Reproductive strategies of two sympatric 'small barbs' (barbus humilis and b. tanapelagius, cyprinidae) in lake tana, ethiopia

Netherlands Journal of Zoology 52 (2-4): 281-299 (2003) REPRODUCTIVE STRATEGIES OF TWO SYMPATRIC ‘SMALL BARBS’ (BARBUS HUMILIS AND B. TANAPELAGIUS , CYPRINIDAE) IN LAKE TANA, ETHIOPIA by E. DEJEN 1 , F.A. SIBBING 1 and J. VIJVERBERG 2 (1 Experimental Zoology Group, Wageningen Institute of Animal Sciences, Marijkeweg 40, 6709 PG Wageningen, The Netherlands, E-mail: [email protected] ; 2 NIOO-Centre for Limnology, P.O. Box 1299, 3600 BG Maarssen, The Netherlands) ABSTRACT The reproductive strategies of two species of ‘small barbs’ (< 100 mm forklength, L F) from Lake Tana (Ethiopia), the benthic-littoral Barbus humilis and the pelagic B. tana- pelagius, were investigated. Monthly samples were collected from Ž ve different habitats over a 1 year period (> 12,000 Ž sh). Both species have a long breeding period (from March to September), and the distinct bimodal size-frequency distributions of eggs suggest multiple spawning for both species. Absolute fecundity increased exponentially with Ž sh size, and was signiŽ cantly higher for B. humilis than for B. tanapelagius. Egg size was similar. Relative fecundity was signiŽ cantly higher in B. humilis . In most habitats B. tanapelagius reached Ž rst maturity at a smaller size (58.5 mm) than B. humilis (64.5 mm). However, in shallow habitats with clear water B. humilis females are much smaller at Ž rst maturity . 48 : 3 mm), possibly due to high tapeworm infection rates. The range of fecundity in small barbs from Lake Tana (172-339 eggs per gram) was low compared with small lacustrine cyprinids and clupeids from other African lakes. The reproductive strategies of the two barbs were discussed in relation to their feeding potential, food availability, parasite infection rate and risk of predation. K E Y WORD S : fecundity, maturity, parasite, sex ratio, small pelagics, spawning. INTRODUCTION Lake Tana, the largest (3150 km 2 / and most elevated (1830 m) lake in Ethiopia, is a circular basin 70 km diam, with a maximum depth of 14 m (average 8.9 m). The lake is thought to have originated by volcanic blocking of the Blue Nile River 2 million years ago (M OHR , 1962). It owes its present form to damming by a 50 km long Quaternary basalt  ow, which Ž lled the exit channel of the Abbay (Blue Nile) river (C HOROWICZ et al ., 1998). The age of this lava  ow is estimated to be some 10,000 years. Lake Tana is characterised as oligotrophic with a chlorophyll content of ca. 3.7 ¹ g l ¡ 1 (R ZÓSKA , 1976; W OOD & T ALLING , 1988). © Koninklijke Brill NV, Leiden, 2003 Also available online - www.brill.nl