I think that catching a fish in trolling would not change your trolling route.Although it doesn't change your trolling route,but in my experience, the trolling path is not fully circular that can return to starting point.Therefore, it may have a chance to step into the area of another ocean.........
I'll try to explain this as clearly as possible... You can't suddenly move off your original trolling route when you actually catch a fish. The Helicoprion trolling route is more shaped like a rectangle than a circle so I think you're still in the correct trolling route. What lure are you using? I caught 2 of them (in one game day) on the orange lure (#1) and one on the #5 (white) lure. I recommend using the orange one since I heard other people had the most luck on that one too. If you're catching halibuts, then, you're definitely in the Frigid Ocean since they can't be caught anywhere else. The route isn't short at all, by the way. It starts just to the east of Sembilan Island then, heads south, turns slightly northeast and then, just straight east until you're in the Frigid Ocean above Tidur River. The route turns straight north to the very top of the map and back west all the way to Sembilan Island before turning back south to where you started from. Just keep trying, PBgamer didn't catch the Dunkleosteus until 10 in game days. The Helicoprion is a legendary too so just keep trying. Hope this helps. I'll try to clear it up some more if it still doesn't make sense.