Complex sentence: North Pacific, Atlantic, and Southern Ocean humpbacks, which have distinct populations which complete a migratory round-trip each year and the Indian Ocean population, which does not migrate, prevented by that ocean's northern coastline.
Split into simpler sentences: North Pacific, Atlantic, and Southern Ocean humpbacks have distinct populations which complete a migratory round-trip each year. The Indian Ocean population does not migrate, prevented by that ocean's northern coastline.