Seite - 110 - in Freshwater Microplastics - Emerging Environmental Contaminants?

remaining solid waste. Gillnets, which compromised more than 80% of all the debris found and 96% of waste in the fourth depth strata, are constructed using syntheticfibers,andalthoughnylonwasusedin the1960s,newermaterials, suchas ultrahigh-molecular-weight polyethylene (UHMWPE) or polyethylene terephthal- ate (PET),arenowcommonplaceas theyarecheaper,aremoredurable,andrequire lessmaintenance.Multifilamentgillnetsareused in thefishingofNileperch,while monofilaments are used for catching tilapiine species, includingNile tilapia.Both species were found to containMPs in their intestinal tracts, as described in Case Study II [17]. Therewereonlyminordifferencesintheabundancebyweightofdebrissampled fromthedifferentdepths,andof thesixwaste types identified, theproportionfound at each depth did not vary to any great degree, with the exception of the bottom strata in which longlines and hooks (67%) were most abundant. Of the three ecological zones, the intermediate zone (20.1–40m) containedmostwaste and is also known to have the highest levels of fishing activities; thus, within this zone, there was a reduced abundance of clothing and plastic bags. Fishing activity appears to be themajor source of solid (plastic)waste inLakeVictoria, but land- basedwastewas not accounted for due to the inability to trawl at shallow depths (<4m) in the nearshore. Land-based waste is often an important component of marinewaste and through tidal action is transported to the lower depths of the sea [46]. However, without strong currents, this mechanism of circulating waste is ineffectivewithin the lakeenvironment.Nevertheless,withinshallowwaters, land- basedwastewouldundoubtedly be important. While not specifically focusing on the abundance ofMPs, this study demon- strates that plastic waste is present at all levels of Lake Victoria and is strongly linked to fishing activities and discarded fishing gear. Though authors do not discount other sources including land runoff and transportation of cargo, the limitations of the studydonot allow these to be investigated further. 3.2.2 CaseStudy II:RecoveryofMPs fromLakeVictoriaNilePerch andNileTilapia Anumber of studies have used the ingestion ofMPs by resident fish species as a marker ofMPpollution.Lusher et al. [47] found thatmarinepelagic anddemersal fishsampledfromtheEnglishChannel readilyconsumeplastics, andSanchezetal. [48] similarly reported, for thefirst time in freshwaters, thatwildgudgeons (Gobio gobio) inhabitingFrench rivers ingestMPs.UsingNileperch (Latesniloticus) and Nile tilapia (Oreochromis niloticus) as proxies for environmentalMP contamina- tion inLakeVictoria, a small-scale studywas conducted in theMwanza regionof Tanzania, locatedon theLake’s southern shore (Fig. 2).Both species are econom- icallyandecologically importantandwere introducedtoLakeVictoria in the1950s and1960swith the aimof supplementingnativefishpopulations that haddeclined due, in part, to overfishing [49].However, this introductionwasdetrimental to the native species, particularly the native tilapiine species such as theVictoria tilapia 110 F.R.Khanet al.