Washington State Ferries returned to regular service Tuesday morning after a staffing disruption left much of the Puget Sound system nearly at a standstill the day before. Thirteen of the agency’s 18 vessels were out of service Monday after 23 engine room employees called in sick. Union leaders said the call-ins were not coordinated and described the disruption as a symptom of a system that relies too heavily on a small workforce and extensive overtime. They said there were no contract negotiations underway and that they did not know why employees had called in sick. Tuesday service resumed after four early cancellations on the Mukilteo-Clinton route and four on the Edmonds-Kingston route. Bainbridge Island-Seattle service, also canceled earlier Tuesday, resumed with a 5:20 a.m. sailing. The Fauntleroy-Southworth-Vashon route operated with two boats, its planned level for the day; a third vessel had been held as a reserve. Other routes, including Seattle-Bremerton, Anacortes-San Juan Islands, Port Townsend-Coupeville and Point Defiance-Tahlequah, operated on their regular schedules. The ferry agency said service levels were back to normal, with only limited early cancellations, as crews were reassigned after Monday’s disruptions. Gov. Bob Ferguson said Monday he expected every vessel on every route to be operating by the first sailing Tuesday. The agency thanked riders for their patience and employees for helping restore service. Edmonds-Kingston passengers should still plan for a separate interruption related to railway maintenance at the Edmonds dock. After the 6:10 p.m. Kingston departure and 7 p.m. Edmonds departure Tuesday, service is scheduled to resume at 9:35 a.m. Wednesday from Kingston and 10:20 a.m. from Edmonds. Riders are encouraged to use Kitsap Transit passenger ferries between Kingston and Seattle during the disruption.