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International Journal of Aquaculture, 2014, Vol.4, No.24 1
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Table 2 A checklist of phyto- and zoo-plankton in lakes Babogaya and Hora-Arsedi during the sampling period. Categories:
1-sporadic, 2-rare, 3-frequent, 4-very frequent, 5-copious
Lake Babogaya
Relative abundance
Lake Hora-Arsedi
Relative abundance
Phytoplakton
Microcystis
sp.
5
Microcystis
sp.
5
Anabaenna
sp.
4
Pseudoanabaenopsis sp.
4
Oscillatoria
sp.
2
Navicula
sp.
Aphanocapsa
sp
.
2
Peridinum
sp.
1
Scenedismus
sp.
3
Phacus caudatus
2
Zooplankton
Brachionus falcatus
2
Cerodaphnia Cornusa
3
Water mite
1
Cyclopoid Copepod
5
Keratella tropica
5
Brachionus calciflorus
4
Trichocerca
sp.
2
Asplanchina
sp.
4
Brachionus angularis
4
Brachionus quadridentatus
5
Hexarthra intermida
3
Histopathological and bacteriological examinations
All the clinical examination: visual observation and
history, gross histopathological and bacteriological
examinations suggested bacterial and parasitic
infection and hepatotoxicity (Figure 3). Parasites were
also isolated from the liver, and from the bacterial
culture,
Aeromonas
sp. and
Plesimonas shigelloides
were identified.
Figure 3 Gross clinical symptom of mass mortality (a) and
lesion (b as indicated by the arrow) during the event of mass
fish kill in lakes Babogaya and Hora-Arsedi
Discussion
The vertical temperature profile for both crater lakes
revealed that the lakes are stratified, which is in
accordance with several previous studies during this
particular period of the year (Prosser et al
,
1968;
Baxter and Wood, 1965; Teshome, 2011). The
clinograde dissolved oxygen profile indicated oxygen
depletion in greater depths, with saturation values
constantly lower than 7% below 5 m in Hora and
8.5 % below 7 m in Babogaya (Figure 2), which is
usual for eutrophic systems (Talling and Lemoalle,
1998). Such DO profiles have been also noted from
other explosion crater lakes located in Cameroon,
Uganda, Nepal and the United States (Melack, 1978;
Kling, 1988; Sharma, 2012; Degefu et al, 2014a) and
usually reflect extended stratification and high
biological oxygen demand (BOD) by decomposition
of sinking labile organic matter and respiration of
organisms in deeper waters (Baxter et al, 1965; Löffler,
1972; Kebede and Belay, 1994; Haberyan et al, 1995).
Consequently, sulfide content which was below the
detection limit throughout the water column reached 8
mg L
-1
at the bottom and at least during complete
mixing, which lead to high biological oxygen demand
during aerobic breakdown of reduced sulphides and
nutrient upwelling which is not the case during this
event. Nevertheless, DO content in the epilimnion was
well oxygenated both during the day and early
morning in both lakes. Therefore, the current selective
mass mortality of
O. niloticus
in both lakes is not
linked to oxygen depletion as it has been widely
thought of the Bishoftu crater lakes (Teshome, 2011).
Apparently, all the key water quality parameters were
within the optimum range for all species inhabiting
the lakes (El-Said, 2006; Popma and Lovshin, 1996;
Kirk, 1972; Chervenski and Herring, 1973). However