The death of about 500 ducks that landed in a pond of oily, toxic sludge operated by Canada's biggest oil sands producer was tragic, the chief executive of Imperial Oil Ltd said Thursday. Imperial has a 25 percent stake in oil sand producer Syncrude Canada Ltd, owner of the tailings pond in northern Alberta, where the ducks died earlier this week because a warning system meant to keep them off wasn't operating. "Without question, I think it was not only an unfortunate event but a really tragic one," Imperial CEO Bruce March told reporters following the company's annual meeting.