Spooked by Halloween, the national box office over the weekend showed a 22% decline from the comparable 1991 frame. The top 10 total dropped to $ 28,163,719 from $ 35,795,994 in 1991, and dropping 23% from the $ 36,736,075 posted a week ago.Clairvoyant distributors counted on the Halloween weekend as a soft spot in the distribution schedule. The last time Halloween fell on a Saturday was in 1987, when national B.O. business dropped 15% from the previous week. With that info stored in the studio memory banks, there was a famine of new studio releases this time around. In a limited break, MGM’s “The Lover” wooed $ 181,147 in its opening clinch. The pic played in eight Los Angeles, New York, Chicago and Canada situations for a per-screen average of $ 22,643. Warner Bros.’ Steven Seagal-starrer “Under Siege” dominated the B.O. for the fourth consecutive weekend, collecting $ 6,195,221, down 31%. The pic passed the $ 50 million marker on its way to a total of $ 51,425,497; Seagal’s earlier top moneymaker was Fox’s $ 44 million grosser “Marked for Death.” Surging into the No. 2 spot was Columbia’s “A River Runs Through It,” which widened from 130 screens to 795 in its fourth week of release. B.O. in such markets as Salt Lake City, Waco, Texas, and Columbus, Ohio, provided solid contributions to the pic’s $ 5,351 per-screen average. “River” lured $ 4,254,015 over the weekend for a 24-day total of $ 7,455,004. TriStar’s “Candyman” continued to show unusual strength for a horror movie, dropping just 21% to $ 3,354,546 in its third weekend of release. The bargain-basement acquisition from Columbia/TriStar Home Video has grossed $ 15, 656,516 for TriStar to date. BV’s “The Mighty Ducks” iced $ 2,688,706 on 1,856 rinks, but was penalized by 36% for its kidpic status on the fright-night weekend. Pic ranked just ahead of BV’s “Consenting Adults,” sliding 35% to $ 2,421,911 in 1,495 venues. There were no significant new entries in the top 10, as WB’s “Pure Country,” U’s “Dr. Giggles” and 20th Century Fox’s “Night and the City” and U’s “Sneakers” ranking seventh through 10th, respectively. Last year, B.O. was bolstered by the $ 5,522,250 opening of Wes Craven’s “The People Under the Stairs,” one of three pix over the $ 5 million mark. Halloween fell on a Thursday last year. The somewhat morbid weekend tallies stood to leave plenty of screens available for hot numbers opening this weekend, including Paramount’s “Jennifer Eight” and Warner Bros.’ “Passenger 57.” Release patterns of such pictures as U’s “Mr. Baseball,” WB’s “Pure Country” and Colpix’s “Hero” are likely to shrink by next weekend.