Officials with Washington State Ferries said employees were “working round the clock” to keep ferry sailings on schedule due to a severe shortage of available ferry workers.

By the afternoon of Christmas Day, WSF said it had made more than 500 dispatch calls, cut sailings on a route and stopped service on several others.

Ferry officials called it a “staffing crisis,” and said it was due to staff who had called in sick because of the flu, increased staffing on some types of vessels due to Coast Guard regulations, and a large number of scheduled vacations during the holiday season by WSF staff.

WSF said it had authorized additional overtime to cover the worker shortage, and has asked licensed administrative staff to work aboard the ferries.

Ferry officials are also meeting daily to come up with options to cover staffing needs, and said the union had asked employees to answer the call to work.

WSF is warning travelers to allow for extra travel time and to make alternative travel plans if possible.