NAB Asset Servicing's position as the biggest player in the Australian custody market is under threat, after it suffered a big fall in assets under custody in the second half of last year.According to the Australian Custodial Services Association's latest industry data, the value of assets NAB Asset Servicing holds for Australian investors fell 16.8 per cent in the six months to December - down from A$702 billion at the end of June to $584 billion at the end of December.JP Morgan has $573.2 billion of assets under custody - an increase of 2.8 per cent over the six months to December.BNP Paribas's custody book grew 24.9 per cent over the same period to $429 billion and Citigroup's grew by two per cent to $324 billion.Other custodians in the top ten include Northern Trust, which holds $306 billion of assets for Australian investors, State Street ($266 billion), HSBC ($170.8 billion), BNY Mellon ($88.9 billion), RBC Investor Services ($79.6 billion) and Bond Street ($68.5 billion).ACSA's figures show that total assets under custody for Australian investors were worth $2.92 trillion at the end of last year - an increase of 11.5 per cent compared with a year earlier.Of that total, the value of Australian assets grew by 10.5 per cent to $1.99 trillion and the value of foreign assets held on behalf of Australian investors grew by 17.2 per cent to $926 billion.In the sub-custody market, where Australian assets are held in custody for foreign investors, the value of assets grew by 9.1 per cent to $1.2 trillion.HSBC is the leader in the sub-custody market, with assets of $805.1 billion (up 10.7 per cent over 12 months), followed by JP Morgan ($160.7 billion), Citigroup ($111.7 billion) and NAB Asset Servicing ($89 billion).