Why, I am sure, if he forfeit, thou wilt not take
his flesh: what's that good for?
To bait fish withal [withal: preposition at the end of a phrase, "with"]: 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.
The answer to Shylock's allusion, if there is one, must come from the text rather than from speculation. Solarino has just said to Shylock that surely, if Antonio's ships fail to come in and his "bond" [loan obligation] comes due, he will not require the absurd condition of a pound of Antonio's flesh, asking, "what's that good for?" Salarino's allusion, and what Shylock correctly understands him to mean, is to food: flesh of animals is used for food. Salarino questions Shylock's condition because, once the pound of flesh is delivered, it is no good; it can't be used for food of any sort.
Shylock's answer, in its essential parts, is that regardless of whether Antonio's flesh is good for food or not, it can at least feed the fishes and it will metaphorically feed his desire for revenge for all the ill Antonio has done to him. If you examine the list Shylock gives as being accurate and not laced with rhetorical hyperbole (exaggeration), the damages Antonio has caused are considerable:
- disgracing Shylock
- hindered his business to the extent that he lost 500,000 ducats
- knew of and thus laughed cruelly at those losses
- mocked him (disparaged, reviled him) when he profits in his business
- hated his "nation" of Jewish people
- actively interfered with Shylock's conduct of his business
- turned Shylock's friends against him
- stirred Shylock's enemies up against him
Now the question remains: While alluding to flesh as food, does "to bait fish with" also allude to the Christian ichthus fish symbol that represents the acronym for "Jesus Christ, Son of God, Savior" or to a pun on the Apostle's occupations as fishermen? While it may be that it does allude to the ichthus or pun on a metaphor, this would be between Shakespeare and his audience rather than a textual allusion or pun. Solarino never answers and he and Shylock are both clearly talking about flesh as food.
As a Jew, Shylock would be unable to even contemplate Antonio's flesh in context of food except for wild beasts or wild fish. Shylock immediately furthers the food allusion by saying that it will metaphorically feed his revenge, a revenge he desperately wants to take against Antonio [here we have Shylock's great tragic flaw: his burning desire for revenge, which goes directly against Judaic law, "O God, to whom vengeance belongeth" (Psalms 94:1)].
According to textual analysis, then, Shylock is not intending Solarino to understand his remark about bait for fish as an allusion to a Christian symbol because the allusion Shylock carries on with is the allusion to food and food's metaphorical importance to revenge. It is even less likely to be a pun, which would be intended to be humorous, referring metaphorically to Jesus' Apostles as fishermen because the speech isn't amusing; Solarino doesn't respond in any way as he would be expected to do to a pun; and the remainder of the speech focuses on reasons for revenge.
As confirmation of this textual analysis against his remark being intended as an allusion or a pun addressed to Solarino about a Christian symbol or metaphor is that Shylock ends his speech, to which Solarino never replies, by re-emphasizing the idea of revenge:
and if you wrong us, shall we not
revenge? If we are like you in the rest, we will
resemble you in that. If a Jew wrong a Christian,
what is his humility? Revenge. If a Christian
wrong a Jew, what should his sufferance be by
Christian example? Why, revenge. The villany you
teach me, I will execute, and it shall go hard but I
will better the instruction.