Complex sentence: The Dutch were bringing cattle into New York at a time when the Witrick was plentiful, while many British ships coming to America were sailing from Bristol, a geographically convenient port for the export of Gloucester and Welsh cattle.
Split into simpler sentences: The Dutch were bringing their Witrick cattle into New York at a time when the Witrick was plentiful. Additionally, many British ships coming to America were sailing from Bristol, a geographically convenient port for the export of Gloucester and Welsh cattle.