Discrimination of the natal origin of young-of-the-year Allis shad (Alosa alosa) in the Garonne–Dordogne basin (south-west France) using otolith chemistry

Abstract –  The otolith composition of age 0+ Allis shad ( Alosa alosa ) captured in two rivers in the Gironde system in south-west France, the Dordogne and the Garonne, was analysed to establish the natal origin of the fish. Inductively coupled plasma mass spectrometry analyses were used to take measurements of 15 isotopes, among which Co, Fe, Sr and Rb were the most important elements in the discrimination process. It was possible to reclassify each sampled fish into its natal river basin (Garonne or Dordogne) with a high degree of accuracy. Linear discriminant analysis correctly reclassified 91.1% of the Garonne samples and 87.8% of the Dordogne samples (mean success in reclassification in 20 runs ± 1 SD: 89.21 ± 8.6%). Artificial neural networks correctly reclassified 90.1% of the Garonne samples and 81.2% of the Dordogne samples (mean success in reclassification after 1000 training cycles ±1 SD: 85.6 ± 9.1%). Using these tools the present results show that it is possible to assign a natal origin to juvenile shad that hatched in the Dordogne–Garonne basin based on the trace elements in the otolith.