Ndjaula, H. O. N., Hansen, T., Krüger-Johnsen, M., and Kjesbu, O. S. 2009. Oocyte development in captive Atlantic horse mackerel Trachurus trachurus. – ICES Journal of Marine Science, 66: 623–630.This 13-month experimental study focused on developing oocyte production in 101 adult female Atlantic horse mackerel. In addition, proxies of energy patterns in relation to oocyte development were investigated. The fish were captured off western Norway and fed to satiation from October 2005 to November 2006 in two replicate adjacent circular tanks. Monthly histological examination of the ovaries indicated that vitellogenic oocytes were present at all times throughout the study period, but that oocyte development did not progress past the incipient migratory nucleus stage. The oocyte diameter (OD) threshold between pre-vitellogenic and vitellogenic oocytes and the mean OD of the leading cohort were investigated, and no hiatus was observed within the OD size distribution around the 185-µm pre-vitellogenic and vitellogenic threshold. Variation in gonadosomatic index, hepatosomatic index, condition factor, and fat content (as measured by the use of a Distell Fish Fatmeter) increased with oocyte development. The observed development of oocyte recruitment and the absence of a hiatus in the oocyte distribution are characteristics of an asynchronous spawner. The species seems to have a prolonged spawning season, judging from the advanced and variety of maturity stages throughout the study period.