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Microplastic pollution in the Thumbprint emperor (Lethrinus harak) from Tanzanian coastal waters: Occurrence, abundance, characterization, and relevance as a monitoring species
Summary
Researchers examined nearly 400 Thumbprint emperor fish from Tanzanian coastal waters and found microplastics in 48% of individuals, with fibers and black-colored particles being most common. While the fish show potential as a pollution monitoring species for East Africa, the inconsistent relationship between microplastics in fish and levels in surrounding water or sediment limits its reliability as a straightforward indicator.
In the present study we collected the Thumbprint emperor (Lethrinus harak) from seven landing sites from the coastal waters around Dar es Salaam and Zanzibar (Tanzania) to (i) quantify and characterize MPs in their digestive tracts and (ii) use previously assessed environmental levels in nearshore surface waters and seabed sediments to determine whether L. harak could be a relevant biomonitor for MP pollution in the region. L. harak (n=387) had an overall frequency of occurrence (FO%) of 48% and displayed spatial variation between sites with Kunduchi (FO=66.7%) and Mijimwena (FO=17.1%) having the highest and lowest FO%, respectively. Fish from Mjimwema had a mean MP content of 0.17 ± 0.38 MPs individual-1 whilst fish from Kizimkazi had the highest MP abundance (1.75 ± 2.33 MPs individual-1). Fibers (overall 64.7%, range across sites 48-86%) and fragments (17.9%, 5-25%) were the most dominant MP types whilst black (46.9%, 40-58%) and blue (22.5%, 7-36%) MPs were the most common colours. Fish length (ρ= -0.09, p=0.09) or weight (ρ=0.07, p=0.18) did not significantly correlate to MP abundance in fish (Spearman rank correlations). Neither MP occurrence nor abundance was linked to MP concentrations in either surface waters or seabed sediments (Spearman rank correlation), but MPs in the fish better reflected MPs in the sediment compared to surface water (two-way ANOVA on ranked data). Whilst L. harak presents as a promising candidate to monitor MP pollution along the East African coast due to its ecology, overall, it lacks reliability. Nonetheless, the present study fills important knowledge gaps both geographically on the East African Coast and with an underrepresented taxonomic family (Lethrinidae ‘Emporer fishes’).