Perhaps you're thinking of the way we sometimes talk about "fish" as a food category. In that sense, "fish" can sometimes refer to the meat of a fish, rather than the animal itself. And, you might be thinking about the distinct ways these two fish are prepared and presented.
Here's why that might lead to confusion:
* Tuna: Often canned, often eaten in sandwiches, salads, and other "everyday" meals.
* Salmon: Often grilled, baked, or smoked, often eaten as a main course, and often considered a more "special" meal.
Because of these differences in preparation and how we talk about them, we might sometimes unconsciously categorize them differently.
But, to be clear: Tuna and salmon are both fish!