Migration and rearing histories of chinook salmon (Oncorhynchus tshawytscha) determined by ion microprobe Sr isotope and Sr/Ca transects of otoliths

Strontium isotope and Sr/Ca ratios measured in situ by ion microprobe along radial transects of otoliths of juvenile chinook salmon (Oncorhynchus tshawytscha) vary between watersheds with contrasting geology. Otoliths from ocean-type chinook from Skagit River estuary, Washington, had prehatch regions with ⁸⁷Sr/⁸⁶Sr ratios of ~0.709, suggesting a maternally inherited marine signature, extensive fresh water growth zones with ⁸⁷Sr/⁸⁶Sr ratios similar to those of the Skagit River at ~0.705, and marine-like ⁸⁷Sr/⁸⁶Sr ratios near their edges. Otoliths from stream-type chinook from central Idaho had prehatch ⁸⁷Sr/⁸⁶Sr ratios ≥0.711, indicating that a maternal marine Sr isotopic signature is not preserved after the ~1000- to 1400-km migration from the Pacific Ocean. ⁸⁷Sr/⁸⁶Sr ratios in the outer portions of otoliths from these Idaho juveniles were similar to those of their respective streams (~0.708–0.722). For Skagit juveniles, fresh water growth was marked by small decreases in otolith Sr/Ca, with increases in Sr/Ca corresponding to increases in ⁸⁷Sr/⁸⁶Sr with migration into salt water. Otoliths of Idaho fish had Sr/Ca radial variation patterns that record seasonal fluctuation in ambient water Sr/Ca ratios. The ion microprobe's ability to measure both ⁸⁷Sr/⁸⁶Sr and Sr/Ca ratios of otoliths at high spatial resolution in situ provides a new tool for studies of fish rearing and migration.