Linking migratory patterns and diet to reproductive traits in female brown trout (Salmo trutta L.) by means of stable isotope analysis on ova

International audience; Many populations of vertebrates display alternative reproductive strategies involving delays in maturation processes and migrations to more productive habitats. Notably for females, migratory patterns and diet could influence reproductive traits such as ova size and number. To test this hypothesis, we targeted anadromous and freshwaterresident female brown trout of various sizes during their return to spawn in tributaries, and carried out stable isotope analyses (d15N and d13C) on ova to identify their feeding areas and diet. The study was performed in the Oir River (France), a small coastal system where trout migrations to sea and within the river basin have been monitored for the last 20 years. General trends were observed in reproductive traits, such as the positive correlation between female fork length, fecundity and ova size. Fecundity was sizedependent whatever the migratory origin of females, but major divergences were seen for ova size and isotopic ratios. Freshwater residents displayed a constant increase in ova size and a 15N-enrichment in ova, suggesting an investment in larger ova and a shift to piscivory as the fish become larger and older. Conversely, anadromous females had smaller ova compared with freshwater-resident females of similar body size and achieved higher fecundity as they grew bigger. Our results suggest the existence of a continuum of reproductive traits for freshwater-resident females whereas anadromous females clearly show a break with this continuum. This major dissimilarity could be explained by the difference in growing environments in terms of food availability and quality.