The average daily trading volume for agency MBS fell to a yearly low of $149.2 billion in December, as trading desks from coast to coast continued to assess how to make money in what’s become a business of tight profit margins. Late this week, Barclays Bank unveiled a massive restructuring of its MBS and whole-loan trading business, cutting the number of employees in the division – including traders – down to 50 from roughly 100. As a structural matter, the bank is moving...