Both salting and then smoking them provides double protection to ensure preservation of the fish. This double treatment is used in making red herring.
The herring is salted heavily, either directly in salt or in a very salty brine, for up to 24 hours. They they are smoked heavily for up to 2 days, then subjected to lighter smoke for up to 6 weeks.
They become rock-hard, salty, and a browny-red. Even dried, salted and smoked, they still have a strong fishy smell.
In factories, thousands would be smoked at once.
Bloaters were considered better, because they were milder.
Red herring is usually soaked before using, both to soften them and to leach some of the salt out.
Red herring keep very well.
Red herrings were particularly produced in Yarmouth, England, as were bloaters. They were eaten a lot by sailors and the poor.
Much of Great Yarmouth was destroyed by bombing during World War II.
Literature & Lore
“Up in the morning, and had some red herrings to our breakfast, while my boot-heel was a-mending, by the same token the boy left the hole as big as it was before.” — Samuel Pepys diary entry of 28 February 1660.
“The quantity of herrings that are caught in this season are diversely accounted for. Some have said that the towns of Yarmouth and Lowestoft only have taken 40,000 last in a season. I will not venture to confirm that report; but this I have heard the merchants themselves say, viz., that they have cured – that is to say, hanged and dried in the smoke – 40,000 barrels of merchantable red herrings in one season, which is in itself (though far short of the other) yet a very considerable article; and it is to be added that this is besides all the herrings consumed in the country towns of both those populous counties for thirty miles from the sea, whither very great quantities are carried every tide during the whole season.” — Daniel Dafoe, Tour through the Eastern Counties of England, 1724.
“Before the late civil wars, at Christmas, the first dish that was brought to table was a boar’s head, with a lemon in his mouth. At Queen’s College, in Oxford, they still retain this custom; the bearer of it brings it into the hall, singing to an old tune an old Latin rhyme – Caput apri defero, etc. [The boar’s head in bring I.] The first dish that was brought to table on Easter-day, was a red herring riding away on horseback — i.e., a herring arranged by the cook, something after the manner of a man of horseback, set in a corn-salad.” — Chambers, Roger. The book of days: a miscellany of popular antiquities in connection with the calendar, including anecdote, biography, & history, curiosities of literature and oddities of human life and character. London: W and R. Chambers Ltd. 1832. Volume 2. Page 440.
Red herring is mentioned in an anonymously-written manuscript dating from the early 1400s:
“A sheld of red goulez he beruþ,
A launce red in on hand,
Of wyne red hys mouth ys ful,
And he eteþ no ffyssh
But heryng red by name.” 
Red herrings were sometimes referred to as “Norfolk Capons”. This was in reference to how they were a common food for the poor, taking the place at their table of better quality food such as capons.
The expression “red herring” now means to throw or lure someone off track.
Sometime in the 1600s, or prior, it was the practice to lay a trail for hounds by dragging a dead animal, or a strongly smelling dried fish such as a red herring, along the desired trail. The trail could have been a true one, or a false one.
Here is a clear 1682 reference to a red herring being suggested for use in creating a false trail.
“Therefore I have heard him say often, That the Act of Oblivion was an Act of the King’s Honour and Justice, but not of his Mercy; it being a Treaty and Agreement, much more sacred than any Act of Parliament can be; and I must tell your Lordship, and your Friends the Papists, that if you consider what Promises, Declarations, and Engagements the dissenting Protestants had, both of his Majesty, his Lords, and his Bishops, at the Time of his coming over, and how they have been since used, and with what Submission and Loyalty they have carried themselves, you will not find a parallel instance.
But your Lordship’s Business is, to keep your Hounds in full Cry against the pretended Association, for since you cannot find one really in being, a Red herring from your own Kitchen must be hunted, and trailed through the Kingdom, to make a Noise.
The Malice is more than the Wit in the Matter. You have broken down your Gates in the Chase, and made so many Gaps in your own Hedges, that your Cattle are broke out and come to the Pound; and what Sort of Beasts you trade in will be discovered. ‘Tis an Impudence beyond the Jesuits, to say that nothing was more exactly proved, nothing more unquestionable and free from Disputes, than that the Association was seized in the Earl’s Closet; Gwyn himself neither does nor dare positively swear it…. ” — Excerpt from: A Reply to the Second Return: 1682. Unknown author. In F. Cogan. A collection of scarce and valuable tracts, on the most interesting and entertaining subjects: but chiefly such as relate to the history and constitution of these kingdoms. London. 1750. Volume 3. Page 342.
The OED first recorded the term in 1884, giving it the meaning of “false scent.”
 Rothwell, William. Editor. Femina (Trinity College, Cambridge MS B.14.40). University of Wales: The Anglo-Norman On-Line Hub, 2005. Page 27. [Ed: “Femina” is a four-part manuscript held at Trinity College, Cambridge, written in three languages (Middle English, French and Latin), whose stated aim is to teach French “rethorice” (language.) It is called “Femina” because the book says, in a Latin portion, that a woman can use it to teach her child French.