by
F.C. Roest
Senior Fisheries Adviser
International Agricultural Centre
Wageningen
The Netherlands
ABSTRACTLimnothrissa miodon is the least important of the pelagic fish species caught by the industrial fishery in the Burundi sector of Lake Tanganyika, making up 4–14% of the catch. Because of this it has not been studied in detail and this paper summarises the available data. During the yeaars 1973–1980 the major clupeid, Stolothrissa tanganicae, decreased in abundance and was largely replaced by Lates stappersii. Although the catches of Limnothrissa fluctuated considerably it also declined during this pariod. Limnothrissa spends much of its life in inshore waters and is therefore likely to be caught in greater numbers by the artisanal fishery. There is an urgent need to standardise the units of effort in both fisheries so that the effect of fishing on its stock can be properly assessed. RESUMELimnothrissa miodon est la moins importante des espèces de poissons pélagiques capturées par les pêcheries industrielles dans le secteur burundais du lac Tanganyika, où elle ne représente que 4 à 14 pour cent des captures. C'est la raison pour laquelle elle n'a pas été étudiée en détail; le présent document résume les données disponibles. Au cours des années 1973–1980, le clupéidé principal, Stolothrissa tanganicae, est devenu moins abondant et a été en grande partie remplacé par Lates stappersii. Bien que les captures de Limnothrissa aient beaucoup varié, au total elles ont elles-aussi diminué au cours de cette période. Limnothrissa passe une grande partie de son existence dans les eaux litorales et a donc des chances d'être capturée en plus grandes quantités par la pêche artisanale. Il est urgent de normaliser les unités d'effort des deux types de pêche de façon que l'on puisse évaluer convenablement l'effet de la pêche sur les stocks. |
The pelagic ecosystem of Lake Tanganyika is highly dynamic (Coulter, 1991; Mann et al., 1973; Pearce, 1985; Roest, 1988). The production cycle is wind-driven and there are considerable fluctuations in productivity because of variations in the quantities of wind and rain from year to year. In addition a complex predator-prey relationship between the two major pelagic fish species causes intricate long-term cycles of abundance which as yet are little understood (Roest, 1992).
The pelagic fish community numbers only six abundant fish species, of which the catches per unit effort (CPUE) of the industrial fisheries in Burundi constitute the only measure of “abundance” available for the northern waters. Stolothrissa tanganicae is the most numerous of the six species and so it has been studied the most (e, g, Roest, 1978; FAO, 1978; Coulter, 1991). Lates (Luciolates) stappersii is the main predator and fluctuates spectacularly in abundance with Stolothrissa. It has been studied by Coulter (1976), Ellis (1978), Chapman and van Well (1978), Nyakageni (1985) and Roest (1988). The three other species of Lates (angustifrons, mariae and microlepis) have received little attention because their numbers declined after intensive exploitation. Limnothrissa miodon is relatively unimportant in the industrial fish catch of Burundi (4–14% by weight) in contrast to the situation in Zambian waters, where it occasionally outnumbers Stolothrissa.
Its successful introduction into other African waters has generated considerable interest in its biology and stock dynamics. The present paper attempts to describe and analyse its role in the pelagic fishery of northern Lake Tanganyika during the period from 1973–1980.
The industrial fish catches were sold in the central market in Bujumbura and the data were relatively reliable in the period from 1973–1980. Fish were sampled in two three day periods during the lunar fishing month as introduced by FAO (Mann, 1975). A handful of fish was taken from each 40 kg crate of small pelagics (“Ndagala”), sorted by species, weighed and finally measured individually (to fork length). Since the total weight and the weight of the sample were both known it was possible to estimate, by extrapolation, the total catch per boat and the total overall catch. This procedure gave two length-frequency tables per lunar month, which were given equal weight in the computation of a final single monthly length-frequency distribution. The length interval in the length-frequency distributions was 4 mm. This methodology is described in more detail in Roest (1991).
Great changes occurred in the size and composition of the pelagic fish stocks during the study period. There was a strongly seasonal pattern in the monthly CPUE of the industrial fishing fleet (Fig. 1). The highest catches of Stolothrissa occurred in the period September-February of each year when the biomass of this short-lived species reached its annual maximum (Roest, 1978). At this time, there is also an increase in the populations of other fish species, including Dinotopterus and Lates, which are attracted by the increase in the biomass of Stolothrissa. Its biomass was exceptionally high in 1973/74 and 1974/75, after which there was a general decrease which resulted in the apparent complete failure of the 1978/79 cohort.
The decline of Stolothrissa coincided with a great increase in the CPUE of its main predator Lates stappersii, which was very abundant in 1976, 1977 and 1978. During this period the stocks of adult clupeids were, in fact, largely replaced by Lates stappersii. In contrast to the situation in Stolothrissa where the evolution of the CPUE represents actual biomass development, the highest catches of Lates stappersii are made during their annual spawning run into the northern waters of the lake. Juveniles of this species occupy the offshore pelagic areas during their first year where they are exposed to industrial fishing. They are marketed as part of the commercial category “ndagala”, from which they are sampled separately as “Lucio” (Fig. 1).
During the study period Limnothrissa made a modest contribution to the catches. It leads a more inshore life and does not recruit fully to the industrial fishery until it reaches 100 mm in length (Fig. 2). At this stage, it is thought to follow Stolothrissa out into the open waters (Henderson, 1976). Coulter (1991) mentions that one of the general characteristics of Limnothrissa is its more stable pattern of abundance compared to Stolothrissa. Since this species is relatively unimportant in Burundi, the data gathered on it have not been analysed and most observations come from Zambia or Tanzania (Pearce, 1985; Ndugumbi et al., 1976).
The irregular monthly variation of Limnothrissa in the Burundi industrial fishery is most clearly seen on a quarterly basis (Fig. 3). Like that of Stolothrissa, its abundance decreased steadily and over a period of eight years its CPUE fell from 120 to 55 kg per boatnight. Seasonal variation in the abundance of Limnothrissa is roughly similar to that of Stolothrissa, with peaks usually in the fourth and first quarters of the year.
The monthly length-frequency tables for Limnothrissa were subjected to Bhattacharya and ELEFAN analyses for the estimation of growth parameters. The preliminary results indicate that there is considerable variation in the von Bertalanffy parameter K, a finding which requires furthjer investigation. Because of the small numbers of Limnothrissa in the industrial catch very few distributions which were obviously normal distributions (chi-squared test) were obtained. The monthly modes were widely scattered and clear progressions could not be easily recognised (Fig. 4). In some of the more obvious cohorts L∞ and K values were in the order of 180 mm and 0∙85 year-1 respectively. This is close to the values of L∞ = 175 mm and k = 0∙0765 month-1 found by Ndugumbi et al. (1976).
Without exception all the pelagic species spend a part of their lives in the littoral zone of the shallow northern part of Lake Tanganyika. Juvenile Stolothrissa (0–55 mm), Limnothrissa (up to 100 mm) and Lates stappersii (130–250 mm) as well as juveniles of the larger Lates spp. which reportedly live in weed beds are frequently seen in the traditional fish catches from the littoral zone in Burundi.
As the industrial fishery operates away from these inshore areas juvenile fish are not adequately sampled by the fishing gear. For a better understanding of their biology samples of these fish should be taken from the artisanal fishery as well as from the industrial one. In addition, the units of effort in the two fisheries should be standardised so that samples can be put together for anlysis of their population parameters. This work requires a considerable amount of sampling, as witnessed by Enderlein's efforts in the 1970s (Enderlein, 1976). For the time being, little or no additional work on Limnothrissa is likely in Burundi because of its modest contribution to the total fish catch.
Chapman, D.W. and P. van Well, 1978. Observations on the biology of Luciolates stappersii in Lake Tanganyika (Tanzania). Trans. Am. Fish. Soc., 107: 567–73.
Coulter, G.W., 1976. The biology of Lates species (Nile perch) in Lake Tanganyika, and the status of the pelagic fishery for Lates species and Luciolates stappersii (Blgr). J. Fish Biol., 9: 235–59.
Coulter, G.W., (ed.), 1991. Lake Tanganyika and its life. London; British Museum (Natural History) and Oxford University Press.
Ellis, C.M.A., 1978. Biology of Luciolates stappersii in Lake Tanganyika (Burundi). Trans. Am. Fish. Soc., 107: 557–66.
Enderlein, H.O., 1976. Biological sampling survey no. 1 of the traditional and artisanal fisheries, Lake Tanganyika (Burundi). FAO FI: DP/BDI/508/6: 17 pp.
FAO, Tanzania, 1978. Fishery biology and stock assessment: a report based on the work of D.W. Chapman, H.Rufli, P. van Well and F. Roest. Lake Tanganyika Fishery Research and Development Project FAO FI: DP/URT/71/012.
Henderson, H.F., 1976. Notes on Luciolates based on a study of length frequency diagrams from the ring-net fisheries of Lake Tanganyika. FAO FI: DP/URT/71/012/29: 1–6.
Mann, M.J., 1975. Catch sampling methods and results in Burundi. FAO/EIFAC Tech. Pap. 23 (Suppl. 1): 36–48.
Mann, M.J., F. Bashirwa, C.M. Ellis, J.B. Ahabakomeye and H.O. Enderlein, 1973. A preliminary report on fish biology and stock assessment in Lake Tanganyika (Burundi). FAO FI: DP/BDI/73/020/5: 1–58.
Ndugumbi, Z., P. van Well and D.W. Chapman, 1976. Biology of Limnothrissa miodon in Lake Tanganyika. FAO FI: DP/BDI/73/020/5: 7pp.
Nyakageni, B., 1985. Biologie et écologie d'un poisson endémique du lac Tanganyika, le mukehe Luciolates stappersii (Boulenger, 1914). Thése Doct. 3e cycle, U.P.S. Toulouse, 108 pp.
Pearce, M.J., 1985. A description and stock assessment of the pelagic fishery in the south-east arm of the Zambian waters of Lake Tanganyika. Report to the Department of Fisheries, Zambia, 5p.
Roest, F.C., 1978. Stolothrissa tanganicae: population dynamics, biomass evolution and life history in the Burundi waters of Lake Tanganyika. CIFA Tech. Pap. 5: 42–63.
Roest, F.C., 1988. Predator-prey relations in northern Lake Tanganyika and fluctuations in the pelagic fish stocks. CIFA Occas. Pap. 15: 104–129.
Roest, F.C., 1991. Statistiques et informations des peches, Burundi. Rapport de consultation du 27 mai au 13 juin 1991. PNUD/FAO/BDI/90/002/8: 6pp + annexes.
Roest, F.C., 1992. The pelagic fisheries resources of Lake Tanganyika. Mitt. Internat. Verein. Limnol., 23: 11–15.
Figure 1. The monthly catch per unit effort of the industrial fishery, Burundi, 1973–1980. 1 = Limnothrissa miodon; 2 = Stolothrissa tanganicae; 3 = Lates stappersii (Juveniles) and 4 = Lates stappersii adults (“Mukeke”).
Figure 2. Cumulative length-frequency distribution of Limnothrissa miodon in the industrial fishery, Burundi, 1973–1980.
Figure 3. The quarterly catch per unit effort of Limnothrissa miodon in the industrial fishery, Burundi, 1973–1980. Note that the line indicates the trend only.
Figure 4. Length-frequency modes of Limnothrissa miodon caught in the industrial fishery. The modes were plotted by the Bhattacharya methd, the value on thr ordinate is the number of days since 1 January 1973.
