Implications for Atlantic Bluefin tuna conservation and management
Conservation of ABFT is challenged by past and future fishing pressure (Fromentin et al. 2014a, Secor et al. 2015), which has sharply increased over the last five years following the rebuilding of the Mediterranean ABFT population (ICCAT 2023), and by changes in environmental conditions (often interacting with fishing pressure), which have been shown to alter population size and productivity, migratory behavior and spatial distribution (Ravier and Fromentin 2004). From a conservation perspective, hybridization between genetically differentiated lineages, in this case between GOM-like and MED-like individuals, could increase each population’s genetic diversity, leading to the incorporation of potentially adaptive genomic variation and reducing vulnerability to environmental changes (Brauer et al. 2023). However, strong unidirectional gene flow could provoke genetic swamping of the western Atlantic spawning areas jeopardizing ABFT genetic diversity (Roberts et al. 2010). In this sense, large effective population sizes, which could increase following a rebuilding of abundance at the different spawning areas (Hoey et al. 2022) would counteract the homogenizing effect of genetic drift. In the absence of accurate estimations of ABFT effective populations sizes (Puncher et al. 2018), further genetic monitoring of temporal samples could help to understand potential ongoing trends in genetic diversity conservation (Hoban et al. 2014, Oosting et al. 2019).
From a fisheries management perspective, the confirmation of ongoing admixture in the Slope Sea challenges the paradigm of two isolated ABFT stocks. However, large knowledge gaps related to the dynamics of Slope Sea individuals, the magnitude of the Slope Sea spawning in terms of recruitment, and its demographic connectivity with other components hinders explicit modelling of it as a distinct stock. Nonetheless, the recently adopted management procedure (ICCAT 2023) does explicitly consider spawning in the Slope Sea. Our study highlights the need for further monitoring combining multidisciplinary data such as larval sampling, tagging, otolith microchemistry signature and genetic origin to understand the Slope Sea population dynamics and the relevance of this spawning area in demographic and evolutionary terms.