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.
It was used to refer to new prisoners as far back as 1863. Memphis daily appeal, October 15, 1863:
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.
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.
Your obedient servant.
PRISONER OF 12 MONTHS' CONFINEMENT.
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.