Golden Walnut exactly 100?

[R3i]

Can some ways to get walnuts be skipped, ie fish or farm for them instead ?

 

[imnvs]

Nope. You can only get 5 from farming, 5 from mining, 5 from fishing. The rest are very specific, and you have to track them each down or complete the requirements.

 

[Giant Octopodes]

Some can be skipped though in terms of getting to 100; there are 130 total so you can always save the 30 associated with activities you don't enjoy and focus on the 100 which suits your preferences.

The exact source of each can be found here (obviously heavy spoilers):

Golden Walnut - Stardew Valley Wiki

Golden Walnuts are the currency of the parrots on Ginger Island. The number of Golden Walnuts that a player has saved is displayed directly underneath the Player's name in the Skills Tab. Walnuts are shared amongst players in Multiplayer.

stardewvalleywiki.com

You also don't need all 130 to unlock everything on the island, only 116 are needed to unlock all upgrades, and some of those upgrades are of questionable merit anyway (farm obelisk vs a return scepter for example, or unlocking a "convenient" bridge which lets me forget to bring my watering can only to realize I don't have it when it's absolutely required to progress further on level 7...)