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.