With Canada's prison service absorbed in an overhaul of its solitary confinement policies, new figures show how similar efforts to limit inmates in solitary over the past five years have failed to make much headway, particularly among aboriginal inmates.

Over all, the number of inmates who have spent at least one day in solitary confinement grew from 6,165 in the 2009-10 fiscal year to 6,758 in 2013-14, according to figures filed in the House of Commons in response to written questions from Liberal MP Irwin Cotler.

That works out to as many as one in five federal inmates spending a day or more in solitary confinement last year.

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"Administrative segregation is supposed to be used as a last resort when there's no reasonable alternative," Mr. Cotler said. "But it seems from these figures that it's being used rather frequently. That's disconcerting."

The increase took place during a span when both the service and its critics placed unprecedented weight on reforming its policies for solitary confinement – referred to as administrative segregation in the Canadian prison system.

In its response to Mr. Cotler, the CSC detailed how its rationale for solitary has shifted in recent years based on 15 separate reports and inquests. Where once it was a key tool in "maintaining the good order of the institution," solitary incarceration is now guided by more Charter-friendly parameters: "to be used as a last resort, where there are no reasonable alternatives, and for the shortest period of time possible."

Despite that philosophical shift, the numbers suggest inmates at some institutions are far more likely to be placed in solitary than they were five years ago. In the case of Joyceville Institution in Kingston, Ont., 201 inmates spent at least a day in solitary in 2009-10. That figure jumped to 263 in the most recent fiscal year, despite the population dropping to 444 from 763 over the same period.

Donnacona Institution near Quebec City held 531 inmates for at least one day last year and yet recorded 333 offenders in its solitary cells.

The hike is particularly steep for aboriginal inmates. Over the past five years, the number of aboriginal offenders who spent at least a day in segregation increased by 22 per cent.

In an e-mailed statement, Correctional Service spokeswoman Sara Parkes said the rise in segregation placements "can be explained by multiple factors that include an increase in inmates being assessed as requiring higher security, and an increase in CSC's efforts to detect and eliminate alcohol and drugs."

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The Correctional Service warned of two factors that could distort the figures. First, numbers for some individual prisons underwent a sudden jump due to the closing of Kingston Penitentiary and Leclerc Institution, whose segregated inmates had to be transferred elsewhere in the CSC system. The agency also said that some inmates could be counted twice if they were transferred between institutions.

Increased solitary confinement here is taking place as it falls from favour elsewhere. The United Nations recently released a draft proposal that would urge member nations to limit solitary stints to 15 days and ban it outright for mentally ill inmates. Several U.S. states have passed laws limiting the use of the incarceration method, which some researchers have linked to long-term mental-health problems. The most high-profile overhaul is taking place at New York's notorious Rikers Island prison complex, where officials announced a prohibition on isolation for inmates 21 and younger in January.

In that respect, Canada is more on trend. The CSC data shows segregation for offenders aged 20 or younger falling by 13.5 per cent over the past five years.

One former CSC warden said age is not generally a consideration when decisions are made to segregate an inmate. "However, in reviewing cases we would always be mindful of maturity and cognitive ability," said Glen Brown, now a criminology instructor at Simon Fraser University.

In a separate response to written questions from Liberal MP Wayne Easter, the agency stated it intends to amend federal segregation policy in the first quarter of 2015 with a goal to "reduce the number and length of segregation placements, prevent unwarranted admissions, and to motivate offenders for release from segregation when risk can no longer be substantiated."