Validation of Back Calculation Estimates to Predict Size and Time of Strontium Inflection in Chinook Salmon Otoliths

Otolith chemistry is often used to reconstruct origin, and migratory history in anadromous fishes, although the accuracy and precision of back-calculated life history parameters are often not well understood. In this paper, we evaluate back-calculated size and timing estimates based on otolith ratios of Strontium:Calcium (Sr:Ca) in Chinook salmon marked with increased concentrations of Strontium Chloride2 (SrCl2). We use a combination of Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA ICPMS), back-calculation models, and daily increments. Overall, back-calculated size at marking based on otolith Sr:Ca was underestimated by < 2 mm. Proportional back-calculations underestimated actual length when somatic growth (%/day) was less than otolith growth (%/day) and overestimated when somatic growth was greater than otolith growth. A negative relationship between somatic growth rate and direct back-calculated estimates was observed such that the size of slower and faster growing fish was over and underestimated, respectively. Overall, the number of otolith daily increments since Sr:Ca inflection underestimated the actual days since marking by a median of 1 (± 0.57) day. Results from this study suggest that life-history parameters estimated using otolith chemistry and structure and back-calculation models are suitable when fish have a similar relationship between somatic and otolith growth.