With global warming, the surface temperature of the Mediterranean Sea has increased by 0.4 °C per decade from 1985 to 2006 (Nykjaer, 2009) and is projected to continue increasing by 2.2 to 3.4 °C until 2080 (IPCC, 2021). Therefore, it is crucial to understand the impacts of this warming on the larval growth of the European sea bass, Dicentrarchus labrax, which is very important for both fisheries and aquaculture. In this study, we investigated the larval growth and differential survival of three natural populations: Atlantic (AT), Western Mediterranean (WM) and Eastern Mediterranean (EM) in different temperature conditions.
Materials and Methods
The production of the three populations was achieved using three different full factorial design with 30 sires per population and 6, 14 and 13 dams for AT, WM and EM populations, respectively. Until 19 days post-hatching (dph), the populations were reared separately in 2 tanks per population at 13 °C. From 19 dph, the populations were mixed in equal proportions and placed in four thermal environments that mimic the temperatures encountered in the Atlantic (eAT, annual average = 13.8 °C, min = 10.4 °C, max = 18 °C), the western Mediterranean Sea (eWM, annual average = 16.6 °C, min = 12.6 °C, max = 22.5 °C), the eastern Mediterranean Sea (eEM, , annual average = 21.7 °C, min = 16.8 °C, max = 27.6 °C) and a temperature generally applied in aquaculture (eAQUA, 16 °C until 70 dph, 23 °C from 70 to 120 dph and then an eastern Mediterranean temperature regime). Each thermal regime was replicated in 4 tanks. Twenty larvae per tank were sampled in each regime at six time points corresponding to equivalent developmental stage from an average notochord length of 8.8 mm to an average fork length of 53.5 mm. Larvae were photographed, weighed, and their photos were analyzed with ImageJ to measure the notochord length (until the fourth sampling) and then, the fork length. The population of origin of each larvae was recovered by parentage assignment using 96 SNPs with the APIS software(Griot et al., 2020). At around 10 grams, 2000 fish per regime were genotyped and assigned to their parents to evaluate the survival of each population in each thermal regime.
From the fourth sampling (≈ 27.8 mm fork length), the AT fish showed a tendency to be longer than WM and EM fish. This difference became significant (P < 0.01, AT > WM ≈ EM) from the fifth samplings (≈ 40.2 mm fork length) in all thermal regimes. Furthermore, the size advantage of AT was larger in the coldest thermal regimes (eAT and eWM, Figure 1.A). Regarding survival, there was a strong interaction between population and thermal regime (P < 2.2e-16, Figure 1.B), and within each regime, there was a significant effect of population on survival (P < 2.2e-16 in eAT, eWM and eEM regimes). The AT population had the best survival rate in the eAT and eWM regimes while the EM population survived best in the eEM regime. The WM population had the worst survival rate in all thermal regimes. We identified three main periods of time where population-specific mortality may occur through linear regressions of survival on temperature at different stages: between 19 dph and the first sampling (6.6-8.8 mm), between the fourth and the fifth samplings (27.8-40.2 mm), and between the fifth and sixth samplings (40.2-53.5 mm).
The results demonstrate the growth of European sea bass is strongly influenced by thermal regimes. However, in terms of growth, we did not observe evidence of adaptation of a population to a specific regime. A local adaptation to low temperatures could explain the better growth potential of AT population than WM and EM populations under the coldest regimes (eAT and eWM). The largest size of AT population in the warmest regimes (eEM and eAQUA) could be explained by the countergradient variation phenomenon, where cold-adapted population may express better growth rates when warmer temperatures occur (Conover and Present, 1990).
The study of survival rate revealed that populations are adapted to their thermal regimes of origin. Specifically, the AT population was the better suited to the coldest regimes in terms of both growth and survival. However, for survival, there was also an adaptation of the EM population to the eEM regime. The WM population had the worst survival rate in all regimes, although it survived better in warm than in cold regimes. These results are consistent with previous research that suggested nonadaptive introgression and maladaptation of the WM population (Guinand et al., 2017).
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