Journal of Fish Biology (2015) 86, 845–853 doi:10.1111/jfb.12605, available online at wileyonlinelibrary.com Vertical habitat shift of viviparous and oviparous deep-sea cusk eels revealed by otolith microstructure and stable-isotope composition N. N. Chang*, E. Y. Liu*, Y. C. Liao† and J. C. Shiao*‡ *Institute of Oceanography, National Taiwan University, 1, Sec. 4, Roosevelt Rd, Taipei, Taiwan, Republic of China and †National Museum of Marine Science and Technology, No. 367, Pei-Ning Rd., Zhongzheng Dist., Keelung City, Taiwan, Republic of China (Received 22 July 2014, Accepted 12 November 2014) Otolith stable-oxygen-isotope composition and microstructure were analysed in order to investigate the vertical habitat shift of deep-sea cusk eels (Ophidiiformes). Otolith  18 O profles suggested that both viviparous blind cusk eels and oviparous cusk eels experienced a pelagic larval stage and then settled to the deep-sea foor over a vertical distance that ranged among individuals from 200 to >1000 m. This result shows that the larvae of viviparous Barathronus maculatus undertake an ontogenetic vertical migration after a period of larval drift that may facilitate their wide distribution on the sea foor. © 2015 The Fisheries Society of the British Isles Key words: Barathronus maculatus; ontogenetic vertical migration; Ophidiidae; oxygen isotope; vertical distribution. Distributions of benthic deep-sea fshes on the sea foor are dependent on their repro- ductive strategies and ontogenetic vertical migration during their early life stages. Ophidiiformes (cusk eels) are common in deep-sea environments and have different reproductive strategies (e.g. oviparous and viviparous) among families. Most ophidiids are oviparous and the larval fshes are hatched in the pelagic zone (Fahay, 1992), yet fshes belonging to the Aphyonidae (blind cusk eels) and Bythitidae give birth to young fshes directly (Nielsen et al., 1999). Larval and juvenile viviparous cusk eels are believed to inhabit adult living depths and undertake little vertical migration throughout their life (Nielsen et al., 1999). A juvenile Barathronus pacifcus Nielsen & Eagle 1974 of 41⋅8 mm total length, L T , was collected from waters shallower than 260m in the Tasman Sea (Okiyama & Kato, 1997). The adults, however, generally inhabit depths from 3334 to 3860 m. This fnding implies a possible pelagic larval stage for aphyonids, particularly those of the genus Barathronus, but the data were still insuffcient to verify the migratory life history of deep-sea cusk eels. The otolith stable-oxygen-isotope composition coupled with microstructure analysis can reconstruct the historical residence depths of fshes. The microstructures of otoliths ‡Author to whom correspondence should be addressed. Tel.: +886 2 33663227; email: jcshiao@ntu.edu.tw 845 © 2015 The Fisheries Society of the British Isles