Jack Mackerel (Trachurus symmetricus) are considered a highly underutilized nearshore commercial purse-seine fishery of the northeastern Pacific Ocean. In 1983, the majority of the jack mackerel commercial catch was fish younger than 3+ years (<400 mm FL). Since then, no investigations of age and growth have been conducted (Mason 1989). Recently, there has been interest by domestic and international trawling fleets to develop a fishery in the offshore region of its range. In anticipation of this development, this study examines the age composition, growth rates, population structure, and possible migratory patterns of both nearshore and offshore components of the jack mackerel population collected from the northeastern Pacific Ocean, 1978-1993. This study indicated that ages were best estimated from transverse thin-sectioned otoliths, and that growth was best described by the von Bertalanffy growth function (K = 0.159). Marginal increment analysis was used to validate annual deposition of opaque and translucent otolith growth zones. Young jack mackerel were found nearshore and grew quickly until 400 mm FL, after which growth slowed in length but weight increased dramatically. Fish greater than 300 mm FL tend to move offshore into deeper, colder waters. This change in growth directly corresponds with their change in environment from nearshore waters to offshore waters. There is some indication that older members of the offshore portion of the population migrate to the north. It remains unclear whether the northern portion of the population is a resident stock, a migrating component, or both. Fish found in the northern limits may simply be displaced due to seasonal oceanographic warming trends or warming trends resulting from an El Niño event.