Age of mature Oreochromis niloticus in Lake Awassa, Ethiopia, was estimated by analysing optical macrozones translucent and opaque in sagittal otoliths from fish sampled over a month period. Seasonal record on the type of macrozone at the edge of otoliths suggested that two translucent macrozones associated with biannuli were formed each year; one during January and February and another during June and July.
Formation of translucent macrozones coincided with minimum water temperature, spawning associated loss in condition and presumably with reduction in the quantity and quality of the food consumed by the fish. Relative marginal increment analysis showed that biannulus formation may be completed in March and in August each year. A concurrent study has confirmed a biannual recruitment in O. A procedure to assign otolith age is described which considers median hatch-dates and uses the number of biannuli in otoliths without discriminating between fish from the two recruitment cohorts.
We also show a procedure to discriminate between the two cohorts by using the number of biannuli, conditions on the edge of the otolith and time of capture to assign age for the two recruitment cohorts separately. This is a preview of subscription content, log in to check access. Balarin, J. National reviews for Aquaculture Development for Africa. Balarin, T. Hatton, University of Sterling, Scotland. Balon, E. Possible fish stock assessment and available production survey as developed on Lake Kariba.
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Age and growth studies on tropical fishes. In Saila, S. Proceedings of an International Workshop. Campana, S. Neilson, Microstructure of fish otoliths: review and synthesis. Casselman, J. Determination of age and growth. In Weatherly, A. Gill eds , The Biology of Fish Growth. Academic Press, London: β Growth and relative size of calcified structures of fish. Caulton, M. Feeding, metabolism and growth of tilapias: some quantitative considerations.
