Reproduction
Male and female rohu attain first sexual maturity at two years of age and 50 percent maturity is attained at three years. Complete maturity of male and female is observed at age four and five years respectively (Khan and Jhingran, 1975). The minimum size at first maturity of male is about 46 cm and in the case of female it is about 51 cm. One hundred percent maturity has been found in 65 cm length group in the case of male while in the case of female hundred cent percent maturity has been found in 70 cm group (Chondar, 1999). During the breeding season the pectoral fins of mature males are rough on the dorsal surface, while those of females are smooth. Rohu is highly fecund (109 000 to 535 000 eggs per kg) (Chondar, 1999). The species breeds naturally in rivers, reservoirs, and in large lakes in which fluvial conditions are simulated. The breeding season generally coincides with the southwest monsoon. The fish can be spawned through hypophysation with 2-3 mg pituitary/kg body weight followed by a second dose of 5-8 mg/kg after an interval of 4-6 hours. Males are given only a single dose of 2-3 mg/kg at the time of the second injection given to females. Fish generally spawn within six hours of the second injection.
The alimentary canal of rohu consists of the mouth, buccal cavity, pharynx, oesophagus, intestinal bulb, intestine and rectum. The free edges of the lips are broad and have four to five rows of conical papillae. The buccal cavity is dorso-ventrally compressed. The mucous membrane lining the buccal cavity contains minute papillae. The pharynx is well demarcated into an anterior respiratory part and a posterior masticatory section, that has well developed molariform pharyngeal teeth and the teeth are all alike (homodont) and arranged in three rows on the inferior pharyngeal bones.
The oesophagus is short and a true stomach is absent. The anterior part of the intestine, behind the oesophagus, is modified into a sac like structure (intestinal bulb) in which food is stored. There are no gastric glands in the intestinal bulb. The intestine is extremely long (Figures 3a and b). The length of the intestine relative to body length (relative length of gut - RLG) increases slowly from fry stage up to the juvenile stage, where after it changes rapidly because of the change in feeding habit (i.e. from zooplankton to plant feeding). For example, the RGL for rohu fingerling is about 4, juvenile is 6, small mature adult 10 and in large mature adults the gut may be 12 times the body length (Das and Srivastava, 1979). For example- fry of rohu have low RLG i.e. (0.7-1.0) and the value varies between 1.1–2.5 for fingerlings and the adults have value ranging from 2.6 – 19 (Sinha and Moitra, 1975). The intestine is more or less uniform in diameter and forms a number of coils (Figures 3a and b). Elongation of intestine and its extensive coiling are related to its herbivorous feeding habit and the intestinal coil may provide a large surface for absorption of food. The terminal part of the intestine is slightly dilated and forms the rectum. Pyloric caecae are absent.
The absence of stomach is compensated by the production of pancreatic trypsin and erepsin as well as enterokinase from intestinal mucosa and the digestive function is performed in the anterior part of the intestinal bulb. The intestinal fluid has a pH ranging from 6.8-7.1 in the intestinal bulb, but decreases to a range of 6.2 to 6.5 in the hind gut. The intestinal bulb serves the role of a store house for the acceptable food materials. Protein digestion occurs under pH 7.0 to 7.1.
