The effect of growth-dependent mortality, external environment and internal dynamics on larval fish otolith growth: an individual-based modelling approach

The daily otolith increment growth of individuals in a cohort of fish larvae was simulated by a simple individual-based model over 30 days. The daily otolith growth of an individual larva was dependent on past growth, within fixed limits common to all larvae. The survival of a larva at the end of each day was either a linear function of larval growth or a random outcome, simulating growth-dependent and growth-independent mortality, respectively, The combined effect of the external environment on growth was also studied. Eleven environmental scenarios, favouring or hindering growth at different stages, were tested and compared to runs with no environmental effect on growth. Growth-dependent mortality induced an increase in the average otolith daily increment width amongst surviving larvae. Such an outcome, however, could be negated by an unfavourable environment. The increase in mean growth rate of the population generated by growth-dependent mortality was directly related to the inherent variability in daily otolith growth. With increased variability, the influence of the environment became relatively less important. The effect of the environment on growth was more critical during the early stages of development. A comparison of results generated by the model with patterns observed in data from a field survey of larval herring was consistent with the occurrence of growth-dependent mortality in the sea. The simulation model provided a useful insight into the way in which various processes controlling larval growth interact.