Fisheries Management Scotland believes at least 3,000 farmed salmon may have found their way into Scottish and some northern English rivers when Storm Ellen struck the Mowi fish farm at Carradale North last August.
It has just released the result of an investigation into the incident when 10 circular net pens containing 550,000 salmon shifted position after sea anchors became dislodged in what was an exceptionally violent summer weather incident.
Some 48,834 salmon managed to escape, but while the vast majority did not get into rivers, anglers reported catching 295 fish, while a further 171 suspected farmed salmon were caught in rivers following visual identification.
The Fisheries Management Scotland report said: “There was strong agreement between visual identification of farmed salmon by anglers and subsequent confirmation of the origin of caught fish by scale analysis.
“Anglers were given guidance on how to distinguish farmed and wild salmon and were asked to provide location of capture, weight, and a photo of suspected farmed fish.
“Fisheries Management Scotland received reports of escaped farmed salmon from 22 rivers. Escaped farmed salmon were verified through scale reading as definitely entering 17 rivers, but the analysis demonstrates that visual identification of farmed salmon by anglers was highly accurate (95-97%).
It adds: “The results demonstrate the speed and high level of dispersal with which the farmed salmon entered fresh water The first capture was nine days after the event. It should be noted that the techniques deployed in this study cannot guarantee that all the salmon caught originated from Carradale North. “
It believes fish appeared in many rivers across the west of Scotland and north west England.
The report continues: “However, given the timing of the captures and that the farmed salmon were all a similar size to those reported by Mowi, we are confident that the majority did come from this specific escape.
“Fisheries Management Scotland is in the process of establishing a work-stream with regulators and industry to assess the feasibility and practicality of recapturing escaped farmed salmon as soon as possible after an escape event and before they enter rivers.
The report says the vast majority of escaped salmon remain unaccounted for and concludes: “Whilst there is variation in the catch efficiency of anglers both geographically and throughout the angling season, it is generally accepted that anglers catch in order of 10% of the wild salmon entering Scotland’s rivers.
“If a capture efficiency of 10% is applied, we can predict that a minimum of 3,000 farmed salmon entered Scottish rivers. It is likely that this is an underestimate of the total numbers of farmed salmon entering rivers.”
The report concludes: “It is clearly in the best interest of both the farmed and wild salmon sectors to ensure that escapes do not occur… we welcome the open and transparent communications from Mowi surrounding the escape incident and their subsequent proactive engagement and cooperation in the monitoring phase. There have been other escape incidents in Scotland since the Carradale North event which have demonstrated the need for a more consistent and strategic approach to managing escapes and a need for more effective communications with wild fisheries managers.”
Following the escape, Fisheries Management Scotland, alongside Marine Scotland Science and funded by Mowi, established a genetic monitoring project. The aim is to determine whether any impact on the genetic integrity of wild salmon populations occurred following the escape.
Data collected in September 2020 will be compared to data collected in 2021 and subsequent years. The study should also show whether the escaped farmed salmon were able to survive and breed the following year and the extent of any genetic impact on the wild population.