Spawning migrations of the endemic Labeobarbus (Cyprinidae, Teleostei) species of Lake Tana, Ethiopia: status and threats
Anteneh, W. ; Getahun, A. ; Dejen, E. ; Sibbing, F.A. ; Nagelkerke, L.A.J. ; Graaf, M. de; Wudneh, T. ; Vijverberg, J. ; Palstra, A.P. - \ 2012
Journal of Fish Biology 81 (2012)2. - ISSN 0022-1112 - p. 750 - 765.
The reproductive biology of the only known intact species flock of large cyprinids, the 16 Labeobarbus species of Lake Tana (Ethiopia), has been extensively studied for the past two decades. Seven species of Labeobarbus are known to migrate >50 km upstream into tributary rivers for spawning during the rainy season (July to October), whereas eight other species are absent from these rivers and probably developed a new strategy of lacustrine spawning (macro-spatial segregation). One species (L. intermedius) probably spawns in the lake as well as in the rivers. Between the early 1990s and 2000s, the riverine spawners showed a decline of 75% in both biomass and number in both fishery independent surveys and in commercial catches. Reproductive migration makes fishes vulnerable to fisheries and other threats like habitat modifications. Lacustrine spawners are probably more resilient as they are not known to form spawning aggregations that can easily be exploited by fishermen. In addition, upstream rivers and catchments around Lake Tana are highly degraded by erosion and recently subjected to intensive habitat modification for irrigation and hydroelectric power generation. This article reviews results of field studies on the Labeobarbus spawning migration from Lake Tana to spawning rivers, giving emphasis on segregation and homing. It also summarizes existing and emerging threats which form potential causes for the decline of the migratory Labeobarbus species. Knowledge gaps on the reproductive biology are identified for further investigation
Past, current and potential production of fish in lake Ziway - Central Rift Valley in Ethiopia
Spliethoff, P.C. ; Wudneh, T. ; Tariku, E. ; Senbeta, G. - \ 2009
Wageningen : Wageningen UR Centre for Development Innovation - 31
The Ethiopian / Netherlands project: Ecosystems for Water, Food and Economic Development aims to strengthen local authorities, development organisations and the private sector in the field of sustainable land and water use and sound environmental planning and management, with the aim to contribute to the sustainable development of the CRV . So far the project has a strong focus on water/ agriculture and over the past few years, the project has developed a sound knowledge base of the problems at stake in the Central Rift Valley (CRV). Major conclusion of the project is that the current way of water and land resources development is not sustainable and will sooner of later result in an environmental and humanitarian crises. One of the main reasons for this deteriorating situation is the surface water extraction for irrigation purposes by smallholders in the upstream areas, resulting in the watershed-wide drop of surface water tables and increased Stalinization of water resources. These changes directly impact the resource productivity of downstream water resources like Lake Ziway and the Bulbula river. Ultimately the drop in surface water will influence livelihood strategies of communities living down streams and reduce the resilience of ecosystems and the biodiversity of lake Abiyata and the national park. There is ample evidence from similar cases in Ethiopia, that the water resources will gradually become saline, will silt up as a result of increased erosion and will become ecologically poor waste lands.
Lake Tana's (Ethiopia) Labeobarbus Species Flock (Cypriniformes: Cyprinidae): a Future of Biodiversity Conservation and Sustainable Exploitation?
Graaf, M. de; Nagelkerke, L.A.J. ; Dejen, E. ; Wudneh, T. ; Osse, J.W.M. ; Sibbing, F.A. - \ 2008
In: Proceedings of the 4th International Conference on African Fish and Fisheries, Addis Ababa, Ethiopia, 22-26 September 2008. - Tervuren, belgium : Royal Museum for Central Africa - p. 31 - 47.
Lake Tana, the source of the (Blue) Nile, is situated in the north-western highlands of Ethiopia and harbours an extraordinary diversity of cyprinid fishes. While cyprinid fishes are common and abundant throughout the world’s fresh water systems, the Labeobarbus species of Lake Tana form the only remaining intact species flock of large cyprinid fishes. Lake Tana and its Labeobarbus species flock provide(d?) an unique opportunity to study the selective forces driving speciation due, among others, to its relatively undamaged state. However, this undamaged state of the Labeobarbus species flock is seriously threatened by anthropogenic activities that have intensified over the past 30 years. Between the 1990s and early 2000s, Labeobarbus stocks decreased by 75%, most likely due to the increased fishing pressure after the introduction of a motorized commercial gillnet fishery. Many of the lake’s Labeobarbus species are highly vulnerable to exploitation during their spawning aggregations and upstream migrations. Erosion due to poor land use might have also contributed to habitat degradation of the upstream spawning sites. Between 2000 and 2010 the commercial fishing fleet has expanded from 5-10 to 50-100 boats, but the Labeobarbus CPUE of the commercial fishery appeared to have declined a further ~50% over the same period. A (final) blow to the survival of the species flock will probably be the planned and realized (Rib River) irrigation dams in the spawning rivers.
|Vulnerability to a small-scale commercial fishery of Lake Tana's (Ethiopia) endemic Labeobarbus compared with African catfish and Nile tilapia: an example of recruitment overfishing?
Graaf, M. de; Zwieten, P.A.M. van; Machiels, M.A.M. ; Lemma, E. ; Wudneh, T. ; Dejen, E. - \ 2008
In: Abstracts of the 4th International Conference of the Pan African Fish and Fisheries Association. - Addis Ababa, Ethiopia : - p. 6 - 6.
Vulnerability to a small-scale commercial fishery of Lake Tana's (Ethiopia) endemic Labeobarbus compared with African catfish and Nile tilapia: An example of recruitment-overfishing?
Graaf, M. de; Zwieten, P.A.M. van; Machiels, M.A.M. ; Lemma, E. ; Wudneh, T. ; Dejen, E. ; Sibbing, F.A. - \ 2006
Fisheries Research 82 (2006)1-3. - ISSN 0165-7836 - p. 304 - 318.
oreochromis-niloticus - species flock - reproductive segregation - perch upsurge - barbs barbus - cyprinidae - victoria - impact
In 1986 a motorised, commercial gillnet fishery was introduced in Lake Tana, Ethiopia's largest lake (3050 km2) in addition to the artisanal, predominantly subsistence fishery conducted from reedboats. The three main species groups targeted by this fishery are a species flock of endemic, large Labeobarbus spp., African catfish (Clarias gariepinus) and Nile tilapia (Oreochromis niloticus). The commercial gillnet fisheries was monitored during 1991-1993 and in 2001 (CPUE, effort allocation, catch composition, yield) and the development of the stocks of the three species groups was determined using an experimental trawl program during 1991-1993 and 1999-2001. In 1991-1993 the vast majority of fishing activities took place in the southern Bahar Dar Gulf (71%). Each species group contributed roughly one third to the total catch. In 2001, 41% of the effort was allocated to the north-eastern shores of Lake Tana. Both the CPUE and the contribution of O. niloticus to the total catch had doubled. However, the proportion of large specimen (>50 cm TL C. gariepinus; >20 cm FL O. niloticus) in the trawl surveys decreased significantly. A three-fold decline in abundance of the anadromous Labeobarbus species occurred. The 15 large labeobarb species are long-lived, ecologically specialised endemics. The seven riverine spawning Labeobarbus species form aggregations in the river mouths in August-September, during which period they are targeted by the commercial gillnet fishery. A sharp decrease in abundance by ca. 75% of the migratory riverine spawning Labeobarbus species in the sublittoral and pelagic zones of the lake, areas where no fishing takes place and the collapse of juvenile Labeobarbus (between 5 and 18 cm FL: by 90%) during the 1990s suggest recruitment-overfishing. To prevent the possible extinction of the unique Labeobarbus species flock all fishing effort should be severely restricted near the river mouths and on the upstream spawning areas during August-September (peak breeding period) to protect the vulnerable spawning aggregations
|Lake Tana Fishery and Sustainable Development
Dejen, E. ; Graaf, M. de; Nagelkerke, L.A.J. ; Sibbing, F.A. ; Wudneh, T. ; Osse, J.W.M. - \ 2006
Bahir Dar, Ethiopia : Amhara Region Agricultural Research Institute & Wageningen University - 84 p.
Declining stocks of Lake Tana's endemic Barbus species flock (Pisces, Cyprinidae): natural variation or human impact?
Graaf, M. de; Machiels, M.A.M. ; Wudneh, T. ; Sibbing, F.A. - \ 2004
Biological Conservation 116 (2004)2. - ISSN 0006-3207 - p. 277 - 287.
ethiopia - ecology - africa - fishes
The only remaining species flock of endemic, large cyprinid fishes is found in Lake Tana, Ethiopia. A monthly experimental trawl program was conducted in 1991-1993 and 1999-2001, sampling 12 stations distributed over three habitats differing in depth and distance to shore. The aim was to compare the total abundance, spatial distribution and proportion of juveniles of the most common Barbus species in the Bahar Dar Gulf between both periods. We found a sharp reduction (75%) in total abundance, both in number and biomass of the Barbus species and even more (90%) in the number of juveniles between the two periods. However, the spatial distribution of the different Barbus species over the three habitats had not changed. High natural variability in fish stocks might be expected in environmentally unstable lakes. Although strongly pulsed (seasonal), Lake Tana is a relatively stable system. No major differences were found in abiotic parameters in 1990s that could have caused the dramatic changes in abundance. The most likely explanation is the negative impact of the motorised, commercial gillnet fishery targeting the spawning aggregations of these barbs. The drastic decline in juveniles points especially towards serious recruitment over-fishing. The results stress the need for the immediate development of a sound management plan focussing on fishing effort restrictions during the Bat-bus breeding season. (C) 2003 Elsevier Ltd. All rights reserved.
Length at maturity and gillnet selectivity of Lake Tana's Barbus species (Ethiopia): Implications for management and conservation
Graaf, M. de; Machiels, M.A.M. ; Wudneh, T. ; Sibbing, F.A. - \ 2004
Aquatic Ecosystem Health and Management 6 (2004)3. - ISSN 1463-4988 - p. 325 - 336.
Lake Tana’s 15 large Barbus species form the only known intact endemic cyprinid species flock left in the world. The barbs contribute around one third of the total annual catch of the motorised commercial gillnet fishery which was introduced in 1986. A dramatic reduction of the adult Barbus stocks and the even lower proportion of recruits at the end of the 1990s, show the necessity for the development, implementation and control of fisheries legislation in Lake Tana. The reproductive biology of the Barbus species, essential for fishery management, is poorly known. This paper presents results on size at maturity, size at harvest and gillnet selectivity curves, which can be used to provide a scientific base for management proposals. Size at maturity varied widely among the Barbus species, ranging from 18.8 cm in Barbus brevicephalus to 44.3 cm in Barbus crassibarbis. Males matured at smaller size and reached smaller maximum length than females. Estimated selectivity curves fitted closely or
were slightly larger than the observed length-frequency distribution of the commercial catch. The vast majority (85%) of barbs landed by the commercial gillnet fishery were mature. Fishing pressure on juvenile, immature fish is unlikely to be the cause of the observed decrease in Barbus stocks. Size control regulations like mesh sizerestrictions, intended to protect the immature part of fish populations are expected to have little positive effects on the Barbus stocks and are therefore not recommended. The drastic reduction in barbs during the 1990s is most like due to recruitment overfishing, that is, poorly regulated high fishing effort by the commercial gillnet fishery on the spawning aggregations of adult barbs during their annual breeding migration in river mouths and
surrounding floodplains. Only effort control regulations limiting the gillnet fishery in the spawning season and/or areas will prevent a total collapse of the Barbus stocks as has happened to other cyprinids in African lakes. Such
measures have to be implemented urgently to guarantee the conservation of Lake Tana’s unique biodiversity as a sustainable source of cheap protein and as a natural laboratory to study the evolutionary processes underlying speciation in freshwater fish.