Understanding the impact of warming surface waters on fish growth is crucial for both fisheries and aquaculture. We investigated the growth of three genetically distinct populations of European seabass: Atlantic (AT), West-Mediterranean (WM) and Est-Mediterranean (EM) in three seasonal thermal regimes mimicking temperatures encountered in the Atlantic (rAT), the western Mediterranean (rWM) and the eastern Mediterranean (rEM).
At 20 days post-hatch, the three AT, WM, EM populations were reared in common garden in three separate seasonal temperature regimes , representing rAT , rWM or rEM. A total of 5148 fish were genotyped with the ThermoFisher AxiomTM Seabass 57k SNP array DlabChip and PIT-tagged (± 10 g average weight) to study their growth over 1.5 years , as their daily growth coefficient (DGC). We used GBLUP models to investigate the genetic bases of local adaptation to temperature by estimating heritability and genetic correlations, to study genotype by environment interaction (GxE) for AT and EM populations (the WM population did not have enough individuals to compute robust estimations).
During the first year after tagging, growth of the AT population was higher than those of EM and WM populations in all regimes, with greater differences in colder regimes (rAT and rWM). As of the 2nd year, the growth of AT remained higher only in rAT . Modelling of growth rate as a function of temperature and fish size revealed that the thermal optimum of the AT population was between 24 and 26 °C whereas the optimum was above 27 °C for the WM and EM populations. At 2 years old, the AT population was on average bigger in all thermal regimes, followed by the EM and WM populations. Heritability of growth rate during the winter (0.55) was higher than in summer (0.33) in both AT and EM populations. During the summer, there were negative genetic correlations between the extreme regimes (rAT and rEM ; Table 1).
These results reveal countergradient variation for growth of the AT population during the first year and, then, a specific adaptation of AT to rAT and EM to rEM. The strong GxE interaction , particularly in summer, indicates an inversion of genotype rankings for summer growth between the warmest and coldest regime . Further analyses are in progress to further explore and explain these observations.
This study is part of FishNess project (ANR-21-CE20-0043).