will a mortar and pestle be sufficient to grind fish bones? As the question states.  I believe a mortar and pestle is more for things like nuts, seeds etc.  is that the case or will it easily make fish bones e.g. sprats, sardines into small particles?If there is a specific  material of mortar and pestle I need?

Just to add, when answering this question you should imagine the bones are raw or have been boiled for 20 minutes i.e. not fried, pressure cooked or other methods. i know for example fried sardine bones crush easily, but i doubt raw or boiled bones break easily.
Small fish like sardines, pilchards, etc., yes you should be able to pestle these down with no problem at all (although they tend to break down naturally when you mix them with other ingredients.  When I eat sardines and pilchards, the bones are hardly noticeable.

Larger fish would be another matter though.

As with any unknowns, waste a little to learn a lot.  Just try mashing up a few fish and see what the state of the bones are afterwards.