State and industry data show the number of rigs actively exploring for or producing oil and natural gas in the top three U.S. producing states declined.

North Dakota state data show 84 active drilling rigs in service as of Monday. That's down from the 86 reported last week, 108 year-on-year and the lowest on record for the state in more than five years.

Oil companies are spending less on exploration and production because crude oil markets have been depressed for nearly a year. That leaves energy companies with less capital to invest in exploration and production.

North Dakota produced 1.17 million barrels of oil per day in February, the last full month for which data are available. That's a decrease of 4 percent from the all-time high reached in December.

North Dakota is the No. 2 oil producer in the nation. In Texas, the No. 1 oil producer, data published last week by oil field services company Baker Hughes show one less rig in service for the week ending May 8. That's down from the 516 rigs reported for the same week in 2014. Gas rig counts were up for the week, while the number of oil rigs was down two and 477 year-on-year.

Some oil companies have reported improved efficiency despite the decline in rig counts. Texas in February produced 2.34 million barrels of oil per day, an increase of 5.2 percent from January.

For California, the No. 3 oil producer in the nation, the rig count fell by one to 13 for the week ending May 8. All of the rigs in service in the state are employed in state oil reservoirs, where state refineries reported a 0.2 percent increase in crude oil inputs for the week ending May 1.