Oocyte dynamics for Atlantic bluefin tuna (Thunnus thynnus) in the southern Gulf of Mexico: a comparative assessment with Mediterranean spawners by two stereological methods

Abstract

AbstractAlthough most of the research have been realized in the northern Gulf of Mexico for western Atlantic bluefin tuna (BFT‐W), newly the histological records of reproductive activity of this species in the southern Gulf of Mexico (Mexican waters) was presented for the first time. This work aimed to study for the first time the oocyte dynamics in Atlantic bluefin tuna caught in the southern Gulf of Mexico by carrying out an appraise and compare them with Mediterranean stock (BFT‐E) through stereology using two different methods. Regardless of Atlantic bluefin tuna females turned up to their respective spawning grounds at different months in the southern Gulf of Mexico and the Mediterranean, both stocks arrived reproductively inactive and remained in these zones during periods of similar length; they started being reproductively active until March for the southern Gulf of Mexico and May for Mediterranean females. The comparison of the size structure between both stocks examined by Kernel Density Estimators (KDE) demonstrated a quite remarkable difference in mean fork lengths between stocks. The ovarian oocyte density, i.e., the number of oocytes per gram of ovary, for each one of the gonad stages predicted by the Weibel & Gomez (W&G) and Oocyte Packing Density (OPD) methods did not significantly differ between stocks, and exhibited that A‐Vit oocytes from spawning capable females are an appropriate indicator to estimate potential fecundity, presenting values of ~1273 and ~ 1355 eggs · g−1 for southern Gulf of Mexico and Mediterranean females respectively. Females caught in Mexican waters (southern Gulf of Mexico) were larger than those caught in the Mediterranean; however, it was demonstrated that the length and weight of females did not affect ovarian oocyte density production. In addition, densities estimated for each gonad stage by W&G and OPD methods did not differ between stocks and presented equal patterns in their oocyte dynamics. These findings contribute to a better understanding of the reproductive biology of Atlantic bluefin tuna, especially in the southern Gulf of Mexico, due to the lack of information regarding this zone and may allow to support strategies for a proper assessment, management, and conservation.This article is protected by copyright. All rights reserved.