From the biology of the individual to the dynamics of the population: bridging the gap in fish early life studies

The mean properties of larval fish populations do not necessarily reflect the properties of the mean individual. For example, the change in mean length in a population with time may not reflect the average individual growth rate, since individual growth rates and survival probability are linked so that slow growing individuals suffer higher mortality. Hence, mean growth rate indicated from population data could be biased upwards. Factors which influence the magnitude and variability of individual growth rates can exert nonlinear effects on population survival. Two categories of process must be considered: first, the variability in exposure of the average individual as a consequence of individual variability in dispersal through a patchy environment; and second, the intrinsic variability between individuals expressed even under equal exposure conditions. These two aspects have been addressed independently, the first by lagrangian modelling of individual fish larvae linked to spatially resolved hydrodynamic models, and the second by strategic biological modelling. In this paper, progress towards the goal of individually based larval fish ecosystem models is reviewed, highlighting the space and time scales which may be important in such systems, and identifying the gaps in current knowledge of larval biology.