The only way to sample fish is to pull them up to the surface, so you need a net, or you need hooks, traps, or whatever it is that fish will go into. We have an entire sablefish fishery on the west coast that's one of the most valuable in Canada. It's highly selective because only sablefish go into traps that they set on the bottom—not only. They get sablefish. They get some rougheye rockfish, but that's about it. If that were your sampling method, you'd think the world was only made of sablefish.
If you take a trawl net, for instance, here on the east coast we found that when the trawl moratorium was put in place, in 1992 I believe, there was such a strong reduction in trawl fishing that the halibut took off—Atlantic halibut took off—because that trawl fishery tended to catch small halibut, and every new year class of halibut that would come out was getting taken out by the cod fishery.
Gill nets are the same. They're very selective for size. With hooks, you can control the size of the fish and the species that you catch, depending on the size of hook you use.