The impact of the introduced sardine Limnothrissa miodon on the ecology of Lake Kariba

Biological Conservation 55 (1991) 151-165

The Impact of the Introduced Sardine Limnothrissa miodon on the Ecology of Lake Kariba

B. E. M a r s h a l l Department of Biological Sciences, University of Zimbabwe, PO Box MP 167, Mount Pleasant, Zimbabwe (Received 8 March 1990; revised version received 15 May 1990; accepted 29 May 1990) ABSTRACT The sardine Limnothrissa miodon was introduced from Lake Tanganyika into the man-made Lake Kariba where it now supports a major fishery. The suggestion that this fish could be introduced into other African lakes has been strongly criticised and this paper outlines its effects on the ecology of Lake Kariba. It may have prevented an indigenous species from becoming a pelagic planktivore but the populations of the predatory tigerfish and some piscivorous birds increased following the sardine introduction. It has caused major changes, similar to those described elsewhere, to the zooplankton. The zooplankton biomass is now very low and it is not clear how it can maintain itself in the face of intense predation by the sardine. Cascade theory suggests that the sardines should have caused changes in the phytoplankton as well but there are too few data available to demonstrate this. The sardines may also have influenced the nutrient economy of the lake and contributed to the decline of the aquatic fern Salvinia molesta, which once covered large areas of the lake. It is concluded that while this species is a suitable candidate for stocking into man-made lakes its introduction into natural lakes is undesirable. This is especially so in Lake Malawi because the sardine's capacity to change the plankton population could threaten the lake's endemic species flocks.

INTRODUCTION The introduction o f exotic fish species, and the translocation o f indigenous ones, has been carried out as widely in Africa as it has on other continents 151 Biol. Conserv.0006-3207/90/$03"50 © 1990 ElsevierSciencePublishers Ltd, England. Printed in Great Britain

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(Welcomme, 1981). In southern Africa alone, about 15 non-African and 27 African species have been introduced or translocated (Bell-Cross & Minshull, 1988; de Moor & Bruton, 1988). Some of them, such as the carp Cyprinus carpio L. and large-mouth bass Micropterus salmoides Lac6p6de in South Africa or red-breasted tilapia Tilapia rendalli Boulenger in Zimbabwe, have adversely affected the systems into which they have been introduced (Junor, 1969; de Moor & Bruton, 1988). Concern about the consequences of introducing alien fish into African lakes has recently increased because of the drastic effects of the Nile perch Lates niloticus L. on the fish stocks of Lake Victoria (Barel et al., 1985). This paper considers the known ecological impacts of one of the most important fish introductions in any African lake--the translocation of the sardine Limnothrissa miodon (Boulenger) from Lake Tanganyika to Lake Kariba. Lake Kariba is a very large man-made lake (c. 5400 km 2) located on the Zambezi River where it forms the border between Zimbabwe and Zambia. Prior to its creation Jackson (1961) suggested that none of the indigenous Zambezi river fish species would be able to colonise the pelagic waters of the newly created lake. It was recommended that a pelagic species from the African great lakes should be introduced. Consequently, Limnothrissa was successfully introduced from Lake Tanganyika in 1967-68 (Bell-Cross & Bell-Cross, 1971). By 1970, it had colonised the entire lake (Junor & Begg, 1971) and it was able to pass through the hydroelectric turbines of the Kariba Dam and establish itself in the Zambezi River. It invaded the Cahora Bassa reservoir, downstream from Kariba, which was completed in 1975 (Bernacsek, 1984). Commercial sardine fishing began on Kariba in 1973 and the fishery grew rapidly. The annual yield currently exceeds 20 000 t, which is several times greater than the total yield from all the other species in the lake (Marshall, 1987a). It is the most significant event in the Kariba basin since the creation of the lake, having brought economic development and job opportunities to an area where none existed before (Marshall et al., 1982; Bourdillon et aL, 1985). From this point of view the introduction of the sardines can be considered an unqualified success. However, this view of Limnothrissa is not necessarily applicable to other African lakes. The suggestion that it could be introduced into Lake Malawi (Turner, 1982) has been strongly criticised because its ability to change the structure of the pelagic ecosystem could threaten the lake's diverse and unique fish fauna (Eccles, 1985; McKaye et al., 1985). Its introduction into Lake Kivu in 1960 has also been condemned on the grounds that it has caused adverse and irreversible changes to the planktonic fauna in the lake. It was concluded that the fish stocks, and any fishery dependent on them, were in danger of collapse as a result of these changes (Dumont, 1986). In

Introduced sardine and Lake Kariba ecology

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view of these criticisms it is useful to evaluate the effects of Limnothrissa miodon in Lake Kariba, emphasising the differences between natural and man-made lakes. IMPACT ON COMPETITORS A N D PREDATORS

Competitors The rationale for introducing the sardines into Lake Kariba was that none of the native species would be able to colonise the open waters of the new lake and occupy the plankton-feeding niche that had been created (Jackson, 1961). This proved to be the case and most of the indigenous fish species live near the bottom in water less than 5 m deep, although some can be found at a depth of about 15 m (Coke, 1968; Mitchell, 1978). A possible exception to this generalisation is the small characid Brycinus lateralis (Boulenger) which appeared to be developing planktivorous habits at the time the sardines were introduced. Because of this, it was suggested that the sardine introduction may have been unnecessary since Brycinus could have occupied the vacant ecological niche (Balon, 1974; Balon & Bruton, 1986). This might have happened because B. lateralis does feed on zooplankton (Mitchell, 1976) but a fishery based on it may not have been as productive as the one based on the sardine. This is because most species of Brycinus are anadromous and must run up flowing rivers to spawn (Paugy, 1985). The most suitable sections of the rivers flowing into Kariba have been drowned by the lake (Begg, 1969), which means that the population of Brycinus might have been limited by a lack of suitable breeding sites. Another possible limitation is that these fish can only breed when the rivers are flowing, which may be for only 3-4 months of the year. Breeding might also have been reduced in drought years when river flow was reduced. From a commercial point of view, the characid is not a desirable species. Compared to the sardine, it is bony and has large scales which do not detach themselves and it has a sometimes bitter gall-bladder. Even though it may have been displaced by the sardine in open water Brycinus lateralis is still numerous in inshore areas (Table 1). There is little likelihood that its population will decrease further because the two species now occupy quite different ecological niches.

Predatory fish The fish populations of Lake Kariba have changed considerably since it was built and several species, especially those which inhabited running water

154

B. E. Marshall TABLE 1 The Mean Abundance of Brycinus lateralis in the Inshore Areas of Lake Kariba, Expressed as a Proportion of the Total Number and Total Weight of Fish Recovered after Sampling with Poison or Explosives

1968-69 1972-74 1984

% Numbers

% Weight

Source

58-8 22.6 53"6

6'5 1.4 4"0

Balon (1973) Mitchell (1976) J . D . Langerman (pers. comm.)

or specialised riverine habitats, have decreased in numbers. Some are no longer found in the lake itself although they still occur in the tributaries (Kenmuir, 1984; Bell-Cross & Minshull, 1988). These changes are a consequence of the lake's formation and the evolution of the lacustrine ecosystem and cannot be attributed to the sardine. However, the population of one native species, the tigerfish Hydrocynusforskahlii Cuvier, increased dramatically as a result of the sardine introduction. The sardines arrived in the eastern basin of the lake, about 150 km from their point of introduction, in late 1969 (Junor & Begg, 1971). They were not found in the stomachs of tigerfish caught during an angling competition in 1969 but they made up 75% of the food items in the stomachs of the fish caught in 1970 (Kenmuir, 1971). This led to speculation that their numbers would increase, which proved to be the case. By 1978 they had risen from about 5 to 10% of the commercial gillnet catch (Junor & Marshall, 1979). Their numbers have since declined, however, because of changes in the sardine population (Marshall, 1987b). Young tigerfish live in shallow, inshore waters and become piscivorous at about 40mm in length (Kenmuir, 1975). The sardines breed in the same waters from September to April (Cochrane, 1976) and thus provide a supply of small fish for the juvenile tigerfish to prey upon. Because of this there is a close correlation between the abundance of young-of-the-year (0 +) tigerfish and sardines (Fig. 1). The numbers of young tigerfish increased from 1970 until 1974 when the sardine stocks began to decline, possibly as a result of increasing commercial fishing activity. The progression of the strong tigerfish year-classes of 1970-74 can be followed through other fisheries which take older fish (Fig. 2). Since the tigerfish also feed on theother species in the lake, the number of 0 + fish did not fall below the pre-1970 level and has, indeed, begun to increase slightly in recent years. The increase in the tigerfish population thus appears to be a relatively transient phenomenon which came to an end when the sardine stocks declined. It seems unlikely that the population will increase by so much again unless sardine stocks increase first (which seems improbable in view of

155

Introduced sardine and Lake Kariba ecology

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the intense fishing that now takes place) or if there is a major change in the inshore fish stocks. Kenmuir (1971) suggested that the abundance of other fish species might increase because the tigerfish were now preying upon sardines and not on them. N o evidence has been found to support this suggestion because the biology of the inshore fish species is still poorly understood. It is, perhaps, more likely that their numbers might have increased because they themselves feed on the sardines. M a n y of the indigenous species tend to be opportunist feeders (Mitchell, 1976; Bell-Cross & Minshull, 1988) and some 1.0 f /

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as two-year running means and the average age of the fish in each fishery is shown on the top right-hand side of each panel. Redrawn from Marshall (1985).

156

B. E. Marshall

of them, including eels Anguilla bengalens& labiata Peters, squeakers Synodontis zambezensis Peters and butter catfish Schilbe mystus L., have been caught in fishing nets whilst feeding on sardines (unpublished observations). Other predators Fish-eating birds are the only other predatory species that are known to have benefited from the sardine introduction. The white-winged black tern Chlidonias leucoptera (Temminck) is able to feed on them in the pelagic zone (Begg, 1973) and has recently begun to feed at night around the fishing vessels (Marshall, 1984). This behaviour has not been previously recorded in this species. The grey-headed gull Larus cirrocephalus Vieillot was a non-breeding vagrant on Lake Kariba but its population has increased in the last few years and it has begun to breed on the lake (Worsley & Worsley, 1986). This has been attributed to an increase in the amount of food available to these birds, in the form of dead and discarded fish, which followed the establishment of the commercial sardine fishery (Hustler, 1986). The gulls have, in fact, begun to steal fish from the racks on which they are sun-dried.

IMPACTS ON THE P L A N K T O N

Limnothrissa is primarily planktivorous although it will feed on a variety of other food items (Begg, 1974). The effects of planktivorous fish have been examined in many parts of the world (Lazzarro, 1987) and tested experimentally (Hall et al., 1970; Lynch, 1979). Because the fish select the largest and most visible prey the biggest members of the plankton community tend to disappear first. Thus, the large cladocerans and copepods, and larvae of the midge Chaoborus are selectively preyed upon and may be driven to extinction (Gliwicz, 1985). They are replaced by smaller species such as Bosmina and rotifers. Changes in the zooplankton should theoretically also be reflected by changes in the phytoplankton, through the cascade or top-down effects (Carpenter et al., 1987; McQueen et al., 1989). The situation in Lake Kariba The impact of sardines on the zooplankton of Lake Kariba is similar to the impact of planktivorous fish on the zooplankton of lakes elsewhere. Prior to the introduction of Limnothrissa it was dominated by large cladocera, such

Introduced sardine and Lake Kariba ecology

157

as Ceriodaphnia and Diaphanosoma, and by calanoid copepods (diaptomids). Chaoborus larvae, which are partly benthic, were the major predators (Fig. 3). The jellyfish Limnocnida tanganicae Giinther was presumably a significant predator when the planktonic medusae were abundant, although this was a sporadic event (Mills, 1973). Small cladocera and rotifers were relatively unimportant. Chaoborus seems to be especially vulnerable to fish predation (Lynch, 1979) and it was rapidly affected by the sardines. In 1967-68 there was an average of 362 larvae m -2 in the benthos (Bowmaker, 1973) but by 1970 there were only 192 larvae m -2 (Mitchell, 1975). Thus, the population of Chaoborus was reduced by half within a year of the sardines becoming established and the animal now appears to be extinct in the lake itself. Larvae have not been found in recent samples nor have adults been seen anywhere around the lakeshore (unpublished data). The jellyfish has not apparently been affected by the sardines and is still

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B. E. Marshall

abundant in the lake from time to time. The sardines appear to avoid areas where this species is numerous and it is possible that the medusae can defend themselves against these small fish. The larger cladocera and diaptomids also declined rapidly (Table 2) and are now restricted to very shallow water where they can escape predation by the sardines (Green, 1985). The zooplankton is now dominated by smaller crustacean species with nauplii larvae and rotifers becoming increasingly important. The biomass has decreased as well, from a mean of 228 mg m - 3 (dry weight) in 1967-68 (Bowmaker, 1973) to a mean of 2.76 mg m-3 in the late 1970s (Magadza, 1980). These values may be slightly distorted because the earlier samples were taken in an estuarine area where productivity is greater than in the open lake. Nevertheless, the highest value recorded by Magadza (1980) was only 16mgm -3 even though the sample was from an area influenced by inflowing rivers. This clearly reflects what has happened to the zooplankton in the lake. The striking changes in the zooplankton might be reflected by changes in the phytoplankton but this has not been clearly demonstrated in Kariba because there are too few data on the phytoplankton. Volvox and Microcystis were the most abundant species in 1967-68 (Bowmaker, 1973), possibly because they are relatively large and were less susceptible to grazing by cladocera and diaptomids. These species were rare by the 1980s and the phytoplankton was dominated by small cyanophytes such as Anabaena, Cylindrospermis and Lyngbya (Ramberg, 1987). This change could have come about in two ways. First, the sardines feed on both Volvox and Microcystis (Begg, 1974) and so could have eliminated them by size-selective predation in much the same way that they eliminated the larger crustacea. Secondly, elimination of grazing pressure from these animals could have enhanced the survival of the small species and allowed them to increase at the expense of the larger ones. An important consequence of the sardine introduction is that the structure of the pelagic ecosystem has been changed, notably by a drastic reduction in the zooplankton biomass (Table 3). The phytoplankton biomass appears to have increased correspondingly but the values should be treated with caution because the sampling methods differed considerably. Despite the low zooplankton biomass the sardine population remains large enough to support an expanding fishery. Little is known about the population dynamics of the species which make up the zooplankton, but they can evidently withstand high levels of predation, perhaps through rapid growth rates and high turnover ratios. Vertical migration is one means by which plankton can escape predation but there is no evidence that this is effective in Kariba because the movements of the sardines closely followed those of their prey (Begg, 1976).

159

Introduced sardine and Lake Kariba ecology

TABLE 2 Changes in the Composition of the Zooplankton in Lake Kariba

Diaptomids Cyclopoids Nauplii Bosmina Ceriodaphnia Diaphanosoma

Daphniids Rotifers

1967-68 ~

1970 b

1972 ¢

1975-76 a

1983 e

18"7 17"6 9'3 4.8 43'0 10.5 0.3 4'0

0.2 12'5 16'4 15.9 14"3 0-2 0-2 40.3

0 10"1 17'1 17'1 0 0 0 55.7

+ 13'8 3'1 10'5 0-3 + 0 72.3

0 1'1 14'8 2.8 0 0 0 81"3

a Bowmaker (1973). b Mitchell (1975). c Begg (1976). a Cochrane (1978). e Green (1985). The values are expressed as percentages of the total. The data for 1967-68 are given as dry weight per unit volume (mg m-3) whilst all the other values are expressed as numbers per unit volume (no. m - 3). + indicates a value of less than 0.01%.

TABLE 3 The Biomass (mg m - 3, Wet Weight) of Components of the Pelagic Ecosystem in Lake Kariba before and after the Introduction of the Sardines

Phytoplankton Zooplankton Sardines

1967-68

1980-83

193a 2 281Y 0

3 ltY' 28d 196e

Bowmaker (1973) gives estimates of Volvox and Microcystis biomass only, so that the total value may be

a

underestimated. b Ramberg (1987). c Bowmaker (1973). a Magadza (1980). e Estimates of mean sardine biomass (31-4 x 1012mg) and lake volume (160-3 x 109 m 3) were derived from data in Marshall (1988). Values given as dry weight in the original sources were converted to wet weight by assuming that dry weight = 10% of wet weight.

160

B. E. Marshall

Comparison between Kariba and the other lakes where Limnothrissa occurs There are now four lakes in Africa where Limnothrissa is present-Tanganyika, where it is one of two endemic clupeids; Kivu and Kariba, where it was introduced; and Cahora Bassa which it invaded. Something is known about the plankton in each of them, which provides an opportunity to compare the effects of this fish in lakes that are very different from each other. Lake Tanganyika is one of the world's oldest lakes and has a stable pelagic ecosystem dominated by sardines and four species of Lates that prey upon them. It has evidently evolved over a very long period but recent changes have been brought about as a consequence of the sardine fishery. The zooplankton consists almost entirely ofa calanoid copepod Tropodiaptomus simplex (Sars), although a raptorial copepod Mesocyelops aequatorialis Keifer is present but in insignificant numbers. This situation has been stable since the first zooplankton collections were made in 1904-5 (Dumont, 1985). In contrast to the other African great lakes Chaoborus is absent, presumably because of predation by the sardines (Turner, 1982). Kivu is a much younger lake, formed by a volcanic dam, and has few native fish species, none of which are pelagic. Typically pelagic zooplankton genera like Tropodiaptomus and Thermodiaptomus have never been recorded from the lake and its zooplankton was dominated by 'pond' forms such as Daphnia curvirostris Eylman (Dumont, 1986). As in Kariba, the introduction of sardines led to the elimination of Daphnia and some other large cladocera and the plankton is now dominated by smaller rotifers and ciliates. The biomass declined from a mean of 7.5gin -2 (dry weight) in the 1950s to 0.15gm -2 in 1981 (Dumont, 1986). The sardines grow much larger in Kivu than they do in Kariba (Marshall, 1987c) and cannibalism is regularly recorded. Dumont (1986) concluded that the system had become unstable and that the fish stocks could collapse. However, Kivu is the only one of the four lakes without a predatory fish capable of feeding on sardines in its pelagic waters. Cannibalism by the larger sardines may simply mean that they are occupying this vacant ecological niche rather than indicating an imminent breakdown in the system. Lake Cahora Bassa was completed in 1975 and the sardines must have invaded it almost immediately. It is shallower and much more turbid than Kariba (Secchi disc visibility is 1.8 m compared to 5.0 m in Kariba) because of a heavy load of suspended clay particles. This affords some protection to the zooplankton and although the larger cladocera have declined (Gliwicz, 1986) they appear to have done so rather more slowly than in Kariba. The high turbidity has also led to a relationship between zooplankton

Introduced sardine and Lake Kariba ecology

161

numbers and the moon phase. Thus, there is a distinct lunar cycle of abundance because fish predation is most severe during the Full Moon period (Gliwicz, 1985). An exceptionally large vertical migration of Volvox has also been described from this lake. Sommer & Gliwicz (1986) suggest that this is a way for the plant to gain access to nutrient-rich layers in the lake but it may coincidentally protect it from fish predation. This could explain why this species has apparently been able to persist in Cahora Bassa longer than it did in Kariba.

N U T R I E N T CYCLING Because the Zambezi River, which supplies most of Lake Kariba's water, is nutrient-poor the lake is oligotrophic (Mitchell, 1973). Nutrients are also lost through the outflow, which can amount to one-third of the lake's volume in a single year (Marshall & Junor, 1981). This means that there is likely to be competition for nutrients between the various communities in the lake and a major increase in one could lead to a decline in another. The expansion of the sardine population in the early 1970s coincided with a decline of the floating fern Salvinia molesta Mitchell. When Lake Kariba began to fill in the early 1960s there was an immediate and explosive outbreak of the plant which, in 1962, covered about 22% of the lake's surface. It played an important role in retaining plant nutrients (Mitchell, 1973) and may have restricted their availability to other communities. Marshall & Junor (1981) suggested that the sardines may have broken this nutrient cycle and contributed to the rapid decline of the weed. The evidence for this is not unequivocal but the rapid decline of the Salvinia mats took place at about the same time that the sardine population was expanding (Fig. 4). This process would probably have taken place in any event, but more slowly, as part of the lake's maturation. Other communities,

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1960

65

70

75

80

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162

B. E. Marshall

such as mussels and submerged macrophytes, have also become established (Machena & Kautsky, 1988) and they too will retain nutrients. The sardine community grew more rapidly than any of the others and these fish must have accelerated the process.

CONCLUSION The ecology of Lake Kariba has been characterised by a process of continuous change to which the sardines have contributed significantly. Some of these changes might have taken place without them. For example, if Brycinus lateralis had become a truly pelagic planktivore it would probably have altered the composition of the zooplankton in the same way that the sardines did. The tigerfish population would also have increased and the nutrient budget of the lake could have been affected in the same way. There are about 40 species of fish in the lake and most are relatively unspecialised in their feeding habits (Mitchell, 1976), which means that they have not been affected by the changes to the zooplankton. None of them appear to have been adversely influenced by the sardine and there is little reason to criticise its introduction. Indeed, a man-made lake as large as Kariba constitutes a major affront to the environment and the development of a productive fishery is one of the few ways of compensating for it. In many cases an alien fish species will have to be introduced so that a productive fishery can be achieved. Limnothrissa may, under these circumstances, be one of the most appropriate species. The dangers of such an introduction into old lakes with a diverse fish community have been emphasised by Eccles (1985) and McKaye et al. (1985), who discussed the situation in Lake Malawi. This lake supports a diverse and complex fish community, the taxonomic position of many species is unknown and their trophic relationships are not fully understood. Because fish introductions are irreversible the risk of inflicting permanent damage to the African great lakes, and effectively reducing their productivity, is too great to justify the introduction of any alien species. The fish population of Lake Victoria has already been permanently altered by the introduction of Nile perch and Lake Malawi is now probably the most vulnerable of the remaining lakes.

ACKNOWLEDGEMENTS I am grateful to R. J. Phelps and J. L. Minshull for their comments on this paper.

Introduced sardine and Lake Kariba ecology

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REFERENCES Balon, E. K. (1973). Results offish population size assessments in Lake Kariba coves (Zambia), a decade after their creation. Geophys. Monogr. Set., 17, 149-58. Balon, E. K. (1974). The success and failure of the clupeid introduction. In Lake Kariba: A Man-made Ecosystem in Central Africa, ed. E. K. Balon & A. G. Coche. Dr W. Junk, The Hague, pp. 524-41. Balon, E. K. & Bruton, M. N. (1986). Introduction of alien species or why scientific advice is not heeded. Environ. Biol. Fish., 16, 225-30. Barel, C. D. N., Dorit, R., Greenwood, P. H., Frs, er, G., Hughes, N., Jackson, P. B. N., Kawanabe, H., Lowe-McConnell, R. H., Nagoshi, M., Ribbinck, A. J., Trewavas, E., Witte, F. & Yamaoka, K. (1985). Destruction of fisheries in Africa's lakes. Nature, Lond., 315, 19-20. Begg, G. W. (1969). Observations on the water quality and nature of the affluent rivers of Lake Kariba, with reference to their biological significance. Newslett. Limnol. Soc. Sth Aft., No. 13 (Suppl.), 26-33. Begg, G. W. (1973). The feeding habits of the whitewinged black tern. Ostrich, 44, 149-53. Begg, G. W. (1974). Investigations into the biology and status of the Tanganyika sardine Limnothrissa miodon (Boulenger) in Lake Kariba, Rhodesia. Lake Kariba Fish. Res. Inst. Project Rep., No. 17. Begg, G. W. (1976). The relationship between the diurnal movements of some of the zooplankton and the sardine Limnothrissa miodon at Lake Kariba, Rhodesia. Limnol. Oceanogr., 21, 529-39. Bell-Cross, G. & Bell-Cross, B. (1971). Introduction of Limnothrissa miodon and Limnocaradina tanganyicae from Lake Tanganyika into Lake Kariba. Fish. Res. Bull., Zambia, 5, 207-14. Bell-Cross, G. & Minshull, J. L. (1988). The Fishes of Zimbabwe. National Museums and Monuments of Zimbabwe, Harare. Bernacsek, G. M. (1984). Cahora Bassa (Mozambique). CIFA Tech. Pap., No. 10, 21-42. FAO, Rome. Bourdillon, M. F. C., Cheater, A. P. & Murphree, M. W. (1985). Studies of Fishing On Lake Kariba. Mambo Press, Gweru, Zimbabwe. Bowmaker, A. P. (1973). An hydrobiological study of the Mwenda River and its mouth, Lake Kariba. PhD thesis, University of the Witwatersrand, Johannesburg. Carpenter, S. R., Kitchell, J. F., Hodgson, J. R., Cochran, P. A., Elser, J. J., Elser, M. M., Lodge, D. M. Kretchner, D., He, X. & yon Ende, C. N. (1987). Regulation of lake primary productivity by food web structure. Ecology, 68, 1863-76. Cochrane, K. L. (1976). Catches of Hydrocynus vittatus Castelnau during sardine fishing operations in Lake Kariba. Kariba Stud., 7, 98-108. National Museums and Monuments of Rhodesia, Salisbury. Cochrane, K. L. (1978). Seasonal fluctuations in catches of Limnothrissa miodon (Boulenger) in Lake Kariba. Lake Kariba Fish. Res. Inst. Project Rep., No. 29. Coke, M. M. (1968). Depth distribution of fish on a bush-cleared area of Lake Kariba, Central Africa. Trans. Amer. Fish. Soc., 97, 460-5. de Moor, I. J. & Bruton, M. N. (1988). Atlas of alien and translocated indigenous aquatic animals in southern Africa. S. Aft. Natn. Scient. Programmes Rep., No. 144.

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