by
Harris Phiri
Lake Tanganyika Research Unit
Department of Fisheries
P.O. Box 420055
Mpulungu
Zambia
ABSTRACTLimnothrissa miodon is one of the most important pelagic fish species in southern Lake Tanganyika. Data collected since 1980 show that the catch and catch per unit effort of this species have declined, especially in the heavily-exploited south-eastern arm of the lake. The definition of fishing effort is discussed and it is concluded that changes in the types of light being used and the introduction of very small mesh beach seines means that the fishing effort has increased substantially. This poses a threat to the stocks of Limnothrissa which are now in danger of being over-exploited in this part of the lake. RESUMELimnothrissa miodon est l'une des espèces pélagiques les plus importantes dans le sud du lac Tanganyika. Les données recueillies depuis 1980 montrent que les captures et les captures par unité d'effort de pêche ont diminué, surtout dans le bras sud-est du lac, où l'exploitation est très intense. La définition de l'effort de pêche est examinée et la conclusion est que les changements de type d'éclairage utilisé et l'introduction de sennes de plage à petit maillage signifient que l'effort de pêche a nettement augmenté. Ceci constitue une menace pour les stocks de Limnothrissa qui risquent aujourd'hui d'être surexploités dans cette partie du lac. |
The commercial fishery in southern Lake Tanganyika began in the late 1950s but the first purse seine was only introduced in 1962 when an industrial fishing company was established. The beach seine was also introduced in 1962. By the end of 1991 there were 17 industrial fishing vessels in the Mpulungu area and one in the Nsumbu area (Fig. 1).
The use of beach and purse seines in the commercial fishery has increased the catch of clupeids of which there are two species, Limnothrissa miodon and Stolothrissa tanganicae. Over the past 30 years (1962–91) clupeids (known as “Kapenta” in Zambia) have, on average, made up more than 50% of the catch of the industrial fishery and more than 90% of that in the artisanal fishery. Limnothrissa is generally the most abundant and is therefore of considerable economic importance since 20 000 people along the lakeshore and the line of rail are now supported by the artisanal fishery. Even more people are employed in processing, fishing or trade in the industrial fishery.
Most of the fishing activity in both fisheries is concentrated around Mpulungu (Hore Bay) which comprises about 40% of the Zambian waters of the lake. Fishing effort in the area has increased since the introduction of electrical generators and lighting on the boats. This paper considers the effects of the increased fishing intensity on the abundance of Limnothrissa miodon in the southern end of the lake and compares the heavily exploited south-eastern arm of the lake with the south-western one where there is less fishing.
From 1962–83 staff of the Department of Fisheries collected data from the industrial fishery every day. The species composition of the catch of each boat was recorded together with the fishing sites, the number of lights employed and the number of draws. After 1983 only the main commercial species were recorded but there was no change in the collection of the other data.
Prior to 1983 catches in the artisanal fishery were recorded daily by Department of Fisheries staff stationed at the major landing beaches. After 1983 two methods were used to collect data. The first was the catch assessment survey, a system introduced by G.P. Bazigos (FAO). In the second, recorders visit the fishing camps twice a week and collect fish while the fishermen are using their gear. On each occasion the recorders note the total catch, the number of lights, the number of draws and the length of time spent fishing. They also collect a sample for analysis which includes length-frequency and the numbers and weight of each species.
Because of inconsistencies in the early data only the only accurate values are those obtained between 1980 and 1991.
In both fisheries at least 50% of the clupeid catch consisted of Limnothrissa with the proportion ranging from 27–90% (mean 50%) in the industrial fishery and from 25–90% (mean 68%) in the artisanal fishery (Fig. 2).
Fishing effort at Nsumbu increased from 1983 to 1986, after which there was a marked decline, especially in the industrial fishery (Fig. 3). At Mpulungu the fishing effort increased almost continuously until 1990 but effort decreased, notably in the artisanal fishery in 1991. The high cost of inputs may have contributed to these decreases by causing a reduction in the number of lights employed by the fishermen.
Industrial catches at Nsumbu increased from 1983 until 1988 after which they fell sharply and the total catch of Limnothrissa miodon was only 123 t in 1991 (Fig. 4). At Mpulungu the catches rose until 1985 but decreased after then although they increased slightly in 1989–90. Catches in the artisanal fishery varied considerably at both Nsumbu and Mpulungu. The data may not be reliable since the values are based on extrapolations.
The average catch per unit effort (CPUE) of the industrial fishery at Mpulungu was about 68 kg per light-night (Fig. 5). Like the catch, the CPUE was stable during the first years of the fishery up to 1985 but it fell in 1986 and remained relatively low. In the Nsumbu area the industrial CPUE was similar to that at Mpulungu until 1987 but it increased in 1988 and 1989, with a sharp increase in 1990. This was probably due to the expulsion of the major industrial fisherman in the area. The CPUE decreased rapidly in 1991 which probably reflects inefficiency on the part of the sole fishing company in the region. The CPUE of the artisanal fishery at Nsumbu was much higher than it was at Mpulungu until 1990 when it declined to a similar level
It is important to define fishing effort with respect to the Lake Tanganyika fishery because the more usual definitions of effort, such as hauls and boat-nights, were not used. The number of hauls in a night depends on the time interval between hauls and the shorter this interval, the more hauls can be made. However, it would appear that the an increased frequency of hauls does not lead to an increased catch. This is probably because the lights are set for a shorter time which is insufficient for fish to gather under them in large enough numbers. This may explain why many industrial companies only draw their nets 2 or 3 times each night in spite of the general increase in the number of lights. If it is assumed that fewer hauls will lead to greater catches then the use of hauls to determine effort when lights are used to attract fish would lead to an underestimation of effort on Lake Tanganyika.
The number of boat-nights in a month is influenced by several factors including moon phase, breakdowns and holidays. By increasing the number of lights in use it is possible to reach a higher level of effort than would be indicated by the number of boat-nights alone. This is especially true when fishermen become desperate because their catches are low and they resort to using more lights to attract fish. Under these circumstances the use of boat-nights would underestimate effort on Lake Tanganyika.
Most industrial fishermen have changed from kerosene pressure lamps to fluorescent lights powered by generators. Some have also placed lights on the main boat so increasing the number of lights in use without increasing the number of light-boats. The artisanal fishermen continue to use pressure lamps, but the number of beach seines has increased. Fishing effort increased further through the use of small meshed netting material (sometimes as small as 4 mm) which may adversely affect the stocks of Limnothrissa.
In view of the modifications made in the fishing industry it was considered that the measurement of effort should include some indication of the changes in light usage. For this reason it was felt that light-nights was a more appropriate measure of effort than either hauls or boat-nights.
The Zambian waters (2 100 km2) make up only 6% of the area of Lake Tanganyika. If fishing pressure is considered as effort per unit area then it can be seen to have increased by 6 times in the last 12 years. All industrial companies in the Nsumbu area were nationalised by the Government in 1989. This led to a shift in the fishing activity so that 95% of the fishing effort is now concentrated in the south-eastern arm of the lake which is only about 2.5% of the its area (c. 875 km2). This may explain the continued drop in the CPUE in this area in 1990 and 1991.
If CPUE accurately reflects fish abundance then the stock of Limnothrissa seems to have decreased in the south-eastern arm of the lake. This coincides with the increase in fishing pressure and may indicate that this species is becoming over-exploited. This may not be true of the Nsumbu area because of the continuing decline in the fishing effort there. Although the CPUE in this area fluctuated there may still be some potential for expanding the fishery in the south-western arm of the lake.
Data on primary productivity and the reproductive biology of Limnothrissa suggest that the southern end of the lake is its main breeding area. Over-exploitation of the stocks in this region is bound to cause concern because of their economic importance. Expanding the fishery in the south-western arm would be a way to reduce fishing pressure in the south-eastern one. Further studies on the clupeids are needed to assist the authorities in making the right management decisions.
Figure 1. The Zambian waters of Lake Tanganyika showing the main fishing zones (shaded).
Figure 2. The proportion of Limnothrissa miodon in the clupeid catches made by the industrial (solid histogram) and artisanal (unshaded histogram) fisheries based at Mpulungu, 1980–1991.
Figure 3. Fishing effort in (a) the industrial and (b) the artisanal fisheries based at Mpulungu (Mp) and Nsumbu (Ns).
Figure 4. The total catch of Limnothrissa miodon in (a) the industrial and (b) the artisanal fisheries based at Mpulungu (Mp) and Nsumbu (Ns).
Figure 5. The catch per unit effort of Limnothrissa miodon in (a) the industrial and (b) the artisanal fisheries based at Mpulungu (Mp) and Nsumbu (Ns).
