At that point people started moving inland more didn't they and moved away from the shores and riverbanks. The shift is bound to have happened.

However, some communities must have stayed at the shorelines and they would have continued to have a fish rich diet.

We mustn't forget that the shorelines these people would have lived on have gone. So has the record of their diet. Sea/water burial might have been common and any shoreline burials have gone too. Thus the bone data is going to be artificially skewed towards the folks that headed inland, because we have more access to their bones.