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 : [email protected] / [email protected] 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 The Ouémé River (Fig. 1) is the largest fluvial basin of many West African countries (OFORI-DANSON, 1992; Bénin, with a catchments’ area of 50000 km², extending OFORI-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- pling station is situated on the Okpara tributary river at (LALÈYÈ et al., 2004). Previous work on the genus in West Kpassa village (09°17’N – 02°43’E). The second sam- Africa has been carried out by DAGET (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, OFORI–DANSON 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, W which blows from November to April, accentuates the Kn = ------------- × 100 b thermic and hygrometric amplitudes. The rainy season aTL 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° ué mé discernible to eye; ovaries occupy about two-thirds of R i ve central cavity; anterior testis whitish with short fingerlike r 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 O O u k é p m light green in colour, completely round and fall from 9° a N I G E R I A é r a ovary with little pressure; eggs oocytes diameter (meas- River 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- Z o u C o rity (L ) was defined as the length at which 50% of the u 50 f fo 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- 7° M o LO KOSS A Agonli n ously determined reproductive season to avoid bias in o #S Lowé $ classifying resting females as immature (PANFILI et al. Lac PORTO Nokoué #S NOVO 2004). The gonado-somatic index (GSI) was calculated #S 0 100 Km AGLER COT ONOU based on the formula suggested by L (1971) which OCEAN ATLANTIQUE is expressed as : 1° 2° 3° Gonad weight (g) Fig. 1. – Map of the study area showing the location of some GSI = × 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.