As posted above this comes up most years and after reading up about it, mackerel like most fish don’t react well to being handled with dry hands, it removes their outer protective skin (slim) and makes them prone to infection.
For the trout fisherman on this board they will know just to well what happens to trout if they have been poorly handled, they look like something out of a horror film with fungi and patches all over.
So try like is suggested above, holding the hook shank and shaking them off, treble hooks will be a pain mind (pain in the finger) for some reason mackerel,herring,shad etc react worse to being handled more than the likes of collie etc.
Nice to hear people are bothered about such things as mackerel, just they seem to be a fish you dont often see being put back no matter how many bags a personn has.
I myself am gulit of killing about 40 a year for bait and more when wreck fishing.