Anthelmintic resistance in sea lice is reducing treatment effectiveness in salmon aquaculture, creating escalating welfare problems as lice burdens increase and treatments become less reliable.
Sea lice resistance to treatments is a welfare escalation problem - as effective treatments become fewer and less reliable, lice burdens increase, treatments must be repeated more frequently, and each treatment causes welfare harm. The cumulative welfare cost is rising as resistance spreads. This is ultimately a consequence of intense treatment pressure in high-density sea cage farming - the same conditions that create disease problems drive resistance development. The welfare solution requires systemic change: reducing treatment pressure through lower density, biological control, genetics and ultimately closed-containment farming systems. The resistance crisis is an argument for accelerating the transition away from open sea cage salmon farming rather than developing more chemical treatment options.