Abstract:

Lake Nakivali is one of the four small lakes that form what is known as the Koki lakes system. It is 14 km long, 6 km wide, 26 km2 in area and has maximum depth of 3.5 m at high water level. The lake is located in a lake-swamp complex with River Rwizi as the principle inflow, and a number of peripheral lakes among which are four major ones, i.e. Lakes Nakivali, Mburo, Kachira, and Kijanebalola.

Lake Nakivali is a controlled lake with four official landing sites, namely: Kikusi, Kahirimbi, Kashojwa and Rukinga. The latter three are located within a Refugee settlement whereas Kikusi is outside. The Nakivali Refugee settlement initially established for Rwandese of Tutsi origin in 1963, now has at least seven nationalities which include people from Rwanda, Democratic Republic of Congo, Somalia, Sudan, Ethiopia, Kenya and Eritrea. By the end of 2006, the lake’s hinterland of about 378 km2 contained 43,448 people of whom
22,448 were refugees. This large population has had stressful impacts on both land and lake resources to the extent that now there is an apparent overfishing on the lake.

Measurements of water quality parameters reveal a change in some of them from the situation that prevailed about 80yrears ago. Water clarity (Secchi reading) has dropped from 0.5 to 0.25m and the pH has risen to 9.7 from 7.7. Water temperatures at 250C have however not changed much. The lake is eutrophic with concentrations of total phosphorus
171,µgL-1, a chlorophyll-a 57.9 and nitrate 25.8 µgL-1.Phytoplankton is dominated by blue green algae, greens and diatoms while benthic organisms consist mainly of molluscs, lakeflies and earthworms (oligochaetes).

Until 1935 when these lakes were stocked with new fish species from other lakes, the lake contained only 5 fish species namely the “Male” (Clarias gariepinus), “Nsonzi” (Clarias werneri), Aplocheilichthys pumilus and 2 species of “Nkejje” the haplochromines. In addition to the above species the lake currently harbours the Lake Victoria native tilapia Oreochromis esculentus, and the stocked Nile tilapia (Oreochromis niloticus), Oreochromis leucostictus and 4 species of haplochromines. Also present but not mentioned as being one of the indigenous fish species is the “Mamba”/Lungfish (Protopterus aethiopicus), an important component of the artisanal fishery on this lake.

Following the introduction of new fish species, annual fish catches from Lake Nakivali rose to a peak of 986 t in 1964 but decreased to 174 t by 1966 and further decline was apparent to the present day. Fishers target 4 fish species namely the haplochromines, P. aethiopicus, introduced tilapiines and C. gariepinus. These are caught using gillnets and hooks. The haplochromine fishery employs small meshed gillnets (1” to 11/2”), the tilapiine fishery utilizes medium sized gillnets (3” and 4”) while the Protopterus and Clarias fishery relies mostly on hooks. In this study, there were 96 canoes all of “parachute” type, each manned by one fisher. 46% of these canoes targeted haplochromines while 40% were fishing for tilapiines. 2906 gillnets were in operation at the gazetted 4 landings in the following proportions: Rukinga (44%) Kashojwa (33%), Kahirimbi (16%) and Kikusi (8%).
The bulk (65%) of these gillnets targeted haplochromines.
Highest catch rates were realized by fishers using small meshed gillnets and targeting haplochromines - 15.2 kg/boat/day while the medium sized gillnets targeting tilapiines and some P. aethiopicus realised rates of 3.9kg/boat/day. Long-line hooks catching mainly P. aethiopicus landed 6.3kg/boat/day.
Overall lake wide catches for September 2010 were estimated at 29.6 t composed of Haplochromines (71.2%), P. aethiopicus (16.4%) and O esculentus (7.5%). Other fishes, i.e. Clarias spp, O. niloticus and O. leucostictus, contributed less than 2% of the total catch. O. esculentus was the most highly priced fish species followed by O. niloticus; haplochromines had the lowest unit price. The lake wide gross landed value of catches for September 2010 was estimated at shs 34 million, distributed amongst the fish species as: haplochromines (65.9%), P. aethiopicus (18.4%), O. esculentus (9.9%) and approximately
2% for each of other species landed.
Among the two options (cage culture within the lake, and tank aquaculture on the shore) for aquaculture, it is the latter option that looks feasible. Lake Nakivali being so shallow with many floating islands (sudds) and a bottom type of organic mud, may not be suitable for cage culture. Land based tank aquaculture is however a viable option for increasing fish production especially within the refugee settlements. Candidate fish species for culture should be restricted to those already in the system to avoid contamination of the lake with foreign fishes.
Socio- economic information was restricted to the native population outside refugee settlements. It revealed that most adult males interviewed were married and had been on the lake for more than 10 years. The population was mostly illiterate as most of them had not gone beyond primary school. This level of illiteracy was attributed to lack of educational facilities but the situation was improving as many of the school going age group could now access educational facilities provided for refugees.
70% of the population around the lake is involved in fishing. Other activities include agriculture (15%) cattle keeping (10%) and business (5%). Most fishers owned “parachute” type of canoes and fished using gillnets and hooks. The Haplochromis fishery is the most important fishery which employed the majority of the people around the lake. The fishes caught were dried on sticks and sold in places as far away as Masaka and Rakai towns. Respondents were concerned about the dwindling stocks of fish which they attributed to the shrinking size of the lake resulting from human activities in the drainage basin and excess use of water in refugee settlements.
The survey therefore established that Lake Nakivali is a healthy ecosystem capable of sustaining fisheries production. While stock enhancement through restocking with fry could boost fish stocks, especially of the Nile tilapia, effective management measures that allow natural regeneration of stocks of indigenous fish species is paramount. This may require closed fishing seasons and control of fishing effort. Tank aquaculture is a viable option for increasing fish production and Ngege (O. niloticus) and Male (C. gariepinus) are candidate fish species for fish farming.