The lice issue is actually several issues... Slice is the only current lice medication. There are already many indications of resistance developing to the medication. Generally the Slice is given with feed. Not all feed gets eaten and there are many questions about how the Slice will affect other critters. All of which leads to the notion of medicating minimally (saves fish farmers money, too) and carefully... Right on the edge of efficacy.

Planting those pens right in the migration route of the downbound smolts leaves them swimming through an artificially inflated population of lice. The lice are there only because the salmon pens are there. This is not an easy fix...
art