“The pens holding cold-water fish in the fjords of Norway and quiet lochs of Scotland, whose depths are normally inky black after dark, now have an eerie night glow. There’s a perfectly rational explanation: Thousands of laser pulses are doing an important job, lighting up fish stocks in the latest bid to control a pesky parasite that can injure or even kill farmed salmon, and devastate an industry estimated to reel in about US $10 billion annually. The problem is sea lice, marine ectoparasites. Two particular sea lice species, Lepeophtheirus salmonis and Caligus elongatus, attach to salmon and sea trout, feeding off their tissues, blood, and protective external mucus membranes. Though the lice find the flesh and blood of wild and farmed fish equally palatable, the problem is particularly acute in densely populated salmon farm pens, where the parasites’ food source are collected in a stationary place. Most pens typically keep 50,000 to 150,000 fish swimming inside a mesh perimeter. So, while the lice are a relatively harmless problem for fast and free-moving wild fish, the captive host in the pens find themselves unable to escape the louse onslaught.”