I believe someone already posted something related to this but I couldn't find the post in the Bug Tracker with the keywords "rotten" and "fish" and so I shall make this one.
Simply put, as the title implies, hammering a rotten fish in the inventory does not yield bone shards. Also, while doing so, your character will do a hammering animation at the ground and/or nothing in particular. Related also with the mechanic that coconuts can only be hacked outside of the inventory which is quite reasonable with the current animation... though, I myself, having been raised in the tropics, can attest that most people just make a gentle tap/crack on the coconut while holding both a machete and the coconut in both hands extract the coconut water, and another tap/crack to completely open it, so I guess being able to hack a coconut in the inventory would be logical... though it would require a new animation...
Acquire a hammer and a rotten fish, and hammer the rotten fish in your inventory...
A developer has marked this issue as fixed. This means that the issue has been addressed in the current development build and will likely be in the next update.