Journal of Fish Biology (2010) 76, 1396–1414 doi:10.1111/j.1095-8649.2010.02571.x, available online at www.interscience.wiley.com Maturation and sexual ontogeny in the spangled emperor Lethrinus nebulosus R. J. Marriott*, N. D. C. Jarvis, D. J. Adams, A. E. Gallash, J. Norriss and S. J. Newman Western Australian Fisheries and Marine Research Laboratories, Department of Fisheries, Government of Western Australia, P.O. Box 20, North Beach, Western Australia 6920, Australia (Received 9 April 2009, Accepted 18 December 2009) The reproductive development and sexual ontogeny of spangled emperor Lethrinus nebulosus popu- lations in the Ningaloo Marine Park (NMP) were investigated to obtain an improved understanding of its evolved reproductive strategy and data for fisheries management. Evidence derived from (1) analyses of histological data and sampled sex ratios with size and age, (2) the identification of residual previtellogenic oocytes in immature and mature testes sampled during the spawning season and (3) observed changes in testis internal structure with increasing fish size and age, demonstrated a non-functional protogynous hermaphroditic strategy (or functional gonochorism). All the smallest and youngest fish sampled were female until they either changed sex to male at a mean 277·5 mm total length (L T ) and 2·3 years old or remained female and matured at a larger mean L T (392·1 mm) and older age (3·5 years). Gonad masses were similar for males and females over the size range sampled and throughout long reproductive lives (up to a maximum estimated age of c. 31 years), which was another correlate of functional gonochorism. That the mean L T at sex change and female maturity were below the current minimum legal size (MLS) limit (410 mm) demonstrated that the current MLS limit is effective for preventing recreational fishers in the NMP retaining at least half of the juvenile males and females in their landed catches. Journal compilation © 2010 The Fisheries Society of the British Isles No claim to original US government works Key words: age estimation; functional gonochorism; gonadal development; Lethrinidae; non-functional protogynous hermaphrodite. INTRODUCTION The emperors (Lethrinidae) are a family of tropical reef fishes that comprise an important component of commercial, recreational and subsistence fisheries through- out the tropical and subtropical waters of the Indo-Pacific Ocean (Carpenter & Allen, 1989). Sex change in the form of functional protogynous hermaphroditism has been demonstrated for the majority of lethrinids (Young & Martin, 1982; Ebisawa, 2006; Sadovy de Mitcheson & Liu, 2008). Sequential hermaphroditism in exploited fish stocks can be of great importance to their management, because size-selective *Author to whom correspondence should be addressed. Tel.: +61 8 9203 0111; fax: +61 8 9203 0199; email: Ross.Marriott@fish.wa.gov.au 1396 Journal compilation © 2010 The Fisheries Society of the British Isles No claim to original US government works