Otolith Sr:Ca and Ba:Ca may give inconsistent indications of estuarine habitat use for American eels (Anguilla rostrata) Brian M. Jessop & Chia-Hui Wang & Wann-Nian Tzeng & Chen-Feng You & Jen-Chieh Shiao & Shih-Huan Lin Received: 30 July 2010 / Accepted: 10 July 2011 / Published online: 28 July 2011 # Springer Science+Business Media B.V. 2011 Abstract Temporal patterns in otolith Sr:Ca and Ba:Ca ratio values of American eels Anguilla rostrata from two sites in western Newfoundland gave insight into the use of freshwater and saline habitats. Mean Sr:Ca and Ba: Ca values at the core zone did not differ between sites, indicative of a common oceanic origin. At the otolith edge, representing continental life, both Sr:Ca and Ba: Ca values varied between sites consistent with ambient element:Ca ratio values and salinity, with typically higher Sr:Ca and lower Ba:Ca values in saline than in fresh waters. Most eels (73%) from Muddy Hole, an estuarine site, were evaluated as estuarine residents while most (70%) eels from Castors River, a freshwater site, were evaluated as freshwater residents, with the remaining eels from each site evaluated as inter-habitat migrants. An otolith element:Ca critical value appro- priate for distinguishing between fresh and saline water residence is fundamental for estimating the proportion of eel residence in freshwater and their subsequent classification into habitat residence groups. Such classification is moderately robust to the critical value selected. For inter-habitat migrants, moderate otolith Sr: Ca values between the elver check and otolith edge suggestive of estuarine residence may coincide with Ba: Ca values suggestive of freshwater residence. No general critical value for separating fresh and estuarine habitats was found for otolith Ba:Ca. Otolith Ba:Ca temporal patterns may assist the use of Sr:Ca in the evaluation of historical habitat residence and inter- habitat movement but the use of otolith Ba:Ca values should be applied cautiously for American eels and perhaps of other estuarine/freshwater migratory fishes. Keywords American eel . Habitat use . Otolith strontium and barium Introduction American eel Anguilla rostrata stocks have declined in many regions of North America (Jessop 1997; Environ Biol Fish (2012) 93:193–207 DOI 10.1007/s10641-011-9905-0 B. M. Jessop (*) Department of Fisheries and Oceans, Bedford Institute of Oceanography, PO Box 1006, Dartmouth, Nova Scotia B2Y 4A2, Canada e-mail: jessopb@mar.dfo-mpo.gc.ca C.-H. Wang : C.-F. You Earth Dynamic System Research Center, National Cheng Kung University, 1 University Road, Tainan 701, Taiwan C.-H. Wang : W.-N. Tzeng Department of Environmental Biology and Fisheries Science, National Taiwan Ocean University, Keelung 202, Taiwan W.-N. Tzeng : S.-H. Lin Institute of Fisheries Science, National Taiwan University, 1, Sec. 4, Roosevelt Road, Taipei 106, Taiwan J.-C. Shiao Institute of Oceanography, National Taiwan University, 1, Sec. 4, Roosevelt Road, Taipei 106, Taiwan Author's personal copy