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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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RESEARCH ARTICLE
Adult Atlantic salmon have a new freshwater
predator
Ste
´phanie Boulêtreau
1
, Adeline Gaillagot
1
, Laurent Carry
2
, Ste
´phane Te
´tard
3
, Eric De
Oliveira
3
, Fre
´de
´ric Santoul
1
*
1EcoLab, Universite
´de Toulouse, CNRS, Toulouse, France, 2MIGADO, Saint-Orens-de-Gameville,
France, 3LNHE, EDF—R&D, Chatou, France
*frederic.santoul@univ-tlse3.fr
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 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
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 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].
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OPEN ACCESS
Citation: Boulêtreau S, Gaillagot A, Carry L, Te
´tard
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 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
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) 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
3
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
st
1993 and December 31
st
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
th
of April to the 26
th
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
´
Pre
´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.
Results
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
shown).
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
th
to July 14
th
, 2015 and from April 29
th
to August 16
th
,
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
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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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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
th
to May 26
th
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.
Discussion
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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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
fishway.
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dam, fisheries), is concerning, especially in the case of declining populations of anadromous
species.
Supporting information
S1 Movie. Adult Atlantic salmon have a new freshwater predator.
(MP4)
Acknowledgments
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,
Fre
´de
´ric Santoul.
Data curation: Ste
´phanie Boulêtreau, Laurent Carry, Eric De Oliveira, Fre
´de
´ric Santoul.
Formal analysis: Adeline Gaillagot, Eric De Oliveira, Fre
´de
´ric Santoul.
Funding acquisition: Ste
´phane Te
´tard, Eric De Oliveira, Fre
´de
´ric Santoul.
Investigation: Ste
´phanie Boulêtreau, Adeline Gaillagot, Laurent Carry, Fre
´de
´ric Santoul.
Methodology: Ste
´phanie Boulêtreau, Adeline Gaillagot, Laurent Carry, Fre
´de
´ric Santoul.
Project administration: Fre
´de
´ric Santoul.
Supervision: Ste
´phanie Boulêtreau, Fre
´de
´ric Santoul.
Validation: Ste
´phanie Boulêtreau, Fre
´de
´ric Santoul.
Writing original draft: Ste
´phanie Boulêtreau, Ste
´phane Te
´tard, Eric De Oliveira, Fre
´de
´ric
Santoul.
Writing review & editing: Ste
´phanie Boulêtreau, Ste
´phane Te
´tard, Eric De Oliveira, Fre
´de
´ric
Santoul.
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A new predator for the Atlantic salmon
PLOS ONE | https://doi.org/10.1371/journal.pone.0196046 April 19, 2018 12 / 12

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... Additionally, the resolution of ISs is high enough that assorted characteristics of fish targets can be observed, including morphological traits (e.g. fins- Langkau et al., 2012 ;Boulêtreau et al., 2018 ) and various behaviours, for example, locomotion (Rose et al., 2005 ;Zhang et al., 2014 ) and predation (Becker et al., 2011a(Becker et al., , 2011b. ...
... Traditionally, four characteristics have been used to discriminate fish in manual processing of IS footage: swimming pattern (e.g. Parsons et al., 2014 ;Keefer et al., 2017 ), body shape (e.g. Rose et al., 2005 ;Becker et al., 2011a ;Boulêtreau et al., 2018 ), length (e.g. Magowan et al. , 2012 ;Gurney et al. , 2014 ), and discrete morphological features (Frias-Torres and Luo, 2009 ;Boulêtreau et al., 2018 ;Jones et al., 2021 ). ...
... Parsons et al., 2014 ;Keefer et al., 2017 ), body shape (e.g. Rose et al., 2005 ;Becker et al., 2011a ;Boulêtreau et al., 2018 ), length (e.g. Magowan et al. , 2012 ;Gurney et al. , 2014 ), and discrete morphological features (Frias-Torres and Luo, 2009 ;Boulêtreau et al., 2018 ;Jones et al., 2021 ). In many circumstances, however, species inferences are made via the collection of alternative evidence that provides information on the composition of the target assemblage, a process sometimes referred to as "ground truth" (McClatchie et al., 2000 ). ...
Article
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Imaging sonars (ISs) are high-frequency acoustic devices that are increasingly being used to study fish in marine and freshwater habitats. Acoustic devices are limited in quantifying species richness, and previous attempts to identify fish species using IS have mostly focused on assemblages of low species richness or high morphological diversity. This study aimed to determine the ability of IS for identifying fish species at a subtropical artificial reef off Perth, Western Australia. Several fish traits that could be defined using IS were identified and described for all fish species observed with simultaneous optical footage. These traits were used to create a clustering algorithm to infer the species identity of IS detections of the five most abundant species at the reef. The identities of all fish from two species (Chromis westaustralis and Neatypus obliquus) were inferred with 100% success, though no individuals from the remaining three species (Seriola dumerili, Coris auricularis, and Pempheris klunzingeri) were correctly identified. An alternative clustering-based approach to categorising fish detected by IS independent of taxonomic inference was also implemented. Overall, this study demonstrates that IS can identify reef fish with variable success, and proposes an alternative method for describing fish assemblages irrespective of species identity.
... The species is well-documented to reach lengths of 2.7 m and weights of 130 kg in Europe [17], and rumored to reach very large sizes (length of 5 m). Catfish are apex predators with a varied diet and ability to adapt to new food resources [2,[18][19][20][21][22]. There are several studies investigating various spatiotemporal behavioral patterns of catfish in Europe. ...
Article
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Fish behavior often varies across a species’ distribution range. Documenting how behaviors vary at fringes in comparison to core habitats is key to understanding the impact of environmental variation and the evolution of local adaptations. Here, we studied the behavior of Wels catfish (Silurus glanis) in Lake Möckeln, Sweden, which represent a European northern fringe population. Adult individuals (101–195 cm, N = 55) were caught and externally marked with data storage tags (DSTs). Fifteen DSTs were recovered a year after tagging, of which 11 tags contained long-term high-resolution behavioral data on the use of vertical (depth) and thermal habitats. This showed that the catfish already became active in late winter (<5 °C) and displayed nocturnal activity primarily during summer and late autumn. The latter included a transition from the bottom to the surface layer at dusk, continuous and high activity close to the surface during the night, and then descent back to deeper water at dawn. During the daytime, the catfish were mainly inactive in the bottom layer. These behaviors contrast with what is documented in conspecifics from the core distribution area, perhaps reflecting adaptive strategies to cope with lower temperatures and shorter summers.
... Furthermore, certain species are notably more sensitive to catfish predation, as demonstrated by the experimental study [16]. Due to its remarkable dietary plasticity, European catfish can quickly switch its focus to ecologically and economically important anadromous species that migrate to rivers in large numbers for short-term reproduction, such as Altantic salmon (Salmo salar) [17], allis shad (Alosa alosa) [18,19], and even sea lamprey (Petromyzon marinus; [20]. The intense predation by European catfish on these anadromous species is noteworthy, and similarly intense predation occurs on common freshwater species, such as common roach (Rutilus rutilus), during their reproduction period [3]. ...
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European catfish is a large-bodied apex predator, a key species in native areas, but invasive in others where it negatively impacts local aquatic fauna necessitates catfish regulation. However, traditional ichthyological methods face challenges in capturing it. The study presents a detailed description of the efficient long-line method, refined through 48 sampling campaigns across twelve European water bodies. This method proves cost-effective and technically undemanding, requiring an average of 5.6 bait fish to catch one European catfish per day. The long-lines outperform other techniques, with the highest Biomass per unit effort (BPUE) of 6.205 kg of catfish per man-hour and minimal by-catch (0.276 kg per man-hour). In contrast, fyke nets, the second most efficient method, achieve a BPUE of 0.621 kg of catfish per man-hour with 3.953 kg of by-catch per man-hour. To optimize long-line catches, a 15 m distance between branch lines and regular relocation is recommended. Live fish is the most effective bait with no significant differences observed among species. However, earthworms, a less controversial alternative, are also efficient, especially for smaller catfish. Our recapture approach using various ichthyological methods revealed no hook avoidance behavior by catfish after a previous catch or avoidance by a certain part of the population. The long-line method is suitable for population regulation, scientific research, and conservation efforts and is the most effective means of capturing live European catfish.
... Both in Germany and in the Netherlands, the abundance of the Wels catfish (Silurus glanis) has increased significantly over the past decade (Schneider & Seufert, 2021;van Rijssel et al., 2021). Wels catfish are large, predatory fish and a study in the South-West of France found that in a fishway of the river Gironne, 35% of the 40 adult salmon passing through it were predated by Wels catfish (Boulêtreau et al., 2018). In the fishways of Iffezheim and Gambsheim in Germany, used by salmon to migrate through, the numbers of Wels catfish have increased from 75 in 2014 to over 300 in 2018. ...
Article
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Freshwater species biodiversity is under threat. The average global decline for migratory fish species is estimated to be more than 75% since 1970. Atlantic salmon is one of these species with a steep decline in northwestern Europe and it even went extinct in the river Rhine in the 1950s. The causes for this decline have been posted to habitat loss, pollution, climate change and overfishing. Annual stocking in the Rhine since the late 1980s resulted in an initial increase in the Atlantic salmon numbers after which numbers collapsed again. In this paper, we lay out the recent decline, estimate losses of smolts and adults at different sections in the freshwater habitat and elaborate on potential causes of the recent decline and these losses. We found that the salmon population of the river Rhine has declined rapidly over the past two decades, with a current estimated spawning population of only $350-800 individuals. The percentage of salmon smolts returning as adults to spawning grounds is estimated at 0.5%-0.6%, well below the 3% supposedly needed to maintain a self-sustaining population. Many individuals disappear during their migrations, with the highest percentage of smolts disappearing in the German tributaries (44%) and the Dutch lower Rhine (71%), while the percentage of disappearing adults is highest in both the Dutch (74%) and the German (78%) Rhine. Causes for the losses per river section remain unclear and possible threats, some specific to the river Rhine, are being discussed. The large losses of smolts and adults in inland waters, compared with open sea losses, indicate that restocking the river Rhine might only result in a self-sustaining population with more ecological restoration than carried out so far along the intensively shipped and highly regulated river course and associated high levels of predation, and might be increasingly limited by future climate change. K E Y W O R D S
... Invasive parasites (e.g. Sandodden et al., 2018) and predators have a strong role in altering fitness of migrating animals, for example the introduction of Wels catfish (Silurus glanis) in France led to increased pre-spawn mortality of salmon trying to use a fish ladder to reach upstream spawning areas (Boulêtreau et al., 2018). The spread of non-endemic diseases is also presenting an urgent threat to migratory species; chronic wasting disease, a transmissible prion disease in ungulates (Edmunds et al., 2018), and avian influenza A ('bird flu'; Olsen et al., 2006), are growing focal points of research on migratory animals that may threaten future viability of many species. ...
Article
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Animal migration has fascinated scientists and the public alike for centuries, yet migratory animals are facing diverse threats that could lead to their demise. The Anthropocene is characterised by the reality that humans are the dominant force on Earth, having manifold negative effects on biodiversity and ecosystem function. Considerable research focus has been given to assessing anthropogenic impacts on the numerical abundance of species/populations, whereas relatively less attention has been devoted to animal migration. However, there are clear linkages, for example, where human‐driven impacts on migration behaviour can lead to population/species declines or even extinction. Here, we explore anthropogenic threats to migratory animals (in all domains – aquatic, terrestrial, and aerial) using International Union for the Conservation of Nature (IUCN) Threat Taxonomy classifications. We reveal the diverse threats (e.g. human development, disease, invasive species, climate change, exploitation, pollution) that impact migratory wildlife in varied ways spanning taxa, life stages and type of impact (e.g. from direct mortality to changes in behaviour, health, and physiology). Notably, these threats often interact in complex and unpredictable ways to the detriment of wildlife, further complicating management. Fortunately, we are beginning to identify strategies for conserving and managing migratory animals in the Anthropocene. We provide a set of strategies that, if embraced, have the potential to ensure that migratory animals, and the important ecological functions sustained by migration, persist.
... Predictably, this can lead to dominance of ariid population, as they have dual competitive advantage over other important benthic predatory species in the bay. Catfish dominance is known to profoundly influence the changes in biodiversity (Copp et al. 2009) and many species may go locally extinct (Boulêtreau et al. 2018). The local habitat of the upper bay will have reduced benthic biodiversity as the dominance will be skewed due to increased ariid population. ...
... Predictably, this can lead to dominance of ariid population, as they have dual competitive advantage over other important benthic predatory species in the bay. Catfish dominance is known to profoundly influence the changes in biodiversity (Copp et al. 2009) and many species may go locally extinct (Boulêtreau et al. 2018). The local habitat of the upper bay will have reduced benthic biodiversity as the dominance will be skewed due to increased ariid population. ...
Article
Full-text available
Benthic predatory catfishes are voracious and opportunistic predators and can easily shift their diet according to the availability of prey. In this study, feeding ecology of catfishes from two adjacent habitats of an estuarine bay is compared. The lower bay was relatively pristine as compared to the upper bay and was represented by two families of catfishes—Plotosidae and Ariidae, while the upper bay represented only ariid catfishes. Gut content analysis revealed that catfish predators from lower bay consumed conventional prey like teleosts and benthic invertebrates with a linear pattern of ontogenetic dietary shift. Plicofollis dussumieri and Plotosus canius occupied the position of top predators in the lower bay and were specialized feeders. Other predators like Plotosus lineatus, Arius arius, Arius jella, and Arius maculatus were generalist feeders occupying the position of mesopredators. However, in the upper bay, the catfish predators represented by Arius maculatus, Arius jella, and Arius arius predominantly fed on human discarded food. The easily available human food in the form of chicken, corn, and rice as noted from the investigated guts shows altered trophic guilds of ariid catfishes wherein only mid to large-sized catfish community was noted in this bay. A distinct “trophic switch” altered the trophic function from predation to scavenging which was observed in their feeding behavior. The anthropogenic impact in the form of unmanaged organic waste alters the role of predatory catfishes thereby restructuring the food web that may lead to unknown changes in the estuarine benthic ecosystems resulting in reduced ecosystem services.
... In southern Europe, it is considered a highly problematic IAS (Copp et al. 2009;Cucherousset et al. 2018), because of its top-predator position and ability to feed on a wide range of prey, its aggressive behaviour during the spawning period and high predation pressure on local fishes (Castaldelli et al. 2013;Vejřík et al. 2017a, b). The major ecological impacts associated with S. glanis invasion are (i) local extirpation or significant reduction in abundance and biomass of native fish (Castaldelli et al. 2013;Ferreira et al. 2019;Boulêtreau et al. 2021), (ii) potential alteration of energy fluxes between freshwater-marine habitats due to the predation on migratory species, such as European eel Anguilla anguilla, shads Alosa spp., salmon Salmo salar and sea lamprey Petromyzon marinus (Syväranta et al. 2010;Guillerault et al. 2017;Boulêtreau et al. 2018;Ferreira et al. 2019;Boulêtreau et al. 2021), and alteration of the nutrient cycles with increases in the amount of nitrogen in lakes and reservoirs due to its massive aggregations (Boulêtreau et al. 2011) (iii) potential competition with native predators, such as pike Esox spp. (Vejřík et al. 2017a) or Perca fluviatilis (Vejřík et al. 2017b) and (iv) potential introduction of new pathogens that adversely impact native fish (i.e. ...
Article
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The management of Invasive Alien Species (IAS) is often hindered by ecological, social and economic factors, resulting in inadequate biodiversity protection and inefficient use of public money. A clear example of such inefficient management in aquatic ecosystems is the European catfish Silurus glanis L. in southern Europe. Native to central Eurasia, S. glanis is an emblematic and controversial freshwater fish, being the subject of extensive and profitable trophy angling in central Europe and of commercial fishing in eastern Europe. Concurrently, in western and southern Europe where it was introduced in the XIX century, S. glanis is considered a problematic invader. The lack of comprehensive information on S. glanis invasive populations has limited effective management, which is critical to successfully control the spread and minimize negative impacts on native ecosystems and species. LIFE PREDATOR, started in September 2022 with a budget of € 2.85 million and a consortium of six partners from three countries, aims at developing a multidisciplinary and transnational approach to control established populations of S. glanis, and prevent further spreading and future introductions in southern European lakes and reservoirs. The project will develop and test an early warning system based on eDNA and citizen science and identify the most effective and selective capture techniques to reduce the abundance of catfish, particularly in Natura 2000 lakes, actively involving anglers and professional fishermen on this. Massive raising awareness campaigns will be conducted targeting anglers but also the general public, and protocols and best practices will be transferred to management authorities. For the long-term sustainability of the project, a South European Management Group will be created. Additionally, in northern Italy, where the catfish invasion is more advanced, a local circular economy will be implemented, involving the increase in fishing pressure by encouraging catfish consumption as food.
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The European catfish (Silurus glanis) is a large apex predator native to Eastern Europe. Increasing populations within and outside the species’ native range in recent years, and its popularity with recreational anglers are fueling discussions about appropriate management. To understand the motivations of anglers and their views on different management strategies, an internet survey was conducted in southern Germany. The results showed that catfish anglers differ in several aspects from those targeting other species. For specialists, catfish fishing is a central part of life, and they invest significant time and money to catch trophy sized fish. Most catfish anglers think that their targeted species has no negative effect on the local fish community and practice catch and release, despite this practice being illegal in Germany. Large catfish are often released under the misapprehension that they are inedible. The findings of this study suggest that new fishery management approaches are needed in order to mitigate the impact of European catfish in southern Germany. A co-production approach actively incorporating anglers’ perspectives will be essential in implementing education and incentives for catfish consumption alongside other aspects of fisheries management.
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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.