"So how does he justify stopping Lundgren and not stopping a fisherman?"

[Thinks it over]

I suspect the difference (At least to whoever wrote this particular story) might lie in Lundgren being a *sportsman* who is killing for the fun of killing whereas the fishermen are killing for their livelihood. Given that fish kill fish to live there's a limit as to how shocked Aquaman could be by the concept were he sane. Likewise, aquariams (At least the Earth-2 variety) would be OK since they provide a steady diet and shelter for their inhabitants in return for their inhabitants being admired.

Then again, the sea is a big place and your second surmise, that's he's friends with some fish but not necessarily all of them might also be the case. A third possibility (although I doubt the DC writers themselves would have thought of this) is that his particular friends are either Sapient or closely pre-Sapient and he's more likely to go to bat for something with close to human intelligence than he is for a critter that doesn't have such intelligence even if they are obstensibly the same species.

But yeah, in Real World terms I suspect what we're seeing here is the problems of a series being written by a series of authors rather than by a single creator. Sooner or later the inconsistencies start coming home to roost. ^_^;