When hours away from civilisation and electricity keeping bait from rotting and falling apart can be a problem. I buy fresh pilchards (frozen works as well if necessary) and bury in rocksalt (covered completely) for around 5-6 days, left outside.
Use around 4kg salt per kilo of pilchard. make sure they have been gutted before salting as this will slow down the lactic fermentation which causes the rotting. Pillies will hold unrefrigerated for a couple weeks and, even better, won't fall of the hook. the left-over oily salt can be blended through berley. have caught some of my biggest snapper this way.