Introduction
Grass carp (Ctenopharyngodon idella), an herbivorous species, is commonly cultured in many parts of the world, especially in east Asia. In China, polyculture of grass carp with other species of different feeding habits is traditionally practiced, where grass carp consume low-value vegetative waste and increase natural food production in the pond by nutrient recycling and fecal production (Yang et al., 1990; Li and Mathias, 1994). This effectiveness is depicted in the Chinese saying "one grass carp raises three silver carps." It is reported that a 5:1 stocking ratio by weight is most suitable for grass carp and filter-feeding species in a polyculture system consisting of silver carp, Hypophthalmichthys molitrix; bighead carp, Aristichthys nobilis; and common carp, Cyprinus carpio (Yang et al., 1990). However, as grass carp are known to feed on a wide variety of plants, the quantity and quality of natural food pro
duction derived from recycling of grass carp wastes depend largely on the type and input of forage provided.
In Nepal, pond fish culture is mostly conducted in the southern subtropical region, where water temperature is between 15 and 20°C during the winter period from mid-December to mid-February (Shrestha, 1999). Polyculture of herbivorous carps is the common practice in Nepal. The major constraints for the small-scale resource-poor farmers are fish feeds and chemical fertilizers, which are expensive and unavailable, while livestock manure is traditionally used for land crops (Shrestha and Yadav, 1998; Shrestha, 1999). Exploration of easily available or easily grown plant material that is not used in human food production is a prime need to solve the problems of these fish farmers. Napier grass (Pennisetum purpureum) is high yielding, perennial, and tropical (Humprey, 1978; Edwards, 1982). It is accepted by grass carp and can produce
Table 1. Stocking and harvest size, survival, growth, and net fish yield (NFY) of grass carp and Nile tilapia in monoculture and polyculture tanks fed with fresh chopped napier grass during the 188-day culture period. Mean values with different superscript letters in the same row were significantly different (P < 0.05).
