It’s nearly impossible to come up with an exact figure to support this claim. A 2016 report estimated that in a “business-as-usual scenario, the ocean is expected to contain 1 tonne of plastic for every 3 tonnes of fish by 2025, and by 2050, more plastics than fish (by weight).” The report was quoted widely in the press but the figures really are just a first attempt to address the issue. Questions have been raised regarding the report’s figures and date of 2050. That’s because there is uncertainty over the methods used to estimate both fish populations,and the amount of plastic in the oceans.
So, the “more plastic than fish by 2050” quote is a useful illustration but it is not verifiable. At least not yet.
 World Economic Forum, Ellen MacArthur Foundation and McKinsey & Company, 2016. The New Plastics Economy: Rethinking the future of plastics. Available at here
 Watson, R.A. et al., 2015. Plenty more fish in the sea? Available here
 Homak, L., 2016. Will there be more fish or plastic in the sea in 2050? Available here
 Galloway, T. and Lewis, C., 2017. Marine microplastics. Current Biology, 27(11), pp.R445-R446.