In terms of Long Lines, they are a hell of a lot less damaging than nets, as obviously they are much more selective in what they catch.
I watched a programme called 'Alaskan Fishing Wars' the other night and they were long lining for halibut and Black Cod (sablefish), and although these boats pulled in thousands of pounds of fish on their sets, the fishery is sustainable because every boat has a quota, and the quota limits that are set are based on the scientific advice, and are set well below the sustainable level, so that there will always be a fishery, whereas our limits are set by the CFP, which ignores the scientific advice, and always ends up way above the recruitment and reproduction levels of the fish they catch.
The only bycatch in this fishery was a fish that was only worth $1 per pound, but those fish were still used, whereas trawled bycatch is discarded back over the side, which includes, many sharks, turtles, cetaceans....in fact the whole ecosystem.
These long liners also had to set lines back from the stern of the boat, with bird scaring devices to protect the sea birds that used to go for the baited hooks, and these guys didn't mind having some of their sets damaged by oportunist sperm whales, as they marvelled at the mammals and simply moved on when they lost a few fish!