Complex sentence: Lascars were barred from entry to British Columbia and other Canadian ports from June 1914 until the late 1940s; therefore, they seldom occur on landing and embarkation records, though they were frequenting the ports but remaining on board the ships.
Split into simpler sentences: Lascars were barred from entry to British Columbia and other Canadian ports from June 1914 until the late 1940s. Therefore, they seldom occur on landing and embarkation records, though they were frequenting the ports but remaining on board the ships.