U-Haul trucks and trailers that brought hundreds of oil field workers and their families to Williston over the past few years are now headed in the other direction.

"It is a good indicator of what's going on here, no doubt about it," Kent Horob, owner of one U-Haul center in town, said about the change in his business. "Last year, it was pretty overwhelming - everything was coming in."

The workers are leaving because of slipping oil prices and slowed drilling in North Dakota's oil patch.

Marie La Veaux, an employee at the town's other U-Haul rental center in town, said there are four reservations this week, all for outbound moves.

"Everyone who has made a reservation this week is going out of state," she said. "They're oil field workers and their families."

Williston Mayor Ward Koeser estimated the city's population has grown from 12,500 to about 15,000 over the past few years because of the increased oil activity in the region.

While people are not leaving Williston en masse, "we have lost some, definitely," he said. "The price of crude where it's at has caused some problems with any company that is oil related, and we're feeling the impact of that."

North Dakota sweet crude was fetching $39.85 on Tuesday, down from a record $136.29 in July.

Fifty-one oil rigs were operating in North Dakota's oil patch this week, down from a record 98 in November, said Ron Ness, president of the North Dakota Petroleum Council. Each active oil rig represents about 40 direct jobs and 80 indirect jobs in the state, he said.

Ness expects at least 15 more oil rigs will be idled soon.

"Hundreds and hundreds" of people have lost their jobs in the state's oil patch, he said. But he believes many of the losses are only temporary.

The last oil boom in Williston began in the late 1970s and went bust a few years later. Times are different now, Ness said.

"The oil industry, for the most part, is trying to keep people on, hoping for better times ahead," he said.

Koeser said the abrupt slowdown in oil activity caught most everyone by surprise. Just last fall, companies were desperately looking for workers, but now, he said, most oil companies have had some layoffs and nearly all have a hiring freeze.

"In oil country, we've grown used to cycles, but this wasn't a cycle that anyone expected to come this soon," the mayor said.

"The industry is portraying this as a temporary blip in the curve instead of a major collapse or bust," said Lynn Helms, director of the state Department of Mineral Resources. "I don't care how you look at it, that's a lot of people who are not working full time or not working at all."

Along with slumping oil prices, North Dakota's harsh winter - which included record snowfall - was responsible for "gobs of wells that were choked back or shut down," Helms said.

"Access to drill rigs and wells has really been hurt," he said.

But while rigs might be idled, they are not leaving the state, he said.

"They are parked here in anticipation of recovery in oil prices," Helms said. "Most people are betting that it will improve sooner than later."

North Dakota has 4,792 oil wells but only 4,151 are producing, Helms said.

"Companies are restricting production because of prices," he said. "We need prices at $50 to $60 a barrel to kick this back into gear."