Growth characteristics in the otoliths of selected South African sparid fish

Growth characteristics in the otoliths of several South African sparid fish (Diplodus sorgus copensis. D. cervinus hottentotus, Chrysoblephus laticeps. Cymotoceps nasutus, Pachymetopon grande. Rhobdosargus holulJi and Sparodon durbonensis) were investigated. The micro-incremental pattern of deposition in the otoliths conformed to the general pattern of otolith structure described in the literature. Sparid otoliths consist of a central opaque nuclear region, composed of a nucleus and multiple acccessory primordia. Daily increments surround the nuclear region, which decrease in width as the distance from the nucleus increase. At the macroscopic level, under transmitted light, narrow opaque and wide translucent annual growth zones were identified in sectioned otoliths. Scanning electron microscope analysis of the annual growth zones revcaled that the micro-incremcnts within the opaque zones are narrowly spaced with prominent discontinuous phases (D-zone). This results in a higher protein content and greater optical density of the zone. The translucent zones are composed of widely spaced, daily increments with prominent incremental phases (L-zone), accounting for the translucent nature of this zone. Opaque-zone formation was shown to be indicative of slow growth and was correlated with periods of reproductive activity in some of the species studied. The translucent zone is formed after the spawning season and represents fast growth.