by
J.H. Magasa
Fisheries Department
P.O. Box 27
Monkey Bay
Malawi
A brief review is given of the predator-prey relationships, population dynamics and fisheries productivity of Lake Malawi.
Approximately one third of the cichlid species at all depth ranges are predators. Rhamphochromis and Diplotaxodon (Cichlidae) are the main predators in the pelagic zone whereas Bagrus and Bathyclarias are the dominant demersal predators.
Growth, mortality rate, recruitment, breeding season, life span and age at first maturity are given for Engraulicypris and various cichlids. Due to a high degree of a speciosity, dynamic pool models cannot be satisfactorily applied to the demersal stocks thus the entire demersal population is treated as a single species for management purposes.
The fish biomass of the pelagic zone of Lake Malawi has been estimated by acoustic methods to be 90kg/ha. The potential yield was estimated at 45kg/ha/year and the total estimated biomass for the lake in 1979 and 1980 was 150,000t and 140,000t respectively. A comparison has been made between Lake Malawi and Lake Tanganyika in terms of fish biomass, potential yield and primary productivity and a hypothesis has been put forward as to why the pelagic fish biomasses of the two lakes differ.
Lake Malawi is the most southerly of the great African lakes. It is about 560km long, 50km wide, has a maximum depth of 695m and covers an area of 30,800km2. The lake is very clear with Secchi disk readings of up to 17m. The water below 250m is anoxic and complete vertical mixing of the lake has never been observed (Eccles, 1974). The lake has a unique fish fauna with more known species than any other lake in the world. Of the 250 species described, more than 200 belong to the family Cichlidae and of these all but six are endemic. The lake supports a wide variety of fisheries, both commercial and artisanal, which, between them, exploit most of the stocks.
Lake Malawi's fisheries differ dramatically from those of its morphometrically similar neighbour Lake Tanganyika. Lake Tanganyika's fishery is based almost exclusively on two species of clupeids and the centropomid predators which feed upon them. In contrast, Lake Malawi has neither clupeids nor centropomids and its pelagic zone currently makes a relatively insignificant contribution towards the total fisheries production. The fisheries of Lake Malawi may be conveniently divided into commercial and artisanal components.
The commercial fisheries component consist of ring-net, demersal trawl and mid-water trawl fisheries. The Oreochromis ring-net fishery was started in 1943 and has been reviewed by several authors (Lowe, 1953; Williamson, 1966; Fryer and Iles, 1972; FAO, 1976). Information on net size used prior to 1956 is limited. The major nets in current use are 102mm stretched mesh, 593m long and 57m deep. The catch consists of three Oreochromis species; O. saka, O. squamipinnis and O. lidole. Fishing is prohibited in November and December to reduce fishing effort and protect nesting Oreochromis for at least part of their breeding season. The ‘usipa’ (Engraulicypris sardella) ring-net fishery was started in 1974 and uses nets 232m long and 55m deep with 13mm stretched mesh. Fishing for Engraulicypris is conducted at night since the species is strongly attracted to bright light. The ring-net fishery for ‘utaka’ (small, zoo-planktivorous haplochromines) was started in 1975, using nets 274m long and 55m deep with 25mm stretched mesh. Fishing takes place during both the day and night. Demersal trawling started in 1968 and currently fourteen units are engaged in a trawl fishery for small demersal cichlids. An exploratory trawling programme was started by the Malawi Fisheries Department in 1969 to monitor changes in the fish population brought about by commercial fishing operations and to assess the stock size and potential fish yield in other parts of the lake (Tarbit, 1972). The mid-water trawl fishery was started in 1973 and fishes for Oreochromis, Diplotaxodon and Rhamphochromis.
Artisanal fisheries consist of open water (‘chirimila’) seines, shore seines, gillnets etc. The chirimila is the most refined of the indigenous fishing methods on Lake Malawi (Howard, 1964) and is used in open water for catching utaka and usipa. Shore seines, which were in existence on L. Malawi before 1859 (Livingstone, 1965), are used to catch both Oreochromis and small cichlids. It is not known when gill nets were first used on Lake Malawi, but they were first introduced on Lake Victoria in 1905 where they spread rapidly (Worthington and Worthington, 1933). To-day gill-netting is widely used for exploiting Oreochromis, catfish and Labeo.
There is a clear indication that some fish stocks are interrelated. Jackson et al. (1963) noted that a good year class for usipa was associated with a good year for utaka (presumably because both depend on abundance of zooplankton) but local fishermen suggest that an inverse relationship exists between the two. This may be because the rapidly-growing usipa respond more rapidly to favourable conditions than the slower growing and slower reproducing utaka.
Little work has been conducted on predator-prey relationships in Lake Malawi and few data are available on the diet and feeding habits of predators or on the interrelationships between predator and prey populations. Approximately one third of the cichlid species at all depth ranges are predators (Lewis, 1981), though these rarely make up a high proportion of the catch in terms of weight or number of individuals. Many are relatively small and probably feed on juveniles of other species. Important pelagic cichlid predators are a number of species of Rhamphochromis and Diplotaxodon which feed on Engraulicypris (Fryer and Iles, 1972). Bagrus and the Bathyclarias species flock constitute the most important predators on the demersal stocks and are known to feed on nesting cichlids (Fryer and Iles, 1972). Opsaridium species are amongst the most important predatory species in the northern part of the lake where they prey on open water zooplanktivorous fishes (Tweddle and Lewis, 1983). Serranochromis robustus and various species of Cyrtocara, such as Cyrtocara lepturus, C. kiwinge and many others, feed on the young of Oreochromis (Lowe, 1952) and other cichlids. C. kiwinge is also an important inshore predator on Engraulicypris.
Growth, recruitment and mortality rates have been studied in selected species but because of the very high degree of speciosity, dynamic pool models cannot be satisfactorily applied to the demersal stocks. Turner (1977) stated that the demersal fisheries exploit a number of species which are so evenly mixed on the fishing ground that it is difficult to single out individual species as dominant. Hence the entire catch is managed as a single unit and the total biomass is treated as though it were a single species.
Demersal cichlid fishes in general have K values (average growth constant) ranging from 0.5 to 0.7. Natural mortality values (M) range from 0.92 to 1.69 with an average instantaneous mortality rate of 1.3 which is equivalent to an annual mortality of 73%. Life spans range from 4 to 7 years, maturity is usually reached after 3 years and one batch of eggs per year is produced (Iles, 1971; Tweddle and Turner, 1977). Fish population densities, and thus catch rates, decline with increasing depth (Turner, 1977).
Turner (1982) analysed the Engraulicypris catches made by the Maldeco (a large commercial fishing company) purse-seine from 1974 to 1980. Catch rates for individual years tend to follow a declining pattern from January to June and then increase from July to December. Turner (op.cit.) noted that, in common with many other prolific, short-lived pelagic species, spawning stock size is probably unimportant in governing stock size in the following year. He concluded that year class size is determined in the early larval phase and is governed by food abundance and predation. The estimated annual mortality varied from 89 to 99% and this is similar, though somewhat lower than the instantaneous rates of 5.2 (Chapman and Van Well, 1978) and 5.5 (Roest, 1977) for Stolothrissa tanganicae in Lake Tanganyika, which correspond to an annual mortality of 99.5 to 99.6%.
The fish biomass of the pelagic zone of Lake Malawi was estimated by acoustic methods in October, 1979 and April, 1980 to be 90kg/ha (Rufli and Vitullo, 1982). Turner (1982a) obtained an independent estimate of 75kg/ha on the basis of purse-seine catches and estimated the potential yield to be 45kg/ha/year. The total estimated biomass in the studied area (Fig.1) in October, 1979 and April, 1980 was 150,000t and 140,000t respectively (Rufli and Vitullo, 1982).
In the October 1979 survey, biomass estimates for the northern (Karonga to Nkhata Bay), central (Nkhata Bay to Domira Bay) and southern (south of Domira Bay) regions of the lake were estimated to be 58,000t, 58,000t and 35,000t respectively whereas the April 1980 survey produced estimates of 42,000t, 47,000t, and 49,000t for the three regions (Rufli and Vitullo, 1982).
Lake Malawi is similar to Lake Tanganyika in morphometry and it has been suggested that the two lakes may be comparable in terms of primary production (Hecky and Fee, 1981; Degnbol and Mapila, 1982). However, recent as yet unpublished research on sediment cores off Nkhata Bay shows that at the time of Degnbol and Mapila's research the productivity in that area was abnormally high and therefore the assumption that the two lakes are similar in primary productivity is questionable (R. Crossley, pers. comm.). Pelagic fish biomass estimates on Lakes Tanganyika are larger than for Lake Malawi, ranging from 175 to 800 kg/ha (Hecky and Fee, 1981) and fish yields from the pelagic zone are in the order of 125 kg/ha/year (Coulter, 1971).
Turner (1982b) suggested that the lack of an efficient zooplanktivorous fish species in the pelagic zone of Lake Malawi could explain the discrepancy between the pelagic fish biomass of the two lakes. There is also a difference between the zooplankton communities of the two lakes. Large populations of the zooplanktivorous larvae of the midge Chaoborus and two species of cladocerans are found in Lake Malawi but are absent from the pelagic waters of Lake Tanganyika. Turner (op. cit.) suggested that this may be due to the presence in Lake Tanganyika of the two endemic and highly efficient clupeids, Stolothrissa tanganicae and Limnothrissa miodon. Chaoborus and large cladoceran species are the first to decline under intense fish predation (Lynch, 1979).
The assertion that the absence of Chaoborus in Lake Tanganyika is due to the presence of clupeids is queried as Lake Kivu, which is the major source of water inflow to Lake Tanganyika, originally had neither clupeids nor Chaoborus, though clupeids have now been introduced.
The explanation for the different productivity levels between the two lakes may well be a function of different ionic compositions and a project designed to investigate the whole pelagic ecosystem of Lake Malawi in depth is planned for 1987.
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