Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

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Hydraulic and operational performance of irrigation schemes in view of water saving and sustainability : sugar estates and community managed schemes In Ethiopia
Dejen, Z.A. - \ 2015
University. Promotor(en): E. Schultz, co-promotor(en): S.B. Awulachew; L. Hayde. - Leiden : CRC Press/Balkema - ISBN 9789462571693 - 165
irrigatie - irrigatiewater-toedieningsschema - hydraulica - duurzaamheid (sustainability) - water - ethiopië - waterbeheer - irrigation - irrigation scheduling - hydraulics - sustainability - ethiopia - water management
Zooplankton, fish communities and the role of planktivory in nine Ethiopian lakes
Vijverberg, J. ; Dejen, E. ; Getahun, A. ; Nagelkerke, L.A.J. - \ 2014
Hydrobiologia 722 (2014)1. - ISSN 0018-8158 - p. 45 - 60.
fresh-water zooplankton - top-down control - trophic relationships - subtropical lake - nutrient state - body-size - sri-lanka - shallow - reservoirs - food
Fish and zooplankton populations of nine Ethiopian freshwater lakes were quantitatively sampled along a North–South gradient. Differences in altitude and latitude resulted in a temperature gradient from North to South. We tested three hypotheses: (1) the degree of zooplanktivory decreases with water temperature, i.e. from North to South; (2) the degree of zooplanktivory increases with the abundance of large-bodied zooplankton; and (3) the pattern of zooplanktivory in eutrophic Ethiopian water bodies differs from other tropical and temperate water bodies. Proportions of zooplanktivory in the fish communities did not show a geographical trend, but mainly depended on fish species, zooplankton density and the availability of large-bodied cladocerans. The degree of zooplanktivory in eutrophic Ethiopian water bodies differs from other eutrophic water bodies, both temperate and tropical. In Ethiopia, the degree of zooplanktivory can be both low and high, in contrast with other tropical water bodies where zooplanktivory is generally low and with temperate eutrophic water bodies where it is generally high. As a result, predation pressure on zooplankton by fish varies dramatically amongst Ethiopian water bodies.
Demonstration of Solar Tent Fish Drying Technology and Market Promotion of Lake Tana Fisheries
Yalew, A. ; Spliethoff, P.C. ; Dejen, E. - \ 2013
Comparative irrigation performance assessment in community-managed schemes in Ethiopia
Dejen, Z.A. ; Schultz, E. ; Hayde, L.G. - \ 2012
African Journal of Agricultural Research 7 (2012)35. - ISSN 1991-637X - p. 4956 - 4970.
The composition of fish communities of nine Ethiopian lakes along a north-south gradient: threats and possible solutions
Vijverberg, J. ; Dejen, E. ; Getahun, A. ; Nagelkerke, L.A.J. - \ 2012
Animal Biology 62 (2012)3. - ISSN 1570-7555 - p. 315 - 335.
rift-valley lakes - fresh-water ecosystems - species flock pisces - reproductive segregation - cyprinidae - tana - barbs - strategies - diversity - example
Fish populations of nine Ethiopian freshwater lakes were quantitatively sampled with a standardized protocol, using multi-mesh gill nets. In total, 27 species were identified, but only 14 species were common. Based on the common species, the fish communities showed large differences in their species composition, except for Lake Abaya and Lake Chamo which were similar. Most fish species were observed in only one or two lakes. Compared with the information reported in literature the present study generally underestimated the species richness. The empirical model of Amarasinghe and Welcomme (2002) for African lakes was used to estimate fish species richness, which was compared with species presence reported in literature. Biodiversity in the two northern highland lakes is low, but not lower than the model estimate. Lake Tana has a high biodiversity which is close to what is estimated by the model, but three Rift Valley lakes have low biodiversity, lower than estimated by the model. There are also strong indications for the Rift Valley lakes that species richness was higher in the past because the species richness reported in the older literature was generally much higher than those observed by us in the present study and those reported in the more recent literature. Threats like overfishing, high sediment load and degradation of habitats were identified. It is recommended that Ethiopia should develop guidelines for fishery legislation and implement it through an enforcement agency. Moreover, catchments management should be practiced to save the water bodies and their fish communities
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
Business opportunities for aquaculture in Ethiopia
Rothuis, A.J. ; Duijn, A.P. van; Kamstra, A. ; Dejen, E. ; Pijl, W. van der; Rurangwa, E. ; Stokkers, R. - \ 2012
The Hague : LEI / IMARES (LEI report 2012-003) - ISBN 9789086155798 - 120
plattelandsontwikkeling - voedselzekerheid - voedselproductie - aquacultuur - bedrijfseconomie - visserij - overheidsbeleid - ethiopië - nederland - rural development - food security - food production - aquaculture - business economics - fisheries - government policy - ethiopia - netherlands
Irrigation performance in community-managed schemes: assessment using comparative indicators and utility analysis
Dejen, Z.A. ; Schultz, B. ; Hayde, L.G. - \ 2011
In: Proceedings of the ICID 21st International Congress on Irrigation and Drainage, Tehran, Iran, 15-23 October, 2011. - Tehran, Iran : ICID - p. 17 - 17.
Growth, biomass, and production of two small barbs (Barbus humilis and B. tanapelagius, Cyprinidae) and their role in the food web of Lake Tana (Ethiopia)
Dejen, E. ; Vijverberg, J. ; Nagelkerke, L.A.J. ; Sibbing, F.A. - \ 2009
Hydrobiologia 636 (2009)1. - ISSN 0018-8158 - p. 89 - 100.
species flock - seasonal-variation - fish - zooplankton - model
Growth, biomass and production of two small barbs (Barbus humilis and Barbus tanapelagius) and their role in the food web of Lake Tana were investigated. From length–frequency distribution of trawl monitoring surveys growth coefficient, F' values were estimated at 3.71–4.17 for B. humilis and 3.70–4.14 for B. tanapelagius, respectively. Values for B. humilis were confirmed in pond experiments. Mean biomass of the small barbs was 13.3 kg fresh wt ha-1, with B. humilis being most abundant in the littoral and sub-littoral zones, whereas B. tanapelagius was most abundant in the sub-littoral and pelagic zones. The two small barbs had a production of 53 kg fresh wt ha-1 year-1. Although their P/B ratios of about 4.0 were relatively high for small cyprinids, both their biomass and production were low in comparison with other small fish taxa in other tropical lakes. Of the zooplankton production only about 29% was consumed by the small barbs. However, they did not utilize calanoid copepods, which were responsible for approximately 57% of the zooplankton production and it is likely that small barb production was food limited during certain periods of the year. Piscivorous labeobarbs consumed about 56% of the small barbs production annually, but additionally, Clarias gariepinus, and many bird species were also preying on them. Therefore, limitation of Barbus production by predation during certain periods in the year cannot be excluded
Lake Tana: Source of the Blue Nile
Vijverberg, J. ; Sibbing, F.A. ; Dejen, E. - \ 2009
In: The Nile. Origin, Environments, Limnology and Human Use / Dumont, H.J., Springer Science + Business Media B.V. (Monographiae Biologicae 89) - ISBN 9781402097256 - p. 163 - 192.
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.
Adaptive radiation of Lake Tana's Labeobarbus species flock (Pisces, Cyprinidae)
Graaf, M. de; Dejen, E. ; Osse, J.W.M. ; Sibbing, F.A. - \ 2008
In: Abstracts of the 4th International Conference of the Pan African Fish and Fisheries Association. - Addis Ababa, Ethiopia : - p. 43 - 43.
Adaptive radiation of Labeobarbus species in Lake Tana
Sibbing, F.A. ; Graaf, M. de; Dejen, E. ; Nagelkerke, L.A.J. ; Vijverberg, J. ; Osse, J.W.M. - \ 2008
In: Abstracts of the 4th International Conference of the Pan African Fish and Fisheries Association. - Addis Ababa, Ethiopia : - p. 76 - 76.
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.
Adaptive radiation of Lake Tana's (Ethiopia) Labeobarbus species flock (Pisces, Cyprinidae)
Graaf, M. de; Dejen, E. ; Osse, J.W.M. ; Sibbing, F.A. - \ 2008
Marine and Freshwater Research 59 (2008)5. - ISSN 1323-1650 - p. 391 - 407.
late pleistocene desiccation - tropical fish assemblage - cichlid fishes - barbs barbus - east-africa - reproductive segregation - ecological divergence - victoria - speciation - origin
Studying species flocks (e.g. Darwin¿s finches, Caribbean anoline lizards, East African cichlid fishes) has proven to be highly successful in understanding the forces driving speciation. The only known, intact species flock of cyprinid fishes, the 15 Labeobarbus species in Lake Tana (Ethiopia), includes eight piscivorous species. Piscivory is a rare specialisation among the highly successful (>2000 species) but mostly benthivorous Cyprinidae. The extent and mechanisms of diversification of this remarkable Labeobarbus species flock, particularly among the unexpected piscivorous species, are still largely unknown. In the present study we demonstrate that all 15 Labeobarbus species are segregated to a great extent along spatial, trophic and/or temporal dimensions. The spatial distribution, diet (prey species but not prey size), time of active feeding and predation techniques differed significantly among the eight piscivores. Lake Tana¿s cyprinids displayed their retained potential for ecological diversification and speciation, including the uncommon specialisation of piscivory. The latter is probably a result of the absence of common African specialist piscivores in Lake Tana. We suggest that the evolution of Lake Tana¿s Labeobarbus species flock at this stage is predominantly structured by ecological selection models. The labeobarbs most likely underwent sequential stages of radiation and speciation: habitat divergence followed by trophic divergence.
Lake Tana, Ethiopia: The Source of the Blue Nile
Vijverberg, J. ; Dejen, E. ; Sibbing, F.A. - \ 2007
SIL-news 2007 (2007)51. - p. 12 - 12.
Seasonal variation in primary production of a large high altitude tropical lake (Lake Tana, Ethiopia): effects of nutritient availability and water transparency
Wondie, A. ; Mengistu, S. ; Vijverberg, J. ; Dejen, E. - \ 2007
Aquatic Ecology 41 (2007)2. - ISSN 1386-2588 - p. 195 - 207.
barbs barbus - turbid waters - species flock - cyprinidae - reservoir
Primary production rates, chlorophyll and phytoplankton biovolume were measured monthly from April 2003 to November 2004 in Lake Tana, a large tropical lake in the highlands of Ethiopia. The lake is characterised by low nutrient concentrations, and a low water transparency due to high silt load of the inflowing rivers during the rainy seasons (May¿November) and daily resuspension of sediments in the inshore zone. The mean chlorophyll-a concentrations varied seasonally and ranged from 2.6 mg m¿3 to 8.5 mg m¿3 (mean: 4.5 mg m¿3) in the offshore zone. Primary production was measured using the light¿dark bottles technique. We incubated only at three depths, i.e. 0.6, 1.2 and 1.8 m. Therefore, we may have missed a substantial part of the depth production profile and probably also frequently missed P max. Gross primary production in the openwater averaged 2.43 g O2 m¿2 d¿1 and ranged between 0.03 g O2 m¿2 d¿1 and 10.2 g O2 m¿2 d¿1; production was significantly higher in the inshore zone. The highest production rates were observed in the post-rainy season (Oct¿Nov), which coincided with a bloom of Microcystis and higher chlorophyll levels. This seasonal high production is probably caused by a relatively high nutrient availability in combination with favourable light conditions. The gross primary production rates of L. Tana are among the lowest compared with other tropical lakes. This will be partly the result of our underestimation of gross primary production by often missing P max. Another cause is the oligotrophic nature of the lake in combination with its relatively low water transparency. The gross primary production per unit chlorophyll in the openwater zone was in the same range as in 30 other tropical lakes and reservoirs. The higher primary production in the inshore zone is probably the result of the daily water column mixing (Z mix ¿ Z t) in this area, enhancing nutrient recycling. A large proportion of the annual primary production is realised in one of the four seasons only. This productive post-rainy season is relatively short (2 months) and therefore efficiency of transfer of matter between the first and second trophic level of the Lake ecosystem will be poor.
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.
Predicting and testing resource partitioning in a tropical fish assemblage of zooplanktivorous 'barbs': an ecomorphological approach
Dejen, E. ; Vijverberg, J. ; Graaf, M. de; Sibbing, F.A. - \ 2006
Journal of Fish Biology 69 (2006)5. - ISSN 0022-1112 - p. 1356 - 1378.
lake tana ethiopia - species flock - sri-lanka - growth - tanapelagius - cyprinidae - reservoirs - humilis - overlap - shifts
Morphometrics on 25 critical feeding structures predicted conspicuous specializations in Barbus tanapelagius (pursuit hunting for zooplankton), Labeobarbus brevicephalus (surface dwelling pump-filter-feeder on zooplankton) and Barbus pleurogramma (particulate feeding on tough, benthic food), whereas for Barbus humilis intermediate values predicted few constraints and specializations in feeding. These potential niches, set by fish size and structural constraints, were tested by comparing gut contents collected during a 24 months sampling programme on Lake Tana, Ethiopia. Zooplankton dominated the diet of B. tanapelagius (75% of gut volume) and L. brevicephalus (39%). The guts of B. pleurogramma and juveniles of the large labeobarbs showed an array of benthic food types, whereas B. humilis had the widest food niche, both zooplankton (40% of gut volume) and benthic invertebrates. Although the pelagic species showed the largest spatial overlap, their size, feeding modes and utilization of zooplankters differed: L. brevicephalus preyed predominantly on the larger zooplankton (Daphnia sp.) and B. tanapelagius also on smaller species (e.g. Bosmina sp. and cyclopoid copepods). The spatial segregation between B. tanapelagius (pelagic) and the juvenile labeobarbs (littoral) indicated the possibility for a small pelagic barb fishery without negative effects on the labeobarb stocks. The ecomorphological approach using the 'Food-Fish Model' appeared to predict competitive positions and resource partitioning appropriately, and is of major importance to evaluate food web interactions
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