Cameroon has formally taken full sovereignty of the oil-rich Bakassi peninsula, ceded by Nigeria in 2008.
The territory was handed over after an International Court of Justice ruling, ending years of border skirmishes.
The five-year UN-backed transition period exempted residents in the area, many of them Nigerian fishermen, from paying tax.
Now the Nigerians must also apply for a residence permit or take up Cameroonian citizenship if they wish to remain.
They were just interested in the oil”
The BBC’s Randy Joe Sa’ah in the capital, Yaounde, says official figures put the number of people living in the peninsula at 300,000, 90% of whom are Nigeria.
It is not clear how many have decided to leave the region, but it is believed most have decided to remain, he says.
Most of those who have left Cameroon are living in camps in Nigeria’s Cross River state, where they have been critical of the authorities for not doing enough to resettle them.
It was agreed that the transitional phase would allow Cameroon to develop an administrative presence in the 1,000sq km (386 sq mile) area, which juts into the Gulf of Guinea.
For Nigerians refusing to change nationality, a residence permit will cost 130,000 CFA ($260; £170) for two years or 250,000 CFA for 10 years, our reporter says.
In Nigeria, small businesses and road-side stall holders do not have to pay tax, but this is not the case in Cameroon, our reporter says.
Each area is targeted once a year by officials working for the Ministry of Finance accompanied by paramilitary police in order to recover taxes from all businesses, no matter their size, he says.