Mercury contamination and life history traits of Allis shad Alosa alosa (Linnaeus, 1758) and Twaite shad Alosa fallax (Lacepede, 1803) in the Gironde estuary (South West France)

Mercury concentration [Hg] was assessed in 20 adult Allis shad Alosa alosa (54-59 cm) and 20 adult Twaite shad Alosa fallax (36-44 cm) collected during their spawning migration in the Dordogne and the Garonne rivers (France). [Hg] was measured in the gills, dorsal muscle, liver and kidney. Twaite shad exhibited higher [Hg] than Allis shad. Median [Hg] values were [Hg](Gills) = 0.33 mu g g(-1) (dw), [Hg](Muscle) = 1.22 mu g g(-1), [Hg](Liver) = 1.99 mu g g(-1), [Hg](Kidney) = 1.93 mu g g(-1) for Twaite shad and [Hg](Gills) = 0.06 mu g g(-1), [Hg](Muscle) = 0.20 mu g g(-1), [Hg](Liver) = 1.18 mu g g(-1), [Hg](Kidney) = 1.08 mu g g(-1) for Allis shad. In order to understand such differences, we investigated some life history traits of the two species: migratory history, age (3-6 years), size at age (an expression of growth) and the number of spawning events (0-2 events). The difference in estuarine residence time between the juveniles of both species, which was assumed to influence [Hg], was investigated using otolith Sr:Ca ratio. The microchemical analysis revealed a significant difference in the residence time of juveniles in the estuary (medians are 21 d and 10 d for Twaite shad and Allis shad, respectively) but this residence time seems too short to influence [Hg] (Allis shad: r (spearman) < 0.128; Twaite shad: r (spearman) < -0.340). As both species show the same age structure, the influence of age on [Hg] was negligible. The literature shows that the differences in growth and in the number of spawning events reported in our study are in favour of a higher [Hg] for Twaite shad than for Allis shad. Although trophic status was not investigated here, the literature reveals that it is another factor that could produce higher [Hg] in Twaite shad, since its diet includes higher trophic levels than Allis shad.