http://www.smogon.com/forums/showpost.php?p=1821938&postcount=5
According to this post, I would say you cannot change/control it, and that it will always be like this on the frame you want on this seed.
I saw in the DPPt information that the monster frame for fishing is 4 though, and I read from you that after some tests you've found out it was 3, maybe try to fish on frame 988, and see what happens ? It might be 4 after all, and you've got nothing to lose considering frame 989 gives you "not even a nibble" message..