When you successfuly catch a fish, there's a chance for it to also give you random materials dependant on the type of fish( and therefore the biome.

This was implemented fairly recently in the same patch that gave us access to being able to finally make the dverger doors ourselves, and also the fishing hat.

It appears that, per the table in the wiki the bonus materials chance to occur seems to range from ~3 to ~16 % depending on the material.