Strong admixture in the Slope Sea as the result of a potential source-sink dynamic
The observed heterogeneously admixed genetic profiles of Slope Sea larvae and YoY ABFT support recurrent interbreeding between migrants from the Gulf of Mexico and the Mediterranean Sea in the Slope Sea, which contributes to the admixed genetic background of this spawning area. This observation is compatible with tagging data, which shows adult individuals that enter the Gulf of Mexico or the Mediterranean Sea also visit the Slope Sea spawning area (Block et al. 2005, Aalto et al. 2023). Otolith microchemistry data provides evidence of individuals with Mediterranean Sea and Gulf of Mexico origin compatible profiles in this area (Siskey et al. 2016).
Our analyses support that the Slope Sea component originated from the Gulf of Mexico population, and that mixing with the Mediterranean population started later. Thus, even if evidence of spawning activity in the Slope Sea dates back to the 1950s (Baglin 1976, Mather et al. 1995) and could have started much earlier, it is most likely that the now observed genetic differentiation of the Slope Sea is due to an increase in the immigration rates from the Mediterranean component towards the Slope Sea. In fact, heterogeneous genetic profiles of individual ABFT from the Slope Sea indicate a diverse genetic composition of spawners, a situation at odds with the scenario of an exclusively self-sustained population at equilibrium. Moreover, previous studies using otoliths have reported highly variable proportions of Mediterranean origin individuals in the western Atlantic across the last five decades (Secor et al. 2015, Siskey et al. 2016, Rooker et al. 2019, Kerr et al. 2020) and Puncher et al. (2022) detected that the proportion of individuals genetically assigned to Mediterranean origin increased over the past two decades at some northwestern Atlantic areas, particularly among individuals younger than 15 years, which is compatible with dynamically changing migratory trends.
Demographic connectivity is of major importance for fisheries management, as it directly affects productivity and a stocks recruitment. Despite the limited knowledge about the spawning dynamics in the Slope Sea, our data suggest asymmetrical genetic connectivity towards the Slope Sea, possibly acting as a sink spawning area which is receiving rather than exporting individuals, although its admixed nature could hamper the detection of gene flow from the Slope Sea towards the Mediterranean Sea and the Gulf of Mexico. This highlights the importance of understanding the demographic interdependence of the Slope Sea with the other components, especially in view of recent studies proposing the Slope Sea as a major spawning ground (Hernández et al. 2022). One important knowledge gap is thus the understanding of Slope Sea born individuals’ life cycle. The currently observed genetic profiles are compatible with Slope Sea born individuals showing i) Slope Sea spawning-site fidelity, ii) limited spawning, iii) spawning in the Gulf of Mexico and iv) MED-like individuals born in the Slope Sea spawning in the Mediterranean Sea. Unfortunately, weak genetic differentiation, typical in marine fishes with large population sizes, together with the presence of intermediate and heterogeneous (and presumably temporally variable in proportions) genetic profiles hamper the clear identification of Slope Sea born individuals based solely on genetic markers. Thus, we suggest that exploration of the dynamics of these individuals may require the use of integrated methods, such as the combination of genetic markers with otolith microchemistry (Brophy et al. 2020). The capability of identifying Slope Sea born individuals and monitoring their presence across the ABFT distribution range, together with an increase in larval sampling efforts in this spawning area, would allow us to obtain and analyze temporal samples to understand their life cycle and estimate admixture rates in the Slope Sea across generations.