Tons of plastic makes its way into the oceans each year, and—according to one statistic that’s been floating around—by 2050 the world’s oceans will contain more plastic than fish by weight. There’s only one problem, BBC reports: The estimates behind the numbers just don’t hold water. The estimate for ocean plastic comes from a 2015 study that only predicts the world’s plastic output up until 2025 and uses data from just one place—San Francisco Bay—to calculate how much of this gets into the ocean. And then there’s the fish. The estimate for fish mass comes from a 2008 report that guesses at the global mass of all marine animals based on how much phytoplankton is on the surface of the sea. Since then, the same research group has concluded that there may be many times that amount of ocean biomass. And, says the lead researcher, it’s still really hard to tease out how much of that biomass is made of fish. There’s no question that accumulating ocean plastic is a problem—but simple factoids can mask the fact that we still have a lot to learn about what’s in the world’s oceans.