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International Journal of Aquaculture, 2014, Vol.4, No.24 1
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2
also been associated with mass mortalities
(Al-Dughaym, 2000; Mohanty and Sahoo, 2007;
Abowei and Briyai, 2011) though factors such as
water temperature, unionized ammonia content and
the associated chemical parameters may take
precedence (Plumb, 1997). Nevertheless, Nile tilapias
are known to tolerate a wide range of environmental
conditions: water temperatures between16 to 40
o
C,
though prolonged extreme temperatures cause stress
induced diseases and mortalities (Uhland et al, 2000);
tolerates a salinity of up to 15 mg L
-1
(Popma and
Lovshin, 1996; El-Said, 2006) and survive under very
low dissolved oxygen levels (e.g. 0.5 mg L
-1
) for short
duration and a pH range of 5 to 10 (Popma and
Lovshin, 1996). Nile tilapia (
Oreochromis niloticus,
Linnaeus), tilapia zilli (
Tilapia zilli,
Gervais) and
African cat fish (
Clarias gariepenis,
Burchell) were
stocked into lakes Babogaya, Hora-Arsedi, Kuriftu
and Bishoftu in the late 1960s in an attempt to
establish pelagic fishery and increase availability of
protein for the local community.
Limnological studies on these crater lakes, a group of
volcanic explosion craters in the vicinity of the
emerging city Debre-Zeit located around 45 km
southeast of Addis Ababa have been conducted by
several authors (Wood et al, 1965; Prosser et al, 1968;
Tefera, 1990, Teshome, 2011). According to these
studies, prolonged thermal stratification occurs
between January and October, while complete
mixing of the lakes occurs once a year between
mid-November and December which coincides with
the cold–dry period around Bishoftu town (Prosser et
al
,
1968; Teshome, 2011). Subsequently, nutrient
upwelling during complete mixing causes oxygen
depletion throughout the water column as dissolved
oxygen is consumed during aerobic breakdown of
reduced compounds such as hydrogen sulfide by
microorganisms (Wetzel, 2001). Recently, Teshome
(2011) has also noted similar evidence, whereby low
dissolved oxygen concentration (0.06 mg L
-1
) at
surface water in Lake Hora-Arsedi was recorded
following Lake turnover in November 2010. During
such events, tilapia exhibit gasping behavior at the
surface exposing themselves to excessive fishing and
predation by birds. But mortality occurs seldom and
only at moderate level, possibly when the anoxic
condition persists for several days. The lakes have
well established pelagic fishery and health breeding
populations of
O. niloticus
for more than five decades
and support a number of subsistence fisheries where
catches are marketed mainly within the Bishoftu town.
The recent tragic episode of mass fish mortality in the
Bishoftu lakes involves only
O. niloticus
populations
(fingerling, juveniles and adults) in these lakes in a
week time intervals with the first sign of mortality
spotted in Lake Babogaya on 11
th
May of 2013. The
event has drawn greater public attention and has
halted fisheries operation at the time. Therefore, the
objective of this investigation is to provide a plausible
explanation to the recent selective mass fish mortality
of
O. niloticus
in lakes Babogaya and Hora-Arsedi.
Materials and Method
Study area
The adjacent lakes Babogaya and Hora-Arsedi (N
8
o
47’6”; E 38
o
59’40’’and N 8
o
45’42’’; E 38
o
59’39’’)
are deep crater lakes formed as a result of volcanic
eruption and located in the vicinity of the emerging
Bishoftu Town, some 45 km southeast of Addis Ababa.
The lakes have a closed basin with no surface inlet
and outlet. It receives water primarily from rainfall
and some ephemeral springs (Mohr, 1961). The region
is characterized by sub-humid climate with an annual
rainfall of around 905 mm with the main rainy season
extending from June to September (National
Meteorological Service Agency of Ethiopia), and diel
air temperature varies from 25.6
o
C to 27.7
o
C during
the day time and falls below 6.1
o
C in the night in
November (Figure 1).
Figure 1 Map showing location of three Bishoftu crater lakes in
the vicinity of Bishoftu town (star showing the location of lakes
Babogaya and Hora-Arsedi)