1) The meaning clearly depends on the context since the two suggested alternatives are contradictory, that is a pickled herring or a fresh herring. The early references here were probably to salted herrings.
1526 to have vij saltfeych, lx whyet heryn and lx red heryn, Skiplam
1729 they used frequently to give her wheat bread ... and white herring made into pastyes, Stainland. Latin references indicate that the term has a very long history: 1377-8 In v barellis allec alby emptis apud Ebor’, Bolton Priory
1395-6 Item eidem de iij barellis allecis albć sibi venditis xxviijs, Whitby. White herring were imported via Hull: 1461 14˝ last’ allecis albi, Hull
1465 4 bar’ white heryng, Hull. An early reference to smoked red herring is in French: 1388 pur j coupill de haranc sor’ et vj pisces sals xvjs, Calverley.