Big money investors are calling a bottom in oil prices, US government data showed, with hedge fund heavyweight Andy Hall predicting "much stronger prices" from the second half onwards.

Money managers and other big speculators in US crude oil futures and options raised net long positions that call for higher prices by some 52 million barrels in the week to April 7, data from the US Commodity Futures Trading Commission showed.

That was the biggest one-week rise in bullish bets since 2011, according to CFTC data.

The data affirmed a growing notion that some of the biggest investors have started betting on a rebound after a seven-month price rout that subsided in January – something Hall asserted in this month’s letter to investors in his hedge fund.

"The seasonal pickup in H2 demand should mean much stronger prices down the road," Hall, who manages the $3 billion Astenbeck Capital Management in Westport, Connecticut, said in the letter, a copy of which was reviewed by Reuters.