According to the NPS website on Visitor Centers in Channel Islands National Park:
As of October 2005, planning is underway to open a visitor contact station in a historic house at Scorpion Ranch on Santa Cruz Island. Please check back here for updates on this project.
And, according to an unsources statement on the Wikipedia article on Channel Islands National Park:
A new island visitor center opened at Scorpion Ranch on Santa Cruz Island on April 6, 2009.
Neither information is very useful. Is there a visitor center on Santa Cruz Island? Does it have a ranger present? When?