Uptake routes of microplastics in fishes: practical and theoretical approaches to test existing theories

Abstract

Microplastics are frequently detected in the gastrointestinal tracts of aquatic organisms worldwide. A number of active and passive pathways have been suggested for fish, including the confusion of microplastic particles with prey, accidental uptake while foraging and transfer through the food chain, but a holistic understanding of influencing factors is still lacking. The aim of the study was to investigate frequently suggested theories and identify relevant biotic factors, as well as certain plastic properties, affecting microplastic intake in fish. Four species of freshwater fish, each representing a different combination of foraging style (visual/chemosensory) and domestic status (wild/farmed) were exposed to different realistic plastic concentrations and polymer types with and without the provision of genuine food. As most previous investigations of microplastic uptake routes consider only particles large enough to be perceptible to fish, the potential for accidental intake via drinking water has been somewhat neglected. This route is evaluated in the current study using a model approach. The results show that visually oriented fish forage actively on microplastic particles that optically resemble their usual food, while fish with a predominantly chemosensory foraging style are more able to discriminate inedible food items. Even so, the accidental uptake of microplastics while foraging is shown to be relevant pathway, occurring frequently in both visual and chemosensory foragers alike. Several factors were shown to increase plastic uptake, including microplastic concentration in the water, foraging behaviour promoted by availability of genuine food, and fish size. Although both wild and farmed fish ingested microplastic particles, cultured fish showed less discernment in terms of colour and were more likely to forage actively on microplastics when no food was available. Drinking has been identified as a possible source of microplastic intake specifically for large marine fish species. Particles smaller than <5 µm can pass the gastrointestinal tract wall and bioaccumulation could arise when uptake exceeds release or when particles are assimilated in tissues or organs. The effects of accumulation may be significant, especially in long-living species, with implications for food web transfer and fish as food items.

Figure 1

Possible uptakes routes of microplastics in fish.

Figure 2

Microplastic uptake (Grand marginal means) directly after exposure. Top: Microplastic prevalence, Bottom: Microplastic abundance (±standard error). Asterisks indicate statistically significant differences between model effects (*P < 0.05, **P < 0.01, ***P < 0.0001). n = number of particles.

Figure 3

Response surface methodology graphs based on the results of a General Linear Model for particle concentration in water versus (a) foraging style and (b) availability of genuine feeding opportunity. n = number of particles.

Figure 4

Effect of particle properties on microplastic uptake (± standard error, all sampling points). Asterisks indicate statistically significant differences between variables (*P < 0.05, **P < 0.01, ***P < 0.0001). n = number of particles.

Figure 5

The effect of feeding and particle appearance on particle intensity (mean ± standard error) in visually oriented fishes (all time points). n = number of particles.

Figure 6

Modelled passive uptake of microplastics via drinking in freshwater fish. (a) Uptake rate (coloured area indicates an intensity >1) and (b) uptake rate combined with egestion rate for different particle concentrations in the water and fish weights respectively (coloured area indicates an accumulation effect). n = number of particles.

Figure 7

Modelled passive uptake of microplastics via drinking in marine fish. (a) Uptake rate (coloured area indicates an intensity >1) and (b) uptake rate combined with egestion rate for different particle concentrations in the water and fish weights respectively (coloured area indicates an accumulation effect). n = number of particles.

Figure 8

Modelled accumulation of microplastics <5 µm in fish tissue via drinking for (a) common carp and (b) cod in dependence on hypothesized translocation probability into tissue and age of fish.