Possible drivers and implications of changes in inter-spawning
area connectivity
The observed levels of genetic differentiation, hardly compatible with
the high level of gene flow suggested by our results in constant
equilibrium, suggest that connectivity patterns between ABFT spawning
grounds could be subjected to temporal changes and that an increase in
migration from the Mediterranean Sea towards the known western Atlantic
spawning areas could have a genetic homogenizing effect. Such a
homogenizing effect is expected to be correlated with migration rates
and the effective population size (Ne) of the recipient population
(Lowe and Allendorf 2010,
Gagnaire et al. 2015), which in turn
relates to the number of adult individuals among other factors
(Waples 2022) and would consequently be
affected by fluctuations in the population’s abundance. The abundance of
ABFT stocks have undergone strong changes during the last
~60 years. After both the western and the eastern
Atlantic stocks reached a critical status, including the collapse of
several fisheries around the 1960-1970s
(Fromentin 2009,
Porch et al. 2019), the western Atlantic
stock has not recovered as rapidly as the eastern Atlantic stock (of
Mediterranean origin), whose estimated abundance has been one order of
magnitude larger for several decades
(ICCAT 2017), despite decades of
conservation efforts. This slow recovery could result from a regime
shift over the last decades, due to the combination of oceanographic
changes in the equatorial Atlantic and overfishing
(Fromentin et al. 2014b) possibly
affecting both migration rates and effective population sizes.
Fluctuations in the abundance and distribution of eastern Atlantic
bluefin tuna during the last century were largely explained by the
Atlantic Multidecadal Oscillation (AMO)
(Faillettaz et al. 2019) and long-term
trends in temperature (Ravier and
Fromentin 2004). Moreover, coinciding with the last negative AMO period
starting in the 1960s, ABFT had disappeared from several North-East
Atlantic areas where it is reappearing during the current positive AMO
phase starting in the mid-1990s (Horton et
al. 2020, Nøttestad et al. 2020,
Aarestrup et al. 2022). Furthermore,
increasing catches of ABFT in Greenland waters show that the northern
limit of ABFT distribution was expanded northwards during the last
decade by mostly individuals of Mediterranean genetic origin
(Jansen et al. 2021). Based on electronic
tagging data, the proportion of individuals of eastern origin present in
the western Atlantic has also increased during the last two decades
(Aalto et al. 2021). Interestingly, AMO
positive or warm phases as well as current global warming involve an
increase in habitat suitability in most of these northern areas
(Fromentin et al. 2014b,
Faillettaz et al. 2019). In summary,
population size, distribution, and migratory behavior of ABFT has been
undergoing changes during the last decades, probably due to changes in
environmental conditions, fishing pressure, conservation efforts or
combined effects of these. These changes could explain migration
intensification from the recently expanded eastern stock towards the
more slowly recovering western Atlantic stock. Our analyses support that
the recent short-term genetic effects of immigration on the variance in
ancestries are much stronger in the Slope Sea than in the Gulf of
Mexico. This could be due to behavioral preferences or favorable
conditions which would make it easier for Mediterranean individuals to
reach and reproduce in the Slope Sea, or due to a smaller effective
population size compared to the Gulf of Mexico. The homogenizing effect
of this unidirectional migration would ultimately lead to the genetic
swamping of the GOM-like genetic component. It is thus possible that
genetic differentiation is detected in the existing samples because gene
flow is relatively recent relative to mean generation times estimated to
average 9.6 years for the Western stock
(Collette et al. 2011), and that this
genetic divergence will be attenuated across generations in the future.
While genetic connectivity does not necessarily equate with demographic
dependence, genetic erosion would not necessarily imply demographic
decline either, but could have more unpredictable demographic
consequences.