The Ontario Teachers' Pension Plan, one of Canada's biggest investment pools, has come up billions of dollars short of what it needs to cover future benefits because too many members are on pension and too few are still working.

Despite good investment gains, a preliminary valuation of the $108.5-billion fund as of Jan. 1 showed a shortfall of $12.7 billion, plan officials said Tuesday.

"This highlights the continuing challenge of managing a mature plan,” Teachers' CEO Jim Leech said in a statement, citing examples:

The ratio of working members to pensioners has shrunk to 1.6 to one, and the plan now pays out $4 billion a year in benefits, almost twice the $2.1 billion it receives in contributions.

"Put simply, a declining proportion of the plan’s members now bear increasing responsibility for keeping it fully funded. All defined benefit pension plans worldwide face this challenge," he said.

Investment gains can help to fill a contributions gap, and Teachers' earned $4.7 billion on its investments in 2007. It is a big player in Canadian markets, currently leading a bid to take phone giant BCE private. 

But a pension plan with a looming deficit can't afford to take risks, Leech lamented.

That means more cautious investing and fewer chances for profit. The portion of the plan's assets invested in stocks fell from 65 per cent in 1995 to 47 per cent in 2007.

It is now up to the plan’s sponsors, the Ontario Teachers’ Federation and the Ontario government, to come up with a plan to make ends meet, the plan statement said.

The options were not specified. They presumably include bigger contributions, smaller pensions or looser accounting rules.

"Teachers' staff and board members have been working with the co-sponsors to help them identify the best solution for eliminating the shortfall," the statement said.