I just recently heard this expression and couldn't quite figure out its meaning from the context, unless it means something like "take the bait."
Is this a common expression? Is it perhaps an abbreviation of another expression? Most importantly, if it is a common phrase, what does it mean?
Edit: The basic context in which I heard this expression used was this: one person asked a technical question of a group, to which a member of the group responded, beginning with "I'll do the fish" and then proceeding to the technical explanation.