Three slots is all How to Fish gives you on Lighthouse. Fish, clams, weapons, and anything else worth keeping fill those pockets fast, and the first shop will not sell you more space.
Quick answer: Unlock the boat, sail to the second island, and buy one extra slot from that shop for $5. Buy one more on the third island for $10.
Starting inventory on Lighthouse
You begin with three inventory slots and stay at that size for the whole first island. The Lighthouse shopkeeper stocks other goods, but not a bag upgrade. If a shop screen has no extra slot for sale, you are still in the starting area.
That limit is the gate. Extra space is sold only after you leave Lighthouse and reach later islands.
Inventory slot prices by island
Each later island sells a single upgrade. You cannot purchase the same shop’s slot twice, and you cannot stack several upgrades in one visit.
| Island | Slots sold | Price |
|---|---|---|
| Lighthouse | None | — |
| Second island | 1 extra slot | $5 |
| Third island | 1 extra slot | $10 |
Note: The second island’s shop is empty of further bag upgrades after that $5 purchase. The $10 slot exists only on the third island.
Unlock the boat for later islands
The second island is across the water. You need the boat, and the boat needs keys from the Lighthouse Keeper. Those keys are held until two conditions are done.
Buy extra slots at island shops
Once you land, look for the shopkeeper. The stall works like the one on Lighthouse, with a short buy list that includes the extra inventory slot.
The purchase worked if your bag holds more than the original three slots. If the shop has no upgrade listed, you are still on Lighthouse or you already bought that island’s one slot.
