Global by-catch – unintended destruction caused by the use of non-selective fishing gear, such as trawl nets, longlines and gillnets – amounts to 20 million metric tons a year (20.000.000.000 kilo). This would be more than enough to feed the entire 3rd world for a while (although this would also have a negative effect on the ocean of course, since other fishes and mammals are living on the thrown back by-catch).

This excellent animation gives you a better idea what by-catch really is.