Dave's pretty much on it: Bait bags,zip locks,with heavy covers are the answer.
Shimano - Bass Pro - Berkley all put them out...........keeps sunlight off them,
individual bait boxs (plano) work for a while, but sunlight will fade them.
Hadn't tried to "cod liver oil" thing, don't know if stomach would handle it....grew up
having to take that crap no matter what the sickness was, also was given as a preventive. (i think)