Otolith age determination for adult tilapia, Oreochromis niloticus L. fromLake Awassa (Ethiopian Rift Valley) by interpreting biannuli and differentiating biannual recruitment

Age of mature Oreochromis niloticus in Lake Awassa, Ethiopia, was estimated by analysing optical macrozones (translucent and opaque) in sagittal otoli ths from fish sampled over a 12-month period. Seasonal record on the type o f macrozone at the edge of otoliths suggested that two translucent macrozon es associated with biannuli were formed each year; one during January and F ebruary and another during June and July. Formation of translucent macrozon es coincided with minimum water temperature, spawning associated loss in co ndition and presumably with reduction in the quantity and quality of the fo od consumed by the fish. Relative marginal increment analysis showed that b iannulus formation may be completed in March and in August each year. A con current study has confirmed a biannual recruitment in O. niloticus in Lake Awassa, from which mid-February and mid-August were taken as median hatch-d ates. A procedure to assign otolith age is described which considers median hatch-dates and uses the number of biannuli in otoliths without discrimina ting between fish from the two recruitment cohorts. We also show a procedur e 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.