Shesher and Welala Floodplain Wetlands (Lake Tana, Ethiopia): Are They Important Breeding Habitats for Clarias gariepinus and the Migratory Labeobarbus Fish Species?

Abstract

This study aims at investigating the spawning migration of the endemic Labeobarbus species and C. gariepinus from Lake Tana, through Ribb River, to Welala and Shesher wetlands. The study was conducted during peak spawning months (July to October, 2010). Fish were collected through overnight gillnet settings. A total of 1725 specimens of the genus Labeobarbus (13 species) and 506 specimens of C. gariepinus were collected. Six species of Labeobarbus formed prespawning aggregation at Ribb River mouth. However, no Labeobarbus species was found to spawn in the two wetlands. More than 90% of the catch in Welala and Shesher wetlands was contributed by C. gariepinus. This implies that these wetlands are ideal spawning and nursery habitats for C. gariepinus but not for the endemic Labeobarbus species. Except L. intermedius, all the six Labeobarbus species (aggregated at Ribb River mouth) and C. gariepinus (spawning at Shesher and Welala wetlands) were temporally segregated.

Full text

The Scientific World Journal
Volume 2012, Article ID 298742, 10 pages
doi:10.1100/2012/298742
The cientificWorldJOURNA
L
Research Article
Shesher and Welala Floodplain Wetlands (Lake Tana, Ethiopia):
Are They Important Breeding Habitats for
Clarias gariepinus
and
the Migratory
Labeobarbus
Fish Species?
Wassie Anteneh,1Eshete Dejen,2and Abebe Getahun3
1Department of Biology, College of Science, Bahir Dar University, P.O. Box 79, Bahir Dar, Ethiopia
2FAO-Su b Re g ion al Office for Eastern Africa, P.O. Box 5536, Addis Ababa, Ethiopia
3Fisheries and Aquatic Science Stream, Faculty of Life Sciences, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia
Correspondence should be addressed to Wassie Anteneh, wassie74@gmail.com
Received 25 December 2011; Accepted 19 January 2012
Academic Editors: S. Brucet and K. Halaˇ
cka
Copyright © 2012 Wassie Anteneh et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
cited.
This study aims at investigating the spawning migration of the endemic Labeobarbus species and C. gariepinus from Lake Tana,
through Ribb River, to Welala and Shesher wetlands. The study was conducted during peak spawning months (July to October,
2010). Fish were collected through overnight gillnet settings. A total of 1725 specimens of the genus Labeobarbus (13 species)
and 506 specimens of C. gariepinus were collected. Six species of Labeobarbus formed prespawning aggregation at Ribb River
mouth. However, no Labeobarbus species was found to spawn in the two wetlands. More than 90% of the catch in Welala and
Shesher wetlands was contributed by C. gariepinus. This implies that these wetlands are ideal spawning and nursery habitats for C.
gariepinus but not for the endemic Labeobarbus species. Except L. intermedius,allthesixLabeobarbus species (aggregated at Ribb
River mouth) and C. gariepinus (spawning at Shesher and Welala wetlands) were temporally segregated.
1. Introduction
The contemporary Labeobarbus species of Lake Tana (Ethi-
opia) form the only known remaining intact species flock
of large cyprinid fishes, since the one in Lake Lanao in the
Philippines has almost disappeared due to destructive fishing
[1]. The vast majority of cyprinids occur in rivers, but some
Labeobarbus and Labeo species are adapted to a lacustrine
environment [2]. However, these lake-dwelling cyprinids
spawn in rivers, by undertaking a single annual breeding
migration up rivers [3]. This spawning strategy makes the
large African cyprinids vulnerable for modern fisheries, since
the fishermen target spawning aggregations at river mouths
by effectively blocking them offfrom the lake, preventing ma-
ture individuals from reaching the upstream spawning areas
[4,5]. Although the causes for the decline are not properly
identified, the migratory riverine spawning species of Labeo-
barbus in Lake Tana have undergone drastic decline (>75%
in biomass and 80% in number) from 1991 to 2001 [6,7].
The other commercially important species in Lake Tana,
C. gariepinus [8], at the beginning of the rainy season (June-
July), moves through the littoral areas towards the inundated
floodplains and upstream inflowing rivers for spawning [7,
8]. Clarias gariepinus is the most dominant species during the
rainy season upstream of the turbid Ribb River probably due
to the availability of extended floodplain [9]. When the water
level starts to decrease (October–December), C. gariepinus
migrates back through the littoral zone towards the pelagic
zone (Lake Tana). Clarias gariepinus is targeted by the com-
mercial gillnet fishery when migrating between the flood-
plains (spawning areas) and the lake [7]. Although the large,
older individuals proved to be vulnerable for increased mort-
ality by the commercial gillnet fishery, it is known that, com-
pared with Labeobarbus spp.,C.gariepinusis only moderately
susceptible to fishing pressure in Lake Tana. This is because
C. gariepinus is found to be more resilient [10]. In the last
decades, as a result of the low monetary value and poor pre-
ference to this species by the Ethiopians, it was not selectively

2The Scientific World Journal
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Figure 1: Map of Lake Tana and Ribb River and associated Shesher and Welala floodplain wetlands (after Atnafu et al. [18]).
targeted by the commercial gillnet fishery and is mainly
landed as bycatch [7]. However, according to Atnafu [11], C.
gariepinus recently has become highly preferred fish by the
commercial fishermen in Lake Tana area for dry fish export,
especially to Sudan.
Various studies [9,12–17] showed that seven (L. macro-
phthalmus, L. truttiformis, L. megastoma, L. brevicephalus, L.
tsanensis, L. platydorsus, and L. acutirostris) of the 15 endem-
ic Labeobarbus species migrate more than 50 km up rivers
during the rainy season to spawn in fast flowing, clear and
gravel bed streams. However, mass spawning migrations for
the remaining eight Labeobarbus species (L. nedgia, L. dainel-
lii, L. gorguari, L. longissimus, L. intermedius, L. gorgorensis, L.
surkis, and L. crassibarbis) were missing from all tributaries
studied so far. According to de Graaf et al. [14], these mis-
sing species may spawn in the lake or adjacent floodplain
wetlands.
The Shesher and Welala wetlands are located 3–5 km
away from Lake Tana (Figure 1) and are valuable for the local
community. They provide fishes, water, and grazing for
livestock. They also harbor large diversity of bird species
including internationally endangered and threatened ones
[18]. They are the buffering zones of Lake Tana [17]. How-
ever, due to unsustainable farming activities by local farmers,
the existence of these floodplain wetlands and associated eco-
logicalservicesaswellassocioeconomicimportanceisun-
der threat [18,19]. It was observed that the local farmers were
draining and pumping the water to expand farming land.
Another potential threat is the large dam under construction
on Ribb River that could minimize the water overflowing to
these wetlands [20]. To have management plans for the two
wetlands and also to conduct environmental impact assess-
ment studies for all future development projects around the
Lake Tana are strongly recommended.
Probably due to remoteness and inaccessibility, data
about the ecological importance of the two prominent wet-
lands, Welala and Shesher, for the migratory fishes of Lake
Tana are totally absent. Ribb River charges these wetlands as
it overflows during the rainy months (July to October) and
form direct connections with the lake during the rainy season

The Scientific World Journal 3
Tab l e 1: Sampling sites, estimated distance from the lake, coordinates, elevation, and bottom types of the sampling sites.
Sampling site
Estimated
distance (km)
from the lake
Coordinates Elevation Bottom type
Ribb River mouth — N11
◦5954.2
E37
◦3306.1 1788 m Mud an d Silt
(mixed)
Welala I 3.5 N11
◦5859.1
E37
◦3612.3 1789 m Mud
Welala II 3.5 N11
◦5838.2
E37
◦3634.9 1789 m Mud
Welala III 3.5 N11
◦5814.1
E37
◦3618.6 1789 m Mud
Shesher I 4 N11
◦5825.3
E37
◦3716.4 1791 m Mud
Shesher II 4 N11
◦5701.3
E37
◦3727.5 1791 m Mud
Shesher III 4 N11
◦5658.2
E37
◦3735.3 1791 m Mud
through fringes. Therefore, the aim of this study was to in-
vestigate whether Labeobarbus species and C. gariepinus use
these wetlands as spawning and/or nursery habitats.
2. Methods
2.1. Description of Study Area. Lake Tana, Ethiopia’s largest
lake and the source of Blue Nile River, has a surface area of ca.
3200 km2. It is situated in the northwestern highlands at an
altitude of approximately 1800 m. It is a shallow (maximum
depth 14 m, mean 8 m) lake. More than 60 small seasonal tri-
butaries and seven perennial rivers (Gumara, Ribb, Megech,
Gelgel Abbay, Gelda, Arno-Garno, and Dirma) feed the lake
[17]. The only outflowing river is the Blue Nile; however, the
ichthyofauna is isolated from the lower Blue Nile by a 40 m
waterfall located 30 km from Lake Tana. Fogera and Dembia
floodplains are the largest wetlands of the country and border
Lake Tana in the eastern and northern parts, respectively.
Welala and Shesher wetlands (Figure 1) are located in the
Fogera floodplain.
Fogera Wore d a (district) is one of the ten Woredas
bordering Lake Tana and is found in South Gondar Admin-
istrative Zone. It is situated at 11◦5800 N latitude and
37◦4100 Elongitude[18]. Woreta, capital of the Fogera
Woreda is found 620 km from Ethiopia’s capital city, Addis
Ababa and 55 km from Bahir Dar, the regional capital,
(Figure 1). Ribb River originates from Gunna Mountains,
at an altitude of above 3000 m and has the length of ca.
130 km and drainage area of about 1790 km2[21]. In its
lower and middle reaches, the river flows over the extensive
alluvial Fogera Floodplains. The river meanders and flows
slowly over this floodplain, and this resulted in river channel
deposition and overflowing of riverbanks and charging water
to Welala and Shesher during the rainy season.
According to the information from the local people, She-
sher dries usually in February or March; whereas, Welala
dries in April or May. In some years, when there is high over-
flow from Ribb River, Welala never dries throughout the year
(personal communication with the local people). This is be-
cause Welala is smaller in size and deeper (maximum depth
2.5 m in the rainy season) as compared to Shesher which is
wider and shallower (maximum depth 1.75 m in the rainy
season). Location, distance from the lake, elevation and bot-
tom types of the sampling sites are summarized in Ta b l e 1.
Coordinates and elevations were assessed with a GPS. The
bottoms of these two wetlands are muddy (Tab l e 1) and now-
adays, during the post rainy season, the local farmers drain
the water by digging canals from these two wetlands to get
fertile land (the muddy bottom) for crop production [18].
The drastic changes in the areas of Shesher and Welala
wetlands in the last two decades are shown in Figure 2.In
1987, the total surface area of Shesher and Welala was
ca. 1557 and 298 hectares, respectively (Figure 2(a))[19].
Whereas, in 2008 the surface area of Shesher shrunk to
136 hectares (91% shrinkage) and Welala shrunk to 159
hectares (47% shrinkage) (Figure 2(b))[19]. These wetlands
are shrinking at an alarming rate, mainly because of unsus-
tainable farming practices by the local inhabitants [18]. The
local farmers drain the water of these wetlands to expand
their farmland and pump water for irrigation. The large
irrigation dam under construction on Ribb River is another
potential threat. This dam prevents overflow from Ribb River
into the wetlands and disrupts the connection with Lake Tana
which is vital for the survival of these wetlands [18–20].
2.2. Sampling. Sampling took place from July 2010 to
October 2010. Three sampling sites were selected in each wet-
land, two shore sites and one site at the middle. In the shore
sites, gillnets were set at the mouth of the inflow from Ribb
River overflow and at the outlets to Ribb River main channel.
The outflow from Ribb first enters to Shesher, from the
north, and when Shesher is filled, it overflows to Welala, and
finally to Lake Tana (Figure 1). Besides Welala and Shesher
wetlands, samples were collected at Ribb River mouth. Fish
and physicochemical parameters were collected nine times
at the seven selected sites, once in July (third week), three

4The Scientific World Journal
++
Welala
Shesher
344000 348000 352000 356000 360000
344000 348000 352000 356000 360000
1312000
1316000
1320000
1324000
1:88000
N
(Kms)
0.8 0.4 0 0.8 1.6 2.4
(a)
Welala
Shesher
344000 348000 352000 356000 360000
344000 348000 352000 356000 360000
1312000
1316000
1320000
1324000
(b)
Figure 2: Extent of Shesher and Welala wetlands in 1987 (a) and their current (2008) extent (b) (after Nemomissa [19]).
times in August (first, second, and fourth weeks), three times
in September (first, third, and fourth weeks) and twice in
October (first and fourth weeks).
2.3. Physicochemical Parameter Measurements. At all the sa-
mpling sites, observation of bottom type and measurements
of depth and Secchi depth (using 30 cm diameter Secchi-
disk) were taken. Similarly, dissolved oxygen, temperature,
and pH were measured (using probes) at the surfaces of all
sampling sites and at all times, in the morning immediately
after overnight gillnet catch collection.
2.4. Fish Collection. Multifilament gillnets (6, 8, 10, 12, and
14 cm stretched mesh size) with a panel length of 100 m
and depth of 1.5 m were used. Gillnets were set usually at
6 : 00 PM, and catches were collected in the following mor-
ning at about 6 : 00 AM. All of the fishes caught were iden-
tified to species level with immediate inspection (for C. gari-
epinus) and with the help of identification key developed by
Nagelkerke and Sibbing [22]forLabeobarbus species. After
identification to the species level, each fish was dissected; the
gonads were examined visually and sexed. The gonad matur-
ity stage of each specimen of Labeobarbus species was de-
termined visually, using the key developed by Nagelkerke
[17], but for C. gariepinus the gonad maturity stage was de-
termined according to Wudneh [8].
2.5. Data Processing. All the statistical computations were
done using Minitab version 14 and SPSS version 11 software.
Pairwise comparison of dissolved oxygen content (mgL−1),
temperature (◦C) and vertical transparency or Secchi depth
(cm) of the seven sampling sites were compared through
one-way analysis of variance (one-way ANOVA) followed by
Bonferroni’s post hoc tests for multiple comparisons if sig-
nificant variance was evident. One-way ANOVA was also us-
ed to investigate temporal segregation of Labeobarbus species
aggregating at Ribb River mouth and C. gariepinus spawning
in Shesher and Welala wetlands. Only fish with ripe and spent
gonads were considered for temporal segregation analysis
as reproductively immature fishes would not be expected
to show temporal variation in aggregation and migration.
Catch per unit of effort (CpUE) was defined number of fish
per overnight gillnet setting.
3. Result
3.1. Physicochemical Parameters. There was a highly signifi-
cant overall variation on dissolved oxygen (F(6,56) =33.85;
P<0.001) but not in temperature (F(6,56) =2.15; P>0.05),
pH (F(6, 56) =0.83; P>0.05), and vertical transparency
(F(6, 56) =0.47; P>0.05) among the sampling sites (Tables 2
and 3). All the sampling sites in Shesher and Ribb River
mouth have significantly higher dissolved oxygen concen-
tration (P<0.001; Tab l e 3) than the sites in Welala. The
pairwise comparison also showed that the sampling sites of
Shesher, except site II, have higher dissolved oxygen concen-
tration (P<0.001) than Ribb River mouth (Tabl e 3 ). The
average dissolved oxygen concentration of Shesher (when the
three sites pooled), 6.13 ±0.22, is higher than Welala, 5.14 ±
0.06 (Tab l e 2 ).
3.2. Fish Species Composition and Abundance. A total of 2403
fish specimens were collected from Ribb River mouth and
Shesher and Welala floodplain wetlands. From this catch,
1725 (72%) specimens were contributed by Labeobarbus
species, followed by C. gariepinus (21%, 506 specimens),
Oreochromis niloticus (7.1%, 170 specimens) and Var i c orh i -
nus beso (with two specimens only). Clarias gariepinus and
Labeobarbus spp. were dominant at the two wetlands and
Ribb River mouth, respectively (Figure 3).
A total of 469 (250 and 219 from Shesher and Welala,
resp.) fish specimens were collected from all the sampling

The Scientific World Journal 5
Tab l e 2: Mean ±SE (Standard Error) values of oxygen concentration, temperature, pH, and Sechi-disk depth at the sampling sites. Nrefers
to number of samplings.
Site name NOxygen
(mgL−1)Tem p . ( ◦C) pH Secchi-disk depth (cm)
Ribb River mouth 9 5.66 ±0.09 21.2±0.14 6.97 ±0.03 4.56 ±1.04
Welala I 9 5.12 ±0.05 21.5±0.22 7.02 ±0.03 5.33 ±0.76
Welala II 9 5.15 ±0.05 20.9±0.21 7.01 ±0.01 5.32 ±0.51
Welala III 9 5.14 ±0.08 21.1±0.23 6.99 ±0.10 6.11 ±0.92
Shesher I 9 6.17 ±0.37 21.6±0.22 6.96 ±0.02 6.22 ±1.06
Shesher II 9 6.02 ±0.09 21.7±0.24 7.02 ±0.01 6.11 ±1.01
Shesher III 9 6.19 ±0.19 21.4±0.21 7.03 ±0.05 6.00 ±0.83
Tab l e 3: Pairwise comparison of dissolved oxygen concentrations (mgL−1) among sampling sites. Abbreviation used: RM =river mouth.
Ribb RM Welala I Welala II Welala III Shesher I Shesher II Shesher III
Ribb RM ×
Welala I ∗∗ ×
Welala II ∗∗ NS ×
Welala III ∗∗ NS NS ×
Shesher I ∗∗ ∗∗∗ ∗∗∗ ∗∗∗ ×
Shesher II NS ∗∗∗ ∗∗∗ ∗∗∗ NS ×
Shesher III ∗∗ ∗∗∗ ∗∗∗ ∗∗∗ NS NS ×
∗P<0.05; ∗∗P<0.01; ∗∗∗P<0.001; NS: not significant; P>0.05.
n=1934 n=250 n=219
0
20
40
60
80
100
(%)
Ribb RM Shesher Welala
V. b e so
O. niloticus
C. gariepinus
Labeobarbus spp.
Figure 3: Percentage composition of fish collected at Ribb River
mouth and its associated wetlands: Shesher and Welala. Data from
all sites (for Shesher and Welala) and months pooled. “n”refersto
the number of fish.
sites of Shesher and Welala wetlands. Clarias gariepinus was
the most numerous (Figure 4) fish species and contributed
about 93% (421 specimens) of the total catch from all sam-
pling sites of the two wetlands. However, Labeobarbus species
were incidentally caught at Shesher and Welala wetlands
(Figure 4). Only 27 (6.2% of the total wetland catch)
S1 S2 S3 W1 W2 W3
Labeobarbus
C. gariepinus
0
20
40
60
80
100
(%)
n=90 n=61 n=85 n=92 n=46 n=74
Figure 4: Percentage contribution of Labeobarbus spp. and C.
gariepinus collected from the six different sampling sites of Shesher
and Welala wetlands. Pooled data collected from July to October. “S”
and “W” stand for Shesher and Welala, respectively. “n”, n u m b e r o f
specimens.
specimens of Labeobarbus were collected from the wetlands.
The Labeobarbus spp. collected from the two wetlands in-
clude L. acutirostris (1 specimen), L. brevicephalus (8 speci-
mens), L. megastoma (3 specimens), L. intermedius (14 speci-
mens), and L. tsanensis (1 specimen). Together with C. garie-
pinus and Labeobarbus spp., only ten specimens of O. nilot-
icus were collected from the two wetlands.

6The Scientific World Journal
Tab l e 4: The abundance of the six most dominant Labeobarbus
species collected from Ribb River mouth, and their catch from
Shesher and Welala wetlands.
Species Number of specimens
Ribb River mouth Shesher Wolala
L. brevicephalus 206 10 3
L. intermedius 675 7 10
L. megastoma 273 2 2
L. platydorsus 81 0 0
L. truttiformis 196 0 0
L. tsanensis 164 1 0
From the 15 endemic species of Labeobarbus described in
Lake Tana, 13 species were collected from Ribb River mouth.
However, from these 13 Labeobarbus species, only six species
(Tab l e 4 ) were the most dominant contributing nearly 95%
(1595 specimens) of the total Labeobarbus catch from Ribb
River mouth. Seven species (L. acutirostris, L. crassibarbis, L.
gorgorensis, L. longissimus, L. macrophthalmus,L. nedgia,and
L. surkis) were incidentally captured and contributed less
than 8% of the Labeobarbus catch from Ribb River mouth.
Two sp e ci e s, L. dainellii and L. gorguari, were totally missing
from the catches of the river mouth.
3.3. Gonad Maturity Stages. From the total of 1668 spec-
imens of the six Labeobarbus species collected from Ribb
River mouth, only 101 (6%) were immature (gonad stages
II and III); whereas 1563 (93%) specimens were ripe (gonad
stages IV and V), and 14 (1%) were spent (gonad stage
VII) (Figure 5(a)). However, no running (gonad stage VI)
Labeobarbus specimen was collected at the river mouth
(Figure 5(a)). However, from the total of 88 specimens of
C. gariepinus collectedfromRibbRivermouthonly11
(12.5%) were ripe or spawning (gonad stage IV), 77 (87.5%)
were immature (gonad stage I–III) and only one specimen
was spent (gonad stage V). Of the gonads of C. gariepinus
collected from Shesher and Welala wetlands, 133 (32%) were
ripe, 197 (46.5%) were immature, and 91 (22.5%) were spent
(Figure 5(b)).
3.4. CpUE and Temporal Segregation. Collectively, the peak
CpUE value for the six Labeobarbus species aggregating at
Ribb River mouth was observed in September (Figure 6(a)).
However, the peak CpUE for C. gariepinus collected from
Shesher and Welala wetlands was in July (Figure 6(b)), and
the slope of the graph remained negative for the whole study
period.
The monthly catches of the six Labeobarbus species ag-
gregating at Ribb River mouth, except L. intermedius, showed
significant temporal segregation (P<0.05; Tab le 5 ). Signifi-
cant temporal segregation (P<0.05; Tab le 5 ) was also evi-
dent for C. gariepinus spawning at Shesher and Welala wet-
lands. The details of temporal segregation trends and appar-
ent overlaps among the six Labeobarbus species aggregating
at Ribb River mouth is shown in Figure 7.Labeobarbus mega-
stoma and L. intermedius were apparently the first to appear
Tab l e 5: One-way ANOVA result on the catch data of the six
Labeobarbus species from Ribb River mouth and C. gariepinus from
Shesher and Welala wetlands, for differences in the four spawning
months (July–October).
Species Source of
variation MS F(3,5)
value P
L. brevicephalus Month 695.19 17.04 ∗∗
L. intermedius Month 2216.72 5.24 NS
L. megastoma Month 873.41 65.51 ∗∗∗
L. platydorsus Month 103.69 6.23 ∗
L. truttiformis Month 589.46 36.02 ∗∗
L. tsanensis Month 176.13 8.32 ∗
C. gariepinus Month 391.54 33.57 ∗∗
∗P<0.05; ∗∗P<0.01; ∗∗∗P<0.001; NS: not significant, P>0.05.
at Ribb River mouth for prespawning aggregation (during
July). The peak CpUE for these two species was in September
(Figure 7), but it declined from August to October. Whereas,
a reverse trend was observed for L. brevicephalus, CpUE in-
creases from August to October. The other three species: L.
platydorsus, L. truttiformis, and L. tsanensis aggregate during
August and September, but their CpUE declined sharply
during October.
4. Discussion
The water temperature and pH values obtained in the present
study (Tab l e 2 ) from Shesher and Welala wetlands lie within
the same range as Lake Tana’s [23]. This is due to the fact
that these wetlands have hydrological connections with Lake
Tana. Dissolved oxygen was significantly higher (P<0.05) in
Shesher than in Welala and Ribb River mouth sampling sites.
This is probably due to high mixing by wind, since Shesher
is shallow and has no shore vegetation cover. Similarly, high
dissolved oxygen concentration was obtained in Shesher by
Atnafu et al. [18]. However, as compared to the Labeobarbus
spawning streams in Gumara [9]andMegech[15], tribu-
taries of Lake Tana, these two wetlands have lower dissolved
oxygen concentration, high turbidity, and lack gravel sub-
strate.
Prespawning aggregations and upstream migrations of
Labeobarbus species to the tributary rivers of Lake Tana was
intensively studied in the last two decades [9,12–16,24–26].
These studies showed that seven species of Labeobarbus,after
making brief prespawning aggregations at the river mouths,
migrate more than 50 km up rivers and spawn in clear, fast
flowing, well-oxygenated, and gravel bed small streams.
Almost all African Labeobarbus, whether lake dwelling or
riverine, require these conditions in their spawning grounds
[3]. However, unlike the other African Labeobarbus,eightof
the 15 species in Lake Tana, are absent in all the seven peren-
nial tributaries. The most acceptable assumption is that like
many other cyprinid genera, the eight missing (species not
found to spawn in river tributaries) Labeobarbus species most
probably breed in the lake and adjacent floodplain wetlands
[14]. The use of marginal vegetation of the lake’s shore and

The Scientific World Journal 7
VII
V
IV
III
II
I
n=206 n=675 n=273 n=81 n=196 n=164
L. brevicephalus
L. intermedius
L. megastoma
L. platydorsus
L. truttiformis
L. tsanensis
0
20
40
60
80
100
(%)
(a)
V
IV
n=421
III
II
I
C. gariepinus
0
20
40
60
80
100
(%)
(b)
Figure 5: Gonad stages of immature (gonad stages I–III), ripe (gonad stages IV and V) and spent (gonad stage VII) of the six most abundant
Labeobarbus species aggregating at Ribb River mouth (a), and C. gariepinus collected from Shesher and Welala wetlands (b). Note that gonad
maturity stage V is ripe for Labeobarbus spp., but spent for C. gariepinus.
OCTSEP
(months)
AUGJUL
0
40
80
120
160
CpUE (river mouth)
Labeobarbus
Claries
(a)
(months)
0
2
4
6
8
CpUE (wetlands)
OCTSEPAUGJUL
Labeobarbus
Claries
(b)
Figure 6: CpUE of Labeobarbus and Clarias at Ribb River mouth (a) and in Shesher and Welala Wetlands (b).
adjacent floodplain wetlands shelters from predators and
provides high densities of prey for larvae and juveniles [27].
Contrary to the above assumption, in the present study,
these missing Labeobarbus species (L. nedgia, L. dainellii, L.
gorguari, L. longissimus, L. intermedius, L. gorgorensis, L. sur-
kis,andL. crassibarbis) were not found to spawn in the most
prominent adjacent floodplain wetlands, Shesher and Welala.
This is probably because none of the requirements for Labeo-
barbus spawning were satisfied in the adjacent floodplain
wetlands. During the spawning months, these wetlands were
so turbid, poorly oxygenated, and the bottom is muddy, in-
stead of gravel (Tabl e 1 ). Unlike C. gariepinus and Nile tilapia,
Labeobarbus species in Lake Tana are ecologically specialized
and highly vulnerable [7]; hence, it is unlikely that the lar-
vae of Labeobarbus will survive under these poor spawning
ground conditions. Another explanation could be the exist-
ence of C. gariepinus in mass in these two wetlands (Figures
3and 4) that predate on fish. The larvae and juveniles of
Labeobarbus couldbepreyeduponbyC. gariepinus [8];
hence, coexistence of C. gariepinus and early life stages of
Labeobarbus in this relatively small wetlands would result in
high mortality rates of juvenile Labeobarbus.
In the present study, a large number of C. gariepinus were
collected from Shesher and Welala Wetlands. Clarias garie-
pinus is well adapted to the environmental conditions of the
wetlands,itishighlytoleranttolowoxygenlevelsandhigh
turbidity. Although the CpUE of C. gariepinus during our
sampling declines from July to October, the presence of a rel-
atively high percentage of spent implies that a substantial
proportion of the fish may not drift back to the lake, rather

8The Scientific World Journal
L. brevicephalus
0
10
20
30
40
50
60
Jul. (1) Aug. (3) Sep. (3) Oct. (2)
Mean catch (number of fish per
overnight gillnet setting)
(a)
L. megastoma
0
10
20
30
40
50
60
Mean catch (number of fish per
overnight gillnet setting)
Jul. (1) Aug. (3) Sep. (3) Oct. (2)
(b)
L. truttiformis
0
10
20
30
40
50
60
Mean catch (number of fish per
overnight gillnet setting)
Jul. (1) Aug. (3) Sep. (3) Oct. (2)
(c)
L. tsanensis
0
10
20
30
40
50
60
Mean catch (number of fish per
overnight gillnet setting)
Jul. (1) Aug. (3) Sep. (3) Oct. (2)
(d)
L. intermedius
0
40
80
120
160
Mean catch (number of fish per
overnight gillnet setting)
Jul. (1) Aug. (3) Sep. (3) Oct. (2)
(e)
L. platydorsus
0
5
10
15
20
25
Mean catch (number of fish per
overnight gillnet setting)
Jul. (1) Aug. (3) Sep. (3) Oct. (2)
(f)
Figure 7: Mean abundance (number of fish per overnight gillnet setting) with 95% confidence limits (CL) of the six Labeobarbus species
aggregating at Ribb River mouth during the spawning months (July to October).
they spend the rest of their life, after spawning, in these wet-
lands. The presence of a relatively high proportion of im-
mature individuals (46.5%) in our catch indicates the pre-
sence of feeding migration of C. gariepinus to these wetlands
as well. It was indicated by the local people that they observe
mass of juveniles of C. gariepinus in these wetlands during
the postrainy season (end of October and November). Most
of the dry C. gariepinus exported to neighbouring countries
such as Sudan is obtained from Shesher and Welala wetlands
[11], and intensive fishing activities by local people using
seine nets takes place in February and March. Probably, most
of the juveniles of C. gariepinus in the nearby shallow in-
undated floodplains move to these wetlands for growth, since
these wetlands get dry usually in April or May. What the local
people intensively catch are those immature feeding migrants
that stayed in these wetlands, young of the year (juveniles),
and those spent C. gariepinus that remained in the wetlands.
Unlike C. gariepinus, only few specimens of Labeobarbus
species aggregating at Ribb River mouth have reproductively
immature gonads, more than 80% were ripe. However, no
running (gonad stage VII; shedding eggs and sperm) Labeo-
barbus was collected from Ribb River mouth. This supports
the suggestion made by Palstra et al. [9] which states that, if
spawning maturity is only reached when the fish arrives at its
spawning ground (more than 30 km up rivers), there could
be a fine-tuning between homing and gonad development.
Figure 7 shows the details of temporal segregation and over-
lapping during the spawning months (July to October)
among the six Labeobarbus species aggregating at Ribb river
mouth. All these six Labeobarbus,exceptL. intermedius,were
temporally segregated in their spawning aggregation at Ribb
River mouth in the four spawning months. Labeobarbus in-
termedius is the only Labeobarbus species in Lake Tana that
spawns throughout the year, and ripe individuals are always
common in the river mouths [14]. Labeobarbus megastoma
migrates from the lake to the river mouth, starting in July,
and the CpUE declined after September. Three species, L.
truttiformis, L. platydorsus and L. tsanensis, start to aggregate
in August and CpUE reached peak in September and then
started to decline. The last species to aggregate was L. brevi-
cephalus, its peak is in October. Strong temporal segregation
during prespawning aggregation at the river mouths among
the migratory riverine Labeobarbus species was reported by
de Graaf et al. [14]. In addition to the six species found to
aggregate at Ribb River mouth in the present study, de Graaf
et al. [14] observed two more species, L. acutirostris and
L. macrophthalmus in four tributaries of Tana (Gelgel Abbay,
Gelda, Gumara and Ribb) from July to October [14]. More-
over, these two species were also found to migrate more-
than 30 km in Gumara River upstream [9]. But, similar to

The Scientific World Journal 9
the present study, both species did not form prespawning ag–
gregations in other tributaries, Dirma and Megech Rivers
[25] and Arno-Garno River [16]. These irregularities proba-
bly originate from the traditional fish migration study meth-
ods used (e.g., CpUE data). Other modern fish migration
study methods such as radio-tracking may supplement the
existing information and clarify the secrecy of the spawning
grounds of those eight Labeobarbus species not found in the
tributaries of rivers.
The hypothesis that the eight Labeobarbus species that
do not migrate to rivers for spawning may spawn in adjacent
floodplain wetlands [14] was not supported in the present
study. The other possibility is that they may spawn in the
rocky shores of Lake Tana [14]. Although the absence of these
species in the tributary rivers and adjacent wetlands does not
automatically mean they spawn in the lake, lake spawning
now seems the best option. However, we again strongly re-
commend the application of radio-tracking or other depend-
able methods to investigate the actual spawning place(s) of
these eight missing Labeobarbus species. Since Lake Tana and
its shore wetlands are under heavy human pressure, mapping
the spawning habitat is essential to conserve this unique
Labeobarbus species flock.
Acknowledgment
The authors thank the Bahir Dar Fish and Aquaculture Re-
search Center for the logistics support. The authors also need
to thank Asratu Wondie (fishing assistance), Endalamaw
Asres (boat driver), Ayenew Gedif (boat driver), and Getnet
Temesgen (fishing assistance). They also thank Dereje Tew-
abe for his support in the field works. The research was fund-
ed by World Bank-financed Ethiopian-Nile Irrigation and
Drainage Project Coordination Office, Ministry of Water
Resources, Addis Ababa, Ethiopia, and International Found-
ation for Science (IFS) Grt A/4922-1 (W. Anteneh).
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