ou partielle obtenues pour quatre populations faisant l’objet d’une surveillance étroite sont aussi intégrées à l’analyse. Le cadre de MBH permet le transfert d’information sur des populations étroitement surveillées aux autres populations. Nos résultats ne font ressortir aucune tendance nette de l’abondance des adultes de retour en Bretagne au cours de la période étudiée. [Traduit par la Rédaction]

This study aimed to describe the influence of different electrofishing engine configurations on electric field dispersion in water. Several experiments with original results are included. The aims were to (i) assess the equivalent electrode resistances and the output voltage at the anode, (ii) assess the influence of electrode misuse on the dispersal of the electric field in water, (iii) assess the influence of the duty cycle and the frequency on the radius of attraction around the anode, and (iv) set the voltage and the duty cycle based on information from models for backpack and bankside electrofishing engines. A synthesis of equivalent electrode resistances from different studies was created. Using an oxidised anode induced a decrease in the radius of attraction. Frequency had no influence on the radius of attraction, in contrast to the duty cycle. Models including the voltage and the duty cycle were created to enable the configuration of electrofishing engines.

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 audience; 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.

The European eel (Anguilla anguilla) is a catadromous species that reproduces at sea and inhabits continental waters during its growth phase. River fragmentation due to obstacles is considered one cause of the decline of this species. However, the colonization process of river catchments by eels is still poorly understood. In this article, we compare two scenarios for the diffusion of eels within river catchments: a path-wise scenario, in which movements are totally random, and a flow-wise scenario, in which movements are partially oriented. Based on these two scenarios, we attempted to predict the distribution of eels within dendritic river catchments, explicitly accounting for the presence of obstacles to movement. The model was fitted to a long-time series of electro-fishing data. The results suggest that the path-wise scenario is more predominant than the flow-wise scenario. Moreover, results show that the distribution of eels in river catchment depends on (i) the types of movements carried out by eels, (ii) the configuration of river networks and (iii) the positions of obstacles within catchments.

Few fish stocks are as influenced by (intentional and inadvertent) human impacts as the European eel, all across the continent. The dynamics of this stock, however, are poorly understood—neither the causes of the historically low abundance, nor minimal protection levels, are beyond discussion. Rather than analysing contemporary processes, this article turns back in time—two centuries or more—unravelling historical abundances and distribution patterns; reviewing historical actions and objectives; and discussing technical developments and scientific advice—picturing the slippery slope the eel stock has come down from. The first claim, that the continental stock was in decline, dates from the early 1800s; stock-enhancement actions were initiated shortly after. Diffuse objectives, technical innovations, eternal optimism, and—above all—no quantification impede the exact evaluation of historical reports. After 1950, when quantification improved, a slow but consistent decline was observed, but it is only two decades after the crash in glass eel recruitment (in 1980), that protection plans addressed the bad status of the stock. A slippery slope, full of pitfalls—yet, we now observe several years of increasing recruitment.

Environmental sex determination (ESD) has been detected in a range of vertebrate reptile and fish species. Eels are characterized by an ESD that occurs relatively late, since sex cannot be histologically determined before individuals reach 28 cm. Because several eel species are at risk of extinction, assessing sex at the earliest stage is a crucial management issue. Based on preliminary results of RNA sequencing, we targeted genes susceptible to be differentially expressed between ovaries and testis at different stages of development. Using qPCR, we detected testis-specific expressions of dmrt1, amh, gsdf and pre-miR202 and ovary-specific expressions were obtained for zar1, zp3 and foxn5. We showed that gene expressions in the gonad of intersexual eels were quite similar to those of males, supporting the idea that intersexual eels represent a transitional stage towards testicular differentiation. To assess whether these genes would be effective early molecular markers, we sampled juvenile eels in two locations with highly skewed sex ratios. The combined expression of six of these genes allowed the discrimination of groups according to their potential future sex and thus this appears to be a useful tool to estimate sex ratios of undifferentiated juvenile eels.

Erratum: Page 262 of this article erroneously refers to Louis Riffardeau Baron de Riviére (1817-1890) as the inventor of eel restocking. This conflates the names of two people living in the same place and period: Baron Louis de Riviére (1789-1871) and Duke Louis Riffardeau de Riviére (1817-1890). While the Baron was the inventor of eel restocking, the Duke was not involved. The article thus incorrectly speculates about the initial transports from St. Gilles in the Camargue (where the Baron was mayor) to lakes around Vernais in Cher (owned by the Duke). Apart from that speculation on page 262 (only), the content of the article is not further affected by this error. Young eel, recruiting from the ocean towards Europe, are most abundant along the Atlantic coast of France. Since 1840, attempts have been made to redistribute them from the areas of highest abundance to other countries and farther inland. This 'restocking' has been troubled by technical constraints (e.g. mode of transport and maximum distance eel can be shipped alive), wars (e.g. the Franco-Prussian War and World Wars One and Two) and, in recent decades, by shortage of supply due to the general decline of the eel stock all across Europe. Though objectives and procedures have changed considerably over the years, the recurring aim has been to increase production and, in that way, to 'faire mieux que la nature'. We document the historical development of these efforts from their inception, and contrast the achievements to the objectives. Except for the 1952-1990 period in Eastern Europe, restocking has probably added only slightly to the natural production. As successful as restocking might have been locally, it has not markedly changed the overall trends and distribution patterns or halted the general decline of the stock and fishery. Poor post-evaluation, frequent technical innovation and a constant renewal of the countries and people involved have kept the promise of a better future alive for 175 years.

A cooperative effort gathered a large European length-at-age data set (N = 45,759, Lat. 36S-61N Long. 10W-27E) for Anguilla anguilla, covering one century. To assess the effect of global warming during the last century and habitat effects on growth, a model was fitted on the data representing the conditions met at the distribution area scale. Two GLMs were designed to predict eel log(GR): one model was fitted to the whole data and the other was fitted to the female data subset. A model selection procedure was applied to select the best predictors among sex, age class, five temperature parameters and six habitat parameters (depth, salinity and four variables related to the position in the catchment). The yearly sum of temperatures above 13 � C (TempSUP13), the relative distance within the catchment, sex, age class, salinity class and depth class were finally selected. The best model predicted eel log(GR) with a 64.46% accuracy for the whole data and 66.91% for the female eel data. Growth rate (GR) was greater in habitats close to the sea and in deep habitats. TempSUP13 variable had one of the greatest predictive powers in the model, showing that global warming had affected eel growth during the last century.

Using the electrofishing database of the French National Agency for Water and Aquatic Environment (Onema), the time trends of 48 freshwater fish taxa at 590 sites monitored for at least 8 years from 1990 to 2009 were investigated. The results demonstrated that species richness increased steadily from the beginning of the monitoring period. This is congruent with the finding that the number of species displaying a significant increase in spatial distribution or abundance was greater than those showing a significant decrease. Some species, however, had declined both in occurrence and abundance, e.g. tench Tinca tinca, common bream Abramis brama, brown trout Salmo trutta and European eel Anguilla anguilla. The species showing the most spectacular colonization were non-native, e.g. topmouth gudgeon Pseudorasbora parva, wels catfish Silurus glanis and asp Aspius aspius. The time trends in population density were related to the maximal body size, habitat requirement, occurrence and abundance and the status (i.e. native or exotic) but not to the spawning temperature.

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.

International audience; Spanning 20 years (1979–2007), this study is the longest time series pertaining to the resident shrimp species Palaemon longirostris in a European estuary. Data from monthly faunal surveys undertaken across the middle part of the Gironde estuary from April 1979 along with data from a statistical analysis of the commercial catches throughout the entire estuary and river were considered in order to explore their inter and intra-annual variability and long-term trends. Long-term densities series as well as environmental series (salinity, discharge, temperature and NAO) were decomposed and the effects of environmental variables were also examined using statistical models (GAMs). This revealed important spatio-temporal variability and a significant overall decrease in abundance of this species in the Gironde estuary since the beginning of the 1980s. This longterm decrease in abundance corresponded significantly to long-term decreases in both discharge and the NAO index, as well a long-term increase in temperature and salinity in the middle part of the estuary. However, models showed that environmental factors explained only a small part of the variability. The upstream shift of the population highlighted in this study, probably due to the intrusion of marine waters into the middle section of the estuary, may also have contributed to its decrease in abundance. Inter-annual variability of densities was also significantly linked with inter-annual fecundity fluctuations, and a significant decrease in both mean female size and fecundity was shown for preserved samples from 1992. Moreover, the breeding period has been temporally stretched out and began earlier in more recent years, potentially due to the increase in spring temperature.

Office National de l'Eau et des Milieux Aquatiques; Larval lampreys were sampled in a small river using a new method modified from the Surber bottom sampler for invertebrates. This method uses a rectangular dredge to sample a constant and small surface of sediments in soft substrate areas. It was compared to a classical point abundance sampling by electrofishing. Both methods generated large numbers of small samples encompassing the spatial heterogeneity of ammocoete distributions. However, the bottom sampler provided samples with higher abundance and was more efficient for small individuals. Furthermore, the equipment is cheap and the method is cost-effective, since it can be operated by only two people. It could therefore be applied at a large scale to collect information on lamprey populations and contribute to knowledge of their distribution, abundance and, in fine, to devise better conservation measures. (C)

Ce rapport présente les données connues sur la pêche et la gestion de l'anguille en France pour la période 2009-2010.

International audience; Among the rivers along the European coast that are still colonized by the sea lamprey (Petromyzon marinus), the Garonne Basin have the largest population of this species. This basin supports the largest fisheries of sea lamprey in France and certainly in Europe. In this article we provide data on the exploitation, the abundance and some life history traits both on the Garonne basin and at the European scale. The mean catch of the Garonne basin, over the period 19852003, is 72 t, confirming the top rank of the Garonne Basin sea lampreys population and of its fisheries. Abundance is estimated using a generalized linear model (GLM) on catch per unit of effort (CPUE) for the 19432005 period. Sea lamprey abundance appears to have peaked strongly between 1952 and 1970. From 1973 onwards, the trend is stable and abundance was only 3540% of the maximum level. Since the end of the 1990s, the CPUE has shown a strong increasea sign of an upward trend in lamprey abundance. This recent trend is confirmed by three other European time series gathered for this study. However a review of the official status of sea lampreys in many European countries shows that this species is still considered to be in decline. A literature review is made and shows that European sea lamprey exhibit an unusual latitudinal variation, with longer animals in the south and an increasing trend of its length at maturity over the years. A focus on Garonne Basin shows stable and distinct life history traits of adult sea lampreys in the two main rivers of the basin, including length and weight at maturity, lengthweight relationship, and female gonad weight. However, the sex ratio shows a slight excess of females in both rivers that seems to be typical of a well-established population of European sea lampreys. Hypotheses explaining differences in the life history traits are discussed, including the possibility of homing in the European sea lamprey, but none of them was fully adequate. We advocate fully addressing the issue of homing in the case of anadromous sea lampreys since this issue can have important consequences on the management of European stock(s).

Most of the European diadromous fish species are endangered and listed in the habitats fauna and flora directive, the bern convention and the IUCN Red List. Current conservation plans do not address global warming issues and consider the 1900 distribution range as the reference without taking into account the potential re-distribution of these species under global warming. However, for other taxa, predictive models have been used successfully to relate conservation assessment with the impact of global warming. We investigated the distribution of the various diadromous species regularly encountered in Europe, North Africa and the Middle East at the beginning of the 20th century. For each species, we related the observed distribution to a set of environmental variables describing the prevailing climate in the basins, the physical nature of the basins and reflecting historical events known to have affected freshwater fish distribution. We successfully built twenty-one accurate species-specific models out of twenty-eight during both the calibration and the validation phases. Longitude, a factor we interpret as related to biogeographical history, was selected in all models and temperature was included in fifteen models, in six it was the most explanatory variable. These models allowed us to separate the roles of ecology and history in explaining distribution patterns at species level. Historical events such as glaciations constrained the continental distribution of all diadromous species and six of these were also highly temperature-dependent. Based on these results, we claim that these models can be used to predict the potential distribution under changing climatic conditions and provide two relevant examples (i.e. for Alosa alosa and Pe tromyzon marinus). Predictive models could be useful as a preliminary tool to prepare long-term conservation plans on European, national and regional scales., La majorité des espèces de poissons migrateurs amphihalins est menacée et listée dans la directive européenne « Habitats - Faune - Flore », la convention de Berne et la liste rouge de l'UICN. Les plans de conservation actuels n'abordent pas la question du réchauffement global et s'en tiennent à la distribution 1900 comme unique référence, sans tenir compte des possibles repositionnements de ces espèces, dans le contexte du changement climatique. Cependant, pour d'autres taxons, des modèles prédictifs ont été utilisés avec succès pour relier les besoins en matière de conservation aux impacts du changement climatique. Nous avons étudié la distribution des espèces de poissons migrateurs amphihalins les plus fréquemment rencontrées en Europe, en Afrique du Nord et au Moyen-Orient et ce au début du XXème siècle. Pour chaque espèce, nous avons relié la distribution observée à un jeu de variables environnementales décrivant le climat prévalant dans le bassin versant, fournissant une description physique du bassin versant et reflétant des événements qualifiés d'historiques et connus pour avoir affecté la distribution des poissons d'eau douce. Pour les 28 espèces répertoriées, nous avons pu construire 21 modèles apparaissant aussi robustes en calibration qu'en validation. La longitude à l'embouchure, un facteur que nous avons interprété comme étant relié à l'histoire biogéographique des espèces, a été sélectionné dans tous les modèles. La température a été incluse, quant à elle, dans 15 modèles et occupe dans six d'entre eux la place de facteur le plus explicatif. Ces modèles nous ont permis de séparer les rôles de l'écologie et de la biogéographie dans les patrons observés à l'échelle de la distribution d'une espèce. Les événements historiques comme les glaciations contraignent la distribution continentale de tous les poissons migrateurs amphihalins, avec pour six d'entre eux, une distribution aussi fortement dépendante de la température. Sur la base de ces résultats, nous considérons que ces modèles peuvent être utilisés pour projeter une distribution potentielle dans le contexte du changement climatique et dans ce but, nous fournissons deux exemples pertinents (i.e. pour Alosa alosa et Petromyzon marinus). Les modèles prédictifs pourraient donc être utiles comme outils préliminaires à la préparation de plans de conservation à long terme, que ce soit à l'échelle européenne, nationale ou bien régionale.

International audience; The aim of this study was to analyse the distribution of 14 diadromous fish at the beginning of the 20th century in western Europe. This study was conducted on a set of 41 water basins. Five environmental variables were selected and we used generalised additive models for explaining the presence absence of species. The richest basins were located in the centre of the study area. Six main assemblage types were identified along a latitudinal gradient; they were constituted of a common species basis but differed by the absence or presence of other species. The 10 single species models produced have moderate to very good discrimination level and they can correctly predict both absence and presence. Temperature is included in all but one model, response curves vary according to the species; surface area is included in six models.

International audience; Visible implant elastomer (VIE) tagging showed no significant effect on survival of either 230 single-tagged or 60 multiple-tagged small European eels Anguilla anguilla. Mean tag retention was 987% during the 5 month laboratory experiments. Multiple VIE tags had no observed effect on European eel locomotor behaviour. VIE appears a reliable method for individually tagging small European eels, and could be useful in capturerecapture field studies.

The continuous and steady decline in glass eel (Anguilla anguilla, L.) recruitment and more generally in the eel stock has risen the ICES concern about the status of the eel stock. The European commission pointed out the need to develop management plans at the water district level and for all stages. We developed a Glass Eel Model to Assess Compliance (GEMAC) with eel recovery objectives. In this study we briefly describe this model already presented in working group on eel (ICES, 2005) and as part of the SLIME project. Then, we apply it on the two well documented Vilaine and Garonne basins, which have contrasted anthropogenic pressures. The model allows to assess the proportion of settled glass eels relative to unimpacted conditions (%S/R). A sensitivity analysis of the model is done showing that the assessment of %S/R is quite accurate. The %S/R is estimated to 0.19 for the Vilaine basin and to 0.78 for the Garonne basin reflecting a contrasted level of anthropogenic pressure. The %S/R is also estimated in a data poor context considering only administration data and no scientific survey and in this case the computation of the %S/R follows the precautionary approach by under-evaluating it (0.19 and 0.64 for Vilaine and Garonne basin). Finally 7 management scenarios that all halve the anthropogenic pressures are tested and we demonstrate that they are far for being equivalent and that the level of increase of %S/R depends upon the estuary. Although no model is needed to take immediate measures, we believe that our model could help in the future to build assessment and management tools along with the national management plans.

International audience; The morphological, physiological and behavioural changes occurring during metamorphosis reveal adaptations to drastic shifts in habitat and life style. We have investigated how dispersal behaviour changed during completion of the larval metamorphosis in migrating European eels Anguilla anguilla, as they reached the limit between the tidal and non-tidal parts of a large river. We show that late-metamorphic glass eels arriving from the sea rapidly migrated in the freshwater zone of the upper estuary by means of selective tidal stream transport. Then, due to the loss of tidal advection and despite the absence of an osmotic barrier, glass eels accumulated at the break point of tidal streams. Newly transformed small yellow eels were homogeneously distributed around the point where they initially accumulated as glass eels. This suggests that completion of larval metamorphosis induced the end of upstream migration (settlement) and a switch to density-dependent dispersal linked to food search. This ontogenetic pattern probably evolved to maximise growth through optimal utilisation of productive marine and estuarine habitats.

Post fishing mortality of the glass eel was monitored in 2007 in the Vilaine estuary. The mortality of the glass eels varied from 2 to 82% (mean 42%) in the two days following the fishing. The mortality of samples collected by hand nets or from the trapping ladder was null. Alterations of the skin mucus were analyzed by the use of Indigo carmine. The mortality was significantly correlated with body injuries, but not to other environmental factors. The presence of a large injury on the body systematically led to the death of the glass eels, and among these dead glass eels, only 3.5% had no external injury. These results are discussed, along with the results from other estuaries, with a view to transport glass eels across Europe, with the objective of restoring the eel stock.

L'UMR Ecologie et Santé des Ecosystèmes (INRAE-Agrocampus Ouest, Rennes) travailledepuis 2013 sur la description des migrations de poissons diadromes à l'aide de camérasacoustiques, en collaboration étroite avec l'Office Français de la Biodiversité dans le cadredu pôle OFB-INRAE-Agrocampus Ouest-UPPA pour la gestion des migrateursamphihalins dans leur environnement. Ces sonars multi-faisceaux à très haute fréquencepermettent d'obtenir des images de poissons sur lesquelles des caractéristiquesmorphologiques et comportementales peuvent être extraites. La mise en œuvre de cesoutils sur les bords des cours d’eau permet une observation du comportement naturel despoissons sans structure porteuse lourde (barrage, seuil), même en eau turbide. Lescaractéristiques des caméras acoustiques permettent l’acquisition de nouvelles donnéespar les gestionnaires qui utilisent couramment des techniques d’imagerie pour suivre lespopulations de poissons migrateurs. À l’heure actuelle, sur une cinquantaine de sites enFrance, des caméras optiques installées sur des passes à poissons permettent decompter le nombre de poissons migrant chaque année. Le développement de nouveauxoutils d’analyse de données issues des caméras acoustiques vise à faciliter l’utilisation deces méthodes de suivi innovantes par les gestionnaires. Pour l’heure, l’analyse des données enregistrées par les caméras acoustiques (DIDSON etARIS, du constructeur Soundmetrics Corp.) passe par une visualisation intégrale oupartielle des images par des opérateurs, procédure fastidieuse et très coûteuse en temps.Les campagnes d'acquisition se déroulant sur plusieurs mois, une automatisation dutraitement de ces images semble une avancée indispensable pour continuer à développerl’utilisation de ces outils en France, en particulier pour les transférer aux gestionnairesdes milieux aquatiques. Les objectifs sont donc de mettre en place i) une détectionautomatique de poissons sur les enregistrements, et ii) une identification des espèces enfonction de critères morphologiques (taille, ondulation...) et comportementaux (direction,vitesse de passage...). Pour développer cette automatisation, des approches innovantes detraitement d'images seront mises en œuvre, via les réseaux de neurones convolutifs(CNN) notamment, en collaborant avec l’UMR Littoral Environnement TélédétectionGéomatique (CNRS – Université de Rennes 2), laboratoire spécialisé en géographiephysique et traitement mathématique d’imagerie complexe. Le développement de cesoutils s’appuiera sur les données acquises dans le cadre d’études préalables du pôlemigrateurs. A terme, l’optimisation des temps d’analyse des vidéos acoustiques élargira lenombre de stations pouvant être suivies ou allongera les campagnes d’acquisition.Pourront ainsi être abordées de nouvelles questions de recherche : dispersion d’uneespèce diadrome sur un bassin versant en multipliant les sites suivis par des camérasacoustiques, étude de l’influence de l’évolution des conditions environnementales sur lesphénologies de migration de plusieurs espèces diadromes qui fréquentent un mêmecours d’eau. Des applications pour la gestion peuvent également en résulter, dont la miseen œuvre temporaire des caméras acoustiques en complément d’une autre méthode desuivi des migrateurs pour estimer les flux de migration (projet Toolbox, 2016-2018).

Qu'elles soient complètes ou partielles, les pêches à l'électricité ont pour objectif d'obtenir un échantillon suffisamment représentatif des populations ou communautés présentes dans les stations prospectées. L'échantillonnage doit être reproductible dans le temps et dans l'espace pour pouvoir évaluer l'état du peuplement vis-à-vis d'un peuplement de référence et permettre une analyse comparative temporelle et spatiale.