Complex sentence: The sea was always an important part of the economy of Nefyn and fishing, particularly for herring, became the prime trade for most of the 18th and 19th centuries: so much so that the town's coat-of-arms bears three herrings.
Split into simpler sentences: The sea was always an important part of the economy of Nefyn and fishing, particularly for herring, became the prime trade for most of the 18th and 19th Century. So much so, that the town's coat-of-arms bears three herrings.