South Korea was close to completing investigations on the 11 men and 20 women, none of whom had expressed an intention to defect to the South, Yonhap news agency reported, citing a Seoul official.
"Since none of them wish to defect, all 31 of them will be returned home by Panmunjom within this week," said the unnamed official quoted by Yonhap, referring to a heavily-fortified land border village.
Their boat will be returned through the maritime border off the west coast, the official was quoted as saying.
The South's defence ministry declined to comment. A spokesman for the unification ministry handling inter-Korean affairs was unavailable for comment.
The fishing workers arrived off Yeonpyeong island near the Yellow Sea maritime border in thick fog on February 5 and were towed in by the South's navy.
Pyongyang three days later called for their repatriation. The South normally returns North Koreans who drift across the border by accident and who express a wish to return.
The contested Yellow Sea border has been a persistent flashpoint. The North's shelling of Yeonpyeong in November, which killed two marines and two civilians, briefly sparked fears of war.
More than 20,000 North Koreans fleeing their famine-stricken home have arrived in the capitalist South since the end of the 1950-53 war, half of them in the past five years, mostly via China.
Two weeks ago a North Korean man in his 20s defected to the South by crossing the heavily-fortified and mined land border