Extreme variability in European eel growth revealed by an extended mark and recapture experiment in southern France and implications for management

WOS:000777836500009International audienceThe European eel (Anguilla anguilla) is endangered due to its peculiar life-history cycle, fishing pressure and difficulty in global population management. To improve our understanding of the population dynamics and refine conservation policies, an extended mark and recapture experiment and glass eel stocking were conducted in the River Rhone Delta (France) over 8 years. Around 1100 yellow eels were PIT-tagged and released in 2007, 2.5 kg of glass eels were released each year from 2008 to 2012, and the population was monitored using fishing between 2007 and 2015. After capture or recapture, the body parameters, sex and maturity were assessed. Age was estimated from otolith growth rings. At the end of the experiment, silver eels were between 352 and 875 mm long (age 17 to 185 months) and yellow eels between 170 and 868 mm long (age 12 to 123 months). Age estimates were validated using mark and recapture and showed 16% age underestimates and 5% inaccurate ages. The growth rates were extremely variable with lengths ranged by 2-fold at a given age. These results highlight the difficulty of eel population management, at least in the Mediterranean area

Diadromous fish run assessment: a double-observer model using acoustic cameras to correct imperfect detection and improve population abundance estimates

Introduction: Diadromous fish populations have strongly declined over decades, and many species are protected through national and international regulations. They account for less than 1% of fish biodiversity worldwide, but they are among the most perceptible linkages between freshwater and marine ecosystems. During their migration back and forth, diadromous fish species are subjected to many anthropogenic threats, among which river damming can severely limit access to vital freshwater habitats and jeopardize population sustainability. Here, we developed a method based on a double-observer modeling approach for estimating the abundance of diadromous fish during their migration in rivers. Methods: The method relies on two independent and synchronous records of fish counts that were analyzed jointly thanks to a hierarchical Bayesian model to estimate detection efficiencies and daily fish passage. We used simulated data to test model robustness and identify conditions under which the developed approach can be used. The approach was then applied to empirical data to estimate the annual silver eel run in the Touques River, France. Results: The analysis of simulated datasets and the study case gives evidence that the model can provide robust,accurate, and precise estimates of detection probabilities and total fish abundance in a set of conditions dependent on the information provided in the data (annual distribution of fish passage, annual number of observation, pairing period, etc.). Discussion: Then, the method can be applied to various species and counting systems, including nomad acoustic camera devices. We discuss its relevance for programs on river continuity restoration, notably to quantify population restoration associated with dam removals

Suivi 2021 des captures d'anguilles pour la DCF : analyse des otolithes

Ce rapport synthétise l'analyse des otolithes d'anguilles prélevés en 2021 dans le cadre du suivi des captures de cette espèce inscrite à la DCF (Data Collection Framework règlement CE n°199/2008 du conseil du 25 février 2008). 578 anguilles ont été prélevées de Juin à Octobre 2021 dans la Loire (303 individus), l’Adour (100 individus), la Dordogne (49 individus) et la Garonne (126 individus). La relation entre la longueur totale (Lt, mm) et le poids total (W, g) des anguilles prélevées en 2021 dans toute la France est du type W = 4.10-7Lt3,232. Ainsi, les anguilles présentent une allométrie isométrique (b=3,232) c’est-à-dire que le poids croit aussi vite que la longueur. L'institut IFREMER à travers le pôle national de Sclérochronologie à Boulogne-sur-Mer a préparé et interprété les otolithes, pièces calcifiées de l'oreille interne du poisson, permettant d’estimer leur âge. Pour chaque otolithe, un polissage mince sur le plan sagittal a été réalisé afin de permettre l’estimation de l'âge de chaque individu à l'aide du logiciel libre Icy. Tous les otolithes et ainsi que les données associées (taille, poids, date de capture, lieu...) sont stockés et gérés actuellement dans la base nationale d'archivage et de gestion des pièces calcifiées développée par l'IFREMER

Difference in factors explaining growth rate variability in European eel subpopulations: the possible role of habitat carrying capacity

This study was funded by the 'Region Bretagne', the MAVA Foundation and 'Bretagne Grands Migrateurs'. The surveys and sampling were organised and operated by the company FISH PASS and the association 'Coeur Emeraude' in the Fremur River, and the INRA-Rennes and ONEMA in the Oir River. We are especially grateful to Frederic Marchand (INRA), Julien Tremblay (INRA), Richard Delanoe (ONEMA), Virgile Mazel (FISH PASS), Sebastien Quinot (FISH PASS) and all the people that helped with sampling and data gathering. We are also grateful to ORE PFC for the salmonids data. We are grateful to the two referees and to Anne Lize for their helpful comments.As a key parameter in the management of fish populations, individual growth rate (GR) variations were examined in the European eel (Anguilla anguilla; >150mm) using extensive mark-recapture surveys in the lotic habitats of two small rivers of western France: the Fremur, supposed to be saturated, and at the same latitude, the Oir with densities fivefold lower than those of the Fremur. In both systems, generalised linear models were used to test whether spatiotemporal factors such as dominant habitat type or local density affect GR variability. In the presumed unsaturated system, the Oir, GR variability is mainly explained by a set of habitat suitability drivers (density, dominant habitat type). In the Fremur, GRs appear independent of differences in habitat density or productivity. Below saturation, an increase in density will decrease the GR through intraspecific competition. At saturation, intraspecific competition reaches such high levels that regardless of eel density and productivity, the resources available by individual are similar throughout the system. In these circumstances, the effect of density on growth was presumed undetectable. Despite these contrasted results, mean GRs observed in both catchments were closed (similar to 20mmyear(-1)). This is an unexpected result as GR is expected to be higher in unsaturated systems. This similarity could be explained by the difference between the two systems in terms of: (i) sex ratio (the Fremur is dominated by male, whereas the female is dominant in the Oir), (ii) habitat type distribution or (iii) possible interspecific competition (important salmonid populations in the Oir)

: Rapport bilan 2013-2015

Ce projet s’inscrit dans le groupe Dispersion et recolonisation (du thème biocénose aquatique) et le présent rapport correspond au bilan de l’état initial pré-arasement pour la partie suivi des flux de poissons migrateurs par hydroacoustique (DIDSON). La première année du suivi pré-arasement (2013) a été essentiellement consacrée à l’acquisition de la caméra acoustique DIDSON et des dispositifs de mesures environnementales, à l’installation des dispositifs, ainsi qu’à quelques observations préliminaires (Rapport d’étape 2013). L’année 2014 a permis de mettre en place une méthodologie adaptée au « tracking » semi-automatique des saumons atlantiques et de traiter les premiers résultats (Rapport d’étape 2014). L’année 2015 a permis de poursuivre les travaux engagés en renforçant les campagnes débutées en 2013 et 2014 et de développer les travaux méthodologiques réalisés en 2014

: Rapport bilan 2013-2015

Ce projet s’inscrit dans le groupe Dispersion et recolonisation (du thème biocénose aquatique) et le présent rapport correspond au bilan de l’état initial pré-arasement pour la partie suivi des flux de poissons migrateurs par hydroacoustique (DIDSON). La première année du suivi pré-arasement (2013) a été essentiellement consacrée à l’acquisition de la caméra acoustique DIDSON et des dispositifs de mesures environnementales, à l’installation des dispositifs, ainsi qu’à quelques observations préliminaires (Rapport d’étape 2013). L’année 2014 a permis de mettre en place une méthodologie adaptée au « tracking » semi-automatique des saumons atlantiques et de traiter les premiers résultats (Rapport d’étape 2014). L’année 2015 a permis de poursuivre les travaux engagés en renforçant les campagnes débutées en 2013 et 2014 et de développer les travaux méthodologiques réalisés en 2014

Testing novel methods for short-term forecasting of European glass eel recruitment

International audienceRecruitment forecasting constitutes a major issue in population dynamics, especially in stock assessments. In many cases, high recruitment stochasticity thwarts the determination of a stock-recruitment (SR) relationship. When no SR relationship is available, recruitments can be projected from past trends using methods such as the geometric means of the past recruitments. This approach implicitly assumes stability in upcoming years and might not be appropriate in the case of regime shifts. Recruitment forecasting is particularly critical when predicting fishing opportunities for recruitment fisheries. The fishery of glass European eels (Anguilla anguilla) is an example of a recruitment fishery. The French glass eel fishery, which is currently guided by two models to forecast upcoming recruitments and derive fishing opportunities, has failed to achieve management targets. This study develops new models that forecast glass eel recruitments without assumptions about the SR relationship, and with flexible assumptions about future recruitment evolutions. Tests based on multiple criteria found the best performance in a random slope model, which provides flexibility to track recruitment dynamics while balancing the interests of fisheries, management and conservation. This model has wide potential for trend projections in natural resource management, economics and other fields