As for bluefin tuna, they find them because they can sell them for many thousands of dollars. There are actually a few tuna “ranching” operations, where they net off a huge area of ocean and fatten up fish in it. This is unfortunately not a sustainable alternative either, as the tuna will eat just about all other living things in the ranch area.

And yes, mercury is an issue with all oily fish, including mackerel, though it’s a comparatively minor one. Depending on how it’s farmed, tilapia can indeed be very sustainable, and it doesn’t have the same mercury issue. There are actually some very cool systems where you can grow crops and raise tilapia and the two sustain each other, kind of like this project: http://gardenpool.org/