As part of our federal election coverage, CBC News is assessing the truthfulness and accuracy of claims made by politicians and their parties.

The Claim: "If we don't act, plastics will outweigh fish in our oceans by 2050."

-- Federal Environment Minister Catherine McKenna on the Liberals' desire to half the amount of garbage Canadians throw away by 2040.

The Facts:

McKenna first made this claim on her Twitter account last November, and it has been reprised — with a conditional "could outweigh" — on the Liberal Party website as part of an attack on Conservative Leader Andrew Scheer's environmental policies.

The estimate comes from a report that British sailor and environmentalist Ellen MacArthur introduced at the World Economic Forum in Davos, Switzerland in January 2016 to highlight the increasing amount of plastic in the world's waters.

"In a business-as-usual scenario, the ocean is expected to contain one tonne of plastic for every three tonnes of fish by 2025, and by 2050, more plastics than fish (by weight)," the report read.

The report, prepared by MacArthur's eponymous foundation, says that the "analysis" is based on a 2015 plastics study by the U.S. environmental group Ocean Conservancy, which was peer-reviewed in the journal Science, and a 2008 assessment of global fish stocks.

Starting with an estimate that 150 million tonnes of plastic are already polluting the world's oceans, and that "leakage" adds at least 9.1 million tonnes more each year — a figure that is said to be growing by five per cent annually — the MacArthur report calculates there will be 850-950 million tonnes of ocean plastic by 2050, versus total fish stocks of 812-899 million tonnes.

"While — by their very nature — there are uncertainties around such estimates, we are confident that this 2050 forecast relies on overall conservative assumptions," says a backgrounder to the report.

It is, however, really a rough calculation based on some educated guesses.

We know how much plastic has been produced over the past six decades: 8.3 billion tonnes, according to a recent study in the journal Science Advances — 90.5 per cent of which has not been recycled.

But after that, things get awfully murky.

Federal Environment Minister Catherine McKenna says that there will be more plastic than fish, by weight, in the ocean by 2050. (CBC)

The 2015 Science study estimates that 6.4 billion people living in 192 coastal countries generated 2.5 billion tonnes of garbage in 2010, and 99.5 million tonnes of that was plastic waste discarded within 50 kilometres of the ocean, meaning it could potentially make its way out to sea.

But that paper's conclusion about how much plastic might already be in the ocean was one to three orders of magnitude greater than what had previously been reported.

Those calculations are based, in part, on a widely used but virtually unprovable estimate that 80 per cent of the plastic garbage in the ocean comes from land, rather than marine sources like fishing vessels, cargo ships or oil platforms.

Then there's the question of how many fish there actually are in the ocean. The expert consensus is not as many as there used to be, but after that, it's really all guesswork, too.

"Managing fisheries is hard: it's like managing a forest, in which the trees are invisible and keep moving around," well-known British oceanographer John Shepherd once quipped.

Fish stock models are based on commercial catches (which are often drastically under-reported) and decidedly old-fashioned research counts, where boats trawl across the same stretch of water year after year and scientists record what comes up in the nets.

There are no definitive numbers for any one species, let alone everything swimming around in the ocean.

What can be said with certainty is that there is already far too much plastic in the world's oceans. But comparing the weight of the garbage to the marine life is, at best, fishy business.

The Verdict: It's complicated