In the old Harper & Brothers edition of _Life on the
Mississippi_ (illustrated by Walter Stewart) that fishy
quotation is way back on page 357, to wit:

"We had dinner on a ground veranda over the water--the chief
fish the renowned fish called pompano, delicious as the less
criminal forms of sin."

But isn't the original date 1874/1875? Indeed, Twain did not
apply for copyright himself until 1883, but for their edition
Houghton & Co. did so in 1874/1875? . . .

I am however no Twain scholar. But your willing student. . . .

(Edd Rowell) senior editor
Mercer University Press
Macon, Georgia USA