ROCKLAND, Maine — The city’s police chief said the continued drop in crime in Rockland last year followed a national trend, due in part to changing demographics.

The Rockland Police Department released statistics Thursday that showed that the number of crimes in the city during 2012 was at the lowest level since the state required departments to file annual uniform crime reports nearly 40 years ago.

Rockland reported 326 crimes during 2012 that are included in the uniform crime reports filed with the Maine Department of Public Safety. Those offenses used to determine crime rates are limited to murder, rape, robbery, aggravated assault, burglary, theft, stolen vehicles and arson.

In Rockland, 284 of the 326 offenses reported in the uniform crime reports were thefts. This includes thefts from motor vehicles, shoplifting and stolen bicycles.

The 326 crimes reported during 2012, were down from 366 in 2011, and 427 in 2010. As late as 2005, there were 500 reported crimes in Rockland.

The most crimes reported in Rockland since the uniform crime reports were mandated in 1973 was in 1980 when the city reported 749 offenses.

Rockland Police Chief Bruce Boucher said that nationally crime also has been falling. He said part of the reason is simple demographics. As the population grows older, those people tend to end their criminal activity.

He said young males account for the largest percentage of crimes nationally and locally.

The chief also said the reports continue to affirm a theory he has.

“I’m a firm believer in the 80-20 principle — 80 percent of the crimes are committed by 20 percent of the people. We focus on those 20 percent and when they go to jail, crime goes down,” Boucher said.

In Rockland during 2012, in addition to the 284 thefts reported there were 23 burglaries, one rape, one robbery, five aggravated assaults, 11 stolen motor vehicles, and one arson.

The department had success in clearing those cases as well, the chief said.

The single rape, robbery and arson cases were cleared with arrests, according to the reports filed by the department. Four of the five aggravated assaults also were cleared.

Twenty-three of the 26 burglaries were cleared.

Eighty-seven of the 284 thefts were cleared. Only one of the 11 stolen car cases was cleared, although all the vehicles were recovered.

The chief said that while drug arrests are not included in the statewide crime report to determine crime rates, the number of drug offenses has remained stable since he became Rockland’s chief in 2005. He said there was a large spike in bath salts offenses a little more than a year ago but that has not continued on that level.

Although Rockland has seen the number of these crimes decline, the city has retained one of the highest crime rates among Maine communities with their own police departments. The 2012 statewide crime figures are not yet available, but in 2011, Rockland has the seventh highest rate in Maine.

The rate has risen in part because the city has seen a continued decline in its population in the past 40 years. Rockland serves as the service center for Knox County, city officials have pointed out, and that the number of people who are in the city each day is far greater than the resident population.