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Adult Atlantic salmon have a new freshwater predator


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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.
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Adult Atlantic salmon have a new freshwater
´phanie Boulêtreau
, Adeline Gaillagot
, Laurent Carry
, Ste
´phane Te
, Eric De
, Fre
´ric Santoul
1EcoLab, Universite
´de Toulouse, CNRS, Toulouse, France, 2MIGADO, Saint-Orens-de-Gameville,
France, 3LNHE, EDF—R&D, Chatou, France
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 pollu-
tion. 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 moni-
toring 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 hunt-
ing 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
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 pred-
ator 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 | April 19, 2018 1 / 12
Citation: Boulêtreau S, Gaillagot A, Carry L, Te
S, De Oliveira E, Santoul F (2018) Adult Atlantic
salmon have a new freshwater predator. PLoS ONE
13(4): e0196046.
Editor: Dennis M. Higgs, University of Windsor,
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
Funding: This study was funded by the Adour-
Garonne 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.
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 popula-
tions 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, intra-
specific 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 specializa-
tion [11].
The Atlantic salmon, considered as an endangered species in Western Europe, was elimi-
nated 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 hypothe-
sized that some specialized catfish individuals could adapt their foraging behaviour to this
restricted and anthropized spatial environment leading to Atlantic salmon predation in the
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) about 270 km from the river mouth (0˚
55’22.3”E; 44˚06’37.6”N), downstream from the confluence with the Tarn River (Fig 1; see [15]
for more details). This diversion-type hydropower facility is the first barrier for upstream
migration of anadromous species in the Garonne River. The Golfech power plant structure
was equipped in 1987 with a fish lift on the right bank of the tailrace. Fish are attracted into a
9-m long, 2.5-m wide and 1.5–4.5-m deep holding pool. At regular intervals (depending on
fish passage frequency), fish are trapped and concentrated into a 3.3-m
tank. This tank is
raised (fish lift) and emptied upstream of the plant into a 250-m long, 2-m wide and 2.5-m
deep transfer canal (Fig 1). Fish pass into this transfer canal before joining the headrace canal.
Fish counting
A permanent video fish-counting station was installed in the fishway to monitor the number
and timing of fish passage since 1993 (Fig 1). Migado, the association in charge of analysing
these records, provided the daily numbers of net passages of European catfish and Atlantic
A new predator for the Atlantic salmon
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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.
salmon in front of the fish-counting station (i.e., for each fish species and each day, the number
of upstream movements minus the number of downstream movements) between January 1
1993 and December 31
2016. The annual (or monthly) number of fish passing through the
fishway was obtained by summing the daily net passages of the year (or month). We also
obtained the number of catfish and salmon upstream and downstream passages for each hour
from 2004 to 2016. We used these long-term data to describe the annual and seasonal timing
of salmon and catfish and their behaviour in the fishway.
A temporary acoustic camera BlueView (Teledyne Blueview M900-2250 Dual Frequency
series) was installed from the 4
of April to the 26
of May 2016 in the headrace canal at the
upstream exit of the transfer canal in order to examine catfish and salmon behaviour in this
unknown place where waters are deep and cloudy (Fig 1). The acoustic camera was placed at
the exit of the fishway in order to the camera view range covered the area of the canal outlet
(2.25 m wide per 1 m high) and therefore ensure that no fish could be missed. Moreover, spe-
cific morphological characteristics of Atlantic salmon and European catfish (body size and
form, catfish head shape and salmon dorsal fin) can allow easily and undoubtedly distinguish-
ing these two species from others. Salmon were counted at this place to be compared with
salmon counted at the video counting station during the same period in order to estimate
Fig 1. Location and overview of the Golfech fishway on the River Garonne.
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catfish predation on salmon inside the fishway. During this period, the transfer canal was emp-
tied twice a week to ensure that no salmon remained in the fishway.
European catfish tagging
A total of 35 European catfish were captured in the transfer canal in April 2015 (n = 10) and in
April 2016 (n = 25) to be tagged in order to monitor their presence inside the fishway. Catfish
were anaesthetised using a benzocaine solution at 10% (0.7 ml/l), measured and tagged with a
32 mm PIT tag (RI-TRP-WR2B, half duplex, 134 kHz, diameter 3.85 mm and weight 0.8 g in
air; Texas Instruments). The procedure took less than 5 minutes. Fish were transferred to a
tank of clean water to recover from anaesthesia and released just outside the upstream exit of
the transfer canal. Fish tagging was ensured by Migado, the association in charge of monitor-
ing fish at the fishway, in accordance with national legislation under the authorisation ‘Arrête
´fectoral 2015–230’. Three antennae were fixed inside the transfer canal to detect tagged fish
and to analyse the periods of catfish presence inside the fishway.
Salmon and catfish numbers and timing
The annual number of returning adult Atlantic salmon averaged 166 (±131 SD), ranging from
a minimum of 45 individuals in 2005 to a maximum of 599 individuals in 2001 (Fig 2A). This
number of Atlantic salmon exhibited a slight peak between 1999 and 2002. First European cat-
fish passages at the video fish-counting station occurred in 1995, with three individuals. This
number progressively increased until 2004 to reach an average of 590 (±232 SD) individuals
per year during subsequent years. Years 2007 and 2012 exhibited the highest annual numbers
of European catfish with 1134 and 956 individuals respectively (Fig 2A).
Between 1995 and 2016, nearly 95% of the European catfish passed between April and July
with proportions reaching 35 and 31% in May and June (Fig 2B). The migration period of
salmon is only slightly earlier with 78% of salmon passing between March and July. A small
peak of salmon passages was first observed in autumn, but since 2003, this peak has disap-
peared and 96% of the salmon were observed to pass between March and July (data not
From 2004 to 2008, the frequency downstream passages by catfish was low. After 2009 it
strongly increased, reaching in 2015 a frequency nearly six times higher than in previous year
(Fig 3B). This suggests that catfish spent more and more time inside the fishway, going back
and forth in front of the video fish-counting station, leaving and entering back the fishway and
not only directly pass towards upstream. Similarly, the number of Atlantic salmon coming
back in front of the video station was particularly high in 2016 as compared with previous
years (Fig 3A).
Salmon preferentially came in front of the video fish-counting station during daytime
between 8am and 9pm (median value around 1pm; Fig 4A). By contrast, catfish preferentially
passed nightly between 11pm and 9am (median value at 4am). In 2014, 2015 and 2016, a
higher proportion of European catfish were observed at the end of the day, between 5pm and
10pm (Fig 4B).
European catfish visiting/occupancy in the fishway
Detections occurred from April 20
to July 14
, 2015 and from April 29
to August 16
2016. Among the 35 catfish tagged and monitored, 30 (86%) were detected at least once by one
of the antennae inside the transfer canal (Fig 5A). 23 catfish (66%) have performed only one
A new predator for the Atlantic salmon
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annual incursion inside the transfer canal staying inside from less than 1 minute to 17.6 hours
(Fig 5A). The other seven individuals (20%) have performed more than one incursion. The
number of incursions they performed, the time of their incursions and the cumulative time
they spent inside during the year of their release differed between individuals, with annual
incursion number ranging from one to 21 (Fig 5B), annual number of detections ranging from
Fig 2. Timings of passages of Atlantic salmon (dark grey) and European catfish (light grey) at the fishway in the River Garonne: (a) annual net number of fish
counted at the video fish-counting station since its installation in 1993; (b) month distribution (in %) of over the period 1993–2016.
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Fig 3. Number of Atlantic salmon (a) and European catfish (b) coming back in front of the video fish-counting station (in white) as compared with the annual net
number of passages of Atlantic salmon (dark grey) and European catfish (light grey).
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A new predator for the Atlantic salmon
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179 to 2012 (Fig 5C) and cumulative annual time inside ranging from 10 hours to 10 days (Fig
5D). These seven most active catfish individuals inside the fishway were detected at night,
mainly at the beginning of the night from 9pm to 11pm (Fig 5E).
European catfish predation on salmon
Salmon migration was observed at the video fish-counting station. Without catfish inside the
fishway, direct upstream migration behaviour, with salmon swimming at the bottom of the
fishway, was observed. With catfish inside, the migration behaviour was disrupted with many
salmon going back and forth, swimming at the surface, staying for a long time inside before
exiting, and sometimes being predated. See “S1 Movie” for direct predation of an adult salmon
(80 cm total length) by a large European catfish (160 cm total length).
Acoustic camera records performed from April 4
to May 26
2016 (51 days) at the exit of
the fishway showed that there were often between one and six European catfish individuals
waiting at the exit of the transfer canal. During this period, a total of 187 European catfish
were observed exiting from the transfer canal and 86 coming back into the fishway.
During this same observation period (51 days), over the 39 salmon counted at the video
fish-counting station, only 25 were observed at the exit of fishway by the acoustic camera. The
remaining 14 salmon—unobserved with the acoustic camera (35%)—were predated by Euro-
pean catfish inside the transfer canal between the video station and the exit. None of them was
detected in front of the video fish-counting nor at the exit of the fishway from 9pm to 2am.
Among the 25 salmon that managed to exit, 12 (48%) spent less than 30 minutes, seven (28%)
spent between 30 minutes and 1 hour, five (20%) spent between 1 h and 6 h and one (4%)
spent near 14 h inside the transfer canal before exiting it. 18 of them (72%) were attacked by
European catfish when exiting but none of the predation acts was successful. Over the 67 other
individual fish of undetermined species that were observed exiting the transfer canal, 31 (46%)
were attacked. Image resolution did not allow us to determine whether attacks on other small-
est fish preys were successful or not.
Most Atlantic salmon populations are declining, conversely European catfish populations are
increasing in western and southern European freshwaters. Despite ambitious rehabilitation
plans, the Atlantic salmon population of the Garonne River remains very low especially since
2003. The European catfish was observed in the fishway of Golfech since 1997 with increasing
passage numbers followed by an apparent stabilization since 2008.
The period of upstream migration of Atlantic salmon that mainly occurs from April to July
coincides with the period of European catfish passages at the dam. The European catfish is not
a migratory species but, as many other freshwater fish, upstream movements can be observed
before the spawning period [16]. The seasonality of this behaviour, from April to July, is linked
to warmer water temperature [17,18] and/or when more prey is available during the spring
migration [19]. Indeed, the present observations proved that returning adults of Atlantic
salmon are a prey for the European catfish in the River Garonne. In a preliminary study, using
DNA metabarcoding, Guillerault et al [20] have found DNA of Atlantic salmon in catfish fae-
ces in the River Garonne, but without certainty that prey were healthy and not already weak or
Fig 4. Hourly timings of Atlantic salmon (a) and European catfish (b) comings in front the video fish-counting
station between 2004 and 2016.
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A new predator for the Atlantic salmon
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dead. Here, we observed predation acts and we report that 35% of the 40 migrating salmon
observed at the dam here were consumed inside the fishway.
Predation inside the fishway is high, despite the observed mismatch between daytime activi-
ties of both species (diurnal for Atlantic salmon, nocturnal for European catfish) that should
limit the predation risk. Individual tagging suggested that predation was due to catfish individ-
uals that were staying at the exit, experiencing upstream and downstream movements inside
the fishway and/or anticipating comings inside the fishway at the end of the day to increase the
probability to encounter salmon inside. Slavı
´k et al [18] demonstrated that the European cat-
fish in a river is not strictly nocturnal in activity. The same authors demonstrated, from elec-
tromyogram biotelemetry records, that there is considerable individual variability in diel
activity depending on different individual behaviour and ability to use energy reserves [21].
Furthermore, the European catfish is known to display individual trophic specialisation
through foraging on terrestrial birds by intentional beaching [11]. Diet plasticity is a common
phenomenon in top predators with high-energy requirements and the ability to learn to utilize
new resources. As a long lifespan species with novel behaviours—massive aggregations [22],
beaching [11], predation in fishway (this study)—European catfish is distinct from many other
freshwater fish, and exhibits adaptations to its environment that are likely to contribute to its
invasive success.
Here, the anthropization of the river provides a local opportunity for European catfish to
exploit migrating Atlantic salmon. The continuous occurrence of several catfish individuals at
the exit of the fishway suggests that predators occupy this strategic location to capture other
fish prey. Indeed, unsuccessful predation acts on Atlantic salmon observed by acoustic camera
at the exit of the fishway suggest that Atlantic salmon escape more easily than other fish from
catfish predation thanks to its high-speed swimming performance. Further investigation
would be necessary to demonstrate selectivity or opportunism to consume prey. Our prelimi-
nary results show that 46% (31/67) of undetermined freshwater fish were attacked compare to
72% (18/25) for salmon. Large adult salmon (mean 80 cm total length), compare to generally
smaller freshwater fish prey, could be preferentially selected by large catfish.
Multiple stressors are known to interact synergistically to amplify the individual effects of
global change drivers on species and ecosystems [23,24,25]. The results suggest that the pres-
ence of dams (and fishways) and a new predator have additional impacts on the precarious
Atlantic salmon population of the River Garonne. In complex narrow fishways, introduced
European catfish can ambush and predate their prey, thus amplifying the ecological conse-
quences of an anthropogenic perturbation [9].
Due to human introduction coupled with future climate change, the range extension of the
European catfish, especially in the north of Western Europe, will continue. In this context,
even if no strong impact may appear for freshwater fish [6], the potential future risks for large
anadromous species should be taken into account. Indeed large anadromous fish species may
be more sensitive to catfish than other freshwater fish that do not need to use the fishway. Con-
versely to other freshwater species, large anadromous species may have not developed defence
strategies against predator before catfish establishment. Moreover, by predating reproductive
adults, the impact of catfish is certainly stronger on anadromous life cycle. The potential novel
predation pressure on these non co-evolved prey, coupled with increased human activity (e.g.
Fig 5. European catfish visiting in the fishway: (a) proportion of tagged individuals detected in the fishway and timings of the presence of the most active individuals
(n = 7) in terms of (b) number of incursions, (c) number of detections by one of the antennae, (d) cumulative time spent and (e) hourly period of their presence in the
A new predator for the Atlantic salmon
PLOS ONE | April 19, 2018 10 / 12
dam, fisheries), is concerning, especially in the case of declining populations of anadromous
Supporting information
S1 Movie. Adult Atlantic salmon have a new freshwater predator.
The authors wish to thank Se
´bastien Delmotte for statistical advice in the first part of this
study and Andrew MacDonald for English revisions.
Author Contributions
Conceptualization: Ste
´phanie Boulêtreau, Laurent Carry, Ste
´phane Te
´tard, Eric De Oliveira,
´ric Santoul.
Data curation: Ste
´phanie Boulêtreau, Laurent Carry, Eric De Oliveira, Fre
´ric Santoul.
Formal analysis: Adeline Gaillagot, Eric De Oliveira, Fre
´ric Santoul.
Funding acquisition: Ste
´phane Te
´tard, Eric De Oliveira, Fre
´ric Santoul.
Investigation: Ste
´phanie Boulêtreau, Adeline Gaillagot, Laurent Carry, Fre
´ric Santoul.
Methodology: Ste
´phanie Boulêtreau, Adeline Gaillagot, Laurent Carry, Fre
´ric Santoul.
Project administration: Fre
´ric Santoul.
Supervision: Ste
´phanie Boulêtreau, Fre
´ric Santoul.
Validation: Ste
´phanie Boulêtreau, Fre
´ric Santoul.
Writing – original draft: Ste
´phanie Boulêtreau, Ste
´phane Te
´tard, Eric De Oliveira, Fre
Writing – review & editing: Ste
´phanie Boulêtreau, Ste
´phane Te
´tard, Eric De Oliveira, Fre
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Supplementary resource (1)

... Several traits have been consistently used to make taxonomic inferences on fishes detected by IS, including body size and shape (Rose et al. 2005, Becker et al. 2011b, Grabowski et al. 2012, Magowan et al. 2012, Parsons et al. 2014, Boulêtreau et al. 2018, Artero et al. 2021, and discrete morphological features, without the need for alternative evidence. Examples of morphological features include the use of head shape and dorsal fin to discriminate European catfish Silurus glanis from Atlantic salmon Salmo salar in Boulêtreau et al. (2018), the distinctive head and fusiform body to discriminate invasive silver carp Hypophthalmichthys molitrix from native species in Ridgway et al. (2023), and the large head and rounded pectoral fins of goliath grouper Epinephelus itajara that discriminate it from other demersal fish species in Frias-Torres & Luo (2009). ...
... Several traits have been consistently used to make taxonomic inferences on fishes detected by IS, including body size and shape (Rose et al. 2005, Becker et al. 2011b, Grabowski et al. 2012, Magowan et al. 2012, Parsons et al. 2014, Boulêtreau et al. 2018, Artero et al. 2021, and discrete morphological features, without the need for alternative evidence. Examples of morphological features include the use of head shape and dorsal fin to discriminate European catfish Silurus glanis from Atlantic salmon Salmo salar in Boulêtreau et al. (2018), the distinctive head and fusiform body to discriminate invasive silver carp Hypophthalmichthys molitrix from native species in Ridgway et al. (2023), and the large head and rounded pectoral fins of goliath grouper Epinephelus itajara that discriminate it from other demersal fish species in Frias-Torres & Luo (2009). Locomotion has also been used as a distinguishing trait (Rose et al. 2005, Becker et al. 2011b, Parsons et al. 2014, Zhang et al. 2014, Artero et al. 2021, and is particularly useful for the identification of eels (Mueller et al. 2008, Doeh ring et al. 2011) and lam- The corresponding measurement tool in the Oculus Viewpoint software ( ...
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Reef fish assemblages across tropical and temperate latitudes are increasingly threatened by human impacts and climate change. Accurate and efficient survey methods are essential for quantifying these communities to inform management strategies. Imaging sonars (ISs) are high-frequency acoustic devices that produce camera-like images of objects. Unlike optical instruments, IS functions effectively in turbid and dark water and has proven valuable in detecting fishes in poor visibility and at night. Abundance, species richness, and fish size are desirable metrics in most reef fish surveys. This review investigates previous attempts to quantify these metrics using IS across different habitats. These metrics are often quantified in comparison with alternative methods (e.g. cameras, extractive techniques). This review examines the causes of agreement or incongruence between estimates from IS and estimates from these alternatives. Any instrument employed to quantify reef fishes should be able to operate in structurally complex habitats, and thus the ability of IS to function in these circumstances is also reviewed. Finally, 5 notable limitations of IS are described and solutions discussed. Overall, this review underlines the net value of IS for surveying reef fishes but advises using alternative methods to complement IS estimates of abundance, species richness, and fish size.
... Sampling sites in non-native areas are represented in green, while sampling sites located in native areas are represented in blue. The grey area represents the extant (resident) native area of the European catfish, according to the IUCN (Freyhof 2008) have a significant impact on endangered anadromous species like the Atlantic salmon (Salmo salar), the sea lamprey (Petromyzon marinus) or shad (Alosa alosa) due to predation (Boulêtreau et al. 2018(Boulêtreau et al. , 2020(Boulêtreau et al. , 2021. Conversely, in other European countries such as Portugal, Spain and Italy this species is now considered as a top priority invasive fish, with significant management efforts starting to be done. ...
... The species shows a great diet plasticity, with some individuals being able to shift their diet towards specific prey depending on their individual size, area of residence and period of the year. For instance, an increasing number of studies suggest that introduced European catfish populations could have a significant impact on endangered anadromous species like Atlantic salmon (Salmo salar), the sea lamprey (Petromyzon marinus) or shads (Alosa alosa) due to predation (Boulêtreau et al. 2018(Boulêtreau et al. , 2020(Boulêtreau et al. , 2021. Several attempts for rearing and introducing European catfish in lakes or ponds for human consumption have been documented in Western Europe between 1850 and 1960, but most were unsuccessful due to high mortality, unsuccessful reproduction and or unfavorable climatic events like frosts (e.g., in Italian pools, Gandolfi and Giannini 1979, in Great Britain in 1853, using individuals from current Moldavia, Lever 1977Davies et al. 2004 or in pools, ponds and some streams in different parts of France; Société Impériale Zoologique d'Acclimatation 1865). ...
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Biological invasions are a major component of global change worldwide. But paradoxically, an invasive species might also have threatened populations within its native range. Designing efficient management policies is needed to prevent and mitigate range expansions of invasive alien species (IAS) in non-native areas, while protecting them within their native range. Characterizing genetic variation patterns for IAS populations and deciphering the links between their native and introduced populations is helpful to (i) assess the genetic state of both native and non-native populations, (ii) reveal potential invasion pathways, (iii) define IAS management strategies in invaded areas, and (iv) identify native populations requiring conservation measures. The European catfish (Silurus glanis) is the largest European predatory fish. Introduced since the seventies from Eastern to Western Europe, it has colonized many waterbodies. Yet, little is known about the genetic status of non-native populations and the invasion pathways used by the species. Besides, some native populations are threatened, requiring conservation actions. Here, we describe current patterns of genetic variability of native and non-native S. glanis populations across Europe. Using microsatellite markers, we first assessed genetic variation within and between native and non-native populations. Second, we performed genetic clustering analyses to determine the genetic structure of multiple catfish populations across Europe and highlight their potential links. We revealed that native populations are more genetically diverse than non-native populations, and highlight complex introduction pathways involving several independent sources of introduction, which likely explain the invasion success of this large predatory fish across western Europe.
... For example; how many animals are ultimately killed by predators, and for what share is this the ultimate compared to the proximate cause of death? How do human landscape changes and restoration efforts contribute to predation (Berekijian et al. 2016, Boulêtreau et al. 2018, Lennox et al. 2022a? Is predation compensatory or additive upon prey populations (Linnell et al. 1995)? ...
... Efforts to separate a tagged species from a predator can be a nuisance when attempting to model behaviour of the prey species and error is introduced by incidentally modelling predator behaviours (Fig. 2). We have provided some insight into the present state-of-the-art for identifying predation events in telemetry data including statistical models for classifying tags that have been eaten (Gibson et al. 2015, Daniels et al. 2019, specific tag sensors that provide better resolution to analysts about animal fate (Boulêtreau et al. 2018) and manual methods for determining fates from tracking studies (Runde et al. 2021 ; Fig. 2). These tools and new ones in the future have great potential to expand both applied and fundamental avenues of field research of predator-prey dynamics; however, as discussed there are pitfalls of using telemetry study designs for ascertaining fates and investigating predator-prey dynamics in the field. ...
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Predation is a fundamental aspect of ecology that drives ecosystem structure and function. A better understanding of predation can be facilitated by using electronic tags that log or transmit positions of predator or prey species in natural settings, however, there are special considerations that must be made to avoid biased estimates. We provide an overview of the tools available for studying predation with electronic tags including the tag types and analytical tools that can be used to identify where, when and how prey are killed by predators. We also discuss considerations for experimental design when studying predation using electronic tags, including how to minimize effects of capture and tagging procedures. Ongoing innovation and integration of sensors for tags will provide more detailed data about the performance of tagged predators and the fate of tagged prey. Where analysts can effectively resolve the timing of predation using state‐of‐the‐art tags and analytical tools, we foresee exciting advances in our understanding of animal demographics, evolutionary trajectories and management systems. Prospects to develop new tools and approaches for tracking predation while designing studies to more effectively limit bias are an important frontier for understanding ecosystems and addressing human–wildlife conflicts. Given great uncertainties about environmental change and intensifying conflicts between humans and predators, effective study designs integrating electronic tagging to study predation have a promising future in fundamental and applied ecology.
... Ces études portent sur des populations natives mais semblent également valables pour des habitats où l'espèce est arrivée récemment(Vagnon et al. 2022). La consommation d'espèces amphihalines semble décrite plus récemment (e. g. saumon atlantique, alose, lamproie marine;Boisneau et Belhamiti 2018 ;Boulêtreau et al. 2018 ;Guillerault et al. 2018 ;Boulêtreau et al. 2020 ;Boulêtreau et al. 2021) ainsi que la consommation d'oiseaux d'eau et de petits mammifères(Carol et al. 2009 ;Cucherousset et al. 2012 ;Haubrock et al. 2020). Les impacts du silure dans ces aires d'introduction sont relativement peu documentés mais le fait est que les silures adultes peuvent être jusqu'à deux fois plus gros que les prédateurs natifs(Cucherousset et al. 2017). ...
Technical Report
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L’analyse des contenus stomacaux des silures sur différents sites et à différentes saisons a pour objectif de décrire la variabilité spatio-temporelle du régime alimentaire de l’espèce. Si cette approche permet d’avoir une idée précise de la diversité des taxa consommés, elle est peu intégrative et pourrait décrire un comportement de prédation opportuniste et potentiellement ponctuel. Le recours aux analyses des ratios isotopiques permet de compléter et approfondir en décrivant semi-quantitativement le régime alimentaire sur des temps de l’ordre de plusieurs semaines. Enfin, le ratio ARN/ADN individuel renseigne le niveau du métabolisme cellulaire, proxy de l’activité éco-physiologique de l’individu.
... In fish passes, catfish efficiently hunt in coordinated groups so that hardly any migratory fish can survive. Others specifically prey upon salmon (Salmo salar, Boulêtreau et al. 2018) or shad (Alosa alosa) (Belhamiti and Boisneau 2015) and in other rivers, even catch pidgeons from the banks. Several radio transmitters have been found as the last remains of radio-tagged salmon in a cave inhabited by a catfish. ...
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This book gives positive examples how humans and rivers have been, and are still in some places, living in harmony. It analyses how this knowledge can be transferred into modern river management schemes and thereby it attempts to mitigate the deplorable trend of the decline of biological and cultural heritages and diversities in and along rivers. A harmonious way to live with the river includes i.a. respecting its natural features and ecosystem services. This means that human land use forms and cultures, including fishing, agriculture, navigation and river works respect the natural hydrological patterns (Flood Pulse, Environmental Flows). It also includes the physical-psychological-spiritual linkage of the people to the river (e.g. worshipping, well-being, detention, and in-spiration), and how these linkages serve as a motivation to take action in favor of the river’s nature. Twenty-nine case studies from Africa, Asia, the Americas and Europe, and 7 papers on overarching themes of sustainable river management are presented. Without claiming its completeness, we understand this book as a first attempt to highlight the interactions between the biological-evolutive populations of non-human biota and the biological-evolutive-cultural populations of human beings, using the dynamic riverscape as the physical background. The target audience of this book includes interdisciplinary scientists from the fields of ecology, geosciences, social and political sciences, as well as urban planners and managers of river ecosystems and riverine heritage sites worldwide.
Managers and stakeholders increasingly ask whether predation is a driving force behind the poor status of many species, and whether predator control is likely to be a successful management action to intervene. We review existing literature on Atlantic salmon Salmo salar predation and predator control, as well as general ecological theory on the role of predation in the life cycle of this iconic fish. Many bird, mammal, and fish predators target salmon at different life stages. In healthy salmon populations, predation is likely compensated for by reduced intra-specific competition during the freshwater stage. There is little evidence that predation alone has been an underlying mechanism for driving salmon populations below conservation limits. However, depending on the predator’s response to salmon abundance, predation may keep decimated populations from recovering, even when the actual causes of decline have been removed. Under such a scenario, predation control may contribute to recovery, but there are no strong examples that clearly demonstrate the efficacy of managing predators to recover threatened salmon populations, challenging further applications.
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This work aims to present an historical review of the ecosystem services provided by anadromous fish (i.e., species that migrate from the sea to the river to spawn) throughout Human time, as well as of the main related threats, focusing on the Iberian Peninsula region. Anadromous fish provide important provision, cultural, regulatory and supporting ecosystem services across their distribution range and have been extensively exploited by humans since prehistoric times. In the Iberian Peninsula, sea lamprey, allis and twaite shads, sea trout, Atlantic salmon and European sturgeon were once abundantly present in several river basins covering what is now Portuguese and Spanish territory. These species have suffered a severe decline across their distribution range, mainly due to habitat loss and overexploitation. Considered regal delicacies, these fishes were once a statement on the tables of the highest social classes, a much appreciated bounty for the poorer population and are still an important part of the local gastronomy and economy. Such high economic and cultural interest encouraged intensive fishing. Currently, management efforts are being implemented, pairing habitat rehabilitation (e.g., construction of fish passes in obstacles to migration such as weirs and dams) with sustainable fisheries. Considering the present climate change scenario, these species are bound to endure increased pressures, demanding novel management approaches to ensure population numbers that are able to secure their sustainable exploitation.
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The current study reports the first occurrence and the spread of the European catfish Silurus glanis (Family: Siluridae) in the Portuguese section of the Douro River, suggesting a potential expansion of its distribution in Portugal either via westward dispersal across international rivers and/or human-assisted introductions into new reservoirs and drainages. The European catfish has unique features (e.g., opportunistic predator, hunting, and aggregation behaviour) that make it highly suitable for establishing self-sustaining populations in new areas and likely contribute to its invasion success. The species may severely affect native prey communities and modify food web structure and ecosystem functioning. Efficient and sustainable management actions are needed to prevent further introductions in the future.
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Most lotic ecosystems have been heavily modified in recent centuries to serve human needs, for example, by building dams to form reservoirs. However, reservoirs have major impacts on freshwater ecosystem functions and severely affect rheophilic fishes. The aim of this review is to gather evidence that aside from direct habitat size reductions due to reservoir construction, competition for food and space and predation from generalist fishes affect rheophilic community compositions in tributaries (river/stream not directly affected by water retention). River fragmentation by reservoirs enables the establishment of generalist species in altered river sections. The settlement of generalist species, which proliferate in reservoirs and replace most of the native fish species formerly present in pristine river, may cause further diversity loss in tributaries. Generalist migrations in tributaries, spanning from tens of metres to kilometres, affect fish communities that have not been directly impacted by reservoir construction. This causes "edge effects" where two distinct fish communities meet. Such interactions temporarily or permanently reduce the effective sizes of available habitats for many native specialized rheophilic fish species. We identified gaps that need to be considered to understand the mechanistic functioning of distinct fauna at habitat edges. We call for detailed temporal telemetry and trophic interaction studies to clarify the mechanisms that drive community changes upstream of reservoirs. Finally, we demonstrate how such knowledge may be used in conservation to protect the remnants of rheophilic fish populations.
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In this study, the results of conventional stomach-content analysis are compared with the recent DNA metabarcoding approach on faeces to identify fish species consumed by non-native European catfish Silurus glanis in the Garonne River (south-western France), with a special emphasis on anadromous prey. Fourteen prey species were identified in the stomach contents or faeces, including four anadromous fish species. Despite higher intestine than stomach emptiness, more species were identified through faecal analysis (11 of 14) than through stomach-content analysis (five of 14) suggesting that DNA metabarcoding on faeces is an efficient, non-intrusive technique to study the diet of predatory fishes.
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Atlantic salmon (Salmo salar) is an economically and culturally important species. Norway has more than 400 watercourses with Atlantic salmon and supports a large proportion of the world's wild Atlantic salmon. Atlantic salmon are structured into numerous genetically differentiated populations, and are therefore managed at the population level. Long-distance migrations between freshwater and ocean habitats expose Atlantic salmon to multiple threats, and a number of anthropogenic factors have contributed to the decline of Atlantic salmon during the last decades. Knowledge on the relative importance of the different anthropogenic factors is vital for prioritizing management measures. We developed a semi-quantitative 2D classification system to rank the different anthropogenic factors and used this to assess the major threats to Norwegian Atlantic salmon. Escaped farmed salmon and salmon lice from fish farms were identified as expanding population threats, with escaped farmed salmon being the largest current threat. These two factors affect populations to the extent that they may be critically endangered or lost, with a large likelihood of causing further reductions and losses in the future. The introduced parasite Gyrodactylus salaris, freshwater acidification, hydropower regulation and other habitat alterations were identified as stabilized population threats, which have contributed to populations becoming critically endangered or lost, but with a low likelihood of causing further loss. Other impacts were identified as less influential, either as stabilized or expanding factors that cause loss in terms of number of returning adults, but not to the extent that populations become threatened. Management based on population specific reference points (conservation limits) has reduced exploitation in Norway, and overexploitation was therefore no longer regarded an important impact factor. The classification system may be used as a template for ranking of anthropogenic impact factors in other countries and as a support for national and international conservation efforts.
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The European catfish Silurus glanis is the largest freshwater fish (excluding anadromous species) in Europe. Its measurements raise many concerns about its potential impact on recipient ecosystems and fish communities and also feed many rumors and fantasies. It is largely accepted that the largest individual ever caught was 5 m long and weighed 306 kg. This information has been reported in the scientific literature for at least a century and is now reported in numerous specialized Web sites. In this manuscript, we test the hypothesis that such a huge European catfish specimen, and other specimens of similar size, have never existed. We demonstrate inconsistency between the reported record body dimensions and the normal species length: weight ratio established from growth curves in several European sites. We examined the original picture of the record specimen and concluded this record specimen is likely misidentifica-tion of a sturgeon. At the present time, the accurate largest individual of Silurus glanis was caught in the Rh^ one River in Southern France and measured 2.73 m and weighed 130 kg. Although still a very impressive megafish, it is nevertheless much smaller than this mythical European catfish.
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Numbers of wild anadromous Atlantic salmon (Salmo salar) have declined demonstrably throughout their native range. The current status of runs on rivers historically supporting salmon indicate widespread declines and extirpations in Europe and North America primarily in southern portions of the range. Many of these declines or extirpations can be attributed to the construction of mainstem dams, pollution (including acid rain), and total dewatering of streams. Purported effects on declines during the 1960s through the 1990s include overfishing, and more recently, changing ocean conditions, and intensive aquaculture. Most factors affecting salmon numbers do not act singly, but rather in concert, which masks the relative contribution of each factor. Salmon researchers and managers should not look for a single culprit in declining numbers of salmon, but rather, seek solutions through rigorous data gathering and testing of multiple effects integrated across space and time.
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The European catfish, Silurus glanis, was widely introduced in western Europe, and it has now established self-sustaining populations in numerous large rivers of western France. Using data collected from surveys conducted by the French National Agency for Water and Aquatic Environment (Onema) from 1989 to 2010 in more than 500 sites throughout the country (10636 electrofishing surveys), we investigated the potential impact of the European catfish on fish communities in French rivers.In the first part of the analysis, we compared trends observed before and after the European catfish was established at given sites (before–after analysis). Species richness, evenness and diversity decreased significantly after the European catfish was established at 1.4%, 1.4% and 5.8% of the sites, respectively. Total fish biomass and density decreased significantly at 6.6% and 2.9% of the sites, respectively. In the second part of the analysis, we compared sites with European catfish against sites lacking European catfish during the same period (with–without analysis). Fish species richness was significantly higher in sites with European catfish. No significant differences in fish diversity, evenness, total fish biomass or density were found between sites with or without European catfish.While our results indicate that the European catfish may in a few cases impact fish communities or populations, it does not appear to be responsible for a countrywide collapse in fish assemblages.
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Fish ladders are a strategy for conserving biodiversity, as they can provide connectivity between fragmented habitats and reduce predation on shoals that accumulate immediately below dams. Although the impact of predation downstream of reservoirs has been investigated, especially in juvenile salmonids during their downstream movements, nothing is known about predation on Neotropical fish in the attraction and containment areas commonly found in translocation facilities. This study analysed predation in a fish passage system at the Lajeado Dam on the Tocantins River in Brazil. The abundance, distribution, and the permanence (time spent) of large predatory fish along the ladder, the injuries imposed by piranhas during passage and the presence of other vertebrate predators were investigated. From December 2002 to October 2003, sampling was conducted in four regions (downstream, along the ladder, in the forebay, and upstream of the reservoir) using gillnets, cast nets and counts or visual observations. The captured fish were tagged with thread and beads, and any mutilations were registered. Fish, birds and dolphins were the main predator groups observed, with a predominance of the first two groups. The entrance to the ladder, in the downstream region, was the area with the highest number of large predators and was the only region with relevant non-fish vertebrates. The main predatory fish species were Rhaphiodon vulpinus, Hydrolycus armatus, and Serrasalmus rhombeus. Tagged individuals were detected predating along the ladder for up to 90 days. Mutilations caused by Serrasalmus attacks were noted in 36% of species and 4% of individuals at the top of the ladder. Our results suggested that the high density of fish in the restricted ladder environment, which is associated with injuries suffered along the ladder course and the presence of multiple predator groups with different predation strategies, transformed the fish corridor into a hotspot for predation.
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Large carnivores face serious threats and are experiencing massive declines in their populations and geographic ranges around the world. We highlight how these threats have affected the conservation status and ecological functioning of the 31 largest mammalian carnivores on Earth. Consistent with theory, empirical studies increasingly show that large carnivores have substantial effects on the structure and function of diverse ecosystems. Significant cascading trophic interactions, mediated by their prey or sympatric mesopredators, arise when some of these carnivores are extirpated from or repatriated to ecosystems. Unexpected effects of trophic cascades on various taxa and processes include changes to bird, mammal, invertebrate, and herpetofauna abundance or richness; subsidies to scavengers; altered disease dynamics; carbon sequestration; modified stream morphology; and crop damage. Promoting tolerance and coexistence with large carnivores is a crucial societal challenge that will ultimately determine the fate of Earth’s largest carnivores and all that depends upon them, including humans.