If you don't declare foreign income, the taxman will assume you have no foreign income and will tax you on the difference between what you earned and what you declared.
If you're a Canadian citizen, you're entitled to claim foreign income on your tax return.
If you're a non-Canadian citizen, you can only claim foreign income if you're a resident of Canada.
If you're a non-resident of Canada, you can claim foreign income, but you'll have to pay tax on it.
If you're a resident of Canada and you don't declare foreign income, you'll be taxed on the difference between what you earned and what you declared.
If you're a non-resident of Canada and you don't declare foreign income, you'll be taxed on the difference between what you earned and what you declared.