Reproductive biology and population structure of the shortfin mako shark (Isurus oxyrinchus) in the southwestern Atlantic Ocean

The shortfin mako shark, Isurusoxyrinchus, plays essential ecological and economic roles in all oceans as a top predator and an important fishery resource. However, knowledge of its reproductive biology is still incomplete on a global basis, mainly due to a lack of sampling of mature females, particularly pregnant ones. In this paper, we studied the reproductive biology of shortfin mako sharks in the southwestern South Atlantic Ocean (SWAO) by describing primary and secondary sexual characteristics (both macroscopic and histologic) and determining the median length at maturity (TL50) using a Bayesian approach. Individuals were collected from pelagic longline fisheries in the SWAO between November 2020 and July 2021. The morphology of the ovaries and their characteristic ovarian pores are described. The histologic analyses showed at least three different follicular development stages for both sexes. The TL50 was found to be 278 cm (95% Cr. I., 257-387) in total length for females and 197 cm (95% Cr. I., 187-207) in total length for males. With these updated parameters, we analyzed the species' use of the SWAO in different life stages between 2018 and 2023. We found that individuals from different life stages are present in the study area at different times throughout the year and over the years, with, generally, larger individuals in spring and summer. The updated reproductive parameters for the SWAO will support future stock assessments of this species. The evidence of a heterogeneous use of the study area can be seen as an indication of a lack of stock structuring in the region.