REPRODUCTIVE SEGREGATION AMONG THE BARBUS-INTERMEDIUS COMPLEX OF LAKE-TANA, ETHIOPIA - AN EXAMPLE OF INTRALACUSTRINE SPECIATION

Reproductive segregation among the large barbs Barbus intermedius comp lex, Cyprinidae, of Lake Tana, Ethiopia, was investigated in the mouth s and upstream reaches of rivers tributary to the lake, during the spa wning seasons of 1993 and 1994. The percentage of running fish of four Barbus morphotypes [Acute (Ac), Bigmouth big-eye (Be), Intermedius (I n), 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 Bigmo uth small-eye (Se)]. Significant differences occur in temporal, but es pecially in spatial patterns of the relative abundance of running morp hotypes, when the lake is compared with the four major river mouths an d their upstream reaches. These different relative abundances are dear est among Ac, Be, In, Sh and Troutlike (Tr). Also, the absence from th e rivers of some morphotypes that are usually abundant in the lake its elf [Barbel (Ba), Dark (Da), Lip (Li), and Zurki (Zu)], and the abunda nt presence in the rivers of Tr, rare in the lake, strengthen the hypo thesis that spatial and/or temporal spawning segregation is present am ong the morphotypes. There are indications that Bighead (Bh) and Zu sp awn in the lake itself, while other morphotypes were only found runnin g in or near rivers. Combined with the morphological, ecological, and genetic differences, these data suggest the existence of several Barbu s species, forming a unique species flock, which probably evolved with in the lake. The presence of several, instead of one Barbus species co mplicates management of sustainable fisheries and protection of the la ke's biodiversity. Present results show the importance of the rivers f or Barbus reproduction and stress the urgent need for careful manageme nt of fishing activities in spawning times and areas. (C) 1996 The Fis heries Society of the British Isles