Why? I gave Willy requested fish, he took one as a gift

[Magpie]

Today Willy asked me for 3 salmon, so I brought them to him the same day in the saloon. Instead of accepting them as the quest items, he took one of them as a gift. I can't restart the day, but I'd sure like to know why this happened. I'd already read I could catch up with him there for quests. No?

[Pixel Peeper]

You have to read the quest goals when they're in your journal.

When Willy asks you for fish, you have to catch them first. The quest goal will change to "Bring Willy the fish" only after you've caught them. If you're not on that step of the quest then the game assumes you're simply giving Willy the fish as a regular gift.

[The Y'All Of Us]

This can happen with both Willy and Clint's 'find X of X for me' quests- the counter ticks down based off of how many you've picked up, and since they aren't taking the items from you you don't need to present them like you would if someone was asking you to bring them something they /will/ take out of your inventory. Specifically, any quest with the words "You get to keep the [object]".

When you're turning in your fisherman's challenge or ore gathering quests, just gather up that much of the item and talk to the quest giver /without/ the thing in your hands. As Tripoteur mentioned above, the questlog will change your task to 'Talk to Willy' which will make the game recognize your next time speaking to that character and trigger the quest completion (So long as you do it within the allotted timeframe). Since talking is what triggers the flag, the 'gifting' flag will still take priority and anything in your hands will be treated like you're giving it to them instead of turning in your quest, even if you've already gifted them something that day (in which case you get the 'You've already given Willy a gift today' popup).

The Slime Hunting quests Lewis gives you also use this flag, but since you're not gathering any items for that one it's not as large a source of confusion.

[Magpie]

Thank you! I'd known about taking ore out of chests, but didn't have any luck with doing the same with the fish. @Charlotte, thank you for the very clear description.