The amount of empty retail space on O'ahu could double this year as the local economic downturn pushes more stores and restaurants out of business, according to a new forecast.

Local commercial real estate firm Colliers Monroe Friedlander predicts that retail space vacancy could rise this year to between 6 percent and 6.5 percent, from 3.23 percent at the end of last year.

Earlier this month, about 4 percent of retail space was empty, representing 467,535 square feet of space out of 11.45 million square feet on O'ahu, Colliers said.

Since the end of last year, 74,282 square feet of retail space have been vacated, partially by such companies as Circuit City, USA Baby, Brew Moon and E&O Trading Co., which have gone out of business this year amid the economic downturn.

The resulting empty space, which Colliers said was also influenced by the closure of five Starbucks cafes, pushed retail vacancies to a five-year high, while the average rent being asked by landlords fell for the first time in six years.

Average asking rent for retail space outside Waikiki and Ala Moana Center is at $3.52 per square foot a month, down 6 percent from $3.72 at the end of last year, Colliers reported. The average asking rent in Waikiki is down 10 percent.

Mike Hamasu, research and consulting director at Colliers, said some tenants struggling to stay in business are asking landlords for rent relief, which is creating downward pressure on rents for vacant space.

"As the longest post-World War II recession continues, an erosion of current market conditions is anticipated," Colliers said in the report.

Despite the rise in vacancies so far this year, O'ahu has maintained one of the lowest retail vacancy rates of major U.S. metropolitan areas, some of which have experienced double-digit vacancy rate increases.

"The tenants are still hanging in there," Hamasu said about retailers on O'ahu.