Popular Fact

[F]ishing nets account for 46 percent of the trash, with the majority of the rest composed of other fishing industry gear.
- National Geographic - The Great Pacific Garbage Patch Isn’t What You Think it Is - 2018

This cites a Nature Article

Reality

The 46% figure is well founded, but the “majority of the rest” seems to be a misinterpretation of a statement that at least half of total plastic is from marine industry sources.

It’s also worth noting that while at least half the total observable plastic in the Great Pacific Garbage Patch is of marine industry origin, it’s plausible that a lot more plastic from land enters the ocean, but washes up or breaks down before it enters the GPGP.

Original sources

Nature - Evidence that the Great Pacific Garbage Patch is rapidly accumulating plastic - March 2018

Original figure and calculation thereof

Over three-quarters of the GPGP [(Great Pacific Garbage Patch)] mass was carried by debris larger than 5cm and at least 46% was comprised of fishing nets.
- Nature - Evidence that the Great Pacific Garbage Patch is rapidly accumulating plastic - March 2018 - Abstract (p1)
and
Plastic type[…] ‘N’ (nets, ropes and lines) represented […] 52% of the total GPGP plastic mass [..] We estimated that 86% of their 42k tonnes contribution was carried by fishing nets. p7

It appears that the 46% figure is taken by using the total tonnage figures of Table 1 p7 and applying the following formula.

N type tonnage * Percentage of N tonnage that is nets / Total tonnage = Percentage of total that is nets
41,376 * 86% / 78,909 = 45%

Which… is not quite 46% but it seems reasonable that we are subject to a rounding error here given we do not have the original numbers in full fidelity.

Methodology

The study combines observations from vessel expeditions with satellite imaging p2 to draw conclusions about the overall composition of the Great Pacific Garbage Patch by sampling a smaller area. Their methodology is essentially gathering all the plastic from a sample area, weighing the various types of plastic in that area is, and then multiplying those weights by the total size of the GPGP.

The majority of the rest

The National Geographic article cites 46% and then goes on to say “the majority of the rest [is] composed of fishing industry gear”.
The closest to a supporting statement for this in the study is:

[W]e predominantly observed debris originated from marine sources p5

Which is expanded upon later with:

[A]t least half of the collected GPGP plastics was composed of objects from marine based sources, while the relative source amplitudes considered in our model predicted that mass contributions from land-based plastics, even though lower than global average, would still dominate in these ofshore environments. This discrepancy could be due to diferences in the magnitude of certain removal processes between land-based and marine-based plastics that were not accounted for in our models. We trust that beaching is one of these processes as it may primarily remove plastics that are discarded in coastal environments through wave, tidal and onshore winds transport. Nonetheless, the GPGP dominance of marine-sourced plastics could also be attributed to their purposely engineered durability in the marine environment (e.g. strong and thick-walled nets, traps, ropes, and foats used by marine industries) as well as overestimations of land-based sources and/or underestimations of marine-based sources. p12

So although ‘at least half’ are from marine sources, this may be because land sources of plastic are more likely to wash up, or because plastic from the land breaks up more easily as it’s not designed to survive in the sea unlike marine plastics.
In other words, while more than half of the observable plastic in the GPGP is from marine sources, that doesn’t mean that most of the plastic going into the ocean is from marine sources.

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