According to the Canada Revenue Agency page on Interest and investment income:
Generally, you report your share of interest from a joint investment based on how much you contributed to it.
Taxtips.ca expands on this a bit more:
When investments are held in a joint account, the investment income (including capital gains) should be reported based on the funds contributed to the account by each spouse. If the funds were provided equally by both spouses, then the investment income would be split equally.
and links back to the CRA page I cited above.
I found a PDF on the website of Goossen Accounting (p3) that provides another detailed description of this procedure:
Very often, bank accounts, or even bonds and GICs, are held jointly by two or more taxpayers — perhaps a husband and wife or an elderly parent and adult child — and the question arises regarding how interest income from joint holdings should be reported and, especially, how to avoid double reporting of such income. The rule in such cases — a rule that is more easily applied in theory than in practice — is that the interest income is to be reported in the same proportions as the original contribution to the investment.
So, in the simplest of cases, where a husband and wife each contributed exactly one-half of the balance in a bank account, each would be required to report and pay tax on 50% of the interest paid or accrued during the year on that balance. Of course, the bank will not be tracking deposits to determine who contributed what; it will simply issue a T5 slip for the full amount of interest earned. The practical solution is for each spouse to report his
or her share of that interest income, perhaps with a note explaining the breakdown of such interest as between joint account holders.
Even though that page refers to bank accounts, the CRA page leads me to believe that this rule applies to all investment income.
Disclaimer: The standard warning applies. I'm not a tax professional, and I'm definitely not a tax professional familiar with Canadian tax law, so consult a tax professional before submitting your tax forms, if you're concerned.