Biological aspects of juvenile great hammerhead sharks Sphyrna mokarran from the Arabian Gulf Hua Hsun Hsu A,C , Zahid Muhammed Nazeer A , Yu Jia Lin A , Premlal Panickan A , Khaled Al-Abdulkader B , Ronald Loughland B and Mohammad Ali Qurban A A Marine Studies Section, Center for Environment and Water, Research Institute, King Fahd University of Petroleum and Minerals, PB No. 2094, Dhahran 31261, Saudi Arabia. B Environmental Protection Department, Saudi Aramco, P.O. Box 18597 Al Midra, Dhahran 31311, Saudi Arabia. C Corresponding author. Email: hsuhuahsun@yahoo.com.tw Abstract. The life history of the critically endangered great hammerhead Sphyrna mokarran was examined using samples collected from commercial fisheries operating in Saudi Arabian waters of the Arabian Gulf. In all, 105 specimens (47 males, 58 females), with a total length (TL) ranging from 67.0 to 236.0 cm, were examined between April 2016 and November 2019. All individuals were immature. A neonate measuring 67.0 cm TL caught in early June suggested the parturition period of this species was during May and June. Vertebrae under the first dorsal fin from 69 individuals (30 males, 39 females) were processed. Three age groups, from 0þ to 2þ years old, were identified by vertebrae analysis. Based on the relationship between vertebral central diameter and TL, the Fraser–Lee approach was used to back-calculate the growth history of TL. A two-parameter von Bertalanffy growth function provided the best fit to describe early growth of great hammerheads during 0–2.9 years of age based on observed and back-calculated length-at-age data. The asymptotic lengths and k values were estimated to be 256.8 cm TL and 0.449 year 1 respectively. This study provides the first life history information of great hammerhead sharks in the north-western Indian Ocean. Additional keywords: band pair, fishery, life history, vertebra. Received 26 November 2019, accepted 24 March 2020, published online 18 April 2020 Introduction The great hammerhead shark Sphyrna mokarran (Ru¨ppell, 1837) is the largest hammerhead shark (Sphyrnidae) species; it can grow to over 6.1 m in length and is distributed in global circumtropical waters (Compagno 1984; Piercy et al. 2010; Ebert et al. 2013). The great hammerhead is a highly migratory shark that inhabits coastal reefs, lagoons, continental shelves and deep waters (Compagno 1984; Cliff 1995; Ebert et al. 2013; Mourier et al. 2013; Guttridge et al. 2017; Gallagher and Klimley 2018). Even though this species does not form a target fishery, it is still caught by various fisheries operating from coastal to pelagic waters (Compagno 1984; Piercy et al. 2010). Because of its large fins, this great hammerhead shark species, in addition to other hammerhead species, was traded in Oriental markets, commanding high values and being the second most abundant species group in the international trade in fins (Clarke et al. 2005; Abercrombie et al. 2005; Piercy et al. 2010). Great hammerhead sharks also suffer very high bycatch mortality (Gulak et al. 2015; Guttridge et al. 2017; Jabado et al. 2017). Because of this, populations have declined markedly in the past three decades. Therefore, this species was placed in the EN (endangered) category of the International Union for Conservation of Nature (IUCN) Red List in 2007, reassessed as CR (critically endangered) in 2019; it was added to Appendix II of the Convention on International Trade in Endangered Species of Wild Flora and Fauna (CITES) in 2012 and was listed in Appendix II of the Convention on the Conservation of Migratory Species of Wild Animals (CMS) in 2014 (CITES 2014; CMS 2015; Jabado et al. 2017; Gallagher and Klimley 2018; Rigby et al. 2019). Despite the attention this species has received from interna- tional and regional conservation groups and bodies (International Commission for the Conservation of Atlantic Tunas (ICCAT) 2010; WildEarth Guardians 2012), detailed catch and life history information for the great hammerhead shark is limited, particu- larly in the north-western Indian Ocean. Parameters of the growth function for the north-western Atlantic population have been estimated. Asymptotic lengths were estimated to be 264.2 cm fork length (FL) for males and 307.8 cm FL for females (Piercy et al. 2010), and age was validated using bomb radiocarbon analysis that supported the hypothesis of annual band pair deposition (Passerotti et al. 2010). For the eastern Australian population, annual formation of vertebral growth band pairs was validated using tag–recapture methods (Harry et al. 2011). The asymptotic length was estimated to be 402.7 cm stretched total length (TLst) for the sexes combined, which suggested that the CSIRO PUBLISHING Marine and Freshwater Research, 2021, 72, 110–117 https://doi.org/10.1071/MF19368 Journal compilation Ó CSIRO 2021 www.publish.csiro.au/journals/mfr