SEBSCC Annual Report, 1998.

Individual-based modeling of walleye pollock in the southeast Bering Sea

S. Hinckley

Development of the individual-based model of young pollock in the Bering Sea has continued in 1998. The focus of the development work this year has been depth distributions by life stage. They strongly affect the horizontal advection of individuals (for example from spawning regions to juvenile nursery areas), and provide detail specific to the Bering Sea to the juvenile life stage section, as this life stage is a major focus of the SEBSCC program.

The depth distribution of eggs in the Bering Sea is probably different than that in Shelikof Strait, at least over the shelf and slope (Kendall, pers. comm.), with most of the eggs found in the upper 20 m of the water column. Eggs may be deeper in the oceanic region. The individual-based model therefore calculates the depth of eggs differently depending on its spatial location. The depth of individual juveniles is calculated based on work by Bailey 1989, Brodeur and Wilson 1996, and Brodeur 1998. If a thermocline is present, small juveniles are assumed to stay above it, whereas larger juveniles go through the thermocline, staying deeper during the day and rising to nearer the surface at night. If no thermocline is present, both large and small fish migrate vertically.

A volume-searched feeding model has been developed for the juvenile stage. It uses average prey densities by hydrographic region of three organisms, Pseudocalanus, spp., Neocalanus, spp. and euphausiids, spp. as derived from a preliminary survey of the literature. The model also calculates the volume searched by an individual based on its size. The number of prey items consumed is calculated using a random deviate from a negative binomial distribution with two parameters: the mean number of prey encountered, and a parameter describing the patchiness of the prey. Energetics of juveniles is modeled using formulations and parameters from Cianelli et al. 1998.

Further additions to the individual-based model include an algorithm for determining sunset and sunrise times based on locations. These are needed in the feeding and depth calculations. Also added is a method of determining in which of ten major hydrographic regions (the southeast or northwest inner, middle or outer shelf, the southeast or northwest slope, the oceanic region or the Pribilof region) an individual is located on any day. Determination of the region an individual occupies is used to set spawning depth and date and average food densities. The area around the Pribilofs has been designated as a separate region, so that more detail may be added in this area later if desired, as it has been the focus of much of the work on juvenile pollock.