The incorporation of environmentally derived ⁸⁷Sr/⁸⁶Sr and Sr/Ca in early otolith formation of Chinook salmon

Effective species management often requires understanding patterns of movement and habitat use. A common approach in identifying where individuals reside relies upon chemical tracers from the environment that are incorporated into an individual's tissues. For fish, isotopes in their otoliths, specifically the portion of their otolith formed during their larval stage, have been used to identify the natal origin. Complicating this work, however, is the fact that during this life stage, there is a shift in the source of isotopes deposited onto the growing otolith from maternally to environmentally derived. The objective of this study was to identify the portion of the otolith representing this transition to environmentally derived isotopes so as to accurately investigate questions of natal origin for a threatened population of fall Chinook salmon (Oncorhynchus tshawytscha). We exposed developing larvae to four treatments that differed in terms of their water strontium isotope ratio (⁸⁷Sr/⁸⁶Sr) and used change-point analysis of otolith ⁸⁷Sr/⁸⁶Sr and strontium to calcium ratio (Sr/Ca) to identify the otolith radius corresponding to the transition to environmentally derived isotopes. Our results indicated this transition occurred, on average, at 132 μm (⁸⁷Sr/⁸⁶Sr; ±50 μm standard deviation) and 127 μm (Sr/Ca; ±29 μm) from the otolith core, which corresponded to the developmental time between hatching and exogenous feeding. A substantial proportion of our otoliths (i.e., 61%) did not show convergence between otolith and water ⁸⁷Sr/⁸⁶Sr by the end of the 113-day experiment, which was likely due to the dietary contribution of marine-based feed. Therefore, we were unable to recommend an otolith radius to target for the purposes of reconstructing natal origin apart from being beyond approximately 130 μm.