Evaluation of Back‐Calculated Size and Timing Estimates for Juvenile Chinook Salmon Using Otolith Structure and Chemistry

Abstract

AbstractOtolith chemistry is often used to reconstruct origin, life history, and migratory pathways in anadromous fishes. Although the accuracy and precision of back‐calculated size at a particular life history transition (such as when anadromous fish move from fresh to saline water) is often not estimated. We evaluated back‐calculated size and timing estimates based on otolith ratios of strontium to calcium (Sr:Ca) in Chinook Salmon Oncorhynchus tshawytscha marked with elevated concentrations of strontium chloride (SrCl). We used a combination of laser‐ablation, inductively coupled, plasma mass spectrometry (LA‐ICPMS), back‐calculation models, and daily increments to evaluate the accuracy of such estimates. Overall, back‐calculated size at marking was underestimated by <2 mm using direct and proportional back‐calculations of FL based on otolith Sr:Ca. Proportional back‐calculations of fish length were underestimated on average by 2.60 mm (SD, 2.09) when somatic growth (%/d) was less than otolith growth (%/d) and overestimated by an average of 4.89 mm (SD, 3.19) when somatic growth was greater than otolith growth. However, when somatic and otolith growth were equal, proportional back‐calculations were more accurate than direct estimates. Overall, the number of otolith daily increments since Sr:Ca inflection underestimated the actual days since marking by a median of 1 d (SD, 0.57) and was similar for individuals sampled 8–79 d after marking. Results from this study suggest that life history parameters for Chinook Salmon estimated using LA‐ICPMS, back‐calculation models, and daily increments are robust estimates suitable for ecological field studies.Received September 22, 2015; accepted December 29, 2015 Published online April 18, 2016