An IBSEM modelling study by Arev shows that the proportion of hybrid salmon in the Icelandic rivers Hofsá and Selá could rise substantially and persist long after sea-cage farming operations cease. The findings are a warning for all stakeholders with an interest in wild salmon populations.
Icelandic wild salmon are among the most valuable in the world — uniquely adapted over thousands of years to the country’s natural environment and climate. As sea-cage salmon farming expands along Iceland’s coastlines, the risk of farmed salmon escaping into rivers and interbreeding with wild fish increases. Arev tölfræði ehf. has carried out a modelling study using the IBSEM model for Hofsá and Selá in Vopnafjörður to estimate how severe those effects could be — and how long they would last.
What is IBSEM?
IBSEM (Individual-Based Salmon Eco-genetic Model) is a simulation model that tracks every individual fish in a population — each with its own genetic profile — through its full life cycle. The model simulates escapes of farmed salmon and the resulting spread of genetic introgression over a 100-year period. Originally developed by Castellani et al. (2015) based on a Norwegian river, the model has been adapted by Arev tölfræði ehf. to Icelandic conditions, including water temperatures, habitat size, and population structure in Hofsá and Selá.
Hofsá: Hybrids can reach 25–30% — even after farming ends
Based on 400 annual escapes reaching East Icelandic rivers, the results for Hofsá (mean of three independent simulation runs) show:
Figure: Proportion of farmed and hybrid fish in Hofsá over 100 years — IBSEM analysis by Arev (Figure S2)
Reading the figure: The graph shows the proportion of farmed salmon (dark red line) and hybrids (light red line) in the Hofsá population over 100 years. „Start of intrusion“ marks when escapes begin; „End of intrusion“ marks when they stop. The key observation is that farmed fish disappear quickly after the intrusion ends — but hybrids continue to increase for decades. This is because hybrids produce offspring that carry farmed genetic material generation after generation, long after the original source has gone.
Selá: Similar dynamics
In Selá, the model produces comparable results. The proportion of hybrids can reach ~15–20% and remain at that level for an extended period after farming ends. Although the peak is lower than in Hofsá — reflecting differences in population size and habitat — the same fundamental pattern holds: the genetic legacy of farming outlasts the farming itself.
Figure: Proportion of farmed and hybrid fish in Selá over 100 years — IBSEM analysis by Arev (Figure S2)
Key findings
- Hybrid numbers continue to grow after farming stops. Farmed genetics propagate through the population via hybrids, generation after generation.
- Recovery takes ~50 years after the last escape — even if farming operated for only a few decades.
- Genes not under strong natural selection can become a permanent fixture in the wild gene pool.
- Early action matters. The longer the intrusion continues, the greater and more lasting the genetic impact.
For river owners and fishing associations
An analysis like this gives river owners and fishing associations the tools to:
- Quantify the actual genetic introgression risk for their river based on proximity to sea-cage farming
- Build evidence for environmental impact assessments and permit proceedings
- Compare conservation measures and model their long-term effects
Wild salmon rivers near sea-cage farming operations — in Iceland and elsewhere — may face the same challenges. Arev can carry out an IBSEM analysis for other rivers. Contact us to learn more or request a quote.
About Arev
Arev tölfræði ehf. is an Icelandic statistical consulting firm specialising in modelling and quantitative analysis. We have adapted the IBSEM model to Icelandic conditions and can extend this analysis to additional rivers. The analysis is based on Castellani et al. (2015) and is consistent with the international scientific literature on the genetic effects of sea-cage salmon farming on wild populations.
Analysis based on: Castellani et al. (2015), IBSEM: An Individual-Based Atlantic Salmon Eco-Genetic Model. Adapted to Icelandic conditions by Arev tölfræði ehf.