I don't think that's a great metric to measure how good eating fish is. If you look at this https://en.m.wikipedia.org/wiki/Environmental_impact_of_meat_production Under "greenhouse gas emissions " you will see that making(or harvesting) fish produces more GHG emissions per gram of protein than anything except for sheep and beef.

Also way, way down at the bottom of that list you will find beans. As in the lowest GHG emitter per gram of protein. Just eat some beans.