The case study presents two different concepts: (1) the cultivation of aquatic macrophytes concurrently (trapa) or in rotation (euryale) with fish and (2) the cultivation of terrestrial grass (napier) on the banks of a pond and its feeding to fish.
In India, trapa (Trapa bispinosa) and makhana (Euryale ferox) are two seasonal, aquatic cash crops which are grown extensively in Madhya Pradesh and Bihar, respectively. While the environment is not congenial for Indian carps, common carp goes well with trapa and air-breathing fish with makhana. Based on the input requirements for a 0.4 ha pond, the procedures to be adopted are given below.
Concurrent trapa-fish culture
1. Transplant trapa (Trapa bispinosa) seedlings in May/June in a perennial pond. These plants make use of the available organic matter for their growth.
2. Stock 800 (50 g) common carp fingerlings in September-October.
3. Trapa fruits ripen in winter and are harvested from November to January. A production of 3-4 t of fruits is obtained.
4. Fish are harvested in April/May when 750-1 000 g fish are available. A total of 400-500 kg fish are harvested.
Rotational euryale-fish culture
1. The seeds of Euryale ferox sprout in February and the leaves cover the pond fully in May/June.
2. The plants start fruiting in August and burst in October, scattering the seeds at the pond bottom which are collected by scanning the bottom mud.
3. Stock 1 200 (8-10 g) air-breathing fish (Clarias batrachus) in November and harvest in April; when about 500 kg of fish can be obtained.
Besides aquatic vegetation such as Hydrilla, Ottelia, Potamogeton, etc., green grass has an important role in feeding grass carp. Hybrid napier, once sown on pond banks, can be cropped continuously for five years, needing little irrigation during summer. A new system utilizing aquatic vegetation/green grass alone for fish production gives high yields at very low costs. It is labour-intensive and highly suitable for small, shallow ponds (0.06-0.15 ha). Based on input requirements for a 0.1 ha pond, the methods to be followed are given below:
1. Prepare the pond in May/June by using urea-bleaching powder method or by draining, if a source of water for filling the pond is available.
2. Seven to 10 days later, stock the pond with 200 (50-60 g) grass carp. Feed them to satiation with Hydrilla. (System of feeding ad libitum: fish are satiated when they have stopped feeding and there are still some feed material left lying about.) Within about a week, the pond is also stocked with 40 each of catla, rohu, mrigal, silver carp and common carp (5-8 g). Grass carp is gradually weaned from Hydrilla to napier grass.
3. Feeding is done regularly to satiation.
4. Silver carp, catla and common carp will be the first to attain a weight of 1 kg each. From the fifth or sixth month onward, these are harvested one after another. Replenish the harvested fish with an equal number of fingerlings.
5. Hybrid napier is planted at 1 root slip/mē and manured with 2.5 t farmyard manure/1 000 mē. Irrigation is done at 10-15-day intervals. The grass is cut after 75 days, followed by 45-day intervals. About ten cuts can be taken from each plant. A production of 12-15 t napier from 1 000 mē is taken. About 2 000 mē land area will produce enough napier to feed the fish in a 0.1 ha pond. This means that to provide sufficient grass to feed the fish, twice the pond area is needed for growing napier.
6. About 400 kg of fish can be harvested from the pond in the course of one full year.
Issues for further consideration
In Asia, the concurrent production of aquatic macrophytes with fish is fairly widely used. Farmer motivation for using their ponds for this purpose is not well understood. Sometimes seasonal constraints prevent the continuation of year-round fish culture, or the plant has a seasonal production period or market. In Binh Chan, near Ho Chi Minh City in southern Viet Nam, lotus is commonly rotated with breeding tilapias in shallow ponds fertilized with sewage during the off-season for tilapia seed production.
Knowledge about the prevalence of the napier grass system today would be valuable as this is a resource-intensive system. Where grass carp are raised as a central part of the resource-poor farmer systems, they normally have to source a high proportion of the feed off-farm from common property areas (e.g in northern Viet Nam). This raises questions of competition between livestock owners and fish farmers, individual households and the community as a whole.
If the focus is on the production of large individual fish, these systems would be used by resource rich farmers who can risk such systems, probably using employees for the considerable labour requirements. There are considerable implications for adoption of vegetation-based systems by poor farmers especially with regard to their labour and how this is organized within the household, even though it appears to be simple.
Depending on local species, different plants may have to be tried for use as direct fish food or for making green water (i.e. pond fertilization).