They believe that the way they fish is the best way. The gear that they use does little or no damage to the bottom of the sea. Nothing is dragged for hours at a time, nothing is left to soak in the water for a day or more. They kill very few small fish, or fish they can't sell. But longliners don't feed the world.
Draggers haul large, heavy nets along the floor of the sea, over and over again.
They catch huge amounts of fish, far more than what longliners catch. But then again, huge amounts of what they catch they throw back because it is undersized, or the wrong kind of fish. Most of that fish dies. So there is tremendous waste. And the bottom of the sea, where they drag again and again, can become stripped of any vegetation, any of the living things that fish need to grow.