Urgent action is needed to avoid “irreversible tragedies” in the world’s oceans caused by plastic pollution and climate change, Prince Albert II of Monaco has said at a major conference that heard there are already up to 500 “dead zones”.

The Prince told the gathering of academics, senior officials and ministers in Edinburgh that the threats facing the oceans were “increasingly alarming” and people must stop believing it was possible to pour “anything and everything” into them “without consequences.”

His outspoken warning was echoed by Peter Thomson, the UN’s Special Envoy for the Ocean, who compared the situation to all the historic architecture in Scotland’s capital being wiped out by a massive earthquake.

He said the world’s oceans were in “deep, deep trouble” and studies had identified between 400 and 500 so-called “dead zones” – areas that do not have enough oxygen to support marine life.

This is usually caused by an increase in chemical nutrients, leading to excessive blooms of algae that deplete underwater oxygen levels.

If unchecked, it is estimated that by 2050 the amount of plastic in the world’s oceans will outweigh fish and the equivalent of four trucks of plastic waste will be added to the sea every minute.