In Scandinavia in particular, there’s a tendency of pescatarians to refer to themselves as vegetarian for social convenience, but that hasn’t changed the definition of “vegetarian”.
We suffocate them to kill them when we pull them from the sea. That's quite mean. Few people would advocate the humanity of killing a cow in the same way.
Fair enough. How much more would it cost / how much more would one have to pay for humanely slaughtered tuna and salmon, I wonder? Would there be a market? After all, we have certified-organic, fair trade, halal and kosher....