A Study on the Age and Grohth of Adult Oreochromis Ni Loticus Linn. (Pisces: Cichlldidae) in Lake Awassa; Ethiopia
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Date
1989-06
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Addis Ababa University
Abstract
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.
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Biology