by
Richard Ogutu-Ohwayo
Uganda Freshwater Fisheries Research Organization
Jinja, Uganda
INTRODUCTION
Fish introductions have been used as a management tool to enhance production in some aquatic systems for aquaculture for sport and ornamental purposes. A register of international fish transfers of inland species compiled by FAO (Welcomme, 1981) shows how widely these transfers have been made. The Nile perch, Lates niloticus (L.) and four tilapiine species, Oreochromis niloticus (L.), O. leucostictus Trewavas, Tilapia zillii Gervais and T. melanopleura, which were introduced into Lakes Victoria and Kyoga during the 1950s and early 1960s, are examples of such fish transfers. Since these introductions were made, populations of L. niloticus and O. niloticus have increased to make them the most important commercial species initially in Lake Kyoga and now in Lake Victoria. The other introduced species were less successful in establishing themselves.
The introduction particularly of the Nile perch has been condemned (Barel et al., 1985). This was because, after its establishment in Lakes Victoria and Kyoga, populations of those fish which were still abundant in these lakes, particularly the haplochromines declined rapidly and others virtually disappeared from the fishery. The Nile perch, through feeding contributed to this decline (Ogari, 1985; Ogutu-Ohwayo, 1984; Hughes, 1986). It is because of this that it has been blamed for destroying the fisheries of the two lakes. Little effort has however been made to assess and describe other factors which have contributed to the changes in the fishery. For instance, by the time the Nile perch was established, the native tilapiine species Oreochromis esculentus (Graham) and O. variabilis Boulenger, which were originally the most important commercial species, had been depleted by overfishing (Jackson, 1971, Benda, 1979, 1981; Marten, 1979; Fryer, 1984). Labeo victorianus Boulenger, which formed the most important fishery along the affluent rivers of Lake Victoria had been destroyed by intensive gillnetting of gravid individuals on breeding migrations (Cadwalladr, 1969). This suggests that other factors which have contributed to the changes in the fishery before and after introduction should be considered before the role of the Nile perch can be assessed.
The condemnation of the Nile perch may give the impression that no economic benefit has been derived from the introductions, particularly of L. niloticus and O. niloticus, into Lakes Victoria and Kyoga. While not denying the reduction in abundance of some fish species due to predation by the Nile perch, there has been some increase in commercial fish production which is worth consideration. This paper is intended first to discuss the present increase in commercial fish production arising from the introduction of L. niloticus and O. niloticus into Lakes Victoria and Kyoga and then to emphasize the need to study and understand the fishery emerging from the introduction in order to provide information for its rational exploitation and management.
The State of the Fishery before the Introductions
Before the introductions were made, the fish stocks of Lakes Victoria and Kyoga had been declining largely due to overfishing (Beverton, 1959; Garrod, 1961; Jackson, 1971; Ogutu-Ohwayo, 1984). The demands on the fishery had been increasing since its development started about the beginning of this century. Originally, fishing and fish consumption was confined to people living near the lakes due to poor communication and inefficient native fishing methods. Improved communication since the introduction of motorized land and water transport extended the fishing grounds further offshore and the distribution of the fish to markets far from the lake. This was supported by the introduction of more efficient fishing gear, particularly the gillnets and seine nets which increased the efficiency of catching the fish. As early as 1927, there was growing concern that the populations of the native tilapiine species of Lake Victoria were declining rapidly. It was this that prompted a fishery survey of the lake by Graham (1929). The native tilapiine species, however, continued to decline as fishermen shifted from larger to smaller mesh gillnets due to a decrease in abundance and the size of individual fish in the population.
By the time the Nile perch was introduced in Lake Victoria the fishery of the native tilapiines had virtually collapsed from a catch rate of 50–100 fish/net/fishing night to a mere one fish/net by 1962 (Jackson, 1971). A more recent trawl survey in the Nyanza Gulf of Lake Victoria where O. esculentus was originally the most important commercial species, revealed that by the time the Nile perch was established in this part of the lake, O. esculentus was virtually absent (Benda, 1981).
The total absence of O. esculentus, not only in parts of Lake Victoria, but also in Lake Kyoga, cannot be explained purely by overfishing or predation. The only tilapiine species which has become abundant in both of these lakes, O. niloticus, has some similarities in distribution, feeding, and breeding habits with the native species which have disappeared. Competition with O. niloticus is likely to have contributed to the elimination of native tilapiine species. O. niloticus seems to have competitive advantages over the native tilapiines. It has a wider food spectrum, it is more fecund, grows to a larger size, has a faster growth rate, and a longer life span than the native tilapiines of Lakes Victoria and Kyoga (Fryer and Iles, 1972). These tilapiine species have been observed to hybridize. O. esculentus and O. variabilis could have lost ground to O. niloticus as a consequence of competition or genetic dominance.
As the populations of the larger species declined, there was a gradual shift to the smaller, originally less preferred species, particularly the haplochromine cichlids and Rastrineobola argentea (Pellegrin). It was partly in the effort to estimate the potential of the haplochromine cichlids that prompted the UNDP/FAO and EAFFRO assessment of the fish stocks of Lake Victoria from 1969 to 1971 (Kudhongania and Cordone, 1974). Until the establishment of L. niloticus and O. niloticus the fishery of Lake Victoria had been shifting to R. argentea and haplochromine (see Figure 2 of Ssentongo and Welcomme, 1985). Of these two, haplochromines do not have the capacity to withstand heavy commercial exploitation (Goudswaard and Witte, 1985; Turner 1977, 1977a). This suggests that even without the Nile perch, the haplochromine fishery would have collapsed especially if the trawl fishery which was being planned on the basis of the UNDP/FAO and EAFFRO survey had been carred out on a lake-wide scale.
Fish Production from the Introductions
Subsequent to the introduction of L. niloticus, its populations increased, first in Lake Kyoga and later in Lake Victoria, followed by increases in the population of O. niloticus. An indication of the contribution of L. niloticus and O. niloticus can be obtained from commercial fish catch statistics collected by the Fisheries Departments of the East African Governments. Although these statistics have been criticized especially in regard to the accuracy of the actual figures, they do provide useful information on the trends in the fisheries.
The quantity of fish landed from Lakes Victoria and Kyoga increased following the establishment of L. niloticus and O. niloticus. In Lake Kyoga, commercial catches increased from 4 500 t about the time of introduction of exotics in 1956 to a record of 167 000 t in 1977 (Figure 1). This increase in yield was largely due to the increase in stocks of L. niloticus and O. niloticus which from 1967 to 1977 together contributed an almost constant 80% to the total annual commercial catches from the lake.
In the Kenyan part of Lake Victoria, the commercial catches had declined from about 20 000 t recorded around 1948 to the lowest level of 4 200 t in 1961 (Kenya Fish. Dept. Ann. Rep., 1961). This had improved to 10 900 t in 1962 apparently due to stimulation of breeding by expansion of the breeding grounds following the rapid rise in lake level in 1961, but stagnated between 15 200 t and 18 680 t for ten years from 1966 to 1976 (Figure 2a) (Kenya Fish. Dept. Ann. Reports 1962 to 1976). When the Nile perch was established in the areas around 1977, the total quantity of fish landed increased to 89 000 t by 1985. The proportion of L. niloticus in this case increased from less than 1% in 1976 to 68% in 1983. The quantity of O. niloticus landed in the area also increased from 200 t to 7 500 t between 1975 and 1985.
The quantity of L. niloticus landed from the Tanzanian part of Lake Victoria also increased from 274 t in 1981 to 37 600 t in 1985. Populations of O. niloticus in this part of the lake have also increased. These increases have resulted in an overall increase in total catches from less than 70 000 t before the introduced species were established to 99 000 t by 1985 over a period of hardly five years (Figure 2b).
Detailed catch statistics for the Ugandan region of Lake Victoria are lacking, but those of Masese fish landing near Jinja show that the total quantity of fish landed at that station alone increased from 328 t in 1979 to 1 800 t in 1985, again due to a higher contribution of L. niloticus and O. niloticus. From 1981 to 1983 L. niloticus contributed 56.8% to the commercial catches at the landing followed again by O. niloticus. In addition, research data collected from the lake near Jinja shows that the catch rates of L. niloticus increased from 0.96 km h-1 recorded during the 1968/71 UNDP survey to 57.5 kg h-1 in 1983 (Kudhongania and Cordone, 1974; Okaronon, Acere and Ocenodongo, 1985).
Although some of the absolute figures quoted above may not be accurate, they indicate that the total quantity of fish landed from Lakes Victoria and Kyoga increased due to increases in stocks of L. niloticus and O. niloticus. The only native species which is still common in both lakes is R. argentea. The economic viability of the fisheries of these lakes will depend upon proper utilization of these species through rational exploitation and management. This would be based upon a sound knowledge of the biology, ecology, population dynamics of the species and the relationship between the key predator and the prey.
DISCUSSION
If properly utilized, the Nile perch and O. niloticus may either together or individually form sustainable fisheries in Lakes Victoria and Kyoga. Both of them are not totally new in the region. The Nile perch was, for example, present in the Lake Victoria region during the Miocene period (Greenwood, 1951). Both L. niloticus and O. niloticus are among the most important commercial species in their native habitats of Lakes Albert, Turkana and Chad (Holden, 1963; Hopson, 1963, 1982). O. niloticus has been the most important commercial species in Lake George and once formed up to 80% by weight of the commercial landings from that lake (Gwahaba, 1973). Even as a predator, L. niloticus was at one time reported as the most important commercial species in Lake Chad (Hopson, 1963).
Although L. niloticus is not likely to maintain the high yields realized during its early stages of colonization of Lakes Victoria and Kyoga, when suitable vulnerable prey were abundant, its populations may settle to a level where they can be maintained by oscillations in those of the predator and the prey. Alternatively, as the amount of vulnerable prey is reduced, population of O. niloticus may increase to dominate the fishery. For instance, in Lake Kyoga, where the Nile perch and O. niloticus have lived for a longer time, the populations of the predator initially increased faster than those of O. niloticus (Ogutu-Ohwayo, 1984). RcEent observations indicate that the populations of O. niloticus have increased to supercede those of the predator (Twongo, per. com.). A stock survey carried out in 1985 at nine most important landings around the lake gave proportions of the different species as 78.5% of O. niloticus, 16.7% of L. niloticus, 4.1% of Protopterus aethiopicus Heckel, and 0.7% of other species, yet some time back the two species contributed almost equally to the total landings (Figure 1). O. niloticus originates from the same lake as the Nile perch and may be less vulnerable to predation. It may have benefited from a reduction in competition for feeding and breeding grounds following the decline in stocks of those species with which it has similar requirements, particularly O. esculentus, O. variabilis and some haplochromines.
The rapid depletion of haplochromine cichlids through predation by the Nile perch supports the observation made in the southern part of Lake Victoria (Goudswaard and Witte, 1985) and in Lake Malawi (Turner, 1977, 1977a), that haplochromines are very sensitive to cropping. They could, even without the Nile perch, have been depleted by the trawl fishery which was being planned on Lake Victoria. Besides the decision whether to preserve the lakes for studies, such as those of evolution of species flocks or develop them to provide food will be determined by the needs of the people in the region. Lake Victoria alone occupies a large area (68 680 km2) and apart from being an important source of water, it has been used for transport and as a source of food. Presently, it is the Nile perch and O. niloticus which provide the biggest part of this source of protein-food in a region where other sources of animal protein are not well developed. Any effort on how this resource should properly be utilized and sustained would therefore be of great benefit to the people in the region.
The Nile perch was not originally widely accepted as a food, particularly by the rural population, mainly because of its high fat content which sometimes caused diarrhoea. As the quantity of food available to the predator decreases, it is becoming less fatty and more acceptable. Processing the fish by smoking, frying it in its own oil or even filleting it has also made the fish gain a wider market. Methods of processing the Nile perch especially for consumption by the local people, are, however, not well developed and need more research. O. niloticus is popular as a food and is eaten either fresh, smoked or sun-dried. It is in fact this introduced Oreochromis species which Barel et al. (1985) quote as fetching 30 times more per unit weight than Nile perch. The native tilapiines are virtually absent in the Nyanza Gulf (Marten, 1979; Benda, 1979; Okemwa, 1984).
It can therefore be concluded that despite the destruction the Nile perch has done to some of the fish stocks of Lakes Victoria and Kyoga, there is a potentially productive fishery comprising L. niloticus, O. niloticus and R. argentea in these lakes. What is needed is to collect information upon which rational exploitation and management of this fishery can be based, for instance by:
tracing the history of the fishery of the two lakes and documenting the factors which have contributed to the changes in the native fishery. This may give some guidance on areas where the fishery was previously mismanaged and allow for future correction;
continue to follow-up the progress of the introduced species to estimate their impact on the fishery and examine the factors which determine the parallel success of the two nilotic species in their native and new habitats and with R. argentea;
estimate the potential of the present fishery and study the biology, ecology and population dynamics of the dominant species, including suitable methods of exploiting them, and
find out the reasons why the Nile perch, initially was, and in some areas may still not be very popular as a food and investigate ways in which it can be processed and marketed, especially for consumption by the local population.
In Uganda, the Uganda Freshwater Fisheries Research Organization (UFFRO), with financial assistance from the International Development Research Centre (IDRC) of Canada, has initiated a programme to assess the impact of the introduction on the fishery and study the biology, ecology and population dynamics of dominant species, especially the interraction and interrelationship between them. This will permit a more accurate documentation of the recent changes in the fishery in relation to the introduced species and provide information upon which rational exploitation and management can be based. Similar programmes in Kenya and Tanzania will allow a lakewide coverage of the problem, which inevitably affects the three East African states.
ACKNOWLEDGEMENTS
This paper forms part of a broad programme being supported by the International Development Research Centre (IDRC) to whom I owe many thanks. I am also grateful to my colleagues at UFFRO, who may have read and commented on the manuscript, and to Miss Harriet Nakiridde for typing it. Some of the information given here may form part of a thesis for a higher degree to be published later.
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| FIGURE 1 Commercial fish production and species composition of Lake Kyoga from 1963 to 1978. | |
| Data for 1972 were not available. A survey by UFFRO in 1985 gave 78.5% Oreochromis niloticus 16.7% L.niloticus, 4.1%P.aethiopicus and 0.7% of other species | |
FIGURE 2: Changes in total commercial fish production for (a) Kenya waters (b) Tanzania waters of Lake Victoria.
