A decline in the stock of residential mortgage-backed securities on issue, combined with lower mortgage servicing volumes, led to a 39 per cent per cent fall in pre-tax profit for Perpetual's corporate trust division in the December half year.The company reported yesterday that trust and fund services' revenue in the six months to December was down seven per cent on the previous corresponding period, and mortgage services revenue was down 30 per cent.Perpetual's mortgage-servicing contract with ANZ expired and was not renewed during 2011, though the full impact of this does not take affect until this year.In the six months since June, trust and fund services' revenue has fallen 10 per cent, but there was a three per cent pick-up in mortgage services' revenue.The division's pre-tax profit was A$9.3 million - down from $15.3 million in the previous corresponding period.One piece of good news for the trust business was that it has been appointed trustee for three out of four of the new covered bond programs.With the covered bond contracts, the level of funds under administration remains stable. The division had FUA of $205.7 billion at the end of the latest half - down two per cent from December 2010.