The siah mahi, Capoeta fusca, is a sub-endemic fish of eastern Iran. We studied morphometric characteristics, diet and reproduction of 600 specimens in qanats of the central zone of Birjand County, South Khorasan Province, from November 2006 to October 2007. The mean total length was 13.5±1.43 cm and maximum length was 21.5 cm. The length-weight relationship of this fish was described by body weight = 0.0101 × total length2.9477 (r2=0.9747). The mean relative length of the gut was 4.42 ± 0.48, suggesting that this species is an herbivorous fish. Similarly, the mean of the vacuity index was 30.95±5.90, and this fish was classified in a relatively gluttonous group. Besides plants as the primary food, molluscs, aquatic insects, and sometimes frog eggs were distinguished as secondary foods. According to the gonadosomatic index, the reproduction period begins in March and lasts into May. Variations of the gastrosomatic index show that feeding is correlated with reproduction. Measurement of salinity resistance in 96 fish showed that this species can withstand up to 10 p.p.t. of salinity indefinitely.
