Page - 111 - in Freshwater Microplastics - Emerging Environmental Contaminants?

(Oreochromis variabilis) and Singidia tilapia (Oreochromis esculentus), which subsequently disappeared from parts of the lake [49, 50]. Thus Nile perch and Nile tilapia have established themselves as dominant commercial and ecological speciesandtherefore represent logicalchoicesbywhichtomonitorMPpollutionin the area.Moreover, their differing feeding habits could provide additional infor- mation bywhich to contextualize plastic ingestion. Nile perch are predatory fish feeding on haplochromine cichlids and gastropod snails, whereas Nile tilapia are omnivorouswith a diet consistingof plankton andfish. InMarch 2015, 20 fish of each species were purchased fromMwanza harbor market,wherefish are caught and solddaily. Thefishing territory for both species extends to Ukerewe Island (the largest island in Lake Victoria) to the North of Mwanza and across the Mwanza Gulf to the neighboring district of Sengerema (Fig. 2). Nile perch andNile tilapia were 46–50 cm and 25–30 cm in length and 500–800 g and 500–700 g inweight, respectively. For each fish, the dissection of the entire gastrointestinal tract (buccal cavity to anus)was conducted on site. All efforts were made to eliminate sample contamination with separate clean dishes used for eachfishand thoroughcleaningofdissectionutensils betweensamples.A preliminaryexaminationwasmadeofeachgastrointestinal tract, and in thecaseof Nile perch, undigested gastropods and cichlids were removed. Gastrointestinal tracts and their contents were then individually preserved in 96% ethanol and transported to laboratory facilities at the University of Dar es Salaam (Dar es Salaam, Tanzania), a journey of approximately 1,150 km. In the laboratory, NaOHdigestion (10MNaOH at 60 C for 24 h) was used to isolate plastic litter fromtheorganic tissue.TheNaOHmethodhasbeenshowntodigestorganicmatter with an efficacy of>90% [51], and the tests of this protocol prior to its use confirmedsuchhighefficiencies(96.6 0.9%,n¼5,datanotshown). Importantly, NaOH digestion has a minimal impact on the chemical and physical states of plastics, especially when compared to strong acid digestion which, while also Fig.2 Mapof theLakeVictoria studyarea showing theMwanza region. (a)NileperchandNile tilapiawerepurchasedfromtheharbormarketatMwanza(regionalcapital).Thelocalfishingarea extends across theMwanzaGulf and toUkerewe Island. Inset LakeVictoria (LV) bordered by Uganda,Kenya,andTanzania.TheMwanzaregionlocatedonthesouthernshoreofLakeVictoria is highlighted. (b)Urbanwaste inMwanza, including plastic debris, collects in drainage ditches whichareapotentialsourceofplasticpollutioninLakeVictoria. (c)NileTilapiausedinthisstudy (photographedprior to dissection)were purchased from themarket Microplastics in InlandAfricanWaters: Presence, Sources, andFate 111