ZURICH (Reuters) - Billionaires on average became poorer last year as their collective fortunes shrank, even as Asia continued to crank out a new billionaire nearly every three days, a study released on Thursday found.

Transfers of assets within families, falling commodity prices and a stronger dollar helped reduce total billionaire wealth by $300 billion in 2015 to $5.1 trillion.

That meant the average billionaire -- there were 1,397 of them, a net gain of 50 over 2014 -- was worth only $3.7 billion, the survey of 14 big markets by Swiss wealth manager UBS <UBSG.S> and advisory group PwC [PWC.UL] discovered.

"After more than 20 years of unprecedented wealth creation, the Second Gilded Age has stalled," the report found.

The United States added only a net five billionaires as 41 joined and 36 dropped out of the ranks of the ultra-rich. China alone, buoyed by its tech sector, minted 80 new billionaires.

It suggested Asia's newly rich could learn from their counterparts in Europe, where old money is especially adept at passing wealth down the generations. Germany and Switzerland had the greatest share of such old wealth.

Two decades of unparalleled fortune generation are about to make way for the greatest wealth transfer in history, the study also found.

It estimated that fewer than 500 people will hand over $2.1 trillion -- a sum the size of India's economy -- to their heirs in the next 20 years.

(Reporting by Michael Shields, editing by Larry King)