Carl POSSEME

SEELURE : Evaluation yellow eel (Anguilla anguilla) predation by the european catfish (Silurus glanis) in a marsh of the Erdre river in France

The « Seelure » project carried by the Muséum National d’Histoire Naturelle is a collaborative project between scientists (Muséum National d’Histoire Naturelle and Université de Rennes 1) and professional fishermen (Association Agréée des Pêcheurs Professionnels en Eau Douce de Loire Atlantique AAPPED44). This project will take place in the Mazerolles marsh, in Loire-Atlantique, in close vincinity from the Loire estuary. It could allow us to obtain reliable data on relationships between european eels (commercialy exploited fish, classified critically endangered by the IUCN but unprotected) and a super-predator newly arrived on several french watersheds and estuaries : the european catfish. Predation by this species on eels has already been studied, but mainly at the silver stage which is described as likely vulnerable to predation on shallow marine habitats such as the estuary mouths, the continental shelf (Béguer-Pon et al., 2012; Righton et al., 2016; Wahlberg et al., 2014; Westerberg et al., 2021), or at a narrow marine passage such as the Gibraltar Detroit (Westerberg et al., 2021). In rivers, and at the growth stage called « yellow eel », predation has been considered to be neglectable for a long time due to the species specific ecology (mostly active at night and hiding). However, since the arrival, sometimes invasive, of the european catfish in numerous rivers of western Europe, this hypothesis should be verified again. Indeed, the european catfish is a potential predator for european eels (Copp et al., 2009; Guillerault et al., 2017). Moreover, catfish is likely to keep spreading due to intentional introductions, especially for sport fishing, associated with the natural expansion of its natural habitat, which has to be linked with climate change (Cucherousset et al., 2018). Thus, to better evaluate the eel stocks and the population dynamics of this species, it seems really important to evaluate and take this eventual predation pressure into account. This pressure could have an impact on all the development stages of eels : juveniles during upstream migration, yellow eels during growth stage and silver eels during downstream migration. In 2021, a study prioritized the silver stage because it concerns the future spawners at the beginning of their migration, thus the impact of a predation on reproduction success is direct and easy to see (GlanisPoMi study, Muséum National d’Histoire Naturelle, currently being published). This study did not demonstrate a predation from catfish on silver eels. This result could be linked to the downstream migration itself, which takes place mainly in automn and winter during strong water discharge, thus, high current speeds and high water levels and turbidity, making preys more difficult to see and catch by predators. On the other hand, during the same study, smaller eels (yellow stage) were found in catfish stomach contents. Thus, it seems meaningful to study a likely predation of yellow eels by catfish.