{"id":2423,"date":"2026-06-27T11:28:04","date_gmt":"2026-06-27T11:28:04","guid":{"rendered":"https:\/\/arev.is\/?p=2423"},"modified":"2026-06-27T13:54:58","modified_gmt":"2026-06-27T13:54:58","slug":"salmon-farming-genetic-introgression-hofsa-sela-ibsem","status":"publish","type":"post","link":"https:\/\/arev.is\/en\/frettir\/salmon-farming-genetic-introgression-hofsa-sela-ibsem\/","title":{"rendered":"Genetic effects of salmon farming can last for decades \u2014 even after farming stops"},"content":{"rendered":"

An IBSEM modelling study by Arev shows that the proportion of hybrid salmon in the Icelandic rivers Hofs\u00e1 and Sel\u00e1 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.<\/p>\n\n\n\n

Icelandic wild salmon are among the most valuable in the world \u2014 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 t\u00f6lfr\u00e6\u00f0i ehf. has carried out a modelling study using the IBSEM model<\/strong> for Hofs\u00e1 and Sel\u00e1 in Vopnafj\u00f6r\u00f0ur to estimate how severe those effects could be \u2014 and how long they would last.<\/p>\n\n\n\n

What is IBSEM?<\/h2>\n\n\n\n

IBSEM (Individual-Based Salmon Eco-genetic Model) is a simulation model that tracks every individual fish in a population \u2014 each with its own genetic profile \u2014 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)<\/a> Based on a Norwegian river, the model has been adapted by Arev t\u00f6lfr\u00e6\u00f0i ehf. to Icelandic conditions, including water temperatures, habitat size, and population structure in Hofs\u00e1 and Sel\u00e1.<\/p>\n\n\n\n

Hofs\u00e1: Hybrids can reach 25\u201330% \u2014 even after farming ends<\/h2>\n\n\n\n

Based on 400 annual escapes reaching East Icelandic rivers, the results for Hofs\u00e1 (mean of three independent simulation runs) show:<\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

Figure: Proportion of farmed and hybrid fish in Hofs\u00e1 over 100 years \u2014 IBSEM analysis by Arev (Figure S2)<\/em><\/p>\n\n\n\n

Reading the figure:<\/strong> The graph shows the proportion of farmed salmon (dark red line) and hybrids (light red line) in the Hofs\u00e1 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 \u2014 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.<\/p>\n\n\n\n

Sel\u00e1: Similar dynamics<\/h2>\n\n\n\n

In Sel\u00e1, the model produces comparable results. The proportion of hybrids can reach ~15\u201320%<\/strong> and remain at that level for an extended period after farming ends. Although the peak is lower than in Hofs\u00e1 \u2014 reflecting differences in population size and habitat \u2014 the same fundamental pattern holds: the genetic legacy of farming outlasts the farming itself.<\/p>\n\n\n\n

Figure: Proportion of farmed and hybrid fish in Sel\u00e1 over 100 years \u2014 IBSEM analysis by Arev (Figure S2)<\/em><\/p>\n\n\n\n

\"\"<\/figure>\n\n\n\n

<\/p>\n\n\n\n

<\/p>\n\n\n\n

Key findings<\/h2>\n\n\n\n
    \n
  1. Hybrid numbers continue to grow after farming stops.<\/strong> Farmed genetics propagate through the population via hybrids, generation after generation.<\/li>\n\n\n\n
  2. Recovery takes ~50 years after the last escape<\/strong> \u2014 even if farming operated for only a few decades.<\/li>\n\n\n\n
  3. Genes not under strong natural selection<\/strong> can become a permanent fixture in the wild gene pool.<\/li>\n\n\n\n
  4. Early action matters.<\/strong> The longer the intrusion continues, the greater and more lasting the genetic impact.<\/li>\n<\/ol>\n\n\n\n

    For river owners and fishing associations<\/h2>\n\n\n\n

    An analysis like this gives river owners and fishing associations the tools to:<\/p>\n\n\n\n