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Comparative ecology of young stages of shads Alosa alosa & Alosa fallax: preferendum, sensitivity to environmental pressures and population restoration Supervisors Directeur: Marie-Laure Acolas, Irstea unité EABX, équipe PMA Co-directeur: Agnès Bardonnet, INRA UMR Ecobiop Host laboratory Irstea UR EABX équipe PMA 50 avenue de Verdun 33612 Gazinet Cestas Doctoral school Université de Bordeaux, école doctorale sciences et environnements ED 304 Abstract To assess aquatic habitat quality and biodiversity erosion, migratory fishes constitute excellent biological models. They allow studying the functionality of the continuum riverestuary-ocean because, to fulfill their life cycle, this continuum needs to be of good quality. Those diadromous species are part of the biodiversity of the three environments they cross or exploit. And among those species, anadromous species, which young stages are born and grow in freshwaters, represent good biological indicators of river and estuarine ecosystems. During species life cycle, first months of life are a critical period where survival rate can be very low. Those small size individuals have to adapt to new environmental conditions, escape predators and find food rapidly. For anadromous fish, the young stages grow in freshwaters and migrate downstream progressively or massively towards sea waters. Then, in addition to current risks associate to this life stage, they have to face additional risks specific to migratory species: mortality (direct or indirect) or migration delay linked to obstacles, important physico-chemical variations with osmoregulation mechanisms to implement, a large distance to cover travelling through unknown habitats involving increase exposure to predators. Alosa alosa, allis shad and Alosa fallax, twaite shad are anadromous species living in sympatry. Historically they were present on European and North African coasts. Those species do not escape from the global context of decline of diadromous species. Their distribution area has been reduced, both are classified as vulnerable, their exploitation is regulated and restauration program have been established in some rivers. According to large scale modelling studies in Europe, the predictions, within the frame of the global change scenario, suggest an extension of distribution range of twaite shad population and a reduction of the allis shad populations. However, this hypothesis does not take in account the effect of local environmental stress encountered in anthropized watersheds. A comparative analysis of those species ecological preferences and of the responses to some environmental stress would bring elements to weigh these broad-scale predictions. These two species belong to the same genus, they can live in sympatry during a critical phase of their life cycle and they can even hybridize. Then, although they are subject to the same constraints, their resilience capacity can vary and being expressed both in response level and within the range of those responses. We propose in this work a study at the individual level in order to characterize both points. In New Aquitania, population status, especially for allis shad, is worrying with a huge decrease in the number of individuals. While the Gironde-Garonne-Dordogne watershed gathered the largest allis shad population in Europe at the end of the 20th century, the number of spawners dropped drastically since 2006 and a moratorium banishing its exploitation in the basin was adopted in 2008. In the meantime, twaite shad population seemed to decline but in a weaker proportion. The causes of decline can be numerous ant it is necessary for the managers to identify and prioritize adaptive managing measures. Using this watershed as a case study constitutes a good alternative because it is representative of the different pressures encountered in the anthropized European watersheds. Within the frame of this PhD project, the question of the two species of shads’populations adaptability and resilience will be tackled. We want to assess the resilience of both species to environmental stress characteristics of the watershed (e.g. connectivity break, oxy-thermal stress, estuarine turbidity maxima). The reproduction sites of the two species differ in low impacted watersheds, allis shad reproducing more upstream. But in anthropized watersheds, spawning ground of allis shad tend to be constrained to more downstream areas where the twaite shad reproduce; then young stages of both species encounter similar environmental conditions. Thus the proposed comparative approach between the individuals of both species makes sense in anthropised watersheds. Moreover both species can hybridize; hybrids being identified in some rivers and their impact on population functioning has not been studied yet. In this research project, we propose an approach at the individual level, the results being interpreted in terms of population functioning. The PhD project aims at comparing individual capacity (e.g. habitat preferendum, metabolic rate, swimming performances) of the young stages of both species and to evaluate their growth performance and mortality risks associated to environmental stresses identified above. The PhD student will prepare a synthesis of the knowledge acquired on young stage of allis shad and twaite shad in Europe: habitat preferendum in the wild, capacity to cope with physico-chemical stresses (temperature, oxygen levels, salinity) and the effect of connectivity break. The student will lead experimentations in controlled and semi-natural environments in order to compare performances and sensitivity of the young stages (from larvae stage to juvenile of 60-90 days post hatch) of both species and eventually of their hybrids. The aim is to identify critical stage or behavior, especially face to risks associated to anthropized watersheds. This approach will provide answers about differential adaptability and resilience of the populations of the two species. Candidate profile The PhD Candidate will have a master degree with knowledge in population ecology, aquatic ecology and behavioral ecology. He/She will have experience with experimentation on fish species. The following capacity will be appreciated: organization, manual skills for experimentation, data analysis, autonomy. During this work the PhD candidate will acquire theoretical skills on young stage ecology and migratory species life history traits. He/She will masteries experimental tools for behavior, preferendum and stress factors assessment on sensitive fish species. He/She will benefit scientific and technical partnership at different level (local, national, international). His/Her research on shad species can be extended at the European watershed level. Contact e-mails: [email protected] and [email protected]