The proliferation of sea lice from aquaculture may substantially aggravate the decline in marine survival of wild salmons. In some countries, this risk has motivated regulators to adopt more precautionary policies; in other countries, however, regulators have disputed the need for stricter regulation. This article compares the sea-lice regulations of Norway, Scotland, Ireland, and Canada (British Columbia), showing how varying interpretations of the science on farm–wild interactions have shaped eforts to scale up regulatory measures for mitigating health hazards and mortality risks for wild salmon. In Norway and Scotland, scientifc consensus has expedited cooperation between research and governing institutions and facilitated ambitious policy reforms. In Ireland and Canada, by contrast, scientifc controversy around the scale of farm-lice impacts on wild salmon populations has led to confict and disagreement between researchers and policymakers, and to failure of reform attempts desired by wild salmon stakeholders.