Journal of Fish Biology (1996) 49, 1244–1266 Reproductive segregation among the Barbus intermedius complex of Lake Tana, Ethiopia. An example of intralacustrine speciation? L. A. J. N  F. A. S Department of Experimental Animal Morphology and Cell Biology, Agricultural University, Marijkeweg 40, 6709 PG Wageningen, The Netherlands (Received 27 April 1996, Accepted 17 July 1996) Reproductive segregation among the large barbs Barbus intermedius complex, Cyprinidae, of Lake Tana, Ethiopia, was investigated in the mouths and upstream reaches of rivers tributary to the lake, during the spawning seasons of 1993 and 1994. The percentage of running fish of four Barbus morphotypes [Acute (Ac), Bigmouth big-eye (Be), Intermedius (In), and Shorthead (Sh)] peaks with the highest water level of the lake. Large differences are apparent in the minimum fork length of running fish among morphotypes [c. 12 cm for male Sh and 28 cm for male Bigmouth small-eye (Se)]. Significant differences occur in temporal, but especially in spatial patterns of the relative abundance of running morphotypes, when the lake is compared with the four major river mouths and their upstream reaches. These different relative abundances are clearest among Ac, Be, In, Sh and Troutlike (Tr). Also, the absence from the rivers of some morphotypes that are usually abundant in the lake itself [Barbel (Ba), Dark (Da), Lip (Li), and Zurki (Zu)], and the abundant presence in the rivers of Tr, rare in the lake, strengthen the hypothesis that spatial and/or temporal spawning segregation is present among the morphotypes. There are indications that Bighead (Bh) and Zu spawn in the lake itself, while other morphotypes were only found running in or near rivers. Combined with the morphological, ecological, and genetic differences, these data suggest the existence of several Barbus species, forming a unique species flock, which probably evolved within the lake. The presence of several, instead of one Barbus species complicates management of sustainable fisheries and protection of the lake’s biodiversity. Present results show the importance of the rivers for Barbus reproduction and stress the urgent need for careful management of fishing activities in spawning times and areas.  1996 The Fisheries Society of the British Isles Key words: spawning; species flock; biodiversity; fisheries. INTRODUCTION Lake Tana (northern Ethiopia) appears to be a natural laboratory for evolution- ary studies (Nagelkerke et al., 1994, 1995a; Sibbing et al., 1994). The large barbs of the lake, previously described as Barbus intermedius Rüppell, 1836, Cyprinidae, can be divided into at least 14 distinct morphotypes (Nagelkerke et al., 1994, 1995a; Osse et al., 1993; cf. Table I for names and abbreviations). The hypotheses about the taxonomic status of the morphotypes are: (1) they all belong to one, phenotypically plastic species: Barbus intermedius (Banister, 1973); (2) they all constitute different species; (3) there is a combination of a number of morphologically distinct species and one or several phenotypically plastic species, the latter showing late ontogenetic divergence into different Tel.: +31 317 483392 (Nagelkerke), +31 317 482715 (Sibbing); fax: +31 317 483955; email: leo.nagelkerke@morf.edc.wau.nl, nand.sibbing@morf.edc.wau.nl. 1244 0022–1112/96/121244+23 $25.00/0  1996 The Fisheries Society of the British Isles