Fishing gear selectivity for Lates niloticus L., Oreochromis niloticus L. and Rastrineobola argentea P. in Lakes Victoria, Kyoga and Nabugabo

Catch effort data on which fisheries management regulations are sometimes based are not available for most lakes in Uganda. However, failure to regulate fishing gears and methods has been a major cause of collapse of fisheries in the country. Fisheries have been damaged by destructive and non-selective fishing gears and methods such as trawling and beach seining, by use of gill nets of mesh size which crop immature fish and by introduction of mechanised fishing. Selectivity of gears used to crop Lates niloticus L.(Nile perch), Oreochromis niloticus L. (Nile tilapia) and Rastrineobola argentea Pellegrin (Mukene) which are currently the most important commercial species in Uganda were examined in order to recommend the most suitable types, sizes and methods that should be used in exploiting these fisheries. Gill nets of less than 127 mm mainly cropped immature Nile tilapia and Nile perch. To protect these fisheries, the minimum mesh size of gill nets should be set at 127 mm. Seine nets of 5 mm caught high proportions in immature Mukene while those of 10 mm caught mainly mature Mukene. When operated inshore, both sizes caught immature Nile perch and Nile tilapia as by-catch. To protect the Mukene fishery and avoid catching immature bye-catch, a minimum mesh size of the Mukene net should be 10 mm operated as Lampara type net offshore, but since most fishermen have been using 5 mm seine nets for over five years the minimum size should not be allowed to drop below 5 mm pending further thorough investigations. Beach seining and trawling are destructive to fisheries and should be prohibited until data that may justify their use is available.

The exploitation of small pelagic fishes of the great lakes of Africa with reference to the mukene (Rastrineobola argentea) fishery of the northern waters of Lake Victoria

Small pelagic fish species have ultimately become important on most of the Great Lakes of Africa, especially after the depletion of the larger, initially preferred fish species. In Lake Victoria, the small pelagic cyprinid Rastrineobola argentea is now the only abundant native species supporting a fast growing light fishery. In Pilkington Bay, off Lingira Island the artisanal light fishery is well established and in the last two years this bay has witnessed a sharp increase in the fishing effort. This has been followed by a modification of fishing method and a reduction in the mesh size of nets used. R. argentea now caught from this bay consist of mainly juveniles and this could result into localized recruitment overfishing. Drawing examples from what is happening to the fishery in Pilkington Bay, it is necessary to carry out research on the stocks, gear and suitable fishing crafts before light fishing spreads to most parts of the lake.

Lake Nabugabo: a potential reservoir for mukene (Rastrineobola argentea)

"Mukene"Rastrineobola argentea occurs in lakes Victoria, Kyoga, Nabugabo and the Victoria Nile. On lakes Victoria and Kyoga the commercial fishery is well developed. While some fishing occurs at isolated places on the Victoria Nile, the species is yet to be exploited on lake Nabugabo although the potential exists. Experimental fishing (light fishing and beach seining) was conducted on Lake Nabugabo based at the Lake Nabugabo Holiday and Conference Center. Specimens obtained were analyzed for biological and fisheries parameters. Mukene on Lake Nabugabo still grows to a large adult size compared to the Lake Victoria populations. Size at first maturity at 42mmSL is close to that observed on Lake Victoria. Peak breeding occurred between August and January. Unlike on Lake Victoria, R.argentea in Lake Nabugabo is not infested by the cestode Ligula intestinalis that is common in these fishes. Catch rates of 9.8 kg/lamp/hr by the lampara net on this lake compare favorably with many parts of Lake Victoria and on Lake Kyoga where a commercial fishery exists. The amount of by-catch species in the catches however is high and does not therefore favour the establishment of the fishery on this lake. Human exploitation of mukene on Lake Nabugabo would also affect the Nile perch fishery which depends on the species as a source of food.

Adoption of catamaran fishing method for mukene (Rastrineobola argentea) in off shore waters of Lake Victoria

This report presents information on the development of a system for commercial harvesting of Rastrineobola argentea (Mukene) in Lake Victoria. The objective of this work is to develop a system for commercial Harvesting of Mukene with: •Targeted output above 1,000 kg per working night per boat. •Target area to be off shore waters of Lake Victoria. •Drying under hygienic conditions for production of high quality poultry and animal feeds. •Supply to be continuous with predicable prices

Some biological and ecological considerations for research in the management of the fisheries of Lakes Victoria and Kyoga

There is little doubt that the exploitation of the current fisheries of Lakes Victoria and Kyoga requires close monitoring with a view to enforce adherence to prudent management practices. Many indigenous fishes have gradually disappeared from the commercial fishery of both lakes. In the Uganda portion of Lake Victoria for instance Okaronon and Wadanya (in press) have shown that:- 1. The once preponderant haplochromiine taxon ceased to feature in the commercial catches in 1979. 2. The lung-fish (Protopterus aethiopicus) which formerly contributed significantly to the commercial landings had declined to minor species status by the mid 1980s. 3. The catfishes Clarias mossambicus and Bagrus docmac, formerly major fish species, contributed insignificant quantities to the commercial fishery since the early and mid 1980s, respectively. Similar trend have been "reported in the Kenya and Tanzania portions of Lake Victoria (Bwathondi, 1985; Mainga, 1985, Witte and Goudswaard, 1985). On the other hand since their establishment all round the lake in the early to mid 1980s some introduced fishes namely Nile perch (Lates niloticus) and Nile tilapia (Oreochromis niloticus) built up impressive stocks in Lake Victoria (Ssentongo and Welcomme, 1985, Okaronon et al. 1985; Okaronon and Wadanya, in press). Togetther with the native pelagic cyprinid Rastrineobola argentea or Dagaa/Mukene the introduced fishes have contributed unprecidented catches, stimulating vibrant commercial fisheries which have yielded significant social economic benefits to the peoples of the three states riparian to the Lake (Reynolds and Greboval, 1988; Kudhongania et al in press). The impressive landings particularly of the Nile perch and Nile tilapia have also led to rapid industrialisation of fish processing in East Africa mainly for the export market. Fish export has now the potential of a major foreign exchange enterprise in the region.

The present status of the fishery for tilapia in Lake Victoria (Uganda)

The commercial fisheries of Lake Victoria are presently dominated by three species: the stocked Lates niloticus and Oreochromis niloticus, and the endemic cyprinid Rastrineobola argentea. The three comprise at least 90% of the commercial catch while the rest of the endemic species mostly occur as by-catch (incidental catch) except in localised areas. Apart from being a major source of food, the three species especially the Nile perch represent the usually recognized main forms of predation, As they exert a "top-down" effect on production, they are important in the trophic dynamics of the Lake Victoria ecosystem. However, another form of predation which is usually unrecognized in the lake productivity mechanisms is one due to fishing mortality. Fishermen essentially behave as predatory elements in the ecosystem. This is manifested in ways that paral1el the effect of fish as predators e.g. some fishermen are habitat restricted and specialised in catching particular species or sizes, others are opportunistic and switch to whatever species (prey) are available which may depend on season, etc. There are also indirect factors that influence fishing mortality as a form of predation e.g. availability on the market of different gears, thefts of nets and of fish from nets, civil strife, market demand etc. The fatter are essentially socioeconomic factors. Application of the principles of fisheries management requires "a data base from which effective options can be generated. It is considered that one of the fundamental requirements for such a data base is information on the spatial distribution of the species fishery. This can be combined with information on landings which can eventually be incorporated into a programme of stock monitoring. The aim of this paper is to highlight information on the Tilapia fishery that may benefit fisheries management.

The future prospects for the Lake Victoria fishery

The annual fish yields from Lake Victoria have been increasing when the species diversity was dramatically declining. The new fishery is dominated by the introduced predator (Lates niloticus Linne), the exotic herbivore (Oreochromis niloticus Linne) and an indigenous zooplanktivore (Rastrineobola argentea Pellegrin). While the three predominant fish species could sustain the high catch levels, the future viability for the fishery will depend on prudent exploitation regimes and effective management and conservation strategies.

Nabugabo lakes and their current conservation status

Fish species diversity in Lake Nabugabo, Uganda, has declined following establishment of the introduced fish species in the lake. Most of the native fish species have disappeared and the lake is now dominated by the introduced Nile perch, Lates niloticus and the Nile tilapia Oreochromis niloticus. The dominant native fish species include Synodontis afrofischeri, Schilbe intermedius, Rastrineobola argentea and Brycinussadleri. Some of the native fish species that have disappeared from Lake Nabugabo were reported to occur in lakes Kayugi and Kayanja, which are adjacent to Lake Nabugabo but separated from it by extensive papyrus swamps. The Nabugabo lakes are satellite water bodies in the Lake Victoria basin, which is known to have experienced fish species changes due to the introduction of the Nile perch Lates niloticus during the 1960s.The Nabugabo lakes comprising of Lake Nabugabo main, and the smaller lakes Kayanja and Kayugi were investigated between 2000 and 2002 with experimental gill netting to evaluate the potential of these lakes in conservation of fish species diversity. Results show that some native fish species especially Oreochromis esculentus, and Oreochromis variabilis and the haplochromine cichlid Prognathochromis venator that have disappeared from Lake Nabugabo still occur in Lakes Kayanja and Kayugi. Inshore habitats with macrophyte cover were also found to be important habitats for the endangered native fish species in the Nabugabo lakes. These lakes and inshore habitats need to be protected to conserve the endangered native fish species and to reduce further decline in fish species diversity.

The changing fisheries of Lake Victoria, Uganda

Lake Victoria had a complex multi-species fishery dominated until the late 1970s by the tilapiine and haplochromine cichlids but with important subsidiary fisheries of more than 20 genera of non-cichlid fishes. Since the 1980s, however, the fishery has been dominated by the Nile perch (Lates niloticus), Nile tilapia (Oreochromis niloticus eduardianus) both introduced into the lake during the late 1950s and early 1960s and Rastrineobola argentea, a pelagic cyprinid. Although the actual landed commercial catch figures from the Uganda sector of the lake point to the increased landings since 1984, the catch rates in the experimental trawl fishery and mean weight of fish in both experimental trawl and commercial fishery have been on the decline.

Impact of stability in prey supply to the stocks of the Nile perch in Lakes Victoria, Kyoga and Nabugabo

The food of the Nile perch has changed since its introduction into Lakes Victoria, Kyoga and Nabugabo and stabilized on Caridina nilotica, Anisopteran nymphs, Rastrineobola argentea, Nile perch juveniles, and tilapiines. For the Nile perch to sustain production in these lakes, its is important that these prey species are properly managed.

An examination of the usefulness of single-species biomass models for the management of Lake Victoria fisheries

Nile perch (Lates niloticus), tilapia (Oreochromis spp), dagaa (Rastrineobola argentea, silver cyprinid), and haplochromines (Tribe Haplochromini) form the backbone of the commercial fishery on Lake Victoria. These fish stocks account for about 70% of the total catch in the three riparian states Uganda, Kenya, and Tanzania. The lake fisheries have been poorly managed, in part due to inadequate scientific analysis and management advice. The overall objective of this project was to model the stocks of the commercial fisheries of Lake Victoria with the view of determining reference points and current stock status. The Schaefer biomass model was fitted to available data for each stock (starting in the 1960s or later) in the form of landings, catch per unit effort, acoustic survey indices, and trawl survey indices. In most cases, the Schaefer model did not fit all data components very well, but attempts were made to find the best model for each stock. When the model was fitted to the Nile perch data starting from 1996, the estimated current biomass is 654 kt (95% CI 466–763); below the optimum of 692 kt and current harvest rate is 38% (33–73%), close to the optimum of 35%. At best, these can be used as tentative guidelines for the management of these fisheries. The results indicate that there have been strong multispecies interactions in the lake ecosystem. The findings from our study can be used as a baseline reference for future studies using more complex models, which could take these multispecies interactions into account.