An otolith-based back-calculation method to account for time-varying growth rate in rainbow smelt (<i>Osmerus mordax</i>) larvae

Abstract

This study develops a new back-calculation method, based on the larvae of rainbow smelt (Osmerus mordax), that takes into account the variation of growth rate over time. Known-aged larvae were reared in four 60-L microcosms during 49 days in order to obtain a large range of individual growth trajectories. We first validated the daily nature of the otolith increment deposition rate. The proposed time-varying growth (TVG) method weights the contribution of each increment in the length calculation using a growth effect factor. A small growth increment contributes less to the length increase of larvae than its relative importance in total otolith growth. On the other hand, a large growth increment contributes more to the length increase of larvae in comparison with its relative importance in total otolith growth. This method provided significantly better estimates of previous length-at-age than the biological intercept (BI) method at the individual level. In addition, the TVG method tended to provide more accurate estimates of previous length-at-age than the BI method at the population level, but the difference was not significant. The importance of using the TVG method instead of the BI method to back-calculate individual and population growth trajectories increases with the magnitude of the growth effect and the variation in growth rates over time.