Persistence of a southern Atlantic salmon population: diversity of natal origins from otolith elemental and Sr isotopic signatures

Abstract

We investigated the use of Sr:Ca, Ba:Ca, and87Sr:86Sr ratios as natural tags for determining the natal origins of juvenile and adult Atlantic salmon (Salmo salar) from 12 tributaries in the Adour basin (southwestern France) and estimated homing on a tributary scale. Geochemical signatures from core regions of the otolith were also used to identify fish from hatchery or naturally spawned sources. Quadratic discriminant function analysis (QDFA) was on average 80% successful at classifying juveniles according to their natal rivers. Adults of unknown natal origin were assigned to their natal rivers using the juvenile fingerprints from QDFA approach. Only 18 adults originated from streams not included in the juvenile database. Although most of the adults showed a marked homing instinct, homing was not perfect, and some wild fish strayed into non-natal spawning areas. Returns of hatchery-reared fish as adult spawners represented 10% of the total sampled fish. Allocation of fish to natal tributaries or hatcheries illustrated the abundance and relative contributions of natal sources, important for the recovery of Atlantic salmon in this area.