What is the origin of the term "fresh fish"?

I'd only ever encountered this term in more modern movies and literature—typically in a prison setting—referring to new, and therefore more vulnerable, inmates. Recently, however, I noticed its use in The Red Badge of Courage:

Still, he could not put a whole faith in veterans' tales, for recruits were their prey. They talked much of smoke, fire, and blood, but he could not tell how much might be lies. They persistently yelled, "Fresh fish!" at him, and were in no wise to be trusted.

This instance led me to wonder, just how old is this usage exactly? Does anyone have information on its usage before Red Badge (1895) or its origin?

Dictionary

War Slang: American Fighting Words and Phrases Since the Civil War (1994, 2004, Second Edition) by Paul Dickson has two listings for fresh fish.

First in chapter 1, "Civil and Uncivil Words: War Slang Before the Great War: From the War Between the States Through the War with Spain":

fresh fish. Cry that went up from veterans as a new regiment showed up for training.

And in chapter 3, "The Many Words of World War II: G.I.'s, Jeeps, Kilroy, and V-Signs from Two Theaters of War":

fresh fish A Navy recruit.

fresh fish squad A squad of new recruits.

New prisoners

It was used to refer to new prisoners as far back as 1863. Memphis daily appeal, October 15, 1863:

The heavy door swung mornfully back, re-echoing the doleful groans of its neighbors, and as it closed behind us, we found ourself in what is termed in prison language, "the yard." surrounded by at least a hundred vagrants, thieves, burglars, etc . whose united voices rent the air with " Fresh fish, fresh fish, I say" Ah ha! thought we, not so bad after all. The keeper, in anticipation, doubtless, of our arrival, or perhaps instructed by the Hon. Judge, had provided a Rad Snapper or Pompano for our entertainment But alas ! we were soon undeceived.

A similar misunderstanding is reported at Libby prison in the Daily intelligencer of November 10, 1863.

It wasn't good to be fresh fish. Cleveland morning leader, April 26, 1864:

An ingenious device for picking pockets has been invented by the rebel prisoners confined at Wheeling Va. When a new prisoner arrives some of the initiated starts the cry of " fresh fish " which is understood to convey the knowledge of the arrival. When the new prisoner is ushered in he is immediately seized by the occupants of the room, placed in a blanket and thrown up. They continue to toss the new comer in this manner until his pocket-book falls out, when he is released and the pocket-book is confiscated.

The Daily Dispatch of November 13, 1863 reports of similar robbings at Libby prison following a cry of "fresh fish, fresh fish".

Although sometimes they were protected (for a while). Daily intelligencer, April 19, 1864, has a similar explanation of the technique as the Cleveland morning leader, and then continues:

Yesterday morning when Robert Y. Conrad and his distinguished friends were unshered into the prison room, the cry of "fresh fish" was started as usual, and the prisoners were about to serve them as they had served others, but as they were confined as hostages and were supposed to have considerable sums of money, Lieut. Moore interfered, made a chalk mark across the floor as the dividing line, and prohibited the thieves from meddling with the newly arrived prisoners, which in our opinion was quite right and proper.

Finally, Daily intelligencer of May 18, 1864, prints a litany invented by Libby prisoners:

RELIGIOUS SERVICES AT THE LIBBY.

From the Enquirer.

A number of the prisoners at the Libby who have been "on hand" for a long time, tired of the hum-drum of prison routine, and with the true spirit of New England morality, have gotten up a burlesque "litany," a copy of which one of them, who bas been there for twelve months, has inclosed in an envelope and addressed to Beast Butler. The document passed inspection on yesterday, and will duly be forwarded by flag of truce. The following is a copy. The "fresh fish" alluded to, it is almost unnecessary to say, means nothing more than-fresh arrivals; the constant introduction of new prisoners having become, to the old ones, au intolerable bore.

Libby Prison, Richmond, Va.
Sunday, May 1, 1884.

PRISONERS' LITANY

Most respectfully submitted to Maj. Gen. Butler, United States Commissioner of Exchange:

From torpedoes and surrounding bayonets and Beast Butler, good Lord deliver us.
From lice and bed-bugs, good Lord deliver us.
From corn bread and black beans, good Lord deliver us.
From rebel favoritism, good Lord deliver us.
From special exchanges, good lord deliver us.
From fresh fish, good Lord deliver us.
From twelve months' confinement, good Lord deliver us.
From truce boats, good Lord deliver us.
Amen, amen, amen.

Most respectfully,
Your obedient servant.
PRISONER OF 12 MONTHS' CONFINEMENT.

New recruits

A report in Dodge City times of February 15, 1879 suggests "fresh fish" was in use as a "new recruit" around the same time as "new prisoner", but given many of the new prisoners during the American Civil War were also soldiers, it's easy to see the use moving from one to the other.

Now, I never saw a meaner private soldier or a more sulky and morose tent-mate than luck gave me in the winter of l864-1865. He came down to us in the fall a recruit having enlisted for the big bounty, and at that time the old vets who had faced shot and shell for several years had an edgewise feeling against these "fresh fish," who had pocketed five or six hundred dollars and came down to spend the winter in a warm hut.