California’s initial attempt to require all ships calling at California ports to meet low-sulfur emissions standard were struck down. California modified the rules to set fuel requirements for the vessels instead, and these were upheld as valid in-use rules by the Ninth Circuit in Pacific Merchant Shipping v. Goldstene. The Vessel Fuel Rules mandate that vessel operators “use cleaner marine fuels in diesel and diesel-electric engines, main propulsion engines, and auxiliary boilers on vessels operating within twenty-four nautical miles off the California coastline,” and apply to a geographical region known as “Regulated California Waters,” which includes the waters within 24 nautical miles of the state’s shoreline.