View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Wageningen University & Research Publications Biology and management of fish stocks in Bahir Dar Gulf, Lake Tana, Ethiopia Tesfaye Wudneh Promotor: dr. E.A. Huisman, Hoogleraar in de Visteelt en Visserij Co-promotor: dr. ir. M.A.M. Machiels Universitair docent bij leerstoelgroep Visteelt en Visserij Biology and management of fish stocks in Bahir Dar Gulf, Lake Tana, Ethiopia Tesfaye Wudneh Proefschrift ter verkrijging van de graad van doctor op gezag van de rector magnificus van de Landbouwuniversiteit Wageningen, dr. C.M. Karssen, in het openbaar te verdedigen op maandag 22 juni 1998 des namiddags te half twee in de Aula van de Landbouwuniversiteit te Wageningen. Cover : Traditional fishing with reed boat and a motorised fishing boat (back-cover) on Lake Tana. Photo: Courtesy Interchurch Foundation Ethiopia/Eritrea (ISEE), Urk, the Netherlands. Cover design: Wim Valen. Printing: Grafisch Service Centrum Van Gils b.v., Wageningen CIP-DATA KONINKLIJKE BIBLIOTHEEK, DEN HAAG Wudneh, Tesfaye Biology and management of fish stocks in Bahir Dar Gulf, Lake Tana, Ethiopia / Tesfaye Wudneh. - [S.I. : s.n.]. - III. Thesis Landbouwuniversiteit Wageningen. - With ref. - With summary in Dutch. ISBN 90-5485-886-9 Tesfaye Wudneh 1998. Biology and management of fish stocks in Bahir Dar Gulf, Lake Tana, Ethiopia. The biology of the fish stocks of the major species in the Bahir Dar Gulf of Lake Tana, the largest lake in Ethiopia, has been studied based on data collected during August 1990 to September 1993. The distribution, reproduction patterns, growth and mortality dynamics and gillnet selectivity of these stocks are described. The major fish categories, Barbus spp., Clarias gariepinus, and Oreochromis niloticus contribute equally to the catches. O. niloticus is most abundant in the shallow littoral zone, while C. gariepinus and the larger piscivorous Barbus species are found mainly in the deeper open water area of the lake. These larger species are mainly exploited by the motorised boat fishery. The catch from the reed boat fishery, confined to the littoral zone, consists mainly of O. niloticus and Barbus tsanensis. The breeding activity of all major species is associated with the rainy period and increase in lake water level. C. gariepinus has a short breeding period in July whereas O. niloticus and B. tsanensis show extended activity with peak breeding during July and September-October respectively. They are fully recruited to the fishery at 2 - 4 years of age well beyond the age of maturity. During the breeding period a higher catch rate is observed for Barbus species. The sustainable potential yield of 32 kg×ha-1×yr-1 estimated for the Bahir Dar Gulf area, indicates that there is scope for some expansion of the fishery. The shortage in fish storage, distribution and marketing facilities, the limited local market and lack of fishing tradition are major constraints to the development of the fishery. PhD thesis, Fish Culture and Fisheries Group, Wageningen Institute of Animal Science, Wageningen Agricultural University, P.O. Box 338, 6700 AH Wageningen, The Netherlands. This research was supported by: Interchurch Foundation Ethiopia/Eritrea (ISEE), Urk. Interchurch Coordination Commission for Development Projects (ICCO), Zeist. Wageningen Agricultural University, Wageningen. Fisheries Resources Development Department, Ministry of Agriculture, Addis Ababa. Bureau of Agriculture Bahir Dar, Amhara Regional Administration. Ethiopian Orthodox Church, Development Department. Table of contents: Summary ----------------------------------------------------------------------------------------------------1 Samenvatting----------------------------------------------------------------------------------------------3 1. General introduction------------------------------------------------------------------------------------5 2. Study area, sampling and data handling-----------------------------------------------------21 3. Reproduction patterns of Clarias gariepinus, Oreochromis niloticus and Barbus tsanensis --------------------------------------------------------------------------------------31 4. Temporal and spatial dynamics in the size-related distribution patterns of Clarias gariepinus, Oreochromis niloticus, Barbus tsanensis and Barbus platydorsus-----------------------------------------------------------------------------------------------43 5. Growth and mortality estimates of Clarias gariepinus, Oreochromis niloticus and Barbus tsanensis--------------------------------------------------------------------------------53 6. The selective impact of a gillnet fishery on the assemblage of Clarias gariepinus, Oreochromis niloticus and polymorphic Barbus species-----------71 7. Temporal and spatial patterns in the gillnet fishery for Clarias gariepinus, Oreochromis niloticus and Barbus spp. and the formulation of the catch effort data recording system.------------------------------------------------------------------------------85 8. Zooplankton distribution and production in Bahir Dar Gulf, Lake Tana.----- 103 9. Options for the management of the fishery in Lake Tana--------------------------- 117 References --------------------------------------------------------------------------------------------- 129 Acknowledgements--------------------------------------------------------------------------------- 141 Curriculum vitae-------------------------------------------------------------------------------------- 143 Summary Since 1992 Ethiopia has become a land locked country depending only on the inland water resources for the supply of fish as a cheap source of protein. Ethiopia has a number of lakes and rivers with important fish resources. Most of the lakes are in the Rift Valley system. The total area of the lakes is about 7500 km2. The exploitation of the different waterbodies is very uneven. Those located near the capital, Addis Ababa, and having good road connections, like Lake Zewai and Lake Awassa, have been heavily exploited to the extent of overfishing. Lake Tana which is located 500 km from Addis Ababa is among the least exploited lakes, in spite of its size of 3200 km2 and fish resources. One of the largest Rift Valley lakes, Chamo, contains Nile perch, Lates niloticus. Although it is as far from Addis as Lake Tana, the fishing activity is high due to the high price for this fish, being more expensive than beef. Total fish production from all lakes in 1996 was about 7000 tons, corresponding to 10 kg⋅ha-1⋅yr-1, to which Lake Tana contributed 17% corresponding to 4 kg⋅ha-1⋅yr-1. For Lake Tana fish, prices are ten times lower than those of beef. The highly priced Nile perch is absent from the lake, which has attracted little attention from fishermen and developers. The fishery is mainly confined to the southern part of the lake, around Bahir Dar Gulf, with an area of ±300 km2. There are two types of fisheries. One type is the traditional subsistence fishery using papyrus reed boats and 80 mm stretched meshed monofilament gillnets, totalling 113 reed boats. These exploit the inshore fish community consisting of Nile tilapia, Oreochromis niloticus and barbel, Barbus tsanensis. The other type is the motorised boat fishery which started in 1986. This exploits a wider lake area including the open water fish community of the large, piscivorous Barbus spp. and the African catfish, Clarias gariepinus using 100 to 140 mm stretched mesh multifilament gillnets. The number of motor boats amounts to 23. The introduction of new fishing technology was expected to result in a quick increase in utilisation of the resource. It was, therefore, decided to conduct an accompanying research programme in order to: 1) get biological information on the fish stocks, stock size, species composition, spatial distribution, size structure, recruitment, growth and mortality and exploitation rate; 2) define the state of the developing fishery with motorised boats, its interference with the traditional reed boat fishery and their combined impact on the structure and diversity of the fish community; and 3) identify and assess the options for management given the potential production of the fish stock in the lake, the exploitation patterns and the socio-economic conditions prevailing in the area. For this purpose research was done on the identification and distribution of the major fish species, their reproduction biology, growth and mortality dynamics, the selective impact of the gillnet fishery and fishing patterns in space and time, in order to improve the catch and effort data recording system. A distinct spatial segregation in the distribution of the fish community was apparent with the large, piscivorous Barbus platydorsus being found only in water deeper than 6 m while O. niloticus was confined to the littoral area less than 3 m deep. C. gariepinus and B. tsanensis, appeared in all depth ranges, with greater abundance of C. gariepinus in areas over 1 6 m deep and B. tsanensis at depths between 3 and 6 m. Reed boats caught an average of 12 kg⋅trip-1 with only 2% C. gariepinus. The catch of the motorised boats contained C. gariepinus with a mean length of 54 cm, O. niloticus of 27 cm and Barbus spp. of 31 cm, totalling an average 177 kg⋅trip-1. The Barbus spp. in the catch consisted about 50% of B. tsanensis and the other 50% contained larger barbs, mostly B. platydorsus and B. megastoma. The catch rates of 150 kg⋅trip-1 in the littoral area and 270 kg.trip-1 in river mouth areas exceeded