Estimating age and growth of fish larvae is often required in connectivity and recruitment studies. When validated, daily growth increment formation in fish larvae otoliths allows such estimations of larval age and growth. However, several experimental studies showed that otolith distinguishable increments are not always formed at a daily scale, which biases age and growth reconstruction. Here, to better understand the factors controlling the formation of a fish larvae otolith, we propose to further develop a bioenergetics-modeling framework previously applied to the seasonal formation of fish otoliths. This model couples both the growth of an otolith and its opacity to the metabolism of the organism. The model relies on well-tested properties of the Dynamic Energy Budget (DEB) theory, which predicts individual growth, development and maintenance as a function of food density and temperature conditions. We apply the model to sagittae of Atlanto-Iberian sardine larvae Sardina pilchardus. Data on otolith diameter and growth increment numbers, together with individual known age and total length, from larvae reared in laboratory under four different food conditions (high, intermediate, low and starvation conditions, see Ferreira S. et al. abstract) showed that increment deposition was significantly less than one per day. We use data at two different food levels (high and low) to calibrate the otolith module. We first test the ability of the model to reproduce the otolith and fish size for the two other food conditions (intermediate and starvation). We then test the assumption that individuals do not deposit sufficient material of contrasted opacity for an increment to be formed at low food levels. Different opacity functions linked to feeding timings and photoperiod are tested to simulate opacity variation of the deposited otolith material. We discuss the use of this model to reconstruct growth and feeding conditions when increments widths are measured.