Each difficulty level has its own range of potential weight of fish, and upgrading your rod increases your chance of catching heavier fish as well.

Using chum increases your chances of hooking a larger target fish, but like bait, chum is single-use. Be sure to have a high percent chance of finding your target fish before using chum, otherwise the fish you catch (while it may be large) it may not be the target fish.

Fish chum can be acquired from Fishing Crates.