In the Puget Sound people will oftentimes catch halibut, lingcod or cabezon when trolling for salmon, and especially king salmon because of how low to the bottom your downrigger ball and bait is as well as because of how slowly you're trolling. We also have what they call a dogfish or sand shark out here that you'll catch all the time when trolling for salmon as they swim from the surface down to near the bottom. You can typically tell it's a dogfish pretty quickly as you'll get one or two head shakes and then it's just deadweight pulling it the rest of the way in. Salmon typically give a bunch of head shakes and then kings generally dive deep while silvers swim up to the surface and jump. Anything else is possible as well from catching a crab to sole, flounder, bullhead and so forth. A buddy of mine was out with us fishing years ago that was using all pink gear to try and catch pink salmon, as they tend to go for either pink or white, and caught a 25lb+ king.