The civilian work forces at Mare Island in Northern California, Long Beach and six other government-owned shipyards will be cut by a total of 5,000, leaving 73,000 workers at the facilities by the end of 1985, the Navy announced. Navy spokesman Lt. Cmdr. Bill Harlow said in Washington that the yards already have a hiring freeze and that layoffs may also be necessary. Harlow said he did not know what the effect of the cutback would be on Mare Island, which has a civilian work force of 10,100. He said a greater amount of the Navy's ship overhaul work will be shifted to private yards because of a slump in commercial shipbuilding and lower costs. "We can't break it out and tell you how much loss there will be through attrition and how much through reduction in force," Harlow said. Other shipyards that will be affected include Pearl Harbor; Puget Sound, Wash.; Portsmouth, N.H.; Philadelphia; Norfolk, Va., and Charleston, S.C.