The world's sovereign wealth funds - pools of capital controlled by national governments - grew at their slowest rate in almost a decade last year, as low oil prices and volatile markets took their toll.

Sovereign funds controlled total assets of $6.59 trillion at the end of March, according to analysts Preqin, which tracks 76 of the biggest ones. That was a little over 1% up from $6.51 trillion at the end of March 2016.