SEASONAL ABUNDANCE, VERTICAL MIGRATION AND THE FISHERY OF DAGAA RASTRINEOBOLA ARGENTEA IN THE UGANDAN WATERS OF LAKE VICTORIA

by

S.B. Wandera
Uganda Freshwater Fisheries Research Organisation
P.O. Box 343
Jinja, Uganda.

1. INTRODUCTION

Rastrineobola argentea (Dagaa) is the only endemic fish species which has remained abundant in Lake Victoria since the establishment of the introduced Nile perch, Lates niloticus L. and Nile tilapia, Oreochromis niloticus L. The decline in numbers and diversity of the native species especially the haplochromine cichlids, as a result of predation by the Nile perch, appears to have altered the lake's food web. This has led to an increase in biomass of R. argentea (Wanink, 1988) presumably due to a reduction in competition for food with the zooplanktivorous haplochromines. The species now supports a major artisanal fishery, ranking second to the Nile perch in Kenya and Tanzania (Arunga, 1991). In the Ugandan waters Dagaa ranks third in importance to Lates and Oreochromis (Ogutu-Ohwayo, 1990).

The demand for Dagaa in this region has continued to rise over the last decade. The species provides a cheap source of fish proteins for both humans and domestic animals. This is becoming more important especially after the drastic decline of the haplochromines and following the current export drive for processed table fish species, the Nile perch and the tilapia. Similarly, the corresponding growth of this fishery has been quite rapid over the same time. Catches of R. argentea from the Jinja area of Lake Victoria show a five fold increase over the last decade (Ogutu-Ohwayo, op.cit.).

It is however not known whether the high catches can be sustained for a long time especially in light of the changing Lake Victoria ecosystem. Stock biomass of R. argentea lakewide is yet to be determined. The species distribution and abundance in the deep open waters is not known. Until the proposed lakewide stock assessment survey is carried out, caution should be taken in interpreting these high catches and in the management of this fishery.

This paper discusses seasonal abundance of R. argentea in the Ugandan region of Lake Victoria based on the landings at Masese, the main fish market near Jinja. Experimental fishing data and samples of commercial catches are used to interpret the vertical distribution, breeding, growth and recruitment of R. argentea in the region.

2. ARTISANAL FISHING GEAR

R. argentea in Lake Victoria is fished by light attraction. On moonless nights, lighted kerosene pressure lamps tied on rafts constructed out of dry reeds (Phragmites sp.) are set on the water surface. In the Ugandan portion of the lake two main fishing methods are used;

2.1 The Beach seine

After burning for one hour or more, lamps are slowly towed towards the beach in singles, in pairs or even more than two. About 100m from the beach, a seine net is cast in a semi-circle, enclosing the lamps and the attracted fish. These are together hauled ashore and the fish retrieved. The net measures on average 50–75 metres long and 2–3 metres deep.

2.2 The lampara or “boat seine”

Lamps are set just like in the above case but instead of towing them to the beach, fishing is done in the open lake. A net is cast round a lamp and together with the lamp, it is towed and eventually hauled into an anchored canoe. This method allows more rapid and continuous fishing as no time is wasted towing the lamps towards the shore. Thus fishermen have nicknamed this method “Hurry up”. Soon after the fish have been retrieved from individual lamps, the lamps are reset. When all the lamps (normally 7–8 per fleet) have been fished, the process is repeated. The Lampara net measures 100 metres long and 3 to 4 metres deep.

The commonest net mesh size in use now is 5mm stretched. The 10mm net is totally abandoned though it had previously been in use (Wandera, 1988).

2.3 Other fishing methods.

For fishermen who cannot afford the more expensive beach seine or Lampara nets, fishing is by scoopnets, described in Mous et al (1991). In the Tanzanian waters of the lake, lift nets are also in operation (Mous, et al. op. cit.). The lift net is however not yet in use in the Ugandan waters.

3. METHODS AND MATERIALS

3.1 Fishing grounds and sampling sites

The data were collected from the artisanal fishery and experimental fishing in the Northern region of Lake Victoria (Fig. 1). Most dagaa fishing grounds are scattered round the main island of Buvuma and nearby islands. Because of the small size of fishing crafts used by artisanal fishermen, fishing operations are restricted to inshore and relatively shallow sheltered bays. The deepest waters under Dagaa exploitation in the region are the grounds around Bugaia Island and off Kasaali. Further than Bugaia Island, limited Dagaa fishing exists on a number of small islands found in these waters. However, the frequency of fishing here is dependent on favourable weather conditions.

Experimental trawling was undertaken in Napoleon Gulf. A 19 mm codend bottom trawl was operated from a research trawler M.V. Mputa. Monthly samples of fishermen's catches were taken at Kikondo-Napoleon Gulf for length frequency and other biological parameters. Catch data from two main fishing villages Lingira and Kasaali were analysed. The fishery at Lingira is mainly based within Pilkington Bay and the nearby waters of 10 – 12 metres depth. In contrast, the Kasaali fishing ground faces the exposed deeper offshore waters, more than 40 metres. These two fishing villages contribute more than 50% of the Dagaa landed at Masese.

4. RESULTS AND DISCUSSION

4.1 Biology of R. argentea

R. Argentea a small pelagic cyprinid feeds on zooplankton during daytime supplementing this diet with insect larvae at night. The species breeds throughout the year with peaks in August and December-January. Preliminary results on growth of the species indicate that R. argentea grows fast and matures at ages varying from 16 to 25 months (Wanink, 1989; Wandera & Wanink, in press; Mannini, 1992), a period which according to recent indications may be unrealistically too long for such small pelagic fish species.

Studies on vertical migrations of R. argentea have not been fully covered in this study. However, the information which is available in the Mwanza region of the lake indicates that these fishes perform diel vertical migration (Wanink, 1988). The adults of the species stay deep down the water column during daylight hours and at night, they migrate to the surface waters where they are fished. However, the juveniles and parasitized adults stay near the surface throughout the day.

The extent of the depth to which these adults descend especially in the deep waters depends on the prevailing physico-chemical conditions. Bottom trawling experiments in Napoleon Gulf during the period of strong thermal stratification, April-May and November-December reveal very few or no R. argentea at depths greater than 10m. During a recent trawling trip in August when stratification had broken down many individuals of R. argentea were recovered at the same depth. Echograms run at an offshore station off Bugaia waters in April 1992, showed strong traces of what was presumed to be shoals of R. argentea concentrated near the surface water at night. During daytime these traces occurred at a depth of about 25 metres, just above the thermocline (Hecky, unpublished data).

The adult fishes that were parasitized by a fish cestode Ligula intestinalis, together with juvenile Dagaa, seem to perform horizontal movements to and from the shores. In the mid-morning these fishes tend to move towards the beaches (mainly sandy beaches) in search of food. Juveniles feed on zooplankton (copepods and young stages of planktonic chironomids). The parasitized adults tend to feed more on insect larvae and some shore dwelling adults such as coryxid bugs (Wandera, 1988). At night these fishes mix with the up-coming adults in the top waters of the lake where they are all likely to be fished out.

4.2 Seasonal Abundance

Monthly landings of Dagaa at Masese for 1990 and 1991, are illustrated in Fig. 2. There are two peak periods of Dagaa catches; the main one is between December and January with a minor one between July and August. Low catches occur between April-May and in October with lowest catches occurring during the former period.

To explain this trend one needs an understanding of the life history of the Dagaa fishery. This involves following the species history right from breeding, growth, recruitment into the fishery and finally to quantities caught.

R. argentea breeds throughout the year but it shows two peaks. A major breeding peak occurs in December-January and a minor one in August-September (Fig. 3). Although it has been difficult to locate Dagaa eggs during these peak periods in the past, some eggs were recovered in a plankton net in August 1992 (Ndawula, pers. comm.). This confirms earlier reports that R. argentea lays pelagic eggs (Greenwood, 1966). Many larval Dagaa occur in Napoleon Gulf in the months of January (Wandera and Wanink, in press), and September (this study).

Trends in growth and recruitment of R. argentea into the fishery during the year 1992 is shown in Fig. 4. The first recruitment into the fishable stock occurred in April-May, while the second and major recruitment occurred in September. The April-May cohort is most likely the brood of August-September 1991. This appeared in the fishery eight months after spawning. These fishes were however immature (mode 38mm SL). The second cohort, most likely the brood of December-January also appeared 8 months after hatching.

It can be seen from Fig. 2 that much more Dagaa is caught during the second half of the year than during the first half. This is also confirmed by many Dagaa fishermen who were interviewed. The August-September brood which joins the fishery in April-May matures fully in July–August and at that time, catches are high. The high catches are followed by breeding in August-September (Fig. 2). Likewise, the December-January brood joins the fishery in September and is expected to be fully mature in December thus giving rise to the second high catches of December-January. The December–January breeding peak normally gives the major brood. When this brood joins the remnants of the August-September cohort, it results in higher catches especially during the last quarter of the year. The above sequence of events is summarized in Fig. 5.

Trends in quantities of Dagaa caught, appear to show at least some uniformity within the Jinja waters. Fig. 6 shows fluctuations in catches of Dagaa from two different stations, the first one Lingira, (inshore) and the other, Kasaali, representing offshore stocks. The two stations show similar trends in fluctuations and therefore it can be assumed that inshore and offshore stocks of Dagaa have a nearly similar life history. More information on the species from the two stations will be available at the end of the IFIP sponsored regional study of R. argentea which has just started in Lake Victoria.

An understanding of the nature of the Dagaa fishery is essential in the management and sustainable exploitation of this vast resource. Restriction of the size of gear has previously been suggested as a measure designed to protect R. argentea from recruitment overfishing (Ogutu-Ohwayo, et. al. 1989). By identifying those critical periods of breeding and recruitment into the fishery, an alternative measure to gear restriction is available. Closed seasons could be suggested. Fishing restrictions imposed in April to May and August to September could effectively limit exploitation of the juvenile Dagaa abundant during those periods.

At present there are no data on which to base other management measures such as closed fishing grounds and control of fishing effort. R. argentea is pelagic and is believed to lay pelagic eggs (Greenwood, 1966). It can therefore be assumed, until lakewide studies are undertaken, that the whole lake acts as a breeding ground for the species. Current levels of fishing effort are still low. A very small fraction of the lake is being exploitated. Further studies are still necessary to ascertain the species distribution and abundance lakewide. The proposed EEC-sponsored Lake Victoria Stock Assessment project should bring to light more unknown factors on the dynamics of R. argentea.

5. ACKNOWLEDGEMENT

Thanks are extended to the Research personnel of the IDRC-Nile perch Project, with whom most of the data collection was undertaken. Other gratitudes go to Mr. F. Moini for recording catch data at Masese, Miss G. Namulemo for assistance in examining samples in the laboratory, and Miss F. Bazanya for typing this manuscript. This work was carried out with the aid of a grant from the International Development Research Centre, (IDRC), Ottawa, Canada.

6. REFERENCES

Arunga, J. 1991. An overview of Lake Victoria Fisheries - Kenya sector. In Sentongo G.W. and J.N. Dampha (eds) IFIP Report of a National Seminar on the development and management of the Kenyan Fisheries of Lake Victoria. Kisumu, Kenya 22–24 July 1991 UNDP/FAO Regional Project for Inland Fisheries Planning (IFIP) RAF/87/099 - TD/27/91 (En) p. 18–20.

Greenwood P.H. 1966. The fishes of Uganda (2nd edition). Uganda Society, Kampala. 131. pp.

Mannini, P. 1992. Some characteristics of small pelagic species and possible affinities with the population of Lake Victoria Rastrineobola argentea in Mannini P. (ed.). The Lake Victoria Dagaa (Rastrineobola argentea). Report of the first meeting of the Working Group on Lake Victoria Rastrineobola argentea, 9–11th December 1991, Kisumu, Kenya. UNDP/FAO Regional Project for Inland Fisheries Planning (IFIP) RAF/87/099 - TD/38/92 (En): 62–79.

Mous, P.J., Budeba, Y.L., Temu, M.M., Van Deusen, W.L.T. 1991. A catch effort data recording system for the fishery of the small pelagic Rastrineobola argentea in the Southern part of Lake Victoria. In Cox, I.G. (ed.) Fishery News Book 335 – 343.

Ogutu-Ohwayo, R. 1990. The reduction of fish species diversity in Lakes Victoria and Kyoga (East Africa) following human exploitation and introduction of non-native fishes. J. Fish Bio. 3: 207 – 208.

Ogutu-Ohwayo, R., Twongo, T., Wandera, S.B., Balirwa J.S., 1989. Fishery gear selectivity in relation to their manufacture and to the management of fisheries of the Nile perch, the Nile tilapia and Rastrineobola argentea (mukene) in Lakes Victoria and Kyoga. A guide to fishnet manufacturers, fisheries managers and fishermen. UFFRO Occasional Pap., No. 16, 16pp.

Wandera, S.B. 1988. The study of Rastrineobola argentea (Pellegrin), (Pices Cyprinidae) and its importance in the fisheries of Lakes Kyoga and the northern waters of Lake Victoria: Presented at the Annual Symposium of the Hydrobiological Society of Eastern Africa (HYSEA) on “The state of knowledge and recent research advances in freshwater and marine biology in East Africa.” Nairobi, Kenya, 13 – 16b December 1988.

Wandera, S.B. and Wanink, J.H. in press. Growth and mortality of Dagaa Restrineobolaargentea in Lake Victoria. Prepared during HEST/TAFIRI/FAO/DANIDA Regional seminar on Fish stocks and Fisheries of Lake Victoria. Mwanza January – February 1989.

Wanink, J.H., 1988. Recent changes in the zooplanktivorous/insectivorous fish community of the Mwanza Gulf. HEST/TAFIRI report No. 45, Leiden, The Netherlands.

Wanink, H.H., 1989. The ecology and fishery of Dagaa, Restrineobola argentea (Pellegrin) 1904. In: Fish Stocks and Fisheries in Lake Victoria. A handbook to the HEST/TAFIRI & FAO/DANIDA regional seminar, Mwanza January-February, 1989.

Fig. 1 Map of Lake Victoria showing the Jinja sector.

Fig. 2 Landings of Rastrineobola argentea at Masese, 1990–1991.

Fig. 3 Breeding periodicity of Rastrineobola argentea in Lake Victoria.

Fig. 4 Length frequency distribution of Rastrineobola argentea from commercial catches in Napoleon Gulf of Lake Victoria.

Fig. 5 Life history of Rastrineobola argentea in Lake Victoria.

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Fig. 6 Landings of Rastrineobola argentea at Masese in 1991. These are landings specifically for Lingira and Kasaau.