Study identifies predation as primary cause of wild salmon smolt mortality with sea trout identified as a key predator.
New research has found that three out of four migrating wild Atlantic salmon smolt do not survive their journey through Norway’s fjords, with predation identified as the primary cause of mortality.
The findings, published in Marine Ecology under the title “High Level of Predation of Atlantic Salmon Smolt During Marine Migration,” challenge long-held assumptions about the primary threats facing wild salmon populations.
The study, highlighted by Krister Hoaas, Head of Farming Region West at the Norwegian Seafood Federation in a recent blog post, involved tagging smolt with acoustic transmitters equipped with predation sensors.
Fish were released at various points in the fjord, allowing researchers to monitor survival rates along the migration route. The data showed that none of the smolt released from the three inner sites survived past Nordhordlandsbroen, a bridge linking Bergen and Alver municipalities, suggesting both high predation levels and possible structural barriers to migration.
Predation and Migration Barriers Identified as Key Risks
The study found that 73% of smolt were consumed by predators before reaching the outer fjord, raising questions about current risk models that prioritise aquaculture-related threats over natural predation. The researchers did not determine the precise species responsible, though cod, pollock, and sea trout were identified as probable predators.
Notably, the study found no correlation between salmon farming and mortality rates in the area, with smolt passing five fish farms en route to the open sea. The three innermost farms were empty during the migration period, eliminating potential interactions with farmed fish. The findings appear to contradict risk assessments from the Scientific Advisory Committee for Salmon Management (VRL), which has not included predation as a major factor in wild salmon declines, Hoaas pointed out.
The study’s findings may prompt a reassessment of the relative impact of predation, habitat barriers, and aquaculture-related threats on wild salmon survival rates. Under the Institute of Marine Research’s (HI) modelling, policy debates that have focused overwhelmingly on sea lice as the primary driver of wild salmon decline.
The study adds to a growing body of research from Denmark, Sweden, Scotland, Canada, and the US, where predation is increasingly recognised as a significant factor in wild salmon mortality. The full findings are available via Aquablogg, which first reported on the study: Aquablogg article.
