Natural growth
Rohu grow well in varied environments and the best growth is observed in large water bodies. Growth is best described by the Von Bertalanffy growth equation described by Lt= 1015[1-e0.276(t+0.333)] (Khan,1972). Growth in natural waters is influenced by food availability, feeding intensity and the reproductive cycle and under culture condition is determined by pond size, stocking density and size at stocking.
Newly hatched rohu larvae are 3.5 – 4.5 mm in length. The yolksac is absorbed within three to four days at which stage the fish are 6.0 to 7.5mm in length. The 3-4 day old larvae are stocked into nursery ponds at a density of 3-5 million/ha and reared for a period of 15 – 20 days by which time they reach 20 – 25 mm in size and are then called fry. Under experimental nursery rearing conditions, Biswas et al. (2006a) reported maximum growth in terms of length, weight and specific growth rate (SGR) of 28.9 mm, 247.7 mg and 33.96, respectively.
The fry (20–25 mm) are reared in earthen rearing ponds at a density of 0.2 – 0.3 million/ha for 2–3 months and reach 80 – 100 mm in size. Under experimental conditions fry can be grown to fingerlings with an average size and weight of 106.6 mm and 11.9 g at a SGR of 5.38 (Biswas et al., 2006b). After 90 days of rearing Jena et al. (1999) reported SGRs of 5.41 and a maximum size of 20.3 gm.
For grow-out, fingerlings are stocked at a density of 5 000–10 000/ha in composite culture systems in which rohu comprise between 15-30 percent of the pond population. The fish are reared for a period of 10 - 12 months and reach a size of about 700 – 800 g (Jena and Das, 2006). Chondar (1999) reported that in well prepared ponds, rohu attain 700 – 1 000 g in the first year, 800 – 2 000 g in the second year and 2 000 – 4 000 g in the third year. Similar growth rates have also been reported by Hora and Pillay (1962). Comparatively lower growth rates have been reported under experimental conditions in earthen ponds (Jena et al., 2001, Jena et al., 2002a, Jena et al., 2002b, Tripathi et al., 2000). Variations in growth rate are mainly due to hydrobiological and geo-climatic conditions as well as stocking density, biological productivity of the water body and the provision of supplementary feeds.
Reddy et al. (2002) and Gjerde et al. (2002) reported no significant differences between the growth of wild and farmed fish and that there is very little growth heterosis in crossbred progeny. A genetically improved variety of rohu (Jayanti rohu) has been developed through a selective breeding program by the Central Institute of Freshwater Aquaculture in collaboration with the Institute of Aquaculture Research in Norway. It has been reported that after 4 generations the new strain shows a 17 percent higher growth rate and this strain now making an impact on production output at the farmer level (Ayyappan and Jena, 2006).
Indian major carps are known to grow faster during the second year. To benefit from this phenomenon farmers in Andhra Pradesh have developed a technique of stunting the fish. Stunted carp are produced by rearing fingerlings for 6 -12 months at densities ranging between 50 000 and 100 000/ha and fed with just enough food to survive until they attain a weight of 50-150 g, where after they are stocked into the grow-out ponds. As a result of compensatory growth, the fish attain 1.0–1.5 kg after a culture period of 10-12 months. Chakrabarti et al. (1988) and Hossain et al. (2003) demonstrated the compensatory growth potential of rohu under experimental conditions.
