Of 13 reservoirs exceeding 300 km2 in surface area four are found in Cameroon. Lagdo Lake (the largest) is found in the basin of the Niger River, Mbakaou and Bamendjing are in the Sanaga River system, and the Maga Reservoir is situated in the Lake Chad basin. Knowledge on the different lakes is variable; their geographic positions are presented in Figure 2.1.
The inflowing as well as the outflowing river of Lake Lagdo is the Benue River. The Benue River has its origin in the Adamaoua Mountains in central Cameroon and has a total length of approximately 1 400 km. The river discharges into the Niger River in Nigeria. The upper course was impounded at Lagdo in 1982 for hydro-electric power generation, irrigation and fisheries.
A pre-impoundment study was executed in 1980. Fisheries potential of the stabilized future lake was estimated at between 2 800 and 3 000 t/yr, applying a production figure of 55–60 kg/ha (De Kimpe, 1980a). De Kimpe realized that during the initial phase higher yields could be obtained. He estimated that the yield of 3 000 t could be realized by 1 000–1 500 fishermen. Over 100 species of fish of all feeding categories (insectivores, predators, omnivores, plant feeders, deposit feeders and ‘Aufwuchs’ feeders) occur in the Benue River; therefore De Kimpe considered it unnecessary to introduce additional species. He pointed out the possibility that small clupeids of 3–5 cm length could contribute to the total fish production considerably.
In 1981 the number of fishermen that would be attracted to the lake was adjusted to 1 600 (De Kimpe, 1981). De Kimpe took into account that this number could be exceeded due to a sudden spontaneous development of the fishery, as was observed in the Maga Reservoir.
The 40 m high dam was designed in such a way that the Full Supply Level (FSL) of the lake would be at 216 m above sea level. The shape of the reservoir is presented in Figure 2.2. Total surface area would be of the order of 700 km2 and the volume about 7.7 km3, while the maximum depth would be 29 m; the mean depth was estimated at approximately 11 m. The annual variations in water level for the period 1983–1987 are presented in Figure 2.3 (after Zwollo, 1988).
The FSL was reached unexpectedly in 1988, in fact, and all available sluices had to be opened to drain the excess water. With the strong influx of water the FSL was exceeded slightly. The amount of water spilled was such that people living downstream were convinced that the dam had broken. Timely opening of the emergency spillways had averted the danger of the dam cracking. The water level rose 8 m in about one month! Cultivated land and all kinds of infrastructure disappeared under water.
Water samples at Lagdo village were analysed and results summarized by SOGREAH (1983) are presented in Table 2.1.
Before the construction of the dam at Lagdo an interesting river and floodplain fishery existed, employing a large variety of fishing gear, e.g. beach seines, gillnets, weirs, traps, castnets, longlines and other traditional artisanal gear (Stauch, 1966).
Before the impoundment fishermen from Nigeria used to fish seasonally in the Cameroonian part of the Benue. After the completion of the dam the lake attracted fishermen from Nigeria, Chad, Mail and the Central African Republic, who then settled down around the lake. They constructed new plywood canoes on the spot. The Cameroonian fishermen continued fishing in their dugout canoes. Ngwa reported that fishermen who arrived at Lagdo Reservoir used to fish on the Benue River and its tributaries, Lake Chad, Logone River, Maga Reservoir and the River Niger. A considerable portion of the newcomers had an agricultural background (39%).
In 1990 the main gear used on the lake were gillnets (various mesh sizes), longlines (baited and unbaited), traps and castnet. The use of beach seines is forbidden. Dugout as well as plywood canoes are used; some fishermen use large floating gourds (calebas) as boxes for longlines or even as rafts. Few fishermen go out fishing on foot.
Generally, crews consist of two fishermen, however, a range from one to three was observed. In 1990 the total number of fishing enterprises (which equals the number of masterfishermen) was of the order of 1 960, while the number of assistants was 1 909. Approximately 10% of the enterprises operate motorized canoes (Van der Knaap et al. 1991a).
The number of fishermen peaked in August 1991 when the total reached 2 348. One year later the number of fishermen was 2 174. In December 1992 Nigerian fishermen without stay or work permits were forced to leave Cameroon which resulted in 1 852 masterfishermen in February 1993 (Postma et al., 1993b). The evolution of the number of fishing enterprises since construction of the dam is presented in Figure 2.4.
The breakdown by nationality of masterfishermen is presented in Figure 2.5. The 1 960 masterfishermen of five different nationalities represent 51 different ethnic groups (Van der Knaap et al., 1991 a). The different groups were clustered into five sub-groups by Hartsuyker (1988); this subdivision was based on fishing methods used. He recognized the Haussa/Kabawa, Djoukoun/Agatou fishermen from the Far North, local fishermen and those from Mali. The numbers of fishermen, boats and gear of each of the sub-groups in 1990 are presented in Table 2.2.
22.214.171.124 Annual yield
Balarin (1985) quotes some figures from various studies by SOGREAH (1983), Laclavère and Loung (1980) and Aubray (1976). The fisheries potential from the Cameroonian part of the Benue River was estimated at 1 450 to 3 000 t (average 2 125 t). The total annual catch was of the order of 3 000 t in 1966 (Lagoin and Salmon, 1969).
Hartsuyker (1988) calculated the fish production for the period June 1985-June 1986; this resulted in a total of 7 656 t. Total production increased sharply in the period July 1986-June 1987 to a total of 13 404 t. From July 1987 to June 1988 the production dropped to 12 876 t (Anonymous, 1988).
In 1988 the lake water reached its maximum level at 216 m above sea level. Ngwa (1990) then arrived at a figure of 3 103 t for 1989. The reason for the drop in production would have been the enormous amount of water spilled. However, this figure was premature and based on gillnet fishery only. Further, Ngwa had used a method suggested by Malvestuto et al. (1978). A preliminary analysis using the same method as Hartsuyker's resulted in an estimated production between 10 000 and 11 000 t for 1989 (Van der Knaap, 1990).
The total yield in 1990 was assessed at 12 289 t (Van der Knaap et al., 1991b). The annual production by enterprise was calculated, based on the available frame survey data. The results are presented in Table 2.3. An interesting exercise was to calculate the number of enterprises per unit area. In 1983 an average of 1.6 enterprises per km2 was observed, which increased to 4.3 in 1987 and then decreased to 3.3 in 1990. With data on estimated total surface the fish production/ha could be calculated (Table 2.4) and it appeared that the production was of the order of 339 kg/ha in 1986/1987 and decreased to 205 kg/ha in 1990 (Van der Knaap et al., 1991b) and to 178 kg/ha in 1991 (Postma et al., 1993a). Kapetsky (1986) mentions a range of annual yields for African reservoirs between 27 and 65 kg/ha.
Taking into account that an average of over 200 kg/ha is rather high, another way of assessing the total yield was employed (compare for instance the situation in Lake Kainji, Nigeria (Ita, 1984), where the annual production dropped from 225 kg/ha in 1970 to 87 kg/ha in 1971, respectively two and three years after the completion of the dam). The infrastructure around Lake Lagdo was only moderately developed until (at least) 1990 and therefore the majority of the production, both fresh and processed, was marketed at Lagdo. After a six months' survey, during which the quantities of processed fish were estimated, it was concluded that at least 9 750 t left from Lagdo in 1990 (equivalent of fresh fish). MINEPIA (Ministry of Husbandry, Fishery and Animal Industries) independently collects information. Basic data are presented in Figure 2.6. Quantities of processed fish peaked clearly in October 1989 and from August to October 1990. It is felt that the data do not reflect the real situation. Various sources could be indicated from which fish was transported to different markets, in and outside Cameroon. This unknown quantity could account for the gap between fish marketed (9 750 t) and sampled (12 289 t) in 1990 (Van der Knaap et al., 1991b). Thus the high fisheries productivity of the reservoir is substantiated. Postma et al., (1993a) estimated the total production at 10 675 t in 1991, a reduction of 13% as compared to 1990. It was observed that the fishermen tended to use smaller mesh sizes than before.
126.96.36.199 Catch rates
Since certain fishermen use more than one type of gear, the CPUE (Catch per Unit of Effort) for gillnet was calculated for those fishermen who were observed to uniquely operate gillnets. Mean CPUE values peaked in June/July 1990 to about 20–21 kg/net/night. Monthly catch rates are presented in Figure 2.7. A difference appeared to exist between gillnet catch rates of motorized and unmotorized canoes. The peak in May and June (see Figure 2.8) shows values of 29 and 37 kg/net/night, well over the peak value for all gillnets.
Catch rates were calculated for different mesh sizes of gillnets. The gillnets were grouped into three categories: mesh between 40 and 100 mm; and between 100 and 130 mm; and between 130 and 223 mm (stretched). Small-meshed gillnets showed lowest catch rates (less than 10 kg/net). The highest CPUE values were observed for nets with mesh of 100 to 130 mm, with a clear peak in June/July 1990. The data set for the largest mesh was too small for firm conclusions. Monthly catch rates are presented in Table 2.5.
188.8.131.52 Fishing effort
Information on fishing effort around Lake Lagdo is based on interviews. Fishermen generally claim to fish six or seven days per week. In practice, however, this value is lower. The high yields on Lake Lagdo still attract fishermen (see Table 2.3). Although the absolute number of fishermen is still growing, density decreased due to the increase in water level (see Table 2.4). According to Henderson and Welcomme (1974) the optimum number of fishermen per unit area is two fishermen/km2. In the case of the Lagdo fishery the density of enterprises was 3.3/km2 in 1990, which implies 6.4 fishermen/km2. This figure largely exceeds the theoretical optimum, but so, too, does the productivity.
184.108.40.206 Mesh selectivity
During the Catch Assessment Survey carried out in 1990 information was collected on mesh sizes used. Figure 2.9 shows the frequency of mesh sizes in 10 mm groups. Data are presented by ethnic groups. It is clear that the 110 mm (4 inch) mesh (stretched) is the most common mesh encountered. The figure shows ethnic differences, e.g. between the Djoukoun/Agatou and the fishermen from Mali. Length frequencies, collected during exploratory fishing under controlled conditions, of S. galilaeus, showed an interesting picture. Figure 2.10 shows observations for 3- and 4-inch mesh nets. The size ranges were surprisingly wide (13 to 34 cm for 3-inch nets and 21 to 35 cm for 4-inch nets).
220.127.116.11 Species composition for different gillnets
It is evident that different mesh sizes catch different size and species groups. The dataset allowed calculation of species composition for three different categories of mesh size. Table 2.6 shows that catches of nets with mesh between 40 and 100 mm consist of 20% Sarotherodon; those of nets with mesh between 100 and 130 mm consist of 88% Sarotherodon; catches by nets with mesh exceeding 130 mm is approximately 58%. Hydrocynus species compose 21% of catches by large meshed nets, while it is less than 1% in smaller meshed nets. A similar situation is found for Citharinus citharus. The inverse is valid for Chrysichthys and Labeo species: a high percentage in catches of small meshed nets and a much lower one in the bigger meshed nets.
18.104.22.168 Small clupeids
The first fishing trials for small clupeids were executed in the period November 1985-June 1986 during the first phase of the EC-funded Lagdo Fisheries Development Project. The species concerned is Pellonula miri; identification was confirmed by the Museum of Natural History of Tervuren, Belgium (Dr Thys Vanden Audenaerde). Light fishing was carried out using various types of net. Results were not very promising. Mean yields of 138, 90 and 72 grams/hour were obtained for three different nets respectively (HASKONING, 1986).
A second series of trials was performed in 1990, during the second phase of the Lagdo Fisheries Development Project. A small-meshed seine net was used and yields of over 12 kg/hour were recorded (Lont, 1991a). The use of an entirely different net makes the results hardly comparable with the yields from the first phase.
In the period March-May 1993 fishing trials were carried out using a small trawlnet and gillnets to catch Pellonula miri to determine its density, relative abundance and biology (Nomo et al., 1993). Nomo and co-workers estimated a potential production of 600 t/yr which corresponds with 10 kg/ha/yr. This estimate should be dealt with cautiously since research was carried out during three months only. From May to August 1991 experimental fishing with liftnets was carried out, which resulted in mean yields of 49.1 kg/night (Lont, 1991b). Lont assumed that minimum and maximum yields in other African man-made lakes could be applied to the situation in Lagdo. This would result in figures of 600 and 4 740 t/yr (viz., in cases of 10 and 79 kg/ha/yr and a mean surface of 600 km2). It is too early at this stage to predict any realistic potential yield figures. More research would be required to determine the potential. According to Postma (pers. comm.) the seine has been transformed into liftnets and in 1992 a limited number of fishermen picked up this new technique. On one occasion 1.8 t of clupeids were caught by one team of fishermen during one night. Apparently there is a market for their smoked/dried product.
Two types of fishing gear commonly used before impoundment have been forbidden: beach seine and castnet (at present the castnet is still used to catch small tilapias as bait for longlines). During periods of poor catches some ethnic groups apply the ‘water beating’ method. The fishermen then drive the fish into the gillnets by splashing and beating the water surface with sticks. Traditional chiefs (‘djauro’), when convinced of the need for management measures, may forbid this method during certain periods. Fishermen are very inventive and instead of the water beating technique they apply the outboard-engine technique. This is a technique used to chase fish into the nets by producing much noise and disturbance at the surface. No other examples of enforcement are known; however, the influence of the chiefs is considerable and could be called upon if management measures are to be taken.
The following management measures were suggested by Van der Knaap et al. (1991b): the sale of nets with mesh smaller than 100 mm (stretched) should be stopped immediately and the number of outboard engines already present on the lake should not be increased.
Postma et al. (1993a) suggested the consideration of 11 species of fish that contribute more than 1% to the total catch and that a minimum size be determined for each species. They furthermore proposed to monitor sizes of fish at Lagdo market. They recommended a minimum mesh size of 4 inches (stretched).
The river has its origin in Cameroon and forms a complex river system with the Chari River along the border between Cameroon and Chad. Its floodplain, Yaèrès, may have a total surface of 4 600–6 000 km2. The Maga Reservoir is fed with water flowing through a diversion canal from the Logone River. Purposes of the dam were irrigation for rice production, and fisheries.
No data were available on pre-impoundment studies. In the first year after impoundment a study was carried out to investigate the composition of the fish population that originated from the Logone River (De Kimpe, 1980b). De Kimpe mentioned in his report that a preliminary study had been made in 1974. Details on this study were not available to the author.
The purpose of the 10 m high dam (dating from June 1979) was to stock water for irrigation of rice fields. At the end of the dry season the lake dries up almost entirely. Its maximum surface is around 360 km2 at an altitude of 312.5 m. The results of water analyses from the Logone River are presented in Table 2.1. At low water (altitude 310.8 m) the surface is approximately 120 km2 (Figure 2.11). In the first year after impoundment the reservoir was fed by the Logone River through a canal at Djafga in the period June-January. The filling took place in the following intervals: from 29 June to 30 July 1979, from 8 to 28 August, from 25 September to 29 November, and finally from 29 December 1979 to 9 January 1980. De Kimpe (1980b) recommended that the use of seines be halted and that the canal at Djafga, when open, should be free of any fishing gear in order to allow fingerlings and juvenile fish to enter the lake.
Fishing activities take place during daytime. Gillnets are set, but often they are used as seines or surrounding nets. At the end of the dry season when the water level drops, fishing effort is increased enormously and fish are easily caught in small pools. Catches are sold fresh in Maga; however, the majority of the catches are transported to Maroua (Dawila of Fisheries Station Maga, pers. comm.). De Kimpe (1980b) reported that 14 seines were used on the lake, with mesh of 10 mm (knot-to-knot) and a height of approximately 4 m. The seines have a length of about 300 m.
The number of fishermen more than doubled in the period 1986–1991. The numbers of canoes and nets did not increase proportionally. In 1988 an important increase in the number of longlines and traps was observed. Frame survey results for the study period are summarized in Table 2.7.
22.214.171.124 Annual yield
Independent yield estimates are presented in Table 2.8 (Dr Baba Malloum Ousman, pers. comm.). It may be observed that the production was low in 1986 and that it peaked in 1988. An important drop was observed in 1990 when production fell from 3 585 to 1 161 t. The yield in 1992 was well within the range of the potential yield estimates, although it is believed that, due to incomplete coverage, the production is underestimated (Dr Baba Malloum Ousman, pers. comm.). Vanden Bossche and Bernacsek (1991) summarized available information from Balarin (1985) who had quoted, in turn, various other authors. Their figures concerning fishery potential are presented in Table 2.9. Existing predictive models for yield estimation were reviewed by Crul (1992) and his estimates are also presented in Table 2.9 for comparison. Estimations of potential annual yields range from 1 500 to 1 870 t.
126.96.36.199 Catch rates
De Kimpe (1980b) reported seine catch rates of 50 to 100 kg per operation in May 1980. Experimental gillnet fishing operations were too small in number to draw firm conclusions. De Kimpe experimented with 25, 35, 40 and 60 mm meshed gillnets. He used nets of different sizes but adjusted catch rates for nets of 100 m2 that were operated for 12 hours. The results are summarized in the following table:
|Net mesh size||Weight in grams per 100 m2/12 hrs|
|Experiment 1||Experiment 2|
188.8.131.52 Fishing effort
The only figure available is the number of seines operated in 1980: 14.
184.108.40.206 Mesh selectivity
No data available.
220.127.116.11 Species composition
The species composition (percentage of total number) of seine catches in May 1980 was as follows (after De Kimpe, 1980b):
|Tilapia (mainly Oreochromis niloticus)||28||%|
|Lates and other||2|
Tilapia contributed 50% to seine catches and Alestes 25% in terms of weight.
De Kimpe summarized a list of species encountered in the Maga Reservoir or Logone River. The list is presented in Table 2.10.
When the water level is low at the end of the dry season fish are easily removed from the small pools. To prevent fish being wiped out during the dry season MINEPIA (Ministry of Husbandry, Fishery and Animal Industries) imposed a closed season of three months from July to May; this occurred for the first time in 1985. The closure was successful, although some cases of poaching were reported. A minimum mesh size of 2 inches is enforced. Furthermore, all fishermen need to be registered in order to monitor the influx of fishermen.
The Sanaga River has its source in the Adamoua Mountains and has a length of 890 km. The river is fed by a number of tributaries (of which the major ones are Djerem, Mbom and Noun) and was impounded upstream at Edea and Song-Loulov. The Djerem tributary was impounded at Mbakaou and the Noun at Bamendjing. The Sanaga River discharges into the Atlantic Ocean.
No information is available on whether pre-impoundment studies had been carried out. The Mbakaou dam was closed in 1968 and the Bamendjing dam closed in 1975. Their purposes were power generation and fisheries.
18.104.22.168 Mbakaou Reservoir
The Mbakaou dam is 19 m high and the lake's altitude is at 870 m above sea level. Its maximum length and width are of the order of 50 km and 16 km respectively, resulting in a mean surface of approximately 500 km2. Although the reservoir can dry up completely, its volume is 2.6 km3 at Full Supply Level. The annual fluctuation in water level may reach 28 m (Vanden Bossche and Bernacsek, 1990a). Results of water analyses from the Sanaga River are presented in Table 2.1.
22.214.171.124 Bamendjing Reservoir
The height of the dam is 17 m. The lake's maximum length is 32 km, its width 27 km, its surface varies between 150 and 300 km2, and its volume is around 1.8 km3 at FSL (Vanden Bossche and Bernacsek, 1990a).
No data available to the author.
126.96.36.199 Annual yield
Table 2.6 shows estimates of the fishery potential of Bamendjing and Mbakaou of 500 t and 3 500 t respectively. Estimates by Crul (1992) for the two man-made lakes are 1 582 t and 2 530 t respectively.
188.8.131.52 Catch rates
No data available.
184.108.40.206 Fishing effort
No data available.
220.127.116.11 Mesh selectivity
No data available.
18.104.22.168 Species composition
No data available.
22.214.171.124 Small clupeids
No data available.
No information available.