A Study on the Age and Grohth of Adult Oreochromis Ni Loticus Linn. (Pisces: Cichlldidae) in Lake Awassa; Ethiopia

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Addis Ababa University


A study was conducted to determine what methods could he us~'d' to' (l~se!>s, the age and grol'/th of .Q. niloticus in Lake Awassa, Ethiopia. Furthermore, factors responsible for the, seasonal growth cycle of the fish vlere investigated. Otoliths (sagittae) and scales I'lere examined in relation to seasonal variation in some biological and environmental parameters. Oto 1 i ths and scales \~ere found to form trans 1 ucent zones and checks twi ce a year, during January to February and June to July. Growth can, therefore, be slow ,in this period. Spawning activity, decrease in water temperature and the probab 1e decrease in the Qu,an t ity of food consumed dur i n9 spawni n9 and the poor qual ity food consumed during these months v/ere considered to be responsible for slol'l growth. Otoliths and scales showed deposition of nell tiSSU2 during f.iarch to nay and August through to D2cember. Growth vias believed to resume at a relatively faster rate in these months due to comD1ption of spawning, and increase in water temperature. The quantity and quality of the food consumed by the fish could also increase during periods of fast growth. The first annulus in otoliths could not be identified. Therefore, average 1engths-at-re1ative age were calculated for fish with 6, 8, 10, 12, 14 and 16 translucent zones in their oto1 iths. These data are each one year older and were used to estimate the parameters of the von Gertalanffy growth equation. Loo was 30.0 cm for the males but, 29.4 for the females. K was 0.46 for the , males and it was 0.38 for the females. The theoretical age, to' \'las found to be 0.15 year for the males and -0.08 year for the females. Relative linear growth rates (S) estimates from otolith method, from lengthfrequency method and from marked fi sh closely agre8d. Thi s verifi ed that results obtained from the otolith method could be used to describe the average growth pattern of O. niloticus population in the lake. The sixth translucent zone in otoliths was believEd to be formed at the end of the first year of life. Ages estimated using Gulland's(1969) method provided supportive evidence. Therefore, the 6th to the 16th trans 1 ucent zones correspond to the J.st to the 6th year of life, respectively. Ages were assigned based on this finclin~. This indicated that more rapid growth in favour of the males was apparent at and after the second year of life. Length-frequency distribution of experimental commercial catch indicated that the commercial gillnet cau9ht fish in their second year of life and older. This gear does not catch immature fish.