Gut microplastics of two non-native freshwater fish species, Geophagus altifrons and Cichla orinocensis, in Singapore reservoirs

Abstract

Microplastics in freshwater environments have intruded into food webs and can impact fish health negatively. This study quantified the abundance of putative microplastics in the water and sediment of two reservoirs, as well as in the gastrointestinal tracts of two fish species (Geophagus altifrons and Cichla orinocensis) captured along multiple littoral transects parallel to shore in these reservoirs. We found significant differences in microplastic concentrations in the water (8.67 items/m³ vs. 46.7 items/m³) and sediment (313 items/kg vs. 2040 items/kg) between the two reservoirs, and between both fish species. A total of 2394 putative microplastic particles were found from the 165 fish sampled, with a frequency of occurrence of 72%. In Reservoir 1, microplastic concentrations were 4.43 ± 7.02 items/g and 5.52 ± 4.04 items/g in C. orinocensis and G. altifrons, respectively; in Reservoir 2, concentrations were 4.65 ± 6.07 items/g and 12.21 ± 11.96 items/g in C. orinocensis and G. altifrons, respectively. The omnivorous, demersal Geophagus altifrons at Reservoir 2 had the highest microplastic uptake (12.21 ± 11.96 items/g) among the four reservoir-fish species combinations possibly due to environmental microplastic concentrations, its winnowing feeding behaviour, and its benthic feeding zone. Microfibres were the most abundant microplastics (57.9%) in Geophagus altifrons at Reservoir 2 while all other reservoir-fish species combinations had fragments as the most abundant microplastic category. Microplastics present in the fish guts could be associated with their specific feeding behaviours, feeding zones and diets. Our findings suggest that the association between environmental microplastic concentrations and fish gut microplastics may be confounded by the specific mechanisms by which microplastics enter the food web. This could be further modulated by the type and sizes of microplastics; characterizing these interactions are paramount to identifying particularly vulnerable components of freshwater food webs threatened by microplastic pollution. Both fish species are introduced to Singapore, providing insights on how non-native species can also be effective indicators of microplastic pollution in lentic freshwaters.