In The Merchant of Venice, what does Shylock mean when he says, "To bait fish withal. If it will feed nothing else, it will feed my revenge."

To understand a particular quotation from a play, you
really have to look at it in context, considering what has happened before the scene you
are referring to and who the character is talking to.

So,
in Act III scene 1 we are presented with a Shylock who is distraught. Jessica, his only
beloved daughter, has fled his household to elope with a Christian. Not only this, but
she also stole his safebox. This massive loss in his life serves to heighten his desire
to gain revenge on Antonio for all that he has suffered as a Jew. Shylock seems to focus
all of his pent up anger and distress on Antonio, repeating again and again the phrase
"Let him look to his bond" to emphasise how determined he is to take his "pound of
flesh." In response to Salerio's comment that he will not surely take the pound of
flesh, for what use would it be, he responds with your
quote:

To
bait fish withal. If it will feed nothing else, it will feed my revenge. He hath
disgraced me, and hindered me half a million, laughed at my losses, mocked at my gains,
scorned my nation, thwarted my bargains, cooled my friends, heated mine enemies; and
what's his reason? I am a
Jew.

Being in such a
vulnerable position and still hurting terribly because of his daughter's desertion,
Shylock now is ever the more eager to gain some kind of relief or comfort from exacting
revenge on Antonio, who he sees as his enemy, because he has disparaged and treated him
badly because of his race. Even if he just uses Antonio's flesh to bait fish, it will
still "feed" his desire for revenge and his thirst to get even in some way and make up
for the massive emotional loss he has suffered.