Why do herring have 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 

https://www.researchgate.net/publication/231918276_The_herring_swimbladder_Loss_and_gain_of_gas/amp