Complex sentence: Historically, herring has been one of the major protein sources in Sweden, however the elite considered herring being lower class food and consumed the herring mainly in its pickled form, staring in the mid 18th century.
Split into simpler sentences: Historically, herring has been one of the major sources protein in Sweden. However, the elite considered it more of lower class food and did not start consuming it in its pickled form, until the mid 18th century.