This paper aims at assessing the reproductive segregation within the endemic Barbus (renamed Labeobarbus) species flock of Lake Tana (Ethiopia). Migration, followed by temporal and spatial reproductive segregation in the upstream tributaries of two inflowing rivers was studied systematically over the 1999 and 2000 spawning seasons. Physical events that may trigger lacustrine migration and characterise suitability of spawning grounds were analysed. Six species migrate 30-40 km upstream Gumara River during declining flow, just after the rainy season. Spawning occurs in the well-oxygenated gravel beds of four Gumara tributaries. Eight 'large barb' species were absent from the rivers, or found only incidentally, thus segregating them at a macro-spatial scale from the six river-spawners. The missing species spawn either in the lake or in other rivers not sampled. Long distance migration and species-specific spawning sites suggest that homing may have facilitated reproductive isolation and speciation. A fine-tuning between homing and gonad development is suggested since females reach spawning maturity only as they arrive at the spawning grounds. This study provides convincing evidence for reproductive segregation, and contributes in unravelling the evolution of this unique Labeobarbus species flock. Collective migration and riverine spawning of six Labeobarbus species makes them very vulnerable for overfishing at the spawning areas.
Lake Tana, located in the north-western highlands of Ethiopia, contains a unique assemblage of cyprinid fishes. In addition to the only known intact species flock of large (max. 100 cm forklength (FL)) Labeobarbus species, the lake harbours three small (<10 cm FL) Barbus species: B. humilis Boulenger, 1902, B. pleurogramma Boulenger, 1902 and B. tanapelagius de Graaf, Dejen, Sibbing and Osse, 2000. Phylogenetic relationships of the small Barbus species of Lake Tana were investigated using the mtDNA cytochrome b gene. All small Barbus species occurring in Lake Tana are part of the 'small African diploid' groups identified by Tsigenopoulos et al. (2002). Barbus pleurogramma, only reported in Lake Tana, and populations of the morphologically similar species, B. paludinosus Peters, 1852, collected in rivers and lakes throughout Ethiopia, comprised a monophyletic group with separate clades congruent with drainage basins. Barbus pleurogramma could well be a cryptic species rather than synonymous to B. paludinosus as previously suggested. The genetic divergence between B. humilis and B. tanapelagius was very low, and without lineage sorting of haplotypes. This is probably due to recent (<16 000 years ago) evolution of the pelagic, zooplanktivorous B. tanapelagius from the benthic, omnivorous B. humilis after the formation of Lake Tana.
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.