I take a cool box with about 6 inches of crushed ice/cubes in the bottom with me, and as soon as they get caught/dispatched, put them straight in the box, as mackerel start to turn quicly particularly on a warm day.
Also great for putting bass in as well, straight on ice, and then when you get them back to shore they are in tip top condition.
As for catching them, I dont worry about the tubes, all 6 feathers used, and just straight into a bucket, then icebox.
I sometimes use a small dexter wedge and troll slowly back to shore, and never worry about the treble, but if you are worried, get a small split ring from a tackle shop, cut the treble hook off, and and re rig the wedge using a single hook of your own choice.
However also for trolling close to shore, just put a 6 ounce lead on the line, and the normal feathers, and troll along at 3 knots or so, works a treat. If you want the feathers deeper, just let out more line, Simples!
Also the smaller mackerel make superb livebait for Bass, or you can keep them and freeze them use them for pike bait in the winter.