Adult Atlantic salmon have a new freshwater predator

RESEARCH ARTICLE Adult Atlantic salmon have a new freshwater predator Stéphanie Boulêtreau1, Adeline Gaillagot1, Laurent Carry2, Stéphane Tétard3, Eric De Oliveira3, Frédéric Santoul1* 1 EcoLab, Université de Toulouse, CNRS, Toulouse, France, 2 MIGADO, Saint-Orens-de-Gameville, France, 3 LNHE, EDF—R&D, Chatou, France * a1111111111 a1111111111 a1111111111 a1111111111 a1111111111 OPEN ACCESS Citation: Boulêtreau S, Gaillagot A, Carry L, Tétard S, De Oliveira E, Santoul F (2018) Adult Atlantic salmon have a new freshwater predator. PLoS ONE 13(4): e0196046. https://doi.org/10.1371/journal. pone.0196046 Editor: Dennis M. Higgs, University of Windsor, CANADA Received: November 30, 2017 Accepted: April 5, 2018 Published: April 19, 2018 Copyright: © 2018 Boulêtreau et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Funding: This study was funded by the AdourGaronne water agency (AEAG) and Electricity of France (EDF). The funder "EDF" provided support in the form of salaries for authors [EDO and ST], but did not have any additional role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. The specific roles of these authors are articulated in the ‘author contributions’ section. Abstract The Atlantic salmon (Salmo salar) is one of the world’s most emblematic freshwater fish. Despite conservation and rehabilitation plans, populations of this species are dramatically declining due to human impacts such as habitat fragmentation, overfishing and water pollution. Owing to their large body size, anadromous adults were historically invulnerable to fish predation during their spawning period migration. This invulnerability has disappeared in Western Europe with the introduction of a new freshwater predator, the European catfish (Silurus glanis). Here we report how adults of Atlantic salmon are predated in the fishway of a large river of SW France, where the delayed and narrow passage created by the structure increases the probability of predator-prey encounter. We assessed predation risk by monitoring salmon and catfish in one fishway of the River Garonne, using video fish-counting from 1993 to 2016. We analysed the predation strategy of catfish using observations made with acoustic camera and RFID telemetry in 2016. Our results demonstrate a high predation rate (35%—14/39 ind.) on salmon inside the fishway during the 2016 spawning period migration. Our results suggest that a few specialized catfish individuals adapted their hunting behaviour to such prey, including their presence synchronized with that of salmon (i.e, more occurrences by the end of the day). Such results suggest that the spread of European catfish will potentially impact migration of anadromous species through anthropized systems. Introduction The main causes of global Salmonid decline are well identified. Habitat fragmentation, habitat alteration, acidification and overexploitation seriously threaten populations of species such as the Atlantic salmon [1,2]. Furthermore, climate change, introduced fish species or predation are now considered as potential threats, but there is limited information on how these factors and their interactions will affect salmonid populations [2]. Introductions of large-bodied predator fish that forage at the apex of food webs are known to impact native fish populations and modify prey assemblages as well as food web structure [3,4]. A well-known example is given by the introduction of the Nile perch in African lakes that negatively impacted cichlid populations and the food web through top-down effects [5]. PLOS ONE | https://doi.org/10.1371/journal.pone.0196046 April 19, 2018 1 / 12 A new predator for the Atlantic salmon Competing interests: The authors have declared that no competing interests exist. This does not alter our adherence to PLOS ONE policies on sharing data and materials. Largely introduced in the 1970s’ the European catfish Silurus glanis is now widespread in western and southern European freshwaters where it has established self-sustaining populations in most large rivers [6]. Large individuals can measure over 2.7 m total length and weigh 130 kg [7]. With its large gape size, the European catfish is a potential predator to many if not all native fishes, including anadromous species so that native species would no longer benefit from the size-refuge that protected them against native top-predators (e.g., pike; [8]). In this context, human activities that affect fish movement may increase the exposure of Atlantic salmon to predators. Artificial structures (e.g., dams and weirs), even where equipped with fish passage devices, are suspected to reduced survival of prey fish species by increasing prey residence time and predator density [9] and therefore, encounter rates. Increased food resource availability, smaller passage width and and simplified structure inside fish ladders may trigger the emergence of trophic specialization among consumers [9,10]. Moreover, intraspecific variation in trophic specialization might explain the ability of introduced species to establish populations. In European catfish populations, some individuals have been observed to adapt their behaviour to forage on novel prey, leading to behavioural and trophic specialization [11]. The Atlantic salmon, considered as an endangered species in Western Europe, was eliminated in the mid 1900s from many large, heavily anthropized and fragmented rivers [12,13], such as the River Garonne (Southwest France). In the 1980s, a sustainable reintroduction plan and restoration programs to facilitate passage over obstacles were begun in the Garonne basin. The lower-most obstacle on the River Garonne (Golfech power plant) was therefore equipped in 1987 with a fish lift as fishway. The Atlantic salmon population was monitored since 1993 in the fishway where adult returns are counted with video [14]. However, potential predation by the European catfish inside the fishway is now raising concerns that introduced predators may challenge conservation efforts. The aim of this study is to assess the risk for the Atlantic salmon to be predated by the European catfish inside an anthropized system. To this end, we hypothesized that some specialized catfish individuals could adapt their foraging behaviour to this restricted and anthropized spatial environment leading to Atlantic salmon predation in the fishway. Materials and methods Study area Located in southwestern France, the Garonne River runs over 580 km from its source in the Pyrenees to the Atlantic Ocean. The Golfech–Malause hydroelectric complex was built in 1971 on the Garonne River (southwestern France) ab (...truncated)