Fish Skins.

[Lurcolm1]

It doesn't have to be great quality, and I don't think you should tan fish skins with their own fat like you can with animals. But, seriously. It'd help a bunch if you go with an islander run, and it's totally real! That's the fun part!

[Brygun]

Well technically the tree bark we collect as an alternate for tanning is really supposed to be gathered in a certain season (AFAIK). Off hand I forget which one, probably spring.

I suppose you could tan the fish skin in nerjpez fat but thats a bit oof  Might help make a bow with a story.