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Diet of Anguilla mossambica (Teleostei, Anguillidae) elvers in the Sundays River, Eastern Cape, South Africa

The western Indian Ocean is host to four anguillid eel species (Smith and Heemstra 1995) which represent the only southern African catadromous fishes with an obligatory freshwater stage (Whitfield 1998). Thought to spawn in the north-eastern waters off Madagascar (Robinet et al. 2003), anguillid larval leptocephali move toward the African continental shelf where they metamorphose into glass eels (Robinet et al. 2008). These glass eels generally enter southern African estuaries in summer at a body length of 50–60 mm (Harris and Cyrus 1995). Upstream migrations by glass eels into freshwater habitats seem to coincide with strong summer river flow into estuaries (Bruton et al. 1987) and once in fresh water, the glass eels develop into elvers while continuing their upstream migration (Whitfield 1998). Anguillids are thought to be trophically important in Eastern Cape rivers because adult eels often represent the only large native piscivorous fish species in these systems, feeding on crabs, frogs and fish (Bruton et al. 1987). The diet of the earlier life-history stages of these fish is poorly documented and there is little published information on the species level of prey utilised by the elvers of anguillid species globally. To contribute towards the knowledge base on elver diets, the present study assessed gut contents from 142 Anguilla mossambica (identified following Réveillac et al. 2009) sampled from the lower reaches of the Sundays River, Eastern Cape, South Africa (33°38′01′′ S, 25°41′54′′ E). This river, receiving supplemental water from the Orange River system through an inter-basin transfer (Mullins 2011), enters the Indian Ocean via a permanently open estuary mouth (Beckley 1984). Anguilla mossambica, the most common anguillid in the rivers of the Eastern Cape, South Africa (Jubb 1961), were sampled from this system during autumn, winter and spring 2009 and in summer 2010 using fyke nets. Detailed descriptions of the fyke net surveys are provided in Wasserman and Strydom (2011) and Wasserman et al. (2011a). On capture, elvers were immediately fixed in 10% buffered formalin and, once in the laboratory, measured to the nearest millimetre total length (TL). To determine their diet, stomachs were removed, opened and emptied into a 1 mm deep tray graduated at increments of 0.5 mm × 0.5 mm.