13 14 2 A B S T R A C T 15 16This study compared plastic ingestion between pelagic and benthic fish populations 17 from two UK watersheds: the Thames Estuary and the Firth of Clyde. The alimentary canals 18 of 876 individuals were examined. Of twenty-one estuarine species investigated, fourteen 19 ingested plastics, including predator (fish) and prey (shrimp) species. Overall, 32% of 20 organisms ingested plastic, mostly fibres (88% of total plastics). More flatfish (38%) ingested 21 plastics than other benthic species (17%). In the Thames, more plastic was ingested by pelagic 22 species (average number of plastic pieces ingested: 3.2) and flatfish (average number of plastic 23 pieces ingested: 2.9) than by shrimp (average number of plastic pieces ingested: 1). More fish 24 from the Clyde ingested plastic than similar Thames species (39% compared to 28% 25 respectively); however, the average amount of plastic ingested did not differ between the sites. 26 ARTICLE INFO 27 Spectroscopy 30 31 Funding: This work was partly supported by The University of London Sheina Marshall 32 Memorial Fund. 33 4 65 litter, mainly plastic, during a three-month fyke net fishing programme in the Thames Estuary, 66 and 65% of debris on the shoreline of the Tamar Estuary was found to be in the form of 67 microplastics (Browne et al., 2010). 68 The Thames Estuary and the Firth of Clyde are comparable with respect to potential 69 plastic pollution: both are in close proximity to several microplastic sources, including major 70 cities and shipping traffic. The 16,000 km 2 catchment of the River Thames includes 15 million 71 residents (Environment Agency, 2016) whilst the River Clyde has a catchment of over 3,000 72 km 2 which encompasses 1.7 million people (SEPA, 2015). 73 The Clyde and Thames are ecologically diverse and are important habitats and nurseries 74 for marine fish. The Thames Estuary supports over 950 species, including 112 fish species, and 75 has been recognised as a key habitat for commercial flatfish (Thomas, 1998). The European 76 flounder (Platichthys flesus) spends most of its lifecycle in the estuary, and juveniles are able 77 to penetrate the entire tidal reach of the river. Consequently, flounder is a key species to 78 measure the health of the Thames Estuary (Thomas, 1998). Recently McGoran et al. (2017) 79 collected European flounder from two sites in the Thames Estuary to measure the extent of 80 microplastic ingested. The results revealed that up to 75% of sampled P. flesus had plastic 81 6 fibres in the gut. Scotland's coastline supports ca. 8,000 species (WWF & Scottish Wildlife 82 Trust Joint Marine Programme, 2004), including 59 demersal fish species (The Scottish 83 Government, 2012). The Firth of Clyde is a fjordic system with deep valleys and steep sills 84 (Edwards et al., 1986; Jardine, 1986) and a weak tidal current (less than 0.5ms -1 ; Wilding et 85 al., 2005; The Scottish Government, 2012) which may aid the accumulation of plastics in the 86 sediment which could be available to these demersal spec...
