Torontosm is right.
If canada has a tax treaty with your foreign country then you most likely wouldn't have to declare the interest. But think logically, govt won't go after people with undeclared interest of $50. But if its $4000 say and they found out then you may be penalised.
I think the process of 'double tax treaty' is that you only pay tax above what the foreign tax is.
So say in tge foreign country you are taxed 5% on the intrrest which is equivalent to cad 100.
Had the savings been in canada and was chargeable ay say 15% which would be equivalent to cad 300, then you would only pay the difference i.e. 300 payable to canandian govt less 100 already paid to foreign govt.
If your savings are in uk, then don't worry..they have highet taxes so you woildnt have any canadian tax liability on the interest.