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
Experimental evidence for the biological species status in Lake Tana’s Labeobarbus flock (Cyprinidae)
Graaf, M. de; Nagelkerke, L.A.J. ; Palstra, A.P. ; Sibbing, F.A. - \ 2010
Animal Biology 60 (2010)2. - ISSN 1570-7555 - p. 183 - 193.
barbus-intermedius complex - reproductive segregation - ecological divergence - sympatric speciation - cichlid fishes - ethiopia - evolution - africa - origin - salmon
Lake Tana (Ethiopia) harbours the only known remaining intact species flock of large (max. 100 cm standard length, SL) cyprinid fishes (15 Labeobarbus spp.). In 'common garden' experiments progeny of the riverine spawning benthivorous L. tsanensis, and of the piscivorous L. truttiformis and L. megastoma was raised under similar environmental conditions to test if interspecific morphological differentiation would occur. Interspecific morphological differences and divergence were clearly observed early in ontogeny (= 40 mm SL). This study is the first to demonstrate direct proof for the genetic basis of morphological differentiation among these labeobarbs, providing further support that Lake Tana's labeobarb species are true biological species
Preliminary insight into the age and origin of the Labeobarbus fish species flock from Lake Tana (Ethiopia) using the mtDNA cytochrome b gene
Graaf, M. de; Megens, H.J.W.C. ; Samallo, J. ; Sibbing, F.A. - \ 2010
Molecular Phylogenetics and Evolution 54 (2010)2. - ISSN 1055-7903 - p. 336 - 343.
late pleistocene desiccation - barbus-intermedius complex - cichlid fishes - molecular phylogeny - reproductive segregation - ecological divergence - dna-sequences - east-africa - cyprinidae - speciation
The high diversity of Cyprinid fish in Ethiopia’s Lake Tana appears to be an example of ecological differentiation and assortative mating leading to rapid sympatric speciation. Lake Tana’s Labeobarbus species flock consists of 15 morphological and ecological distinct species. This is the first attempt to determine the age and origin and inter-species relationships of Lake Tana’s Labeobarbus species using the mtDNA cytochrome b gene. Analysis of cytchrome b sequences shows that Lake Tana’s species flock appears to be young but the present dataset did not unequivocally support monophyly of Lake Tana’s species. Additional markers are needed to determine whether Lake Tana’s labeobarbs originated from a single or multiple incursion(s) of ancestral L. intermedius in the Lake Tana drainage basin, or the disruption of an ancient continuous riverine population by the emergence of the Tissisat waterfalls. Adaptive radiation and speciation within Lake Tana’s Labeobarbus species flock may have occurred in the last 10,000–25,000 years, following the desiccation of Lake Tana around 17,000 years ago, at the same time as Lake Victoria, however, obtaining more data using other (nuclear) markers is urgently required
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.
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