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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]