After posting the usual first-quarter declines, the number of working U.S. oil and gas rigs made a turnaround this week, increasing by 53 to total 946. The rig count climbed by a dozen or more in three states, and other areas also posted slight gains, Baker Hughes Inc. reported Monday. This week's increase reversed the rig count's downward slide _ a traditional trend in the first three months of the year. Last week, the count dropped by 15 to 893. Baker Hughes' rig count _ the widely watched industry index of drilling activity _ reflects the number of rotary rigs actively exploring for oil and gas as of last Friday, not the number of producing wells. A year ago, the rig count totaled 779. Houston-based Hughes Tool Co., an oil toolmaker company that merged with Baker International Corp. of Orange, Calif., has kept track of the rig count since 1940. In December 1981, at the height of the oil boom, the rig count reached a peak of 4,500. But oil prices plunged to less than $10 per 42-gallon barrel in the summer of 1986, prompting a collapse in the rig count that bottomed out at 663 _ the lowest since Hughes began compiling figures. Among major oil-producing states, Texas had the greatest increase with 15, Oklahoma followed with 13 and Lousiana's count climbed by 12. Wyoming's count was up six, North Dakota's increased by four and Colorado and New Mexico each gained three more rigs. Ohio's count was up two and California was up by one. Michigan lost three rigs and Pennsylvania lost ten rigs.