Cutting open a fish tells you what bait to use.

Wrong, it will only tell you what the fish will eat not what the fish wants to eat. The stomach contents are an indication of what food is readily available but remember fish have excellent digestive systems, & most soft-bodied animals consumed will be unrecognisable. This is why cod always seem to be stuffed with crabs, shrimp and the odd small half digested fish. The most available food is not always the fishes first choice. Fish will seek out prey that is easy to catch, has a higher nutritional value or is simply tastier. What is the point of always using a bait which is in abundace anyway?