Reproductive Biology and Distribution of the Blue Shark (Prionace glauca) in the Western Indian Ocean

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 evaluated 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.

Seasonal distribution patterns and conservation gaps of blue sharks in the Indo‐Western Pacific Ocean

“…Our predictions on the large‐scale seasonal distributions of blue sharks in the IWPO are generally consistent with previous findings (Coelho et al., 2018; Fujinami et al., 2021; Kai et al., 2017; Zhu et al., 2023), and the seasonal divergences may be caused by the migration of blue sharks. The predicted distribution contraction of both the northern and southern populations towards the equator during the September to February (next year) in the Indian Ocean is likely due to the synchronous migration of adult blue sharks from both hemispheres for mating as shown by fishery‐based studies (Coelho et al., 2018; Zhu et al., 2023). The unchanged distributions in the temperate waters in the southwest Indian Ocean and southwest Australia are likely driven by the juvenile sharks given they mainly concentrate in these high‐latitude zones and do not change habitats over the year (Coelho et al., 2018).…”

“…Our predictions on the large-scale seasonal distributions of blue sharks in the IWPO are generally consistent with previous findings (Coelho et al, 2018;Fujinami et al, 2021;Kai et al, 2017;Zhu et al, 2023), and the seasonal divergences may be caused by the migration of blue sharks. The predicted distribution contraction of both the northern and southern populations towards the equator during the September to February (next year) in the Indian Ocean is likely due to the synchronous migration of adult blue sharks from both hemispheres for mating as shown by fishery-based studies (Coelho et al, 2018;Zhu et al, 2023). The unchanged distributions in the temperate waters in the southwest Indian Ocean and southwest Australia are likely driven by the juvenile sharks given they mainly concentrate in these high-latitude zones and do not change habitats over the year (Coelho et al, 2018) tagging study (Fujinami et al, 2021).…”

“…Our study encourages the use of open-access data and SDMs to predict seasonal distribution patterns for the highly-mobile blue shark, which is otherwise normally examined through expensive fishery-dependent surveys or satellite tags (Maxwell et al, 2019;Zhu et al, 2023). However, due to data limitations, we are unable to predict distribution patterns for different sexes and life history stages.…”

“…Oceanic sharks are difficult to study, given they have dynamic distribution patterns (Sequeira et al., 2012). To address this challenge, scientists have applied fishery‐dependent surveys and satellite‐tag tracking projects, which have significantly advanced our understanding of the distributions and movement patterns of some oceanic sharks (Queiroz et al., 2016; Zhu et al., 2023). We now know that the distribution of oceanic sharks is often correlated with environmental factors such as temperature, salinity, and currents, which exhibit seasonal variations.…”