Rapid egestion of microplastics in juvenile barramundi: No evidence of gut retention or tissue translocation

Despite many reports of large microplastics being isolated from fish muscle, there are limited exposure studies documenting the transport of microplastics >10 μm from the gastrointestinal tract (GIT) to surrounding tissues. Moreover, egestion rates of microplastics are not commonly studied, especially for carnivorous fish. In this study, experimental data and a literature meta-analysis were combined to understand microplastic translocation to fish tissue and egestion rates. Juvenile barramundi (Lates calcifer) were exposed through their diet to polyamide (PA) fibres and polyethylene terephthalate (PET) fibres and fragments (8-547 μm in length) to determine if shape, size, and polymer type influence microplastic translocation and egestion rates. Despite the high concentration (∼5000 microplastics g^-1) and variable range of PET sizes and shapes used, their translocation from the GIT into other tissues was not observed, thus demonstrating PET fragments and fibres are unlikely to accumulate within barramundi. Moreover, more than 90% of all ingested PET microplastics were egested in less than 24 h, with only one small fragment persisting to 96 h post exposure. Elimination half-lives ranged from 9.2 to 12.2 h, with small PET fragments egested at a faster rate than the larger PET fragments and fibres but with no significant differences. Due to methodological challenges, PA fibres were unable to be quantified amongst the digesta. The meta-analysis of published fish egestion rates revealed that, when considering multiple fish, gut morphology (i.e., presence of a true stomach) rather than microplastic size and shape influenced egestion rates across species. The results presented here demonstrate no concrete evidence for GIT accumulation or translocation into tissue with rapid and efficient egestion of ingested microplastics by fish. These results suggest microplastics are not likely to bioaccumulate in barramundi and/or directly impact their associated food web.