Bank of Nova Scotia sold $2.5bn of covered bonds Thursday, pushing issuance of that kind of mortgage security to a record high as foreign banks step up to satisfy US investors starved of a supply of traditional mortgage-backed bonds. Foreign banks have been able to raise $11.04bn through sales of covered bonds to US investors so far this year, already exceeding the previous full-year record of $10.7bn in 2007. Covered bonds are similar to traditional residential mortgage securities but require issuers to hold the collateral on their own books rather than spin them off into quasi-independent special-purpose vehicles.