Citation: Zhu, J.; Geng, Z.; Zhu, J.; Richard, K. Reproductive Biology and Distribution of the Blue Shark (Prionace glauca) in the Western Indian Ocean. Biology 2023, 12, 1128. https://doi.org/10.3390/ biology12081128 Academic Editors: John R. Turner and José Pedro Andrade Received: 31 May 2023 Revised: 3 August 2023 Accepted: 5 August 2023 Published: 14 August 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). biology Article Reproductive Biology and Distribution of the Blue Shark (Prionace glauca) in the Western Indian Ocean Jizhang Zhu 1 , Zhe Geng 1 , Jiangfeng Zhu 1,2,3,4, * and Kindong Richard 1,2,3,4, * 1 College of Marine Sciences, Shanghai Ocean University, Shanghai 201306, China; zjz2373@gmail.com (J.Z.); zgeng@shou.edu.cn (Z.G.) 2 Key Laboratory of Sustainable Exploitation of Oceanic Fisheries Resources, Ministry of Education, Shanghai 201306, China 3 Key Laboratory of Oceanic Fisheries Exploration, Ministry of Agriculture and Rural Affairs, Shanghai 201306, China 4 National Engineering Research Centre for Oceanic Fisheries, Shanghai Ocean University, Shanghai 201306, China * Correspondence: jfzhu@shou.edu.cn (J.Z.); kindong@shou.edu.cn (K.R.) Simple Summary: Blue shark populations are currently declining, caused mainly by the impact of overfishing from industrial fishing vessels in the open oceans. However, there is a paucity of comprehensive biological and habitat data concerning the reproductive characteristics of blue shark species in the Indian Ocean. In response to the call made by the Indian Ocean Tuna Commission (IOTC) to provide valuable parameters for the conservation of blue shark resources, this study collected observer data from the Indian Ocean longline fishery targeting tuna between 2010 and 2020. Through rigorous statistical analysis, it was determined that the estimated length at sexual maturity for male blue sharks is 161.4 cm and 179.3 cm for females. Moreover, the average litter size was found to be 33.7 pups. The study suggests the existence of a possible seasonal migratory pattern for pregnant blue sharks, with the first and fourth quarters of the year being potential mating grounds in proximity to the equatorial region of the Indian Ocean. Additionally, the temperate marine areas of the Indian Ocean were identified as crucial habitats for parturition and juvenile development of blue shark species. Consequently, it is strongly recommended to implement more scientifically informed and ecologically sustainable operational strategies in these designated areas. Abstract: Due to the limited biological research on the blue shark in the Indian Ocean, such as the lack of a clear understanding of its reproductive biology and distribution, our study analyzed and evalu- ated the fork length distribution, sexual maturity length, reproductive capacity, and spatiotemporal distribution of blue sharks based on biological data and capture location information collected in the western Indian Ocean from 2010 to 2020. The objective of this study is to provide reliable biological information important in performing future stock assessments vital for species conservation in this region. A total of 791 male (33–249.5 cm FL) and 803 female (12–349.6 cm FL) blue sharks were collected in the West Indian Ocean. We used the morphology of the sexual organs to ascertain their sexual maturity. Results show that the observed size at 50% sexual maturity of male blue sharks in the West Indian Ocean was 161.4cm FL (192.4 cm TL) for males and 179.3 cm FL (213.9 cm TL) for females based on logistic curve analysis. The average litter size of pregnant blue sharks was 33.7 pups. There were significant differences in the distribution of blue shark individuals with different sexual maturity levels in different quarters (p < 0.05). This study suggests that the area near the equator in the Indian Ocean from October to March of the following year may be the mating ground for blue sharks, while the temperate waters in the Indian Ocean are the nursery ground and parturition ground for pregnant and juvenile throughout the whole year. Therefore, it is recommended to adopt a more scientific and reasonable operational method in these areas. Biology 2023, 12, 1128. https://doi.org/10.3390/biology12081128 https://www.mdpi.com/journal/biology