Microplastics in fish gut, first records from the Tom River in West Siberia, Russia.pdf Introduction In recent decades, plastic debris have been found in aquatic ecosystems around the world as a direct consequence of industrial, consumer waste and wastewater emissions [1]. Pollution of the marine environment with microplastics (particles < 5 mm) is especially intensively studied. The number of studies analyzing the abundance of microplastics in the marine environment began to grow rapidly after 2004, when the seminal paper by Thompson et al. [2] had been published. Microplastics are currently defined as polymer particles smaller than 5 mm [3], or 1 mm [4] in the largest axis. Some authors also use the terms “large” and “small” for microplastics (2-5 mm and 0.5-2 mm, respectively) [5]. The adverse effects of plastics when swallowed by hydrobionts, suffocating or entangling them, have been documented for a variety of marine species, so these Microplastics in fish gut, first records 131 materials were found hazardous to marine fauna [6]. Since plastic breaks down into smaller pieces in an aquatic environment to form microplastics, it is believed that it can enter food chains [6-7]. Both field and laboratory studies suggest that fish absorb micro-sized plastic particles, e.g. originating from synthetic clothing and cleaning products containing plastic granules [7-8]. Ingestion of microplastics by hydrobionts and their accumulation in food chains provides a potential pathway for the transfer of other pollutants and potentially toxic additives to living organisms up to humans with uncertain consequences for their health [9-12]. Most studies on microplastics abundance have focused on marine organisms. Microscopic plastic particles have been detected in marine benthic organisms, especially in bivalves [7, 13-15]. Several reports describe microplastics in the gut of marine fish. Microplastics were detected in semipelagic fish bogue (Boops hoops L.) around the Balearic Islands [16]. Microplastic ingestion is documented in commercially relevant fish species from the Spanish Atlantic coast and Mediterranean Sea - Scyliorhinus canicula L., Merluccius merluccius L. and the Mullus barbatus L. [17-18]. Ingestion of anthropogenic microfibres and microfragments by the European anchovy (Engraulis encrasicolus L.) of the Mediterranean Sea has been recently studied [19]. Much less attention is paid to riverine fish. First evidence of microplastics ingestion by fish from the Amazon River was received not so long ago [20]. McNeish et al. [21] measured microplastic abundance in fish from three major tributaries of Lake Michigan, the Muskegon River, the Milwaukee River, and the St. Joseph River. The results obtained from these two and several other [22] studies suggested microplastic pollution is common in river food webs. The aim of this research is to assess the ingestion of microplastics by fish from the Tom River, a large tributary of the Ob River in West Siberia. It should be noted that the abundance of microplastics in fish of the Ob River system has not been studied to date, as well as in fish in other rivers of Russia. Materials and Methods The object of the study was common dace, (Leuciscus leuciscus L.) from the Tom River in Western Siberia, Russia. L. leuciscus is a widespread freshwater fish of Cyprinidae family [23]. Thirteen specimens of common dace were caught using a fishing rod on the right bank of the Tom River within the city of Tomsk (56°27'33''N, 84°56'056''E) on April 01, 2020. Fish were frozen, transported to the laboratory and stored at -20 °C before the laboratory analysis. Subsequently, each fish was defrosted and examined. The total length of the body (L) and the standard length (from the tip of the snout to the posterior end of the midlateral portion of the hypural plate, l) were measured using a caliper to the nearest 1 mm. Total weight (Q) and body weight without viscera (q) (wet weight, ±1 g) were determined using an electronic balance. Scales were taken in the region of the dorsal fin (10-15 pcs. in each Yu.A Frank, E.D. Vorobiev, I.B. Babkina et al 132 specimen). The fish age was determined by the number of annual rings on the fish scales using a dissecting microscope. The sex of the fish was determined visually by gonads as described by Pravdin [24]. The fish was dissected, the gastrointestinal tract (oesophagus, stomach, and intestine) was removed for further processing and stored at -20 °C until analysis according to the method published by Bellas et al. [17]. To extract microplastics from the gut, we used modified protocol developed by Claessens et al. [25] based on acid digestion of the soft tissues. The digestion procedure consisted of 12 h destruction of the fish guts in 25 mL of HNO3 (22.5 M) at room temperature, followed by 2 h of boiling in a water bath. Then the mixture was diluted to 100 mL with 26% NaCl solution for total salt concentration of 20% and left for additional 12 h for the density separation. After separation, the upper fraction was vacuum filtered using 0.45 цш mixed cellulose ester membrane filter (MF-Millipore). Filters were rinsed with 2% KOH solution for saponification of fats and inspected by light microscopy (stereomicroscope Micromed MC2) using digital camera and ToupView 3.7.6273 software. The abundance of microplastics of different shape and sizes was evaluated as the number of particles per fish specimen. The microplastics particles extracted from fish guts were classified into four groups by their shape [26]: spheres, films, fibers/lines, and fragments of irregular shape (including foams). The particles of microplastics were also classified by their major dimension into seven groups:

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