With the increase in catch and release in recent times, it may seem like the released fish actually get wise to given baits after being released. That's because some of them do.
A study of rainbow trout in British Columbia lakes compared abundance and catchability, and did find some evidence that the fish became harder to catch in heavily-fished lakes. It isn't the case for all species, but Berkley's fish behavior lab has also found that largemouth bass learn to avoid baits after being caught, but will resume biting in a couple months.
So what does that mean for us anglers? By no means are the fish out-thinking us, but when the bite drops off, you may want to throw them something they haven't seen before. Food for thought.