A MULTICOHORT STOCK PRODUCTION-MODEL

An age-structured production model of multicohort fish population dyna mics based on the mean metabolic state of individuals and the average abundance of year-classes is proposed. The model allows variations in life stage basal metabolic rates by parameterizing their effects on to tal population abundance through a mortality function which itself dep ends upon an individual's status relative to an optimal metabolic stat e. The metabolic equations are derived in terms of age-specific indivi dual weight. The interaction of individual status and intraspecific co mpetition is allowed by coupling the metabolic equations for growth to those for population abundance. This coupling yields a second-order n onlinear equation in abundance or biomass. In model simulations, (1) i ncreased numbers of cohorts decreased the stability of population biom ass, (2) numerically ''strong'' year-classes damped and sometimes caus ed collapse of proximal age-classes, (3) exploitation reduced competit ive interactions and increased model stability, and (4) increases in n atural mortality introduced greater recruitment variability. The trans itional and steady state behaviors of the multicohort model are explor ed and compared to traditional fishery management models for a small p elagic species, northern anchovy Engraulis mordax.