Investigating environmental influence and temporal changes in sole (Solea solea) larvae condition using histology

In the eastern part of the English Channel, common sole (Solea solea) has strong interests in fisheries research. Low recruitment along with a decline in spawning stock biomass have been observed for several years. According to the recruitment hypotheses, larval survival may play an important role that needs to be considered. The fish larval condition can be assessed using histology which has been recognised as the most appropriate method to provide a reliable index of the nutritional status. Based on this approach, this study aimed to identify critical periods of wild-collected sole larvae and to determine sources of variations of their condition between two periods separated by more than 20 years. In line with other studies, the transition from endogenous to exogenous feeding was identified as the most critical period with the lowest proportion of healthy larvae observed. During this first feeding stage, good larval conditions were located in sampling stations close to the coast and at the end of the spring season, in relation to higher temperature and fluorescence values. This highlights the need for sole larvae survival to cross the coastal front, which splits the central and coastal waters, to reach more stable and productive areas. This coastal migration pattern was consistent between 1995 and 2017, with however significantly lower larval abundances in the recent period. Multivariate analyses showed that the spring environmental conditions of 1995, characterised with lower temperature and higher fluorescence values were more favourable to larval condition, compared to the spring in 2017. Areas providing suitable environmental conditions in 2017 were more restricted and limited to sampling stations in front of estuaries. Since small differences in larval survival can lead to large fluctuations in recruitment, the larval condition should be studied in a more long-term approach. This would provide a better understanding of the environmental influence on larval su