Complex sentence: The third introduction took place in 1925, of three males and four females at Husvik Harbour in Stromness Bay, the nucleus of the Busen herd the numbers of which increased slowly to 40 in 1953 before climbing rapidly to about 800 head by the early 1990s.
Split into simpler sentences: The third introduction took place in 1925; three males and four females were released at Husvik Harbour in Stromness Bay. These became the nucleus of the Busen herd, the numbers of which increased slowly to 40 in 1953 before climbing rapidly to about 800 head by the early 1990s.