As a portuguese I can tell you that sardine stocks are heavily controlled and regulated and there are years where the catchable tonnage is severely reduced. As the fish markets are centralized, meaning fishermen need to sell their catch in a government auction house, you cannot really do industrial fishing out of allowed stock quotas. Sardines quotas are the target of heavy diplomatic negotiations with EU.
South Africa has a pretty strong S. sagax stock, but it all gets canned or ground into fish meal. Eating fresh sardines just isn’t part of the culture — so sad.
Yes, kind of. That said, as far as I know anything that is more complex to digest like dark bread helps prevent type 2 diabetes.
(But maybe it's anyway better to talk to a real doctor or at least a food expert if that is any concern...)
Overfishing of sardines happen because most of it is ground up and used as feed in aquaculture (which is very inefficient). Instead, if you consume sardines directly, it will be a more sustainable enterprise.