Using natural strontium isotopic signatures as fish markers: methodology and application

To distinguish Atlantic salmon (Salmo salar) populations in tributaries of the Connecticut River, we studied the incorporation and stability of Sr iso topes in juvenile salmon. We established the geologic basis for unique isot opic signatures in 29 salmon sites. Stream-specific Sr isotopic ratios (Sr- 87/Sr-86) were found in calcified tissues of salmon parr within 3 months of stocking. We found little seasonal variation in the Sr signatures of strea m water or fish tissue. There were no significant differences among the Sr signatures of otoliths, scales, and vertebrae. For mature salmon raised und er constant conditions, 70% of the Sr isotopic signature in calcified tissu es was derived from food sources. We developed a criterion for identifying moving fish based upon the isotopic variability of genetically marked fish. Applying this criterion to our streams, 7% of the fish in our study had in corporated Sr from multiple streams. Strontium isotopes distinguished all 8 regions in the White River basin and 7 of the 10 regions in the West River basin. When watersheds are considered together, Sr isotopes differentiated 11 unique signatures from 18 regions. We conclude that Sr isotopes are an effective marking tool and discuss ways in which they can be combined with other marking techniques over larger spatial scales.