Interannual variability of the early life history of walleye pollock near Shelikof Strait as inferred from a spatially explicit, individual-based model

A coupled biophysical model is used to hindcast the early life history of a population of walleye pollock (Theragra chalcogramma), to assess possible physical causes of interannual Variability in recruitment. The modelling approach combines a primitive equation, rigid-lid hydrodynamic model with a probabilistic, individual-based biological model of growth, development, and mortality. Individuals are tracked through space using daily velocity fields generated from the hydrodynamic model, along with self-directed vertical migrations appropriate to each life stage in the biological model. The hydrodynamic model is driven with wind and runoff time series appropriate to each year. Biological model output compares favourably with observed spatial distributions for specific years. Lloyd's index of patchiness, calculated from model output, was similar to values calculated from field data. Five noncontiguous years were chosen for hindcasts to span a wide range of meteorological conditions (winds, runoff) and recruitment success. Interannual comparisons suggest that two years of above average recruitment (1978 and 1988), and one year of below average recruitment (1991), experienced flow fields which carried many individuals into the Alaskan Stream. At the same time, the vigorous flow fields generated in each of these years carried some individuals onto the shelf area to the south-west of the spawning site. A year with low runoff and weak winds (1989) exhibited weak circulation, with extended retention of larvae near the spawning site. A year with high runoff (1987) was notable for the strength and frequency of mesoscale eddy activity. Eddies appear capable of both enhancing patchiness of early larvae (through retention) and dissipating patchiness of juveniles (through mesoscale mixing), Larvae retained in an eddy feature exhibit a narrower range of sizes than the population outside that feature.