FISHERY RESOURCE BASE FOR THE UGANDA SECTOR OF LAKE VICTORIA

John Obbo Okaronon (SENIOR RESEARCH OFFICER, UFFRO)

Jackson Wadanya (BIOSTATISTICIAN, FISHERIES DPT.)

INTRODUCTION

Lake Victoria which is located around the equator (3° 00'S, 0° 30'N; 31° 40" and 34° 50'E) occupies an area of about 68,800 km² 51% of which lies in Tanzania, 43% in Uganda and 6% in Kenya. The entire lake has a shoreline which exceeds 3200 km, consisting of rocks, boulders, pebble beaches, sand or papyrus (Cyperus papyrus L.).

Lake Victoria had a complex multispecies fishery dominated until the late 1970s by the tilapiine and haplochromine cichlids, but with important subsidiary fisheries of more than 20 genera of non-cichlid fishes such as Bagrus, Clarias, Mormyrus, Barbus, Protopterus, Synodontis, etc. The naturally occurring fish fauna was modified during the 1950s following the introduction of 4 non-indigenous tilapiine species and was further altered about 1957/58 by the introduction of Nile perch (Lates niloticus). Of late, fish stocks in many parts of the Lake are tending to be dominated by the two introduced species L niloticus and O niloticus and one endemic cyprinid Rastrineobola argentea (Okaronon, 1990). Most of the traditional fish species, including the once abundant haplochromines, have either declined or almost disappeared from the lake altogether.

Despite all the above events, developments continue to take place to increasingly exploit the fish resources of Lake Victoria, especially for export. A number of fish processing and/or handling plants have been constructed along the shores of the Lake. More recently (w.e.f. 1st July 1990) the liberalisation measures on trade, especially Border trade, have further increased demand for fish and consequently increased pressure on the stocks.

In view of the clear situation where there is continued increasing demand of fish and given that there is currently little knowledge on the magnitude and potential of the available stocks from which this demand is to be met, it is highly necessary to institute management strategies aimed at exploiting the fish resources of the Lake while conserving the stocks. This paper, therefore, focusses on the “fishery resources base for the Uganda sector of Lake Victoria”. It particularly addresses itself to four aspects (i) a thorough review of the fishery resources base by types of fisheries and exploited fish species; (ii) the known distribution pattern of available stocks by depth and season; (iii) the problems of multispecies exploited by different gears and factors affecting recruitment; and (iv) suggested or recommended suitable methods of exploiting the resources while conserving the stocks.

I. THE FISHERY RESOURCES BASE (BY TYPES OF FISHERIES AND EXPLOITED FISH SPECIES

The tilapiine fishery.

The fishery for tilapiine cichlids is among the oldest fisheries in the Lake. At the beginning of this century the species exploited were mainly Oreochromis esculentus and O. variabilis, both native to the Lake. The catch of O. esculentus contributed 60% by weight of the total annual fish yield in 1908 (Graham, 1929). This figure dropped to 25% in 1958 and to only 8% in 1959. The catch per unit of effort of the accessible stocks, particularly the tilapiine cichlids O. esculentus and O. variabilis, declined from 30 fish per net of 127mm (5 inch) mesh in 1921 to 6 fish in 1928 and 2.9 fish in early 1940s. The above decline in catches followed uncontrolled entry of fishermen into the fisheries in early 1920s.

Exotic tilapiines (O. niloticus, O. leucostictus, Tilapia zillii and T. rendalli) were introduced into Lake Victoria during the late 1950s (EAFFRO 1964, Welcomme 1967). Following these introductions, the species exploited currently is mainly O. niloticus although O. variabilis, O. leucostictus and T. zillii also occur occasionally. The bulk of the commercial catch during 1960s and 1970s was contributed by the tilapiine cichlids (Fig. 1B) particularly O. niloticus. In 1965 the tilapiine cichlids contributed 86% by weight of the annual commercial catch from the Lake; this figure dropped to 23% in 1980 and 23% in 1985 before beginning to rise again to 15.3% in 1989 (Fig.1B).

The Nile perch fishery.

The Nile perch (Lates niloticus) and the Nile tilapia (Oreochromis niloticus) are native species to Lake Albert, the River Nile below Murchison Falls, Lake Turkana, the Chad basin and rivers of West Africa (Lowe-McConnell, 1988). The Nile perch was introduced into Lakes Kyoga and Victoria basins in the mid-1950s but its presence in Lake Victoria was first noted in 1960 (Gee, 1965). Although Lates was introduced into Lake Victoria in the early 1960s it took more than 10 years to get fully established in the new ecosystem.

When the stocks of Lake Victoria fishes were defined during a joint survey conducted by UNDP/FAO and EAFFRO in 1969/71, the contribution of Lates niloticus to the total demersal ichthyomass of the Lake was then insignificant (Table 1); L. niloticus had not shown the success it had by that time shown in Lake Kyoga. Catches of Lates became significant from 1975, 1977 and 1978 in Uganda, Kenya and Tanzania sectors of the Lake, respectively (Fisheries Department Annual Reports, 1960 onwards; Bergstrand and Cordone, 1971; Ssentongo and Welcomme, 1984). Following the establishment of Lates in Lake Victoria, total fish yield increased significantly (Fisheries Department Annual Reports, 1970–1980; Fig. 1A). Lates contributed more than 50% by weight of the commercial catches at most landings around the Lake (Ssentongo and Welcomme, 1984; Fig 1B). The minimum average total fish production in the Uganda sector of the Lake in recent years (1984–1989) has been estimated at 80,000 metric tons per annum of which over 60% was Lates (Fig.1).

At present there is strong evidence from the commercial and experimental fishing that L. niloticus is well established in Lake Victoria. At Masese fish landing near Jinja, for example, the proportion of L. niloticus in the commercial catches increased from 0.4% of the total catch by weight in 1981 to 62.7% in 1983, settling around 50% thereafter (Table 2(a)). Experimental trawl catches from the UFFRO research vessel IBIS in the Jinja area of the Lake show a similar trend in catch composition (Table 2(b)).

Lates niloticus grows to a large size. In its native habitats individuals over 120 kg have been recorded in the Ugandan part of Lake Albert (Kinloch, 1956) and specimens of over 100kg in Lake Chad (Durand and Louben, 1969). In Lake Victoria where it was introduced specimens weighing over 200kg have been recorded in the Musoma area (Bwathondi, 1984).

However, while the total catches from Lake Victoria appear to have been increasing during the 1980s (Fig. 1A) following the explosive increase in the Nile perch stocks, the mean size of the individual fish (particularly Nile perch) caught and/or landed from the Lake continued to decline during the same period (Okaronon, 1990). The mean weight of Nile perch in the commercial landings dropped from about 9kg in 1982 to about 2kg in 1989 (Table 3). In the trawl catches the mean weight of Nile perch fish similarly dropped from about 5 kg in 1982 to less than 1 kg in 1985 (Table 3). The apparently sharp decline in the mean size of L. niloticus in both the trawl and landed commercial catches may be due to the influx of juvenile Lates in the fishery soon after 1983 and the increasing use in the commercial fishery of seine nets and small-mesh gillnets (Okaronon and Kamanyi, 1986).

The Rastrineobola argentea fishery.

The fishery of Rastrineobola argentea is much more developed in the Kenyan and Tanzanian waters of Lake Victoria where the species now ranks second only to Nile perch in commercial catches (Ogutu-Ohwayo et al, 1988; Wandera, 1988). In the Ugandan portion of the Lake, R. argentea locally known as Mukene has until quite recently been minimally harvested. It was not until the decline in catches of many preferred native species that attention shifted to exploiting Mukene. On Lake Kyoga where the introduced Nile tilapia (O. niloticus) is abundant, no fishing for R. argentea was done before 1991 though the lake supports large stocks of this species (Proude, 1963). The increasing demand for Mukene as a cheap source of fish protein and as an ingredient in the animal feeds has encouraged such a rapid growth in its exploitation on Lake Victoria that the species now ranks third to Nile perch and the Nile tilapia in the commercial catches (Fig. 1B).

Rastrineobola argentea first featured significantly in commercial catch records in 1987 when it contributed 2.15% of total catch by weight from the Uganda sector of the Lake (Fig. 1B). In Masese fish landing the contribution of Mukene to the landed commercial catches rose from 0.47% in 1982 to 30.08% in 1989 and was second to Nile perch in 1989 (Table 2).

Mukene fishing employs the use of the 10mm and 5mm mesh nets. Over 70% of Mukene caught by the 5mm nets are immature but the 10mm nets crop mature individuals (Ogutu-Ohwayo et al, 1988, Wandera, 1988). The 5mm nets capture fish ranging from 19mm standard length (SL) while the 10mm nets catch those from 26mm SL to about 60mm SL. The size at first maturity, i.e. the size at which 50% of the fish are mature is 42 mm SL for males and 44mm for females; and males of more than 45mm SL and females of more than 47 mm SL are mature (Wandera, 1989).

The Haplochromine fishery

The Lake Victoria fish community was originally dominated by haplochromine cichlids (Fryer & Iles, 1972; Witte, 1981). These comprised at least 80% of ichthyomass (Kudhongania and Cordone, 1974) classified under about 11 trophic groups which are dominated by phytoplanktivores and detritivores (Greenwood, 1974; Witte 1981).

With the constantly declining catches of the traditional table fish species, the exploitation of the small haplochromines was intensified during the 1960s. This involved the increased use of beach seines. Catches of haplochromines increased from unknown figure in 1950s to 18.75% by weight of total commercial yield from the Lake in 1968, dropped sharply to 2.6% in 1970, rising steadily again to around 10% in 1978/79 before tailing off (Fig. 1B).

The abundance of the haplochromines in the northern part of the Uganda sector of the Lake decreased dramatically from 668kg/hr in 1968/71 to 294 kg/hr in 1982 and down to less than 5kg/hr in 1985 (Table 1). The contribution of the haplochromines in the commercial and experimental catches in the Jinja area of the Lake also decreased drastically during this period (Table 2). Presently the stocks of haplochromines could be considered to have been drastically reduced to very low levels in the inshore waters but not necessarilly wiped out.

Other fisheries

The potamodramous species of Labeo, Schilbe, Alestes, Barbus and Synodontis have almost disappeared from the catches in the main lake. Historically these were the major fisheries in the Lake but are now occasionally caught with gillnets. Labeo victorianus (popularly known as Ningu) formed the most important commercial fish species along the affluent rivers of the Lake Victoria basin (Cadwalldr, 1965 and 1969). This fishery deteriorated steadily following the intensive gillnetting of gravid individuals on their breeding migrations. In Lake Victoria Labeo catches declined from 10.5% of the total landed catch in 1958 to less than 1% in 1970. Over-fishing of Labeo affected 13 other anadromous or anadromous-like fish species (Whitehead, 1959). These included Barbus altianalis, Schilbe mystus, O. variabilis, Alestes spp, Synodontis spp, Clarias mossambicus, Bagrus docmac and Protopterus aethiopicus. However, Bagrus and Protopterus species continued to feature significantly in the commercial fishery (Fig. 1B).

General Observation

Although the actual landed catch figures of the fish from the Uganda sector of the Lake may be questionable, at least the evidence points to increased landings since 1984. However, the continued abundance of the fish resources in this Lake should not be taken for granted given the changing environment and the continued smuggling at landing of large quantities of fish (from Uganda region) in the neighbouring states, among other factors.

With the rising thermocline especially in the 40–80 metres depth zone, there is likely to be some cause for uncertainty in the stocks of certain species in this zone especially the high oxygen-demanding Nile perch. Nile perch and other fish species feed on lake flies which themselves feed on phytoplankton; the production of phytoplankton will itself be affected by changes in the environment. Quite recently the water hyacinth was detected in considerable quantities in the shallow sheltered inshore waters of Lake Victoria where most of the fish stocks inhabit and where the artisanal fishery currently is based. The continued presence of this weed in this zone will obviously affect the stocks and, consequently, the catches in this zone. Floating water weeds such as water hyacinth (Eichhornia crassipes), water fern (Salvinia auriculata) and water lettuce (Pistia stratiotes) in dense mats have been reported to create deoxygenated conditions in tropical lakes especially the man-made lakes, Lakes Kariba and Volta (Little, 1966). Furthermore, the local press recently reported the alarming rate at which the fish from the Uganda sector of the Lake is being smuggled to a neighbouring state. A conservative figure of 10 metric tons is reportedly smuggled daily and the smuggled fish is from waters in between Mukono District in Uganda and the Kenyan border.

The increasing investment in the fish processing industries and the liberalisation of trade, especially the border trade, have resulted in increased pressure on the fish stocks. Almost all the fish processing plants in Uganda are located along the lake shores and each of them is capable of handling more than 10 metric tons of fresh fish daily.

II. DISTRIBUTION OF AVAILABLE STOCKS

The maximum species diversity in Lake Victoria is supposed to be in the shallow inshore areas of the Lake. Kudhongania and Cordone (1974), for example, observed that half of the demersal ichthyomass of the Lake inhabited waters less than 30 metres deep (Table 4, Fig. 2). Most of the commercially desirable species such as Bagrus docmac, Clarias mossambicus, and all the Oreochromis species were abundant in this area and their catch rates decreased with depth (Table 4). Although the haplochromine species were observed to be euribathic, their catch rates were also highest in waters less than 30 metres deep.

In Lake Victoria, trawling surveys during the early 1970s showed that, at that time, Lates was generally limited to waters less than 30 metres deep (Okedi, 1970; Kudhongania and Cordone, 1974; Table 4). Recent trawl surveys in the Mwanza Gulf, however, indicate that Lates might have spread to deeper waters (Kudhongania et al, 1988); trawl catches from the research vessel MDIRIA in the Mwanza Gulf in 1984 indicated catches of 80–150kg/hr for Nile perch in the 50–60 metres depth zone. Further recent observations on the Nile perch suggest that it occurs in virtually every habitat of the Lake, with possible exceptions of rocks, swamps and the pelagic zone (Witte, 1984). According to other authors Lates niloticus was mainly restricted to shallow waters over sandy bottoms where the oxygen concentration is relatively high (Okemwa 1984). Greenwood, (1966) and Hopson (1972) mention mass mortalities of Lates niloticus in Lake Albert and Lake Chad, respectively, which were probably due to low oxygen concentrations. Greenwood (1966) further quoted the experimental work of Fish (1956) which proved that L.niloticus has a relatively high oxygen demand compared to other freshwater fishes. It is, therefore, argued that the occurrence of Nile perch in deep water is (1) possible only during the dry season and periods during which the stratification (for waters > 5 metres deep) normally breaks down (Talling, 1966) > and (2) due to alleged the presence of two species, one preferring shallow water and the other living in deep water.

Since about 1985 Nile perch has predominated in the landed commercial catches all over the Lake with other fish species taking varying positions (Table 5).

Table 5: The order of abundance of the various fish species in the commercial catch landings from the various areas of Lake Victoria (Uganda)

Peak landings of Nile perch and Nile tilapia are recorded during March-June and October-November while peak landings of processed Rastrineobola are obtained during January-March and July-September.

III. THE PROBLEMS OF MULTISPECIES EXPLOITED BY DIFFERENT GEARS AND FACTORS AFFECTING RECRUITMENT

The combined impact of prolonged overfishing, lack of effective management policy, competition among species, damage to the reproductive strategies for a number of important fish species and predation are expected to have had profound detrimental effects on the fisheries of Lake Victoria as these factors generally affected recruitment.

The gradual collapse of the native fisheries has been attributed to the systematic over-exploitation of the inshore stocks through the intensification of the gillnet fishing and progressive decrease in mesh size (Cadwalldr, 1969). At the beginning of this century fishing was for subsistence needs and the fishing effort was intially low, consisting of locally made traps and hooks. The introduction of the more efficient flax gillnets during 1916 stimulated higher catches mainly of Oreochromis esculentus around the entire lake. But uncontrolled entry into the fishery soon resulted in the decline of the catch per unit of effort in the legal 127mm mesh gillnets to unprofitable levels consequently forcing the fishermen to progressively reduce the mesh size of the nets in use. This practice led to localised over-fishing of certain stocks, particularly O. esculentus, to levels so low that they could not recover because of the drastically reduced rate of recruitment.

In 1933 the minimum mesh gillnet of 127mm was implemented for Lake Victoria with the aim of protecting the declining O. esculentus fishery. Because of management and logistical problems, given the trinationality status of the Lake, Tanzania and Uganda in 1956 and Kenya in 1961 repealed this restriction. The justification for lifting the regulation were (1) that in a multispecies fishery where the various species mature at different size ranges, a management policy based mainly on single species posed constraints and (2) that the move would enable the capture of the relatively smaller fish species (O. variabilis, mormyrids, etc.) to make up for the declining O. esculentus catches. Although the relaxation of the mesh regulation initially led to short-lived increase in catches of fish, especially O. variabilis, the move resulted in the increased cropping of immature fish. The continued removal of immature fish from the Lake resulted in increasingly reduced recruitment to the breeding stocks of the species affected and this eventually led to the collapse of certain species including O. esculentus. As already stated, overfishing for Labeo through intensive gillnetting of the gravid individuals on their breeding migrations affected 13 other anadromous or anadromous-like fish species (Whitehead, 1959).

With the constantly declining catches of the traditional table fish species the exploitation of the small but abundant haplochromines was intensified during the 1960s. This involved the increased use of the beach seines. Unfortunately beach seines have damaging effects on the haplochromines and tilapiines, especially to their eggs and fry and to their breeding and nursery grounds (Welcomme, 1964). The 10mm and 5mm mesh seine nets are being used for the exploitation of Rastrineobola argentea. These seine nets have been observed to take heavy tolls of juveniles of tilapiines and Nile perch (Wandera, 1988) in addition to their own juveniles; they also destroy and/or disturb the breeding and nursery grounds.

Another management measure, expected to replenish the declining catches due to over-exploitation, was the introduction of four tilapiine species (O. niloticus, O. leucostictus, Tilapia zillii and T. rendalli) into Lakes Victoria and Kyoga (EAFFRO 1964, Welcomme, 1967). The establishment of four exotic tilapiines into the ecosystem suddenly increased interspecific competition with the two indigenous species (O. esculentus and O. variabilis) and enhanced the likelihood of genetic dilution due to hybridization (Lowe, 1958, Welcomme 1967). Competition and/or hybridization appear to have been instrumental in accelerating the decline in tilapiine stocks in favour of only one exotic species (O. niloticus).

Mention must also be made of the predation by the voracious Nile perch which has had Obvious effects on other species especially the haplochromines. Nile perch larger than 50cm total length is reportedly dependent on a piscivorous diet (Ogutu-Ohwayo, 1988).

IV. SUGGESTED/RECOMMENDED SUITABLE METHODS OF EXPLOITING THE RESOURCES WHILE CONSERVING THE STOCKS.

Nile tilapia are 50% mature at 26cm total length (TL) and harvested by the 101.6mm (4 inch) mesh gillnets. The harvest of Nile tilapia from the 127mm (5 inch) mesh nets is over 75% mature. For Nile perch the males mature at a smaller size than the females. The size at which 50% of the fish are mature is 50cm TL for males and 95–110cm TL for females (Ogutu-Ohwayo, 1988). Nile perch after 50cm TL shifts to a piscivorous diet.

Gillnets are the major fishing gear used by fishermen in the Uganda sector of the Lake. Other gears generally used include seine nets, cast nets and hooks on long lines. Gillnets of mesh sizes ranging from 101.6mm (4 inch) to 304.8mm (12 inch) were in common use during January-March 1989 (Okaronon and Kamanyi, 1989). The most popular nets in use were the 203.2mm (8 inch) mesh (45.5% of the total number and in 32.4% of the fishing canoes) and 127mm mesh (22.8% of the total number and in 33.8% of the fishing canoes); these were used for catching Nile perch and Nile tilapia. The 127mm mesh nets retained Nile perch of 53cm mean TL and Nile tilapia of 31cm mean TL (Okaronon and Kamanyi, 1989).

The mosquito nets of 10mm and 5mm mesh are used to harvest Rastrineobola species. The nets are used as beach seines and may catch many juveniles of Nile perch, Nile tilapia and other fish species. They are also used as Lampara lift nets in offshore fishing. In the Lampara fishing method minimal effect on the juveniles of other fish has been recorded except at certain periods when these juveniles leave the inshore waters for offshore waters (Wandera, pers.comm.).

Suggested recommendations

1. The minimum gillnet mesh size should be 127mm (5 inch).

   This regulation is aimed at

   • protecting the Nile tilapia.

   • harvesting the Nile perch above 50 cm TL because above this size the species becomes increasingly dependent on a piscivorous diet. As a management measure and in order to reduce predation pressure on other species, it is recommended that the species should be selectively cropped at the stage when it feeds more on other fishes (Ogutu-Ohwayo, 1984). Although gillnets of 127mm mesh catch many immature Nile perch especially among the females, the species has a very high reproductive potential (Ogutu-Ohwayo, 1988).

2. There should be no beach seining at all. It is noted that beach seining destroys the nests of the fish in their breeding areas and catches many juvenile fish. Rastrineobola should be exploited by the Lampara lift net operated offshore.

3. Trawling should be prohibited until the status of the offshore stocks has been reviewed during the stock assessment exercise to be carried out as soon as the research vessels have been repaired/reconstructed. It is, however, to be noted that

   1. even if trawling was allowed in waters of depths greater than 20 metres, there would not be any means of ensuring that the trawlers would not operate in waters less that 20 metres deep;

   2. Codend meshes of trawl nets usually smaller than that set for the gillnets in (a) above, thus these trawlers will catch even smaller immature fish;

   3. stocks in offshore waters are unknown. The trawler may not leave inshore waters with fish to go to offshore waters where little or no fish may be caught; and

   4. trawls dragged at the bottom destroy the breeding grounds of the fish.

ACKNOWLEDGEMENTS

We are greatly indebted to Dr. T.Twongo and Mr. Mbahinzireki of UFFRO for their constructive criticisms and useful suggestions. A number of other UFFRO scientists and staff made significant contributions towards the successful preparation of this paper. Our great thanks also go to Mr. Osako of Uganda Electricity Board for the preparation of the figures and Mrs. Ruth Byekwaso for kindly typing the manuscript.

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Table 1. Trawl mean catch rates (kg/hr) of the various fishes in the northern part of the Uganda waters of Lake Victoria.

Table 2. Estimated fish catches in the Jinja area of Lake Victoria

Ha=Haplochromis
Ra=Rastrineobola
Ti=Bagrus
Bd=Bagrus docmac,
Cm=Clarias mossambicus,
Pa=Protopterus aethiopicus
Ln=Lates niloticus

Table 3. Average size (kg) of fish landed at Masese

Ov=Oreochromis variabilis,
One=Oreochromis niloticus eduardinanus,
Bd=Bagrus docmac,
Cm=Clarias mossambicus,
Pa=Protopterus aethiopicus,
Ln=Lates niloticus
* Specimens obtained by experimental bottom trawling in the Jinja area of Lake Victoria.

Table 4. Bottom trawl mean catch rates (weighted by depth interval) of the various fishes in Lake Victoria (in kg hr).

Source: Kudhongania and Cordone 1974.

Fig. 1. Estimated fish production from Lake Victoria, Uganda, during the period 1965–1989.

Fig. 2. Bathymetric map of Lake Victoria, Uganda.

Paper presented at the National Seminar on the Management of the Fisheries of Lake Victoria, 6 – 8 August 1991, Jinja, Uganda.