Bottom trawling is one of the most hideously destructive things we do to this planet. Imagine if you wanted to catch rabbits, so you hooked a giant nylon net to the back of two humvees. Put them 100 yards apart and drive through a forest, knocking down trees, shredding the underbrush, catching anything and everything in its path. At the other end of the forest you carefully step over all of the dying deer, moose, mice etc and pick out the 30-40 rabbits that you want. Then you leave the net and everything else you just happily killed lying at the edge of the forest so nothing can grow under it. That's what bottom trawling is. People would be horrified by it, except that it literally happens under the surface.
This is a huge part of the reason that we're missing a billion crab. This is a huge part of why the king salmon are dying, which is a huge part of why the orcas are starving.
Commercial fishing in general has massive, glaring environmental problems, from bycatch to emissions to illegal overfishing to everything else under the sun. But if I had to pick one issue to focus on it would be bottom trawling.