It's not the spot. Each character has one of each group of fish that is deliberately rare (or they couldn't code out and then justified leaving it rare), with the idea being to encourage people to swap fish. For me, Shad is the Deep Sea fish I get in quests constantly, but catch at a lower rate.
Alas, many of the honesty-system based Fishing Co-Ops had people loot the fish for themselves, so most keep their catches to themselves now and just grind out the missing ones. That's all you can do, unless you can find someone who'll swap like for like now.