Goal: The objective is not "catch a rare fish." It is turning a cheap shop item or ground pickup into the one bait that actually summons that boss, then keeping the body when the next boss needs it. Players keep burning the wrong item and having to re-farm the whole chain.

1Spider Crab - Empty Beer Can (Lighthouse)

What people mess up: they drink the beer.

Decision: If the crab never appears, you used the full can or a normal lure. Only the empty can works.
After the kill: Keep the Spider Crab Shell. Hand that to the keeper for boat keys. Selling the shell strands you on island 1.

2Giant Piranha - Modified Leech (Forest / Island 2)

What people mess up: they try to fish for leeches.

Decision: Beginner Boss Lure is for Sunfish and Old Pike, not this fight. Do not spend the Modified Leech until you have a gun and cooked fish ready. The bait is consumed on the attempt.
After the kill: Keep the Giant Piranha Skeleton and hand it back to the same NPC. The hand-in, not the kill, unlocks the next island.

3Mutated Bowhead Whale - Bowhead Carcass (Volcano)

This is two bosses. The first whale is the bait for the second.

Decision: Farm extra Fish Buckets if you can before the first whale. Failed mutated attempts are common. Run downhill immediately after tossing.
After the kill: This is the end of the story chain. The carcass was the bait; you already spent it.

Common Mistakes to Avoid

Video Walkthrough