A stock collapse in a fluctuating environment: the chub mackerel Scomber japonicus (Houttuyn) in the eastern central Pacific

Abstract The population dynamics of the stock of chub mackerel Scomber japonicus in the eastern central Pacific off Ecuador are assessed using both age-structured and simple biomass-dynamic methods. A least-squares virtual population analysis (VPA) fitting procedure, ADAPT, was used in a Monte Carlo simulation to assess the uncertainty in the stock assessment. Such uncertainty was found to be large on account of poor sampling and marked inter-year variations in catchability, yet the assessment of a marked decline in biomass was robust to these errors. The results from the two methods, which generally corroborate each other, indicate that the stock has been declining rapidly over the period 1982–1988. Estimates of population size and fishing mortality over the period are presented. Catchability is shown to vary with environmental conditions and with stock size. Two simple models describe this response, and it is proposed that they could be used to improve the precision of future assessments.