Complex sentence: The US stock breeds mainly at the Channel Islands (although some breeding sites may be established in northern California, and females are now commonly found there) while the Western Baja California stock mainly breeds near Punta Eugenia and at Isla Santa Margarita.
Split into simpler sentences: The US stock breeds mainly in the Channel Islands, although some breeding sites may be established in northern California, and females are now commonly found there. The Western Baja California stock mainly breeds near Punta Eugenia and at Isla Santa Margarita.