Prime Minister Justin Trudeau and President Donald Trump have held brief discussions following the signing of the U.S.-Mexico-Canada Agreement (USMCA). Tariffs on steel and aluminum were a source of controversy during the negotiations and may yet dog passage of the enacting legislation through the Canadian parliament. Aluminum is the main sticking point for Canada whose $7.1 billion of exports to the U.S. are 1.3x its steel shipments and have fallen by 8.8% in the three months to Oct. 31 on a year earlier. In the meantime U.S. seaborne imports of aluminum have continued to climb with a 1...