Importance of spawning location and timing to successful transport to nursery areas: a simulation study of Gulf of Alaska walleye pollock

We conducted a model experiment to examine the hypothesis that the spatial and temporal specificity of spawning of walleye pollock (Theragra chalcogramma )i n Shelikof Strait, Alaska, evolved to optimize physical transport to the juvenile nursery area near the Shumagin Islands some 375 km to the southwest. The alternative hypothesis is that factors other than physical transport alone must also be important in the choice of spawning location and timing. We used a coupled biophysical model consisting of a three-dimensional hydrodynamic model of the currents in the region, an individual-based model of the early life stages of pollock, and a nutrient– phytoplankton–zooplankton model that provides a spatially and temporally dynamic source of food for larval pollock. Results showed that fish spawned to the south of Kodiak Island, or too early (February) or too late (July), did not reach the Shumagin Island nursery area by early September. However, the potential region and time of spawning that did allow successful transport to the nursery area was much broader than the observed spawning region and time. Therefore, factors other than physical transport alone must be considered in explaining the specificity of the location and timing of spawning for this stock.