The coastal lagoons in the Northwestern Gulf of Lions are considered to be important 2 nurseries for several coastal species with high commercial value, such as the gilthead 3 seabream Sparus aurata Linnaeus 1758. Juvenile seabream spend a part of their first year of 4 life growing in the lagoons, generally from spring to autumn. This study compared condition 5 and life history traits of juveniles after their first summer in four lagoons of different 6 hydrological and physico-chemical characteristics. Mauguio and Bages are shallow, brackish 7 lagoons with high summer temperatures and primary productivity, while Thau and Salses-8 Leucate are deeper, with higher salinities but lower summer temperatures and productivity. At 9 the end of their first summer, juveniles of Mauguio and, to a lesser extent those from Bages, 10 were in better condition than juveniles of Thau and Salses-Leucate. Hence they were larger, 11 heavier, with a higher Le Cren condition index (only for Mauguio, KMauguio=1.10±0.09 while 12 KThau, KBages and KSalses < 0.98) and higher tissue lipid stores, assessed as the triglyceride to 13 sterol ratio in white muscle (TAG/STMauguio=35.7±20.1, TAG/STThau=15.1±1.2, 14 TAG/STBages=23.2±9.8, TAG/STSalses-Leucate=7.4±7.9). Otolithometry revealed that, although 15 the date of capture differed between lagoons, all the juveniles analyzed were of the same age 16 (on average 299±17 days). Furthermore, retrocalculation of individual hatching dates 17 indicated that the peak of spawning was significantly different between all lagoons, 18 suggesting the existence of several sub-populations of S. aurata in the Gulf of Lions. This 19 hypothesis was further supported by subtle differences in geometric morphometrics in 20 juveniles among the lagoons with, in particular, Thau having different form from all other 21 lagoons. Juveniles of these sub-populations enter lagoons at the same age but over different 22 periods of the year. Calculation of individual growth rates during open-sea (larval) and lagoon 23 (post-larval) life demonstrated that the larger size of Mauguio and Bages juveniles was a 24 result of higher growth in the lagoons. Indeed, otolith growth rates of open-sea stages were 254similar for all lagoons (2.8±0.4 μm.day-1) while they were different for lagoon stages 26 (10.1±0.9 μm.day-1 for Mauguio, 8.4±1.2 μm.day-1 for Thau, 9.7±1.0 μm.day-1 for Bages, 27 8.9±0.8 μm.day-1 for Salses-Leucate)
