Journal of Fish Biology (2012) 81, 427–441 doi:10.1111/j.1095-8649.2012.03357.x, available online at wileyonlinelibrary.com Regional variation in otolith Sr:Ca ratios of African longfinned eel Anguilla mossambica and mottled eel Anguilla marmorata : a challenge to the classic tool for reconstructing migratory histories of fishes Y.-J. Lin*, B. M. Jessop†, O. L. F. Weyl‡§, Y. Iizuka‖, S.-H. Lin¶, W.-N. Tzeng** and C.-L. Sun* *Institute of Oceanography, National Taiwan University, Taipei 10617, Taiwan, †Department of Fisheries and Oceans, Bedford Institute of Oceanography, Dartmouth, NS, B2Y 4A2 Canada, ‡South African Institute for Aquatic Biodiversity (SAIAB), Grahamstown 6140, South Africa, ‖Institute of Earth Sciences, Academia Sinica, Taipei 11574, Taiwan, ¶Institute of Fisheries Sciences, National Taiwan University, Taipei 10617, Taiwan and **Department of Environmental Biology and Fisheries Science, National Taiwan Ocean University, Keelung 20224, Taiwan Otolith Sr:Ca ratios of the African longfinned eel Anguilla mossambica and giant mottled eel Anguilla marmorata from nine freshwater sites in four rivers of South Africa were analysed to reconstruct their migratory life histories between freshwater and saltwater habitats. For A. mossam- bica, the Sr:Ca ratios in the otolith edge differed significantly among rivers and had large effect sizes, but did not differ among sites within a river. Otolith Sr:Ca ratios did not differ among rivers for A. marmorata. When rivers were pooled, the edge Sr:Ca ratios of A. mossambica were not significantly different from those of A. marmorata. According to the river-specific critical Sr:Ca ratio distinguishing freshwater from saltwater residence, most A. mossambica and A. marmorata had saltwater habitat experience after settlement in fresh water. This was primarily during their elver stage or early in the yellow eel stage. During the middle and late yellow eel stage, freshwater residency was preferred and only sporadic visits were made to saltwater habitats. The data also sug- gest that regional variations in otolith Sr:Ca ratios affect the critical Sr:Ca value and are a challenge for the reconstruction of migratory life histories that should be explicitly considered to avoid bias and uncertainty. © 2012 The Authors Journal of Fish Biology © 2012 The Fisheries Society of the British Isles Key words: catadromy; elver; fresh water; life history; residency; South Africa. INTRODUCTION Among 15 freshwater eel species (genus Anguilla ), five are found in South Africa, namely the African longfinned eel Anguilla mossambica (Peters 1852), giant mot- tled eel Anguilla marmorata Quoy & Gaimard 1824, African mottled eel Anguilla bengalensis labiata (Peters 1852), Pacific shortfinned eel Anguilla obscura G¨ unther 1872 and Indonesian shortfinned eel Anguilla bicolor bicolor McClelland 1844 (Jubb, §Author to whom correspondence should be addressed. Tel.: +27 46 603 5800 5834; email: o.weyl@saiab.ac.za 427 © 2012 The Authors Journal of Fish Biology © 2012 The Fisheries Society of the British Isles