Biophysical mechanisms underlying the recruitment process in walleye pollock (Theragra chalcogramma)

Recruitment variability in marine fish species is not well understood, yet is very important as a component of fishery management. This dissertation describes a set of coupled biological and physical simulation models designed to examine mechanisms underlying the recruitment process for walleye pollock, Theragra chalcogramma, in the western Gulf of Alaska. These models consist of (1) a three-dimensional hydrodynamic model of the region driven by winds and fresh-water runoff, (2) an individual-based model of the early life stages (egg through 0-age juvenile) of pollock, which uses a Lagrangian float-tracking scheme to follow fish through space and which keeps track of many characteristics of individual fish, and (3) a three-dimensional nutrient-phytoplankton-zooplankton model which includes the dominant prey species of pollock larvae. Methods of coupling large simulations models are discussed. Six sets of model experiments and hindcasts are described. Important findings of the work include the following. An explanation for the consistency of the timing and location of spawning of pollock in Shelikof Strait is presented, as it relates to transport to the juvenile nursery area and spatial and temporal dynamics of the prey. Support is found for the hypothesis that eddies (specifically cyclonic eddies) are good feeding environments for larvae and aid in transporting them to the nursery areas. The depth of an individual is shown to be an important life history trait which makes a significant contribution to growth and survival. This modelling shows that spatial variability in physical factors such as temperature and salinity at the scales resolved by these models, as well as the spatial and temporal dynamics of the prey resources, have important effects on individuals and significant consequences at the level of the population. The match-mismatch theory of recruitment success is supported. A possible mechanism for the effect of wind-induced turbulence on growth and survival is explored. It is hoped that what has been learned from this of modelling can be integrated into our growing knowledge of the recruitment process for walleye pollock and other marine fish species.