WASHINGTON (AP) _ Pacific islands under U.S. jurisdiction gained population last year while the number of people in Puerto Rico and the Virgin Islands declined slightly, the Census Bureau reported Wednesday.

The 1986 population of Puerto Rico was estimated at 3,274,000, down from 3,282,000 a year earlier. By comparison, it had a population of 3,197,000 in the 1980 census.

The Virgin Islands also showed slight losses between 1985 and 1986, the bureau said, falling from 110,800 people to 109,500. But that is well ahead of the 96,600 counted in 1980.

The Virgin Islands total includes 54,300 in St. Croix and 55,200 in St. Thomas and St. John combined. St. Croix was down about 1,000 people over the year, while the other two islands lost about 300 people.

The major reason for the drop was migration from the Caribbean islands, said Sam Davis of the Census Bureau. Births in Puerto Rico and the Virgin Islands were only slightly fewer than in 1985, he said.

By contrast in the Pacific, American Samoa's population was estimated at 37,300, up from 36,300 in 1985 and above the 32,300 counted in 1980.

Guam has a 1986 population of 126,800, up from 123,900 in 1985 and 106,000 in 1980.

In the Northern Mariana Islands, the 1986 total was 19,700 people, up from 19,100 in 1985 and 16,800 in 1980.