Perhaps biometric "nets" could be made to fish sustainably and efficiently.  By that, I mean an apparatus that is dragged behind the boat as if it were a net; but it is actually a large grid, that kills only the suitably sized right breed of fish, and then scoops it up.  It would be much more cost efficient and it would save a lot of time, as well as human effort, to fish in such a manner.   Also, all of the other fish could either swim through the grid if they are small enough or around it if they were too large - like the porpoise, shark, or whale.