Formulation and application of an efficient optimized biophysical model

The formulation of an efficient optimized biophysical model is described, and the model is applied to the simulation of the climatological 0-group distribution of Icelandic cod Gadus morhua larvae. The method is based on representing the results from particle tracking as drift probability density functions describing the probability that particles released from a given spawning ground are found at a specific downstream grid location some time later. Spawning is considered to take place from 15 spawning grounds, and the model is used to determine 45 egg production model parameters as the solution of a bound constrained optimization problem that minimizes model-data misfits in abundance and age distributions. The problem is solved using a direct search minimization routine. Two cost functions are used. One penalizes misfits in the gridded abundance and age distributions (Model 1). The other directly penalizes the misfit in the spatial age gradient (Model 2). A simple age-based settlement module is tested to determine whether it improves the model fit. Results from Model 1 show a large error in the spatial age gradient. Model 2 achieves a 20-fold reduction in this error, with only a small degradation of the gridded abundance and age distributions. The settlement model does not improve the model fit. The results indicate that the addition of more processes to a model does not always improve model performance, while focusing on gradients in age instead of simple age distributions can lead to overall improved performance. The technique presented in the present paper allows quantitative evaluation of various model processes in a computationally efficient framework.