Prison chiefs are struggling to explain why the number of criminals and suspects locked up in Hong Kong's jails has fallen to fewer than 9,000, a record low. At the same time, suspects are spending significantly more time on remand compared with three years ago. In 2008, the average time a person spent in remand was 60 days. This jumped to 80 days in 2010, and new Correctional Services Department figures show that last year, it was 99.5 days. As of June, the penal population was 8,906, made up of 7,249 inmates - those in prisons, drug treatment and rehabilitation centres - and 1,657 people on remand awaiting trial. In 2004, the number was 13,138, with 1,555 people on remand. Each year over the past decade, Hong Kong's overall penal population has decreased, while the number of inmates in drug addiction treatment centres has risen, suggesting a less draconian approach to sentencing. Possible reasons for the downward trend include fewer mainlanders overstaying their visas and more non-custodial sentences being passed, especially on young, first-time offenders. A departmental spokeswoman said it was hard to explain the trend. "The penal population depends on a number of social, economic, demographic and crime factors and the changing trend cannot be fully accounted for without an in-depth investigation," she said. Of the penal population, 81 per cent is male. In the early 2000s, the government projected that by 2024, the city's jails would not be able to hold the expected 15,000 prisoners. This prompted a controversial plan to build a HK$12 billion super prison on Hei Ling Chau, an island off southern Lantau, but it was scrapped in 2004 because of community opposition. In a recent meeting with the department, Karen Laidler, director of the University of Hong Kong's Centre for Criminology, and juvenile-crime expert John Bacon-Shone discussed possible reasons for the downward trend. Immigration figures show that between 2010 and 2013, the number of mainlanders overstaying their visas more than halved - from 3,381 in 2010 to 1,567 last year. "Frankly, they can get jobs on the mainland," Bacon-Shone said. "There used to be large numbers of women who would come here to work as prostitutes … and get arrested, but it's not attractive to come on an illegal basis any more. That's … going to become more and more the case. "Even for prostitution, I suspect there may be more attraction to going [elsewhere]." In 2010, offenders from the mainland, Taiwan and Macau made up 17.9 per cent of the prison population. By last year, this figure had dropped to 12.9 per cent. Currently, the majority of inmates - 72.6 per cent - are from Hong Kong. Bacon-Shone said it was difficult to pinpoint the exact reasons for the steady drop in inmate numbers because of the disconnect between government departments. "So the Correctional Services Department see the consequences of [actions of] the judiciary or immigration but I don't think they necessarily know why," he said. "Each part tends to know only their own part so the police, immigration, correctional services and the judiciary each do their own thing and, in many cases, it's not very well joined up." According to the International Centre for Prison Studies in Britain, Hong Kong ranks 125th in terms of its prison population rate, with 123 people per 100,000. With fewer prisoners, the average occupancy rate of correctional facilities was about 80 per cent last year. The department said no facility was facing long-term overcrowding. Siu Lam Psychiatric Centre and Lai Chi Kok Reception Centre - which houses both convicts and those on remand - are the exceptions. The Siu Lam centre holds 275 people although it is built for just 261. The Lai Chi Kok centre, which can hold 1,484, is full.