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Otolith increment widths have often been used to infer somatic growth history when studying the influence of temperature and food supply. However, there is no functional relation- ship between otolith accretion rate ('inherent otolith growth') and somatic growth rate; although both may be mediated by temperature and food, the uncoupling of these rates under changing environmental conditions or during ontogenetic transitions can undermine the sensitivity of incre- ment width analyses. We used otolith microstructure analysis to model the daily growth of anchovy Engraulis encrasicolus juveniles collected from the North Aegean Sea in July 2007, December 2007 and February 2009. To assess the environmental causes of seasonal variability in growth we used daily outputs of environmental variables from a coupled 3D hydrodynamic-bio- geochemical model (POM-ERSEM) as a proxy for the potential environment that fish experienced during their development. According to the simulation, juveniles that hatched in late autumn- winter developed in unfavorable conditions (low temperature, low productivity); however, they exhibited wider otolith increments before metamorphosis. To specifically study environmental influences on the daily growth rates, we used the simulated environmental variables in gener- alised additive model (GAM) analysis of otolith increment widths, while also taking into con - sideration inherent otolith growth, expressed by the explanatory variables 'previous increment width' and 'age'. The inclusion of the interaction term 'age' × 'sampling period', rather than 'age', accounted for the seasonal effects on ontogenetic changes (timing and duration of metamorphosis) and the differences in growth-selective mortality among samples. Results showed that the daily growth rate of anchovy is influenced by temperature, exhibiting an optimum at 24.5°C, and increases significantly with increased mesozooplankton concentration (i.e. food availability). The analysis highlights that, in order to infer the daily somatic growth from fish otolith increment data and understand the environmental influences, inherent otolith growth has to be disentangled and otolith width trajectories standardised for between-sample differences in ontogenetic changes and growth selective mortality.

Disentangling the effects of inherent otolith growth and model-simulated ecosystem parameters on the daily growth rate of young anchovies