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International Journal of Aquaculture 2012, Vol.2, No.10, 65
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66
flooding (Tadesse, 1997). Abundance of food has also
been considered as an important factor in timing of
breeding in some species (Tadesse, 1997). Demeke
Admasu (1996) has found the peak breeding activity
of
Oreochromis niloticus
in Lake Awassa to be
coincident with increasing in phytoplankton biomass.
The size of first sexual maturity of tilapia
(
Oreochromis niloticus
) is extremely variable, and
depends on the species, growth rate and environment
(Wootton, 1998).
Oreochromis niloticus
individuals,
which are in poor body condition, mature at a small
size than in good condition (Wootton, 1998). In
contrast
Cyprinus carpio
exploit large and small man
made and natural reservoirs and pools in low and fast
moving streams. They prefer larger, slower-moving
bodies of water with soft sediments, but they are
tolerant and hardy fish that thrive in a wide variety of
aquatic habitats (Dadebo and Tugie, 2009). Under
tropical conditions carp breeds through out the year,
but it is a season spawner in temperate waters and
breeds at a temperature range of 15
℃
to 20
℃
(Dadebo and Tugie, 2009).
The principal fish species found in Fincha reservoir is
probably
Barbus pludunosis
, a fish species which has
no commercial value in Ethiopia. Nile tilapia
(
Oreochromis niloticus
) and common carp (
Cyprinus
Carpio
) were introduced into the reservoir in late
1970-ies to fill an empty pelagic niche of
planktivorous fish and to provide cheap protein to the
local people (Tedla and Fisseha, 1981).
Apparently there exist comparatively very few
scientific investigations on the reproductive biology of
carp species in Ethiopia (Dadebo and Tugie, 2009;
Tedla and Fisseha, 1981). It is not know also if these
introduced fish species established breeding
populations in the reservoir. Hence, this paper is the
first attempt to investigate the adaptability status and
reproductive success of introduced fish species,
Oreochromis niloticus
and
Cyprinus carpio
in Fincha
reservoir.
2 Results
2.1 Length-weight relationships
The Length-Weight relationship of
O. niloticus
and
Cyprinus carpio
in Fincha reservoir was curvilinear
(Figure 1). Length-Weight relationships were calculated
for all tilapia (W=0.8×10
-2
L
3.20
; n=87; r
2
=0.96) and
carp (W=1.4×10
-2
L
2.97
; n=558; r
2
=0.99) collected.
The relationship was first determined separately for
male and female
O. niloticus
and carp respectively,
however when an analysis of covariance indicated no
apparent difference in the length-weight regression
coefficients and adjusted means for either sex, the data
for both sexes were pooled.
Figure 1 Length-weight relationship for
Oreochromis niloticus
(top) and
Cyprinus carpio
(bottom) in Fincha reservoir.
2.2 Fulton’s condition factors (k)
The average K value of
O. niloticus
in Fincha reservoir
was 1.82 for female and 1.74 for the male while mean K
value was 1.97 and 1.88 for female and male
Cyprinus
carpio
respectively. The values were found to be
significantly different between months (ANOVA,
P<0.05). Lower values were observed during January,
March and July (Figure 2) which seem to coincide with
the peak breeding season of the fish. However, no
significant difference was observed between sexes
(ANOVA, P>0.05).