You had to get new levels to buy new bait(earthworm = level 4, fly = level 9, caterpillar = level 16. Each new bait improves the catch chance compared to first one like:
earthworm = 2x, fly = 3x, caterpillar = 4x. Later I noticed that it is too difficult for players to get so high XPs as it requires too much time. I am really sorry for thatβ¦