Study of chemical contamination pressures in allis shad (Alosa alosa) spawning grounds in the Gironde-Garonne-Dordogne catchment

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Inglés. The allis shad (Alosa alosa) is a diadromous migratory fish that breeds in rivers. A decline in this species, on a European scale and in the GGD catchment (historically the largest population), has been observed since the late 1990’s. Several theories explaining this decline have been investigated, including the recent hypothesis of a link between embryo mortality and water contamination. However, information on water quality in allis shad spawning grounds is limited. Thus, using the SIEAG database, this study first categorized the contaminants present (quantified or detected) in these spawning grounds. However, only the presence of contaminants does not provide information on toxicity. For this reason, we used the PAF (Potentially Affected Fraction) method, which is an indicator of potential toxicity that compares environmental concentrations (here, annual averages) with toxicity values. These toxicity values were extracted from the ECOTOX Knowledgebase. The PAF method was applied to the fish taxon and its various life stages. Using quantified environmental data, the Garonne spawning grounds presented a potential toxic risk associated with the fish taxon over the entire study period (msPAFF-T > 5%), unlike the Dordogne spawning grounds (msPAFF-T < 5%). The larval stage was the most at-risk life stage in terms of development and physiology (msPAFSE-T > 5%), and embryonic stage for mortality effect (msPAFSE-T > 1%) using quantified environmental data. Embryos appeared to be lethally more sensitive to metals, and larvae to industrial pollutants. Finally, a number of substances contributed particularly to the toxic risk (PAFi > 5%): metals (zinc, lead, copper and calcium), phenols, agrochemicals (simazine and cypermethrin) and drugs (metformin and carbamazepine). This method has made it possible to fill a data gap on contamination of allis shad spawning grounds in the GGD catchment, and to show that allis shad and other diadromous fish could face a potentially significant toxic risk.