Is there biologically-facilitated transfer of billions to trillions of microplastics between oceans in the tissues of the European anchovy?

There is a paucity of comparative microplastic (MP) data in fish between adjacent oceans. We analysed MPs in muscle tissue of the European anchovy Engraulis encrasicolus from the South Atlantic Ocean (SAO) and Indian Ocean (IO) off the South African coast (n = 58 and n = 45, respectively). SAO anchovy had a significantly higher MP concentration than IO anchovy (1.4 and 0.58 n/g, respectively), with no discernible differences between oceans in MP dimensions or colour compositional proportions. SAO anchovy were younger, smaller (6.5 g), and filter feeding. Migrating toward the IO, they grow larger (7.2 g) and switch to selective feeding, partially accounting for lower MP concentrations. Due to this ontogenetic diet shift, anchovy may not be the best indicator of oceanic concentrations of MPs without accounting for their biology. Based on this ontogenetic diet shift and knowledge of stocks and movement, we estimate that between 460 billion to 2.8 trillion MPs (equivalent to 20.6 – 223 kg) could be transported within the tissues of the anchovy from the SAO to the IO. Our findings underscore the need to incorporate biology when interpreting biota as MP indicators. This study highlights a possible and previously unrecognised biologically-facilitated transport of MPs between oceans, a phenomenon requiring further study for confirmation.