For a salmon in Canada, the fish harvest ends with a slippery slide through one of five black chutes. Halfway down, a piston squeezes out a puff of exhaust as a pneumatic hammer strikes the fish on the back of the head at more than sixty miles per hour. The salmon slides sideways onto the bleeding-table, its visible eye unnaturally still. That’s how you know it’s stunned, explains Jason Stalker, the harvest master for Marine Harvest, a multinational aquaculture company in Campbell River, British Columbia, that slaughters thirty thousand fish every day. Now the salmon is ready for a quick cut of the gills, through which blood is pumped out of its body by a still-beating heart. The fish are dead before coming to, and, silent as loaves of bread, they push one another into the freezer in the ship’s hold.

The salmon harvest begins when an “air stone” is put into the water at the front of the trough, which is a huge pocket of netting where a few thousand salmon are slowly being corralled. The air stone releases a surge of bubbles that creates motion in the water, and the salmon turn to face it, riding the current. “You see, they’re lined up good, like firewood,” Stalker points out. One by one, the fish are sucked through a tube and into the minivan-sized SI~5 stunner bolted to the stern of a nearby ship.

Above the water, Marine Harvest’s Thurlow Point fish farm is little more than a grid of floating steel walkways. A few strands of rope divide the water beneath them into ten soccer field–sized pens, and the only structure is a fish-feed warehouse. The salmon are military-fatigue green, with phosphorescent scales, black spots along their backs, and slick white bellies. Every few minutes, a fish jumps across the surface of the water—there are half a million of them between nets in this stretch of ocean.

The harvest must be highly choreographed because calm fish yield higher-quality flesh. Panic in the water means bruising and a product that doesn’t last as long on the shelf. Until they’re stunned, the thick bodies of the salmon also pose a danger to the workers.

For every fifteen or so stunned salmon, one emerges from the harvest box still writhing, arching its body into a crescent and slamming its muscular tail against the table. The fish-farm workers spend the day grabbing these frantic exceptions and shoving them headfirst into a smaller manual stunner, which makes a sound like a staple gun. A few get their gills cut while still very much unstunned.

A few feet from the bleeding table, I see a bucket of “morts”: fish either dead or deemed unfit...