(Reply to the Note of H. Renson entitled “Consideration Relating to Fisheries Research in Malawi”, received on 23.6.66 from Mr. D.H. Eccles, Senior Fisheries Research Officer.)
At present, four water bodies are the centre of intensive fishing effort in Malawi, namely the Lower Shire River, Lake Chilwa, Lake Malombe and Lake Malawi. Of these, research work has been carried out in Lake Malawi since 1954 and on Lake Chilwa since 1964, while prior to this a number of collections and two surveys had been carried out on Lake Malawi.
Lake Chilwa
Lake Chilwa is a relatively small shallow lake (about 250 square miles) with a fish fauna of about 12 species. The fishery is based on the exploitation of three species, Clarias mossambicus, Barbus paludinosus and a species of Tilapia, and no other species appear to be present in sufficient quantity to form a significant proportion of the catch. Research on this lake is concerned particularly with the Tilapia and is directed towards obtaining the parameters of a yield equation for this species. At present we are able to give estimates of the selection characteristics of the gill nets used in the lake, of length/weight relationships, and of the mean size at first maturity for this species. No work has been done on the basic productivity of the lake, nor has any significant effort been expended on Barbus paludinosus which may prove to be the most abundant species (in terms of weight) in the lake.
Lake Malawi
Lake Malawi is a very large (10,000 square miles) deep (380 fathoms) lake, with a fauna of over 200 species of which about 20 are significant in the fishery. Of these, three species of Tilapia are the most valuable, followed by a number of plankton-feeding species of Haplochromis (Cichlidae) known collectively as ‘Utaka’. Other important species are Labeo mesops, Bagrus meridionalis, Engraulicypris sardella (fished only in the North), various species of Rhamphochromis and some species of Bathyclarias.
A general survey of the fisheries was carried out in 1939 by Bertram, Borley and Trewavas, and in 1945–47 Lowe undertook a study of the Tilapia fishery. In 1953 a Joint Fishery Research Organisation was set up, in conjunction with the Government of the then Northern Rhodesia. In 1954 this organisation established a station, staffed by four scientists, at Nkata Bay to undertake a survey of the northern part of Lake Malawi. In November 1955, the staff of this station was reduced to one scientist and in 1960 an additional research officer was recruited. In 1962 a new research station was opened at Monkey Bay, at the southern end of the lake, and in 1963 a third research officer was recruited. In 1964 the Nkata Bay laboratory was closed and the research officer was moved to Lake Chilwa.
Work has been concentrated on the physical limnology of Lake Malawi, and on Bagrus meridionalis, Labeo mesops and the ‘Utaka’ group of Haplochromis. Some work has also been done in Engraulicypris sardella and a little experimental trawling has been carried out. General taxonomic studies of the fauna of the lake have also been made to form a background to further work. Very little has been done on the important Tilapia because of the virtual impossibility at our present state of knowledge of making specific determinations of all but the largest specimens or breeding males.
Physical Limnology
Lake Malawi is permanently stratified, water below 250 metres being at a constant temperature and devoid of oxygen. As a consequence, the bottom over a large area of the lake is completely anaerobic. The water above 250 metre level is seasonally stratified, and the epilimnion shows marked nutrient depletion while the bottom water is enriched. The intermediate zone (metalimnion) shows gradients in nutrient and oxygen concentrations. Occasionally the stratification of the epilimnion and metalimnion may break down, but this is not an annual event. At the southern end of the lake, particularly in the South East Arm, water from the metalimnion upwells under the influence of the south east winds of the cool season. Furthermore, in this region nutrients not lost to the bottom water at the rate at which this occurs in the deeper regions and turbulence associated with internal waves on the thermocline appear to play an important part in the re-circulation of nutrients from the bottom. The consequence of this is that the South East Arm is considerably more productive than the rest of the lake, though the South West Arm also shows some enrichment. No work has, however, been done on primary productivity (the rate of fixation of solar energy) in the lake, and quantitive estimates of the potential fish production from different areas of the lake cannot be made with any degree of reliability.
Labeo mesops
This important species of cyprinid fish has been the subject of considerable study. Net selection characteristics, length-weight curves and mean size at first maturity have been obtained for it. Breeding has been studied, in the field, and the early stages of the life history have been followed, both in the field and from artificially fertilised ova. Breeding success of this species appears to be largely dependant on meteorological factors since the eggs are laid in ephemeral streams during floods. The growth rate for the first five months of life has been determined and later growth rates estimated, from field observations.
Bagrus meridionalis
Work has been carried out on Kampango both at Nkata Bay and at Monkey Bay. This catfish is present in considerable numbers in the deeper water and appears to be the most important predator in the lake, at least inshore and on the bottom. Net selection characteristics, size at first maturity, length/weight relationships and information on the breeding season have been obtained, and an estimate of growth rate has been attempted. This species appears to feed to a large extent on ‘Utaka’.
‘Utaka’
This group of 15 zooplankton-feeding species of Haplochromis probably form collectively the basis of the second most important fishery in Malawi. Some work has been done on their taxonomy, and a little work was done on their biology at Nkata Bay, but little else has been attempted.
Tilapia
Our knowledge of this important genus has scarcely progressed since Lowe's survey in 1945–47. It is felt that little progress can be made until it becomes possible to recognise the three important species at all stages of their life histories.
Miscellaneous
Much data has been accumulated on a number of other species, and on the fishing potential of gill nets of differing mesh sizes. Some knowledge has also been gained of the plankton-feeding cyprinid, Engraulicypris sardella. Some experimental trawling has been done which indicates that there may be a large stock of small bottom-dwelling cichlids of any as yet undescribed species which are at present unexploited. These are caught in large numbers in the day-time, when very few Tilapia are caught. However, trawling at night gives many Tilapia and might seriously affect the ring net fishery for these.
Lake Malombe
This is a shallow lake about 150 square miles in area, formed by an extension of the Shire River some 10 miles south of Lake Malawi. The fishery is based largely on Tilapia but at some times of year a species of utaka is important. Virtually no work has been done on this lake or on the upper section of the Shire River from Lake Malawi to Matope, the head of the precipitous section of the river.
Lower Shire River
The Shire River from Chikwawa to Nsanje (Port Herald) is the site of a considerable fishery. Only two collecting visits have been made in this area by research staff. The fishery is based largely on two species of tilapia, T. mossambica and T. placida, but the relative importance of these is not known. Zoogeographically this region is unrelated to the other fisheries of Malawi, but forms part of the Lower Zambesi system.
ON RESEARCH PROJECTS WHICH COULD PROFITABLY BE UNDERTAKEN IN MALAWI
Fish Biology
Biochemical Taxonomy
One of the important problems of the fishery in Lake Malawi is precise ecological relationships of the four very closely allied species of Tilapia described from the lake, three of which co-exist in the South East Arm. Difficulty is experienced in distinguishing individual specimens at all stages of the life history. A reliable method of identification is fundamental to studies of the biology of these species. Biochemical taxonomic techniques, such as have been developed for certain aquatic snails, might provide a method for separating the species.
Age Determination
It is essential that a reliable method of age determination should be worked out for each of the important species of fish, so that management policies can be based on sound knowledge of the parameters of the yield equations. At present, age and growth rate can only be inferred from progression of modes in length frequency distributions. This would require a scale projector and at least one scientist. Methods of a determination are also required for the scaleless catfishes.
Ecology
Lake Malawi contains an endemic fauna closely adapted to lacustrine conditions. There are specialised phytoplankton feeding species as well as many zooplankton feeders and bottom and mud-feeders. Many of these are of direct economic importance in Malawi, but they also offer a potential means of utilising the resources of large tropical impoundments, a number of which have already been constructed. In such artificial lakes the natural fauna consist of riverine fishes not well adapted to lake conditions. The most immediately valuable studies would be of the Tilapia species and the Utaka.
Physical and Chemical Limnology
Basic Productivity
No information is available on the basic productivity (rate of fixation of solar energy) in Malawi waters, and very little is known for any tropical fresh water. Malawi, with its variety of types of water body in a relatively small area, and with a University and Government Research Stations conveniently situated, would be an ideal area in which to undertake fundamental studies of basic productivity to compare the productivity of different water bodies under similar climatic regimes. The basic pattern of physical and chemical events in Lake Malawi is understood, and could form the groundwork for such a study.
Chemistry
The chemistry of the bottom deposits of the lakes should be investigated. Very little is known of the processes of ion exchange in the muds of tropical lakes, although work on Lake Victoria has shown that large quantitites of nutrients are absorbed in the muds and the deposits in the S.E. arm of Lake Malawi appear to be highly organic. Such studies should be carried out for each lake.
The chemistry of the lake waters should also be investigated, particularly in the case of Lake Malombe, where differences between the incoming and outflowing water can be observed, and the effect of the lake on the river chemistry can be evaluated.
In Lake Chilwa, a closed system with about 250 square miles of lake and a similar area of swamp, the fish catch is at present of the order of 10,000 short tons per annum. An investigation of the possible effect of removal of nutrients from the system at this area is urgently required, together with work to determine the degree to which nitrogen fixation is effected within the system by bacteria and Cynophyceae and the effect on the availability of phosphate of a body of phosphatic rock in the area. The chemistry of the swamps should be compared with that of the lake.
Plankton and Bottom Communities
Studies of standing crops of the plankton and bottom communities, and of their utilisation by other species are needed.
Benefits Expected to Accrue to Malawi
Of the above projects, those on biochemical taxonomy and ageing are essential before rational management policies can be adopted to ensure the most desirable level of production. The project on ecology would also yield data valuable for management of the fisheries. Direct benefits are not so obvious in the case of work on more fundamental problems such as productivity and the associated physical, chemical and plankton work, but such work forms a necessary background of knowledge for understanding. Indirect benefits of any research work would include the stimulus to the intellectual life of the country by the provision of opportunity for contact with the University.
