Adult Atlantic salmon have a new freshwater predator

<div><p>The Atlantic salmon (<i>Salmo salar</i>) 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 (<i>Silurus glanis</i>). 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.</p></div

Adult Atlantic salmon have a new freshwater predator - Fig 5

<p><b>European catfish visiting in the fishway</b>: (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.</p

Adult Atlantic salmon have a new freshwater predator - Fig 2

<p><b>Timings of passages of Atlantic salmon (dark grey) and European catfish (light grey) at the fishway in the River Garonne</b>: (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.</p

Adult Atlantic salmon have a new freshwater predator - Fig 3

<p>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).</p

Location and overview of the Golfech fishway on the River Garonne.

<p>Location and overview of the Golfech fishway on the River Garonne.</p