Title: Age determination of Cape Horse Mackerel (Trachurus capensis) using otoliths

The Cape horse mackerel (Trachurus capensis) is one of the important commercial fish species of Namibia. The fishery is assessed using an age-structured production model (ASPM), which requires catch at age data that can be derived from the otolith microstructure. For this study, a total of 1604 otolith were randomly selected from annual acoustic surveys conducted in February or March of 2011, 2012, 2014 and 2015, covering the coast of Namibia. Whole otoliths were examined under a binocular microscope. Age was determined by identifying and counting annuli on otoliths. Annuli consisted of opaque and translucent zones, each of which were interpreted as one year of growth. To estimate the age of the fish, translucent zones were counted. Eight age classes were determined, ranging from 0 to 7 year olds. Most fish belonged to the age 3 year class and fewest fish belonged to the age 7 year class. In young fish (0 to 4 year olds), annuli were relatively clear as the zones were wide and distinctive, however in older fish the growth zones were closely packed and often indistinct. Corresponding length data were combined with derived ages in order to construct an age-length key for Cape horse mackerel. The age-length key showed a wide variation in length at age data.