Belg. J. Zool., 136 (2) : 193-201
July 2006
Studies on the biology of two species of catfish Synodon-
tis schall and Synodontis nigrita (Ostariophysi :
Mochokidae) from the Ouémé River, Bénin
Philippe Lalèyè1, Antoine Chikou1, Pierre Gnohossou1, Pierre Vandewalle2, Jean Claude
Philippart2 and Guy Teugels3
1 Université d'Abomey Calavi. Faculté des Sciences Agronomiques, Laboratoire d'Hydrobiologie et d'Aquaculture. 01 BP 526
Cotonou. Bénin.
2 Université de Liège, Laboratoire de Morphologie fonctionnelle et évolutive. Institut de Chimie, B6 Sart Tilman. B-4000 Liège
et Laboratoire de Démographie des Poissons et d'Aquaculture, 8 Chemin de la Justice. B 4500 Tihange Belgique.
3 Musée Royal de l'Afrique Centrale (MRAC), Laboratoire d'Ichtyologie, B - 3080 Tervuren. Belgique.
Corresponding address : Philippe Lalèyè, e-mail : laleye@bj.refer.org / laleyeph@yahoo.fr
ABSTRACT. The abundance and distribution, length-weight, condition factor, diet and reproduction of Synodontis
schall and S. nigrita from the Ouémé (Bénin) are described. S. nigrita is less abundant than S. schall in the river.
Both species are euryphagous with their diet containing a wide variety of food items that include various types of
plankton, invertebrates and plants. This high diversity of the food composition indicates a wide adaptability to the
habitats in which they live. This is an important strategy for survival and an advantage over the fish species compet-
ing for a specific food item. Size at maturity differs between species for both males (15 cm TL for S. schall and 21
cm TL for S. nigrita) and females (16 cm and 22 cm, respectively). Fecundity range is higher for S. schall (1841 -
15076 oocytes) compared to that of S. nigrita (2647 - 9212). Peak values of GSI for males and females in both spe-
cies occurred from mid- to late July, indicating one season of major spawning activity per year.
KEY WORDS : Synodontis schall, Synodontis nigrita, biology, Ouémé river, Bénin
INTRODUCTION
MATERIEL AND METHODS
Study area and sampling sites
Catfishes support the thriving commercial fisheries in
many West African countries (O
The Ouémé River (Fig. 1) is the largest fluvial basin of
FORI-DANSON, 1992;
O
Bénin, with a catchments' area of 50000 km², extending
FORI-DANSON et al., 2002). Catfishes of the genus Syno-
to about 510 km in length originating in from the Tanéka
dontis, are small to medium-sized fish belonging to the
mountains (north of the country) (COLOMBANI et al.,
family Mochokidae. These are a highly valued food-fish
1972). Input waters originate from two main tributaries,
in Benin (BARAS & LALÈYÈ, 2003) and contribute an
Okpara (200 km length) and Zou (150 km length). Peak
unquantified but significant proportion to the fishery of
discharge is rapid and occurs in August-September. It
the rivers. The genus contains approximately 110 species
crosses many agro-ecological zones draining to the down-
(POLL, 1971), and hence, have more species than any tele-
stream side of the Nokoué Lake and Porto-Novo lagoon
ost genus in Africa other than Barbus and Haplochromis
complex connected to Atlantic Ocean. It has an average
(WILLOUGHBY, 1979). In Bénin, about 11 species of Syno-
slope of 0.9 m/km, except along the upstream area of the
dontis have been identified and 3 species, S. schall, S.
basin where it measures 20 m/km. For the purpose of fish
nigrita and S. sorex, are known from the Ouémé River
sampling, 4 stations were selected (Fig. 1). The first sam-
(L
pling station is situated on the Okpara tributary river at
ALÈYÈ et al., 2004). Previous work on the genus in West
Africa has been carried out by D
Kpassa village (09°17'N ­ 02°43'E). The second sam-
AGET (1954) in Lake
pling station at Atchakpa (08°04'N ­ 02°22'E) on the
Chad, BISHAI & GIDEIRI (1965a, 1965b, 1968) in the Nile
Ouémé River is located along a coarse, rocky zone with
River, WILLOUGHBY (1976, 1979) in Lake Kainji, HALIM
swift water currents. Toué (07°12'N ­ 02°17'E) is the
& GUMA'A (1989) in the White Nile, OFORI­DANSON
third sampling station on the Zou tribrutary. This station
(1992) in the Kpong Headpond (Ghana), OLOJO et al.
marks the transition between the zones of swift water and
(2003) in the Osun River (Nigeria). In Bénin, there is
the delta. The forth station at Agonlin Lowé's village
however, no information on many aspects of the genus.
(06°39N ­ 02°28) is situated in the Ouémé Delta.
This paper investigates the abundance, growth condition,
The river is influenced by two distinct climates due to
reproduction and diet of S. schall and S. nigrita in the
its geographic location. The northern basin (near the
Ouémé River basin, Bénin.
sources), a tropical tendency of dry and rainy seasons and
194
Philippe Lalèyè, Antoine Chikou, Pierre Gnohossou, Pierre Vandewalle, Jean Claude Philippart and Guy Teugels
high varying temperature (10-40°C) are observed
(WOOTTON, 1994) was estimated using the formula pro-
throughout the year. From November to March, rains can
posed by TESCH (1971) :
be rare or turbulent. Furthermore, the harmattan wind,
which blows from November to April, accentuates the
W
Kn = ------ × 100
thermic and hygrometric amplitudes. The rainy season
aTLb
extends from May to September. The southern basin is
characterized by a sub-equatorial climate with two rainy
Maturity stages gauged according to the macroscopic
and two dry seasons. The great rainy season occurs from
evaluation of gonads (LAGLER, 1971 and LALÈYÈ et al.,
April to July with the greatest amount in June. The second
1995) were recognized for Synodontis species as : I ­
rainy season occurs in September. Temperature remains
immature (very small sexual organs close under the verte-
relatively constant varying from 18 to 35°C.
bral column; testes and ovaries colourless; eggs invisible
to naked eye); II ­ developing (Testes and ovearies
traslucent; small eggs can be seen with aid of magnifying
glass); III ­ mature (Ovaries orange-reddish; eggs clearly
O
10°
uémé
discernible to eye; ovaries occupy about two-thirds of
Riv
central cavity; anterior testis whitish with short fingerlike
er
PARAKOU
processes and those in posterior appear slightly
#
S
$
Kpassa
translucent; milt drops from whitish process under slight
pressure); IV ­ ripe (ovaries filing ventral cavity; eggs
Ou
O
é
k
m
p
9°
N I G E R I A
light green in colour, completely round and fall from
é
ara
ovary with little pressure; eggs oocytes diameter (meas-
R
i
ver
ured from fresh material) varied from 0.5 mm to 1.0 mm
(mean 0.8 mm ± 0.2) in S. schall and from 0.8 mm to 1.5
mm (mean 1.1 mm ± 0.3) in S. nigrita); V ­ spent (not yet
Atchakpa
T O G O
$
fully empty; no opaque eggs left in ovaries; ovaries large
8°
but flabby; testes thread-like with no granules and are
pink-white shrivelled bodies). Average size at first matu-
Zo
C
u
o
rity (L ) was defined as the length at which 50% of the
u
50
f o
females are at an advanced stage of the first sexual cycle
Toué
ABOMEY
$
(at least in stage III of the maturity scale) as suggested by
#
S
TWEDDLE & TURNER (1977). This is based on the previ-
M
7°
on LOKOSSA
Agonli
ously determined reproductive season to avoid bias in
o
#
S
Lowé
$
classifying resting females as immature (PANFILI et al.
Lac
PORTO
Nokoué
#
S
2004). The gonado-somatic index (GSI) was calculated
NOVO
#
S
0
100 Km
based on the formula suggested by LAGLER (1971) which
COTONOU
OCEAN ATLANTIQUE
is expressed as :
1°
2°
3°
Fig. 1. ­ Map of the study area showing the location of some
GSI =
Gonad weight (g)
× 100
towns, villages and the sampling sites
Total body weight (g)
Sampling and analysis
Fecundity was determined from 27 and 26 mature
gonads of S. schall (13.5 cm - 21.7 cm TL, 30 g - 100 g
Sampling of both S. schall and S. nigrita was con-
TW) and S. nigrita (14.0 - 23.5 cm TL, 32g-125 g TW),
ducted on a monthly basis from May 1999 to April 2000
respectively. Absolute fecundity, probable number of
by using monofilament gillnets of various mesh sizes (10,
oocytes which will be released at the following spawning,
12, 15, 20, 25, 30, 35, 40, 45 mm) per sampling site. Two
was determined by taking two portions of the ovary (500
nets of each mesh size were used. Each net measures 30.0
fresh eggs ± 50) which are then weighed and initially
m length and 2.5 m depth. Floats are used on the head line
fixed in modified Gilson's fluid (BAGENAL & BRAUM,
while lead weights are used on the bottom line. Nets are
1971) for about 2 - 3 weeks until oocytes obtained a free
set in the water and inspected every 3 hours for two days.
and hard texture. The relationship between fecundity and
Total length (TL, to the nearest mm) and body weight (to
some morphometric measurements were determined by
the nearest 0.1 g) were measured and the sex of fish deter-
relating total fecundity (F) data to total length (TL), total
mined according to LAGLER (1971), using macroscopic
weight (TW) using the following formulae :
evaluation.
F = a x TLb; F = a x TWb
A total of 3646 specimens of both species were col-
lected. These were dominated by S. schall (77%) and data
The degree of stomach fullness of the all samples was
collected were used to length-weight relationships based
estimated by the same person by an arbitrary 0-4 points
on the method of LE CREN (1951) and is expressed as
scale defined as follows : 4 points for full, 3 for ¾ full, 2
follows : Log W = Log a + bLog TL, with W fish weight,
for ½ full, 1 for ¼ full and 0 for empty stomachs. The
TL fish total length and a and b the constants. Relative
fullness index (FI) was considered as the percentage of
growth condition factor (Kn, coefficient of condition),
stomachs completely filled, as well as those considered
which measures physiological well-being of the fish
75% filled. Stomach contents were sorted into groups and
Synodontis biology in Ouémé River
195
items identified accordingly. Occurrence percentage (F)
1199) is not significant (P > 0.05) (Fig. 3). In S. nigrita,
(HYSLOP, 1980) was estimated using the formula :
the total length ranged from 6.0 cm to 33.5 cm. No signif-
icant (P > 0.05) length differences were observed between
Nei
F = ----- × 100
males (range 6.0 cm - 31.9 cm TL, mean 16.5 cm 4.1 cm
Nt
TL, N = 436) and females (range 6.2 cm - 33.5 cm TL,
Where Nei = number of stomachs containing a type of
mean 17.0 cm ± 5.3 cm TL, N = 399). Comparing the two
prey i and Nt = total number of full stomachs examined.
species, the difference in fish length is not significant (P >
SCHOENER'S (1970) index was used for establishing diet
0.05).
overlaps between the two species and is calculated as :
n



S = 1­0.5
Pij


­ Pik
i = 1

where n = number of food categories, P = proportion
ij
Number
Weight (g)
(% by weight) to food category i in diet of species j and
P = proportion (by weight) of food category i in diet of
1400
12000
ik
species k. This is considered as the most satisfactory
1200
S. schall
10000
method in the absence of any food estimate data. Diet
1000
8000
overlaps were considered to be biologically important
800
6000
when S exceeds 0.60 (WALLACE, 1981).
600
Number
4000
400
Catch weight (g)
200
2000
RESULTS
0
0
l.
.
.
.
n
Abundance and distribution
May
J
un.
Ju
Aug.
Sep
Oct.
Nov.
Dec.
Ja
Feb
Mar.
Apr.
1999
2000
The total number, and weight of Synodontis caught
during the 12-month sampling period is shown in Table 1.
Synodontis abundance decreases from southern to north-
ern basin. The numbers and weights of the two species in
Agonlin Lowé were significantly higher than in other
Number
Weight (g)
locations (P < 0.05). No S. nigrita have been caught in
200
2500
Kpassa by experimental fishing during the study. S. schall
180
S. nigrita
was observed to be more abundant at all the stations com-
160
2000
pared to S. nigrita. Seasonal variations in numbers and
140
weights are shown in Fig. 2. For both species, peak
120
1500
catches occurred in May and December. In S. schall, peak
100
catches also occurred in July and in January-February and
Number
80
1000
in April (2000). Significant yields catches peak were also
60
Catch weight (g)
observed in August for S. nigrita. Significant reductions
40
500
20
in catch were observed for S. schall in August (1999) and
0
0
in September (2000) for both species.
l.
.
.
.
n
May
J
un.
Ju
Aug.
Sep
Oct.
Nov.
Dec.
Ja
Feb
Mar.
Apr.
Size range and population structure
1999
2000
Total lengths of S. schall ranged from 6.2 to 34.3 cm.
Fig. 2. ­ Change in seasonal abundance of S. schall and S.
The difference in fish length between males (range 6.6 cm
nigrita in Ouémé river basin
- 31.5 cm TL, mean 15.1 cm ± 4.7 cm TL, N = 1314) and
females (6.2 cm - 34.3 cm, mean 15.9 cm ± 5.03, N =
TABLE 1
Numbers and weights of Synodontis caught at sampling station (May 1999-April 2000)
Synodontis nigrita
Synodontis schall
Sampling
% Total
% Total
stations
% Total no
Total weight
% Total no
Total weight
Number
weight
Number
weight
of fish
(g)
of fish
(g)
of fish
of fish
Kpassa
0
0
0
0
42
0,32
1098
0,64
Atchakpa
9
0,16
1039
0,63
54
0,95
6029
3,68
Toué
165
0,87
2764
0,87
910
4,82
11926
3,76
Agonlin Lowé
715
2,43
8506
2,96
4845
16,44
35670
12,41
Total
889
­
12309
­
5851
­
54723
­
196
Philippe Lalèyè, Antoine Chikou, Pierre Gnohossou, Pierre Vandewalle, Jean Claude Philippart and Guy Teugels
that the tendency of the two species is to increase more in
12
size than in mass.
10
SCHALL N = 2513
NIGRIT A N = 835
Condition factor (Kn)
8
s
h
The mean monthly relative condition (Kn) value
f
fi
r
o
obtained was 1.513 ± 0.005 for S. schall while 1.741 ±
e
6
b
m
u
0.088 in S. nigrita (Fig. 4). Significant difference in aver-
% N
4
age Kn values was observed between species (P < 0.01; F
(1,22) = 60.55). Peak of condition factor occurred during
2
the flooding season which falls in September for S. schall
and in October for S. nigrita. In both species, a relative
0
0
5
10
15
20
25
30
35
40
increase of condition factor has also been observed in
T otal length (cm )
July (1999) and from December to February (2000). Low-
Fig. 3. ­ Length frequency distribution of S. schall and S.
est value was observed in June and November for both
nigrita in the Ouémé river basin
species.
Trophic biology
2,4
Of the 2016 stomachs of S. schall (3.0 - 34.3 cm TL)
examined, about 44% (886) were observed full, 20%
2,2
(410) were partially filled and 36% (720) were empty. In
2,0
the case of the 675 S. nigrita (3.4 - 33.5 cm TL) stomachs
Kn)
examined, 42% (283) were full, 24% (159) were partially
1,8
t
i
on (
filled and 35% (233) were empty. In general, stomachs of
1,6
both species were more than half full (Fig. 5). During the
Condi 1,4
year, the mean stomach fullness index estimated as 50.4 ±
9.6 in S. schall while 53 ± 16.7 in S. nigrita. This indi-
1,2
cates a constant level of feeding activity by the two spe-
1,0
cies. Highest index value was observed in June for both
l.
.
.
.
n
species. Lowest values were observed in July and
S. nigrita
May
J
un.
Ju
Aug.
Sep
Oct.
Nov.
Dec.
Ja
Feb
Mar.
Apr.
November for S. schall and S. nigrita, respectively. Simi-
1999
2000
lar food items were consumed by both species (Table 2)
though varying proportions among preys (Table 3) were
2,4
observed. Macrophytes and algae were the most frequent
2,2
food items observed in the stomachs of the two species.
n) 2,0
Animal prey types are larvae and adults of various
(K
insects, crustaceans, rotifers, as well as possible parasitic
n 1,8
nematodes. Mud and some unidentified particles were
1,6
nditio
also observed. For S. schall, however, molluscs seems to
1,4
Co
be a preferred prey following macrophytes and algae. The
1,2
proportion of eggs and fish scales is more important in the
1,0
stomach contents of S. schall (frequence of occurrence,
l.
.
.
.
.
n
40.35%) compared to that of S. nigrita (1.56%). How-
May
J
un.
Ju
Aug.
Sep
Oct.
Nov.
Dec.
Ja
Feb
Mar.
Apr.
ever, in both species, complete fishes were never found in
1999
2000
their stomachs. Diet overlap is biologically important
between the two species (S = 0.755).
Fig. 4. ­ The mean monthly relative condition of S. schall and
S. nigrita in the Ouémé river basin (95% confidence limits are
S. schall
S. nigrita
indicated)
100
x
Length-weight (L-W) relationship
80
This relationship was described for the two species
ss inde
60
based on the linear equations :
S. schall : Log TW = - 4.212 + 2.832 Log TL (r =
40
h fullne
0.982)
20
S. nigrita : Log TW = - 4.047 + 2.779 Log TL (r =
Stomac
0.973)
0
p.
b.
No significant difference in L-W relationship (P >
May
Jun.
Jul.
Aug.
Se
Oct.
Nov.
Dec.
Jan.
Fe
Mar.
Apr.
0.05) was obtained between these two catfish species. In
1999
2000
both species, the constant b, which describes the slope of
the regression line, is smaller than 3 (p < 0.01) : S. schall,
Fig. 5. ­ The monthly stomach fullness index of stomachs of S.
t(2511) = 5.78; S. nigrita, t(831) = 3.459. This implies
schall and S. nigrita in the Ouémé river basin
Synodontis biology in Ouémé River
197
TABLE 2
Plant and animal species identified in the stomachs of either Synodontis schall or S. nigrita
Food category
Species
Machrophytes
Maize, Azolla sp., leaves.
Algae
Actinocyclus sp, Amphora commutata, Anabaena affinis, Ankistrodesmus fusiformis, Anabaena sp, Calothrix bervis-
sima, Anabaenopsis circularis, Centritractus belonophorus, Anemoeoneis sphaerophora, Closterium aciculaire, Cer-
atophallus natalensis, Closterium acutum, Ceratophallus sp, Closterium ehrenbergii, Chrococcus limneticus, Closte-
rium leibleinii, Closteriopsis longissima, Closterium monoliferum, Closterium lanceolatum, Ferrisia eburneensis,
Closterium parvulum, Closterium lineatum, Closterium pseudolunula, Closterium tumidum, Closterium strigosum,
Closterium venus, Cocconeis placentula, Closteropsis sp, Cosmarium brebisonii, Coelomoron pusillum, Cosmarium
granatum, Cosmarium connatum, Cosmarium vexatum, Cosmarium contractum, Cyclotella meneghiniana, Cosmarium
decoratum, Cymbella silesiaca, Cosmarium lundellii, Diploneis ovalis, Cosmarium pseudodecoratum, Euastrum glazio-
vii, Cosmarium pseudopyramidatum, Eunotia didyma, Cosmarium quadrum, Eunotia monodon, Cosmarium retusi-
forme, Eunotia soleirolii, Cymbella caespitosa, Frustulia rhomboides, Euastrum germanicum, Gomphonema granuli,
Euastrum pseudopectinatum, Hantzschia amphioxys, Euastrum sphyroides, Merismopedia elegans, Euglena proxima,
Micrasteria sp, Fragilaria virescens, Micrasterias orux melitensis, Gomphonema gracile, Microcystis sp, Gomphos-
phaeria naegiliana, Monoraphidium sp, Gonatozygon monotaenium, Navicula epypto, Lyngbya bourrellyana, Navicula
pygmaea, Lyngbya cebennensis, Navicula splendida, Lyngbya contorta, Nitzschia recta, Merismopedia punctata, Nitzs-
chia scalaris, Micrasteria foliacea, Oedogonium globosum, Micrasterias crux-melitensis, Oscillatoria ornata, Micras-
terias radians, Pinnularia brauniana, Microcystis aeruginosa, Pinnularia gibba, Microcystis aquatilis, Pinnularia
mesolepta, Microcystis delicatissima, Pinnularia neomajor, Microcystis elachstica, Scenedesmus microspina, Microcys-
tis sp, Scenedesmus obliquus, Microcystis weenbergii, Spirogyra sp, Monoraphidium griffihii, Spondylosum planum,
Mougeotia sp, Spondylosum secedens, Navicula cebennensis, Spondylosum tetragonum, Navicula cryptocephalla, Stau-
rastrum forficulatum, Navicula cuspidata, Staurastrum polymorphum, Navicula placentula, Stauroneis phoenicenter-
aon, Navicula pygmaea, Navicula sp, Nitzschia sigma, Nostoc entophytum, Nostoc piscinale, Oscillatoria platensis,
Oscillatoria pseudolabyrinthiformis, Oscillatoria sp, Pandorina sp, Pediastrum, Pediastrum duplex, Phacus longi-
cauda, Pinnularia acrosphaeria, Pinnularia borealis, Pinnularia brebissonii, Pinnularia cardinalis, Pleurotaenium
cylindricum, Pleurotaenium trabecula, Pseudanabaena catenata, Scenedesmus ecornis, Senedesmus naegelii, Senedes-
mus quadricauda, Spirulina gigantea, Spirulina linearis, Stauroneis phoenicenteron, Stigeoclonium aestivale, Surirella
splendida, Synechocystis aquatilis, Synedra ulna, Tetraedriella gigas, Tetraedron muticum, Ulothrix sp, Ulotrix zonata.
Rotifera
Asplanchna girodi, Brachionus platulus, Keratella cochlearis, Lapadella patella, Lecane leontina, Pompholyx sulcata,
Proales deaprens, Cepadella
Insect larvae
Ceratopogonidae larva, Chironomidae larva and pupa, Coleoptera larva, Other Coleoptera, Euparyphus larva,
Ephemeroptera larva, Heteroptera larva, Hydropsychidae larva, Odonata larva, Culicidae larva and pupa, Lepidoptera
larva, Trichoptera pupa, Neotrichia larva, Piralidae larva, Plecoptera larva, Tabanidae larva, Mosquito larva and pupa.
Aquatic insects
Chaoboridae, Chironomidae, Elmidae, Grillus sp, Pleidae, Hymenoptera, Heteroptera, Mosquito, Orthoptera, Taban-
idae,
Crustacea
Copepoda, Cladocera, Ostracoda, Macrobrachium.
Mullusca
Bellamya unicolor, Bilinus sp, Bulinus jousseaumeis, Bulinus senegalensis, Biomphalaria pfeifferi, Biomphalaria
sudanica, Eupera parasitica, Caelatura sp, Cleopatra bulimoides, Gabiella senaariensis, Limnea natalensis.
Nematoda Miscellaneous
Pisces scales and eggs, Sand particles, Mud, decomposed matter
Reproduction
TABLE 3
Percentage frequency of occurrence of different category in
Sex-ratio
stomachs of Synodontis schall and Synodontis nigrita caught
from the Ouémé River, May 1999-April 2000
Sampling of fish from the different stations in the river
S. schall
S. nigrita
Food category
ensured a representative distribution of males and females
(%)
(%)
species. In general, in both species, males were observed
Macrophytes
59.65
43.70
Algae
35.10
45.63
numerically dominant than females (P < 0.05). According
Insect larvae
17.54
8.40
to the months, the sex ratio is in favour of the males
Aquatic insects
7.02
3.78
Crustacea
5.26
2.94
(Table 4, P 0.01, ÷² (1,11) = 29.06 for S. schall and 40.50
Rotifera
5.45
0.02
for S. nigrita) except in October for S. schall and in June,
Mollusca
19.30
2.10
Nematoda
12.28
2.52
July, December, January and February for S. nigrita
Fish eggs and scales
40.35
1.56
where the females were slightly dominant numerically.
Unidentified decomposed matter
26.32
2.94
Sand particles
3.51
0.00
The same result was obtained in different sizes of the fish.
Mud
7.02
12.18
198
Philippe Lalèyè, Antoine Chikou, Pierre Gnohossou, Pierre Vandewalle, Jean Claude Philippart and Guy Teugels
TABLE 4
was investigated and described by the following
equations :
Number of males and females of Synodontis schall (a) and Syno-
dontis nigrita (b) in the monthly samples
F = 1.978*TL2.817 (r = 0.717) and = 18.53*TW1.425 (r =
0.8088) for S. schall.
(a) S. schall
F = 5.951*TL2.3008 (r = 0.835) and = 190.068*TW0.7591
Months
No. of males
No. of females
Sex ratio M :F
(r = 0.7605) (r = 0.7605) for S. nigrita.
May
117
104
1.0 :1.1
June
96
80
1.0 :1.2
High positive exponential fecundity-length and fecun-
July
157
148
1.0 :1.1
dity-weight correlations (p < 0.001) were obtained in the
August
38
26
1.0 :1.5
two species. No difference were observed between the
September
80
77
1.0 :1.0
October
71
80
1.0 :0.9
two species related to each relationship ((t, 45), P > 0.05).
November
177
179
1.0 :1.0
December
112
88
1.0 :1.3
January
153
145
1.0 :1.1
3,5
February
114
98
1.0 :1.2
March
153
135
1.0 :1.1
3,0
S.schall
April
60
45
1.0 :1.3
males
2,5
Total
1328
1205
1.0 :1.1
2,0
I
(%)
(b) S. nigrita
1,5
GS
Months
No. of males
No. of females
Sex ratio M :F
1,0
May
22
17
1.0 :1.3
0,5
June
21
27
1.0 :0.8
0,0
July
23
30
1.0 :0.8
August
55
43
1.0 :1.3
20
September
66
55
1.0 :1.2
October
63
44
1.0 :1.4
15
S. schall females
November
91
77
1.0 :1.2
December
29
31
1.0 :0.9
10
I
(%)
January
19
36
1.0 :0.5
February
1
6
1.0 :0.5
GS
5
March
44
34
1.0 :0.2
April
5
2
1.0 :1.3
0
Total
439
402
1.0 :2.5
.
.
r.
Ju.
ug.
ov.
May
Jun.
A
Sep
Oct.
N
Dec.
Jan.
Feb
Mar.
Ap
Size at maturity
For both females and males, the sizes at first maturity is
higher for S. nigrita (21 - 22 cm) than S. schall (15-16
3,0
cm). This observation is the same when considering the
2,5
S. nigrita
smallest sizes at maturity for both species : S. nigrita at
males
2,0
10 cm (male) and 12.5 cm (female) while 7.8 cm (male)
1,5
and 8.4 cm (female) for S. schall.
I
(%)
GS 1,0
Gonado-somatic index (GSI) and spawning period
0,5
GSI values varied from 0.12% ± 0.1 to 1.854% ± 1.47
0,0
and 0.43% ± 0.25 to 9.874% ± 7.38 in S. schall males and
20
females, respectively. For S. nigrita, values obtained
S. nigrita females
range from 0.29% ± 0.05 to 0.808% ± 0.565 and 0.33% ±
15
0.01 to 9.88% ± 9.01 in males and females, respectively.
10
GSI values for the two species were found to vary consid-
GSI (%)
erably with time, and this variation was at its maximum
5
during July to October (Fig. 6). Peak values for males and
0
females occurred from mid- to late July, indicating one
.
t
.
.
.
ec.
season of major spawning activity per year for both spe-
May
J
un.
Ju
Aug.
Sep.
Oc
Nov.
D
Jan
Feb.
Mar
Apr.
cies. Major spawning activity coincided with the high
1999
2000
water period (August to October).
Fig. 6. ­ Seasonal variation in gonadosomatic index of males
Fecundity
and females of S. schall and S. nigrita (95% confidence
limits are indicated).
The fecundity ranged from 1841 to 15076 eggs in S.
schall and from 2647 to 9212 eggs in S. nigrita. Between
species, fish observed with the highest fecundity had a
DISCUSSION
length of 20.8 cm TL and weighed 85 g in S. schall. For S.
nigrita, fecundity is highest in species of 23.5 cm TL in
Based on results obtained from experimental and arti-
length and 100 g in total weight (TW). The fish with the
sanal fishing, S. nigrita is less abundant than S. schall in
lowest number of eggs had 13.5 cm TL and 35 g TW in S.
the Ouémé River (Table 1). This species was absent in the
schall and 14.0 cm and 32 g TW in S. nigrita. The rela-
experimental fishing at Kpassa, but is present in the cap-
tionship between fecundity (F), fish length and weight
tures of the fishermen when traps are used.
Synodontis biology in Ouémé River
199
In general, abundance and distribution of both Syno-
species (OFORI-DANSON, 1992) complemented by their
dontis are similar to what was observed earlier on the ich-
high reproductive rates.
thyofauna of the Ouémé River (LALÈYÈ et al., 2004).
About size structure of the populations studied, the
According to LALÈYÈ et al. (2004), the stations of Agonlin
maximum size for S. schall is superior to that indicated by
Lowé and Toué are, by far, the richest in species (Agonlin
A
Lowé, 71 species, = 59.2% of the total ichthyofauna;
LBARET (1982) for Ivory Coast (22 cm) while lower to
the value indicated by O
Toue, 67 species, = 55.8% ichthyofauna). These stations
FORI-DANSON (1992) for S. schall
(26.7 cm SL, or about 33.5 cm TL) in Kpong Headpond
are situated in a vast floodplain whose ecological charac-
(Ghana). This difference of sizes is even better illustrated
teristics favour the colonization by fish. Floodplains
by the condition factor for S. schall (1.49 ± 0.19), which
present a great variety of habitats of which distribution
is lower than that indicated by O
and dynamics vary according to hydrological seasons
FORI-DANSON (1992),
2.54 ± 0.002 for male and 2.91 ± 0.018 for females.
(WELCOMME & DE MERONA, 1988).
Among factors which can explain the difference obtained
OFORI-DANSON (1992) reported that S. schall contrib-
in the average condition factor (see LE CREN, 1951) of a
uted about 50% of the biomass of the 5 Synodontis spp
fish species in two or more habitats, the fact that the fish
found in Kpong Headpond (Ghana). ARAOYE (1999) indi-
are in better condition of feeding in a habitat than in the
cated that S. schall was caught abundantly in As lake
second one can play an important role in this specific
(Nigeria). S. schall seems to be an ubiquitous species,
case.
being found in all aquatic habitats including headwaters
of tributaries, pools in dry sandy river beds, as well as in
Estimated growth factor indices (Kn) for both species,
river and marshes. The high occurrence of S. schall
S. schall = 1.49 and S. nigrita = 1.73, are lower than the
within different ecological niches can be attributed to its
estimated range of mean values (2.65 - 3.32) indicated by
diverse feeding habits.
BAIJOT et al. (1997) for some slow-growing important
fishes in Africa.
The seasonal fluctuations in the numbers and weights
of the Synodontis spp caught using gillnets suggest four
The food items in the stomach of both Synodontis spe-
interrelated factors : changes in the behaviour of the fish,
cies suggest that they are omnivorous feeders as the diet
fishing activities, rainy season and recruitment. High spe-
covers a wide spectrum of food ranging from various
cies yield in catches during April and May corresponds to
types of plankton to invertebrates and plants. This is in
the beginning of the rainy seasons when food availability
agreement with the finding of LAUZANNE (1988) who con-
is highest due to the flood-introduced nutrients and mix-
siders the Synodontis genus as eclectic. OLOJO et al.
ing of water body by rapid currents. Such ecological con-
(2003) observed the same food habits in S. nigrita from
ditions are favourable to fishes and may allow them to
the Osun River (Nigeria). This high diversity of the food
leave their hiding places making them vulnerable for fish-
composition of both Synodontis species indicates a wide
ing. Increase in fish abundance due to the combination of
adaptability to the habitats in which they live. Many cat-
physico-chemical properties and the presence of food
fishes, such as the Chrysichthys spp, are benthic omni-
items has already been reported by FAGADE & OLANIYAN
vores with a strong tendency to predation (BARAS &
(1974) for the Lagos lagoon (Nigeria). In May (1999), the
LALÈYÈ, 2003). This is an important strategy for survival
abundance of vulnerable fishes reached its maximum.
and an advantage over the fish species competing for a
This situation can justify the importance of catches
specific food item (PAUGY, 1994; LÉVÊQUE, 1997). A
obtained in this month. The stock of fishes falls down in
clear morphological explanation for its feeding versatility
June due to the intensity of the fishing.
may be due to the ventral location of the mouth of both
This numerical and ponderal increases of Synodontis
Synodontis species which encourages a detritivorous
are then followed by a decrease in June which will
mode of feeding while the simple horny structures around
increase again during July and August for S. schall and S.
the mouth enable it to adapt to filter feeding (OLOJO et al.,
nigrita, respectively. The lowest numerical abundances
2003). These structures also help Synodontis to gnaw at
are registered in August ­ September for S. schall and in
any hard plant tissue which form part of its rich diet.
October - November for S. nigrita when water level is at
WINEMILLER & KELSO-WINEMILLER (1996) stated that
its highest. The one-month gap noted between the two
Synodontis leopardinus, S. nigromaculatus, S. woosnami,
species could be due to behavioural differences aside
S. macrostigma, S. macrostoma of the Upper Zambezi
from other factors. Decrease in fishing vulnerability may
River floodplain (Zambia) were omnivores, but interspe-
be due to the increased catchments area allowing these
cific differences were however noted. In Ouémé River,
species to disperse and hide during spawning. Catch rates
the most frequent food items in the stomachs of both spe-
increase progressively starting from December when
cies were macrophytes and algae. The diversity of algae
water level and catchments are decreasing. Catch rate for
consumed by both species is high (more than 100 species,
S. nigrita starts to decline in January. Decrease in catch of
Table 2). In Kpond Headpond (OFORI-DANSON, 1992), the
S. schall, however, starts only in March.
frequent food item in S. schall were chironomids. The
insects, fish eggs and scales clearly originated from both
Decrease in abundance after flooding may be due to the
the bottom (with organic sediment and pieces of wood
decrease of river margins suitable for feeding and for
often present) and from periphyton of flooded trees,
spawning (HÅKANSON & BOULION, 2002).
grasses and aquatics plants. These categories of food
Increase in abundance is highly correlated with success
items were more frequent in S. schall stomachs compared
recruitment. The viability of these species may be largely
to that of S. nigrita, suggesting that the habitats used by
enhanced by the reproductive behaviour (guarders),
the two species for their food are not rigorously the same.
enhancing survival due to decreased predation, of these
The presence of sand grains and mud in the stomachs
200
Philippe Lalèyè, Antoine Chikou, Pierre Gnohossou, Pierre Vandewalle, Jean Claude Philippart and Guy Teugels
indicates that these species browse on benthic deposits in
HALIM & GUMA'A (1989) from the White Nile River
the river. The presence of nematodes in many stomachs
(10000-90000 eggs), OLATUNDE (1989) from Zaria
almost in S. schall could explain some diseased and para-
(Nigeria) (2014 - 13262). The great variations in the
sitized specimens observed during the study. Size-
number of eggs produced by the individual of a certain
dependent (ontogenic) variation in occurrence of different
species were demonstrated for a large number of other
categories of food in stomachs of the two species is not
tropical fishes (NAWAR, 1959; AWACHIE & EZENWAJI,
clear. The overall picture of the diet of S. schall and S.
1981; LALÈYÈ et al., 1995; BARAS & LALÈYÈ, 2003 and
nigrita that emerges from this study is that of two species
many others). Apart from the environmental factors, the
which are largely unspecialized in their feeding habits.
differences observed may be, in part, attributed to the
Unspecialized dietary habits are an optimal strategy for
methods used for fecundity estimation.
survival in habitats where food sources are subject to
Eggs diameters at ripe stage were greater (0.8 mm-1.5
fluctuation (WELCOMME, 1979; PAUGY, 1994; LÉVÊQUE,
mm) in S. nigrita than in S. schall (0.5 mm-1.00 mm). For
1997).
S. schall, HALIM & GUMA'A (1989) reported a range of
There is an important difference in reproductive biol-
0.6 mm to 0.9 mm from White Nile in Khartoom. The
ogy between the two species of Synodontis. Size at matu-
greatest estimation (1.15 mm to 1.20 mm) was given by
rity was observed higher for S. schall compared to that of
ALBARET (1982) for S. schall from Ivory Coast. Such var-
S. nigrita. In Lagoon Ébrié (Ivory Coast), ALBARET
iations in the eggs diameters were similarly reported for
(1982) obtained 15.5 cm of size at maturity for S. schall.
other tropical fishes (i.e. AWACHIE & EZENWAJI, 1981).
HALIM & GUMA'A (1989) observed different values (14.0
- 15.0 cm SL) for S. schall in White Nile (Sudan). In Lake
CONCLUSION
Kainji (Nigeria), WILLOUGHBY (1979) reported smaller
sizes at maturity for S. schall, 10.4 and 11.8 cm for male
The feeding versatility of S. schall and S. nigrita cou-
and female, respectively. The highest values (20.0 cm)
pled with the high fecundity enables these species to
were obtained by OFORI-DANSON (1992) for female of S.
overcome perturbations, natural or human induced, in the
schall in the Kpong Headpond (Ghana). Maturation of
Ouémé River. Life-history of species is influenced by var-
fish may be affected by several physical and biological
ying ecological conditions and highly tolerant species,
factors, and these may account for the discrepancies
such as the catfishes, are promising candidates for com-
observed between the findings of different authors. The
mercial exploitations.
influence of varying environmental conditions on matu-
rity and reproductive traits have been shown in studies by
LAË (1997), LÉVÊQUE (1997), DUPONCHELLE et al. (1998),
ACKNOWLEDGEMENTS
DUPONCHELLE & LEGENDRE (2001) and PANFILI et al.
(2004) in other West African aquatic systems.
This research has been financially supported by the Belgium
Development Cooperation (DGCD, Biodiversity and Aquacul-
It is clear from the drop in monthly G.S.I that spawning
ture of Catfishes in Benin' Project). Our gratitude goes to all the
of S. schall and S. nigrita in the Ouémé River occurs from
fishermen for their cooperation and assistance. We are also
August to October which coincides with the flooding
grateful to the anonymous referees for their constructive reviews
period of the river. This, however, is not in agreement
of this manuscript.
with other observations on fish reproduction ecology in
other floodplain environments. WELCOMME (1979) and
REFERENCES
BARAS & LALÈYÈ (2003) indicated that the majority of
floodplain fish species initiate spawning after horizontal
ALBARET, J.J. (1982). Reproduction et fécondité des poissons
flooding has begun, i.e. well into the rainy season. In
d'eau douce de Côte d'Ivoire. Revue d'Hydrobiologie Tropi-
Lake Chad, the spawning activity for S. schall occurs
cale, 15 (4) : 347-371.
from mid-July through September (BLACHE, 1964). The
AWACHIE, J.B.E. & H.M.G. EZENWAJI. (1981). The importance
different environmental and climatic conditions in the
of Clarias spp in the fisheries developpement of the Anam-
habitats could explain the discrepancies observed. As
bra river basin, Nigeria. CIFA Technical Paper, 8 : 212-224.
conditions change during the year with the change in sea-
ARAOYE, P.A. (1999). Spatio-temporal distribution of the fish
son, the suitability of the environment for the vulnerable
Synodontis schall (Teleostei : Mochokidae) in Asa lake,
early life-history stages will vary. A fish should reproduce
Ilorin, Nigeria. Revista de Biologia Tropical, 47 : 1061-
at that time of year that will tend to maximize its lifetime
1066.
production of offspring. The larval fish must hatch into a
BAGENAL, T.B. & E. BRAUM (1971). Eggs and early life history.
In : R
world that can provide appropriate food, protection from
ICKER W.E. (ed), Methods for assessment of fish pro-
duction in freshwaters (2nd ed.). International Biological
predators and benign abiotic conditions (WOOTTON,
Programme, 3. Oxford and Edinbourgh. pp 166-198.
1994).
BAIJOT E., J. MOREAU & S. BOUDA (eds) (1997). Hydrobiologi-
The two species have different fecundities. Though, S.
cal aspects of fisheries in small reservoirs in the Sahel
nigrita produces more oocytes compared to S. schall,
region. C.T.A. Publ. 252 p.
when comparing the smallest sizes at maturity, egg pro-
BARAS, E. & P. LALÈYÈ (2003). Ecology and Behaviour of Cat-
fishes. In : A
duction by S. schall out-numbers that of S. nigrita after
RRATIA G., B.G. KAPOOR, M. CHARDON & R.
DIOGO (eds), Catfishes. Vol. 2, Science Publishers, Inc.
reaching the size of 20 cm TL. These values varied
USA. pp 525-579.
greatly from counts given for S. schall by OFORI-DANSON
BISHAI, H..M. & Y.B.A. GIDEIRI (1965a). Studies on the biology
(1992) from Kpong-Headpond (14000-165000 eggs),
of the genus Synodontis at Khartoum. 1. Age and growth.
NAWAR (1959) from the Nile River (7000-130000 eggs),
Hydrobiologia, 26 : 85-97.
Synodontis biology in Ouémé River
201
BISHAI, H.M. & Y.B.A. GIDEIRI (1965b.) Studies on the biology
LÉVÊQUE, C. (1997). Biodiversity dynamics and conservation :
of the genus Synodontis at Khartoum. 2. Food and feeding
the freshwater fish of tropical Africa. Cambridge University
habits. Hydrobiologia, 6 : 98-113.
Press, United Kindom, 438p.
BISHAI, H.M. & Y.B.A. GIDEIRI (1968). Studies on the biology
NAWAR, G. (1959). On the fecundity of the Nile catfish, Syno-
of the genus Synodontis at Khartoum. III. Reproduction.
dontis schall (Bloch-Schneider, 1801). Sudan Notes and
Hydrobiologia, 31 : 193-202.
Records, 40 : 139-140.
BLACHE, J. (1964). Les poissons du bassin du Tchad et du bassin
OFORI-DANSON, P.K. (1992). Ecology of some species of catfish
adjacent du May Kebbi. Etude systématique et biologique.
Synodontis (Pisces : Mochocidae) in the Kpong Headpond in
Paris. Cahiers Mémoires ORSTOM 4 (2) : 1-485.
Ghana. Environnemental Biology of Fishes, 35 : 49-61.
COLOMBANI J., J. SIRCOULON, F. MONOD & J. RODIER (1972).
OFORI-DANSON, P.K., G.J. DE GRAAF & C. VANDERPUYE (2002).
Monographie du delta de l'Ouémé (tome 1). Rapport
Population parameter estimates for Chrysichthys auratus
O.R.S.T.O.M., Hydrol., France, Bondy, 200p. (Ronéo).
and C. nigrodigitatus (Pisces : Cloroteidae) in Lake Volta,
DAGET, J. (1954). Les poissons du Niger supérieur. Dakar.
Ghana. Fisheries Research, 54 : 267-277.
Mémoires I.F.A.N, 36 : 1-391.
OLATUNDE, A.A. (1989). Some aspects of the biology of Syno-
D
dontis schall (Block-Schneider). Journal of Aquatic
AGNELIE, P. 1973. Théorie et méthodes statistiques. Press.
Agron. de Gembloux, 1, 378 p.
Science, : 49-54.
D
OLOJO, E.A.A., K.B. OLURIN & O.J. OSIKOYA (2003). Food and
UPONCHELLE, F., L. POUYAUD & M. LEGENDRE (1998). Evi-
dence effects on reproductive characteristics of Nile tilapia
feeding habits of Synodontis nigrita from the Osun River,
(Oreochromis niloticus) populations from man-made lakes
SW Nigeria. Naga, WoldFish Center Quaterly, 26 (4) : 21-
of Ivory Coast. Aquatic Living of Resources 11 (3) : 137-
24.
144.
PANFILI, J., A. MBOW, J.-D. DURAND, K. DIOP, K. DIOUF,
D
D. THIOR, P. NDIAYE & R. LAË (2004). Influence on the life-
UPONCHELLE, F. & M. LEGENDRE (2001). Rapid phenotypic
changes of reproductive traits in response to experimental
history traits of the West African black-chinned tilapia
modifications of spatial structure in Nile tilapia (Oreo-
(Sarotherodon melanotheron) : Comparison between the
chromis niloticus). Aquatic Living of Resources, 14 : 145-
Gambia and the Saloum estuaries. Aquatic Living Resources,
152.
17 : 65-74.
F
PAUGY, D. (1994). Ecologie des poissons tropicaux d'un cours
AGADE, S.O. & C.I.O. OLANIYAN (1974). Seasonal distribution
of the fish fauna of the Lagos lagoon. Bulletin de l'I.F.A.N.
d'eau temporaire (Baoulé, haut basin du Sénégal au Mali) :
Tome I Série A : 45-67.
adaptation au milieu et plasticité du régime alimentaire.
Revue Hydrobiologie Tropicale, 27 : 157-172.
HÅKANSON, L. & V.V. BOULION (2002). The lake foodweb :
P
modelling predation and abiotic/biotic interactions.
OLL, M. (1971). Révision des Synodontis africains. Annales des
Sciences zoologiques du Musée royal de l'Afrique centrale,
Backhuys Publishers, Leiden, 344 p.
Tervuren, 191 : 497.
HALIM, A.I.A. & S.A. GUMA'A (1989). Some aspects of the
SCHOENER, T. W. (1970). Non-synchromous spatial overlap of
reproductive biology of Synodontis schall (Bloch-Schneider,
lizards in patchy habitats. Ecology, 52 : 408-418.
1801) from the White near Khartoum. Hydrobiologia 178 :
TESCH, W. (1971). Age and growth. In : RICKER W.E. (ed), Meth-
243-251.
ods for assessment of fish production in fresh waters (2nd
HYSLOP, E.J. (1980). Stomach contents analysis- a review of
ed.). International Biological Programme. Oxford and Edin-
methods and thier application. J. of Fish Biol., 17 : 411-429.
bourgh. 97-130.
LAË R. (1997). Effect of climatic changes and development on
TWEDDLE, D. & J.L. TURNER (1977). Age, growth and natural
continental fishing in West Africa : the examples of the Cen-
mortality rates of some cichlid fishes of lake Malawi. Jour-
tral delta of the Niger in Mali and coastal lagoons in Togo.
nal of Fish Biology, 10 : 385-398.
In : REMANE K. (ed), African Inland fisheries, aquaculture
WALLACE, J. (1981). An assessment of diet-overlap indexes.
and environment. FAO, Italy. pp 66-86.
Transaction of American Fisheries Society. 110 : 72-76
LAGLER, K.F. (1971). Capture, sampling and examination of
WELCOMME, R.L. (1979). Fisheries Ecology of Floodplain Riv-
fishes. In : RICKER W.E. (ed), Methods for assessment of fish
ers. Longman. 317p.
production in freshwaters (2nd ed.). International Biological
WELCOMME, R.L. & B. DE MERONA (1988). Fish communities of
Programme, 3. Oxford and Edinbourgh : 7-44.
rivers. In : LÉVÊQUE, C. M.N. BRUTON, G.W. SSENTONGO
LALÈYÈ, P., J.C. PHILIPPART & P. PONCIN (1995). Biologie de la
(eds), Biologie et écologie des poissons d'eau douce afric-
reproduction de deux espèces de Chrysichthys (Siluriformes,
ains. ORSTOM : 251-276.
Bagridae) du lac Nokoue et de la lagune de Porto-Novo au
WILLOUGHBY, N.G. (1976). The buoyancy and orientation of the
Bénin. Journal of African Zoology, Vol. 109(3) : 213-224.
upsidedown catfishes of the genus Synodontis (Pisces : Sil-
LALÈYÈ, P., A. CHIKOU, J.C. PHILIPPART, G. TEUGEULS &
uroidei). Journal of Zoology, 180 : 291-314.
P. VANDEWALLE (2004). Etude de la diversité ichtyologique
WILLOUGHBY, N.G. (1979). Some aspects of the ecology of Syn-
du bassin du fleuve Ouémé au Bénin (Afrique de l'Ouest).
odontis (Pisces Siluroidei) in Lake Kainji during its early
Cybium, 28 : 329-339.
years. In : Proceedings of the International Conference on
LAUZANNE, L. (1988). Les habitudes alimentaires des poissons
Kainji Lake and River Basins Development in Africa, Vol. 2.
d'eau douce africains. In : LÉVÊQUE C., M.N. BRUTON &
Kainji Lake Research Institute, Ibadan : 376-386.
G.W. SSENTONGO (eds), Biologie et écologie des poissons
WINEMILLER, K.O. & L.C. KELSO-WINEMILLER (1996). Compar-
d'eau douce africains. ORSTOM, Paris : 221-242.
ative ecology of catfishes of the Upper Zambezi River. Jour-
LE CREN, E.D. (1951). The length-weight relationship and sea-
nal of Fish Biology. 49 : 1043-1061.
sonal cycle in gonad weight and condition in the perch,
WOOTTON, J.R. 1994. Ecology of Teleost Fishes. In : CHAPMAN
Perca fluviatillis. J. of Animal Ecol., 20 : 201-219.
& HALL (ed). Fish and Fisheries Series 1. (4th ed). 404 p.
Received: April 14, 2005
Accepted: June 30, 2006

Document Outline