Abstract
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 (~0.7080.722). 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.
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