Probably by doing it when it's too cold out for the bugs. They also dip it in brine to neutralize the pathogens on the surface of the fish.

What I'm curious about is what they do if they put a batch out and then a warm front blows through unexpectedly. Do they take the fish down and stuff them in a freezer until the weather cools off again? I can't imagine they let the whole batch get ruined.