FOR these are sacred fishes all
Who know that lord that is the lord of all;
Come to the brim and nose the friendly hand
That sways and can beshadow all the land.
Nor only so, but have their names, and come
When they are summoned by the Lord of Rome.
Here once his line an impious Lybian threw;
And as with tremulous reed his prey he drew,
Straight, the light failed him.
He groped, nor found the prey that he had ta'en.
Now as a warning to the fisher clan
Beside the lake he sits, a beggarman.
Thou, then, while still thine innocence is pure,
Flee swiftly, nor presume to set thy lure;
Respect these fishes, for their friends are great;
And in the waters empty all thy bait.

Robert Louis Stevenson

http://www.poemhunter.com/poem/ad-piscatorem/