Most fish have largish bones which support the core muscles. Around the abdomen, they act as a sort of framework protecting the organs. With herring, the bones are really fine: almost thread like and there are lots of them: more than on any other fish I've eaten. I would like to know why herring have such fine bones.
Most likely to assist in maintaining neutral buoyancy. Many fish have reduced weight of heavy structures (evolutionary wise)like bone to assist in maintaining nuetral buoyancy. The swimbladder struggles to maintain buoyancy near the surface for some herring