Interactions entre comportement et variations de la croissance des juvéniles de la sole (Solea solea) dans les nourriceries des pertuis Charentais

This thesis aimed to identify and analyse processes involved in the functioning of sole Solea solea (L.) nurseries in the Pertuis Charentais, the most important oyster cultivation basin in Europe and a major nursery for the Bay of Biscay sole. Juveniles concentrate in coastal and estuarine habitats, which are essential in the biological cycle of this species but implies (i) adaptation to environmental variability and (ii), accessibility to habitat resources. Assuming that effective nurseries account for what fish must perform in response to habitat constraints, especially in selecting feeding, resting or refuge areas, we used both field works and mesocosm experiments to obtain integrative indicators of the nursery quality (growth rate, Fulton's condition index, endocrine status), and behavioural responses of how sole use space (acoustic telemetry techniques) and food (feeding regime and carbon daily ration estimates) in a habitat influenced by shellfish culture. Juvenile growth curves in the pertuis Breton were close to the species maximum in summer but they tended to a plateau around the autumn equinox. A mesocosm experiment confirmed that in situ temperatures could not explain this growth arrest. At the same time, juveniles had low condition indices, the weakest thyroid hormone levels and the lowest feeding activity. This status suggested various constraints, which could be specific to Pertuis, semienclosed bays under moderate river plume influence. It seemed that the Pertuis environment is less favourable at the end of summer and/or that cumulated effects on the trophic web do not allow the entire 0 group soles to grow according to species potentialities. Nevertheless, in the present climatic context, some of these juveniles were shown to over-winter in these nurseries, where they recovered high hormonal levels and restored their feeding activity. Under the Pertuis environmental constraints, sole growth was modulated, but behaviour did not appear modified. A mesocosm experiment, whose results still need field assessment, demonstrated that access to areas under oyster culture influence was not restricted, either by oyster-trestle effects or by water and bottom changes due to shellfish biological activity. From two 30h sampling cycles performed in the Pertuis d'Antioche, we demonstrated that young sole were able to adjust their activity rhythm and feeding rates, probably due to hydrodynamics, in relation to fortnightly tide cycles or wind stress. These cycles allowed us to evaluate the daily ration of organic carbon of sole juveniles, results which would contribute to the development of trophic web models. Finally, as spin-off of this thesis, a heavy infestation of the Pertuis Charentais by bucephalid metacercariae was revealed. Cercariae being propagated by mussel culture, this parasitosis gave a new point of view concerning the interaction between nursery function in coastal habitats and shellfish culture.