Trophic-level investigations of microplastics in marine pelagic fish and squid

Abstract

Microplastics (MPs) are ubiquitous in marine environments, with a large number of species constantly being exposed to contamination. Fish ingest MPs, and much research has gone into the reporting on MP ingestion by different species from different environments. There are, however, very few studies on MPs that have discoursed on MPs translocated into the tissues of marine organisms. Each environment provides a new perspective on how MPs contaminate biota. There is, however, a lack of information on conspecifics in two adjacent oceans, each ocean having different MP concentrations and MP characteristics. South Africa is bordered by two oceans, the South Atlantic Ocean associated with the cold Benguela Current, and the Indian Ocean associated with the warm Agulhas Current. The two oceans are uniquely different, making them ideal for comparative studies on shared species. Available water data show that the Indian Ocean has two-to-three-times higher concentrations of MPs than the South Atlantic Ocean. In this study, two species occurring in both oceans were sampled from both oceans: anchovy (Engraulis encrasicolus) and chokka squid (Loligo reynaudii). These two commercially important species make up a large proportion of South Africa's fisheries industry. Samples were analysed for MPs that have translocated into their tissues. I compared the MP concentration differences in the two organisms between the two oceans. This was done vide two manuscripts, one for each species. Microplastics from 2.39 μm to 5000 μm were found in the muscle tissue of anchovy, and mantle tissue, gills, and digestive glands of the chokka squid, regardless of ocean. Anchovy from the Indian Ocean showed lower concentrations of MPs in their muscle tissue compared with South Atlantic Ocean anchovy. The South Atlantic Ocean anchovy were younger and filter feed instead of particulate feeding, compared with the larger anchovy from the Indian Ocean, possibly explaining the difference in MP concentrations. Fibre lengths and colours were largely the same between the two oceans, only fragment lengths had noteworthy differences. There is therefore a difference in how fibres and fragments are translocated to muscle tissue. Anchovy migrate from the South Atlantic Ocean to the Indian Ocean, growing as they travel. At the same time, they carry with them incorporated MPs in their tissue. Assuming 1 million tonnes of anchovy migrating west to east every year, approximately 3.3X1011 MPs are transported from one ocean system to another. As far as I know, this is the first recognition of bio-facilitated transport of MPs between ocean systems. Chokka squid from the Indian Ocean were significantly larger than South Atlantic Ocean chokka squid due to the colder temperature of the South Atlantic Ocean, affecting the growth of the chokka squid found there. MP concentrations in gills were consistently higher than the digestive gland, and lowest in mantle tissue. All three organs in the South Atlantic Ocean chokka squid had higher MP concentrations than the Indian Ocean chokka squid organs. Chokka squid from the South Atlantic Ocean had significantly higher MP concentrations in their mantle tissue than the Indian Ocean chokka squid. Fibre lengths increased in tandem with squid size in both oceans, therefore fibres from the Indian Ocean squid were longer, though only significantly so in the digestive gland. I cannot explain this phenomenon. The three organs of squid from the same ocean had no difference in the fibre lengths found in them. Considering all the results, three possible reasons were argued as to why chokka squid from a region with lower MP concentration in its waters had higher MP concentrations in their mantle, compared with squid from the Indian Ocean with higher MP concentrations. However, none of the reasons were satisfactory. After analysing the MPs in chokka squid and anchovy separately, I compared the data between the two species within each ocean. South Atlantic Ocean anchovy had 50% higher

MP concentrations than chokka squid from the same ocean in their respective muscle tissue. The difference in MP concentrations may be attributed to the different trophic levels of the organisms. Available data suggests that organisms of lower trophic levels are more likely to have higher levels of MP contamination, as MPs do not biomagnify as do organic chemical pollutants. However, this was not the case in the Indian Ocean anchovy and chokka squid --there were no differences in MP concentrations. I have no plausible explanation for this dichotomy. There was therefore also no evidence of trophic transfer of MPs from anchovy to chokka squid in either ocean. Human consumption of either species do not constitute a human health hazard. The dominant type of MP in both organisms and both oceans were fibres. The chokka squid from the South Atlantic Ocean had significantly longer fibres in their mantle than the anchovy from the same ocean (p = 0.0172). I posit that the differences in depths the two species inhabit may be the cause for the difference in fibre lengths. However, and again, this was not found in the Indian Ocean -- no significant differences in fibre lengths between the two species. I cannot explain this phenomenon. Blue was the dominant colour of MPs in both oceans and species, suggesting either that blue was the most abundant colour or that blue was preferentially taken up. There was no significant difference in MP colour proportions of fibres between chokka squid and anchovy in the South Atlantic Ocean, but the proportion of colours differed in the Indian Ocean. Again, I cannot explain this. Both anchovy and chokka squid showed dilution by growth in terms of MP concentrations. I suggest the MPs in the tissues of anchovy and chokka squid are poor biomonitors for MPs in seawater. My findings, though, will also be useful in other ways: - I confirmed translocation of MPs to various organs of two different phyla, inviting more research into translocation mechanisms, given the expected increase in MP concentrations in the two oceans. - I identified inter-ocean transfer of MPs. - The differences in muscle tissue concentrations and characteristics of MPs in anchovy and chokka squid organs can be used as a tracer to track stocks and migration of marine organisms.