Complex sentence: There are only two main populations of the breed, one on the island of North Ronaldsay itself, the other was established in 1974 when the RBST took 178 sheep off the island to establish new populations.
Split into simpler sentences: There are only 2 main populations of the breed, one on the island of North Ronaldsay itself, and the other was established in 1974. This was when the RBST took 178 sheep off the island and attempted to establish new populations.