Water hyacinth is a potential ingredient in farm-mixed feeds for herbivorous or omnivorous freshwater fish in simple production systems and where it is available at low cost (Hasan et al., 2009a). Considerable research has been dedicated to investigate the use of water hyacinth in fish feeding, and a comprehensive review proposed by Hasan et al., 2009a is the basis for the present text. Water hyacinths are fed to fish either fresh, as a dried meal in pelleted diets, or composted as feed and fertilizer. Attempts are also made to feed water hyacinths to fish by processing them with other techniques. However, many of these studies were conducted under laboratory conditions and reports of on-farm utilization as fish feed are limited.
Inclusion rates, feed conversion and digestibility
Suitable inclusion levels are 25-50% when supplementing basic feed (rice bran, broken rice, chicken manure, etc.), or 5-10% when replacing protein sources in formulated feeds (fish meal, vegetable oil meals/cake) (Hertrampf et al., 2000). A feed conversion ratio of 3.0 is an acceptable level for fresh or processed water hyacinths. For all practical purposes, the protein digestibility of water hyacinth may safely be taken as 70-80% at 15-30% inclusion rates in the diet, while it may be around 50-60% at inclusion rates of 45% or above.
The use of fresh water hyacinth as fish feed has achieved limited success, principally because of its high moisture content and because of the presence of calcium oxalate crystals. Hyacinth leaves are generally cut into small pieces and fed to grass carp or other macrophytophagous fish. Generally, grass carp (Ctenopharyngodon idella) feed on this plant only when no other macrophytes or feeds are available, though they have been found to readily accept roots and leaves (but not the swollen petioles). Grass carp grew well when fed exclusively with water hyacinth, though fish above 80-100 g were able to better utilize it (Riechert et al., 1977). In Vietnam, mixtures of fresh whole water hyacinth and rice bran (2:1), fermented or not, have replaced rice bran in nursery ponds of Nile tilapia (Oreochromis niloticus), common carp (Cyprinus carpio), grass carp and Java barb (Barbonymus gonionotus). Nile tilapia performed better than the other species, exhibiting a specific growth rate of 4.3-4.8%/day (Nguyen Anh Tuan et al., 1994).
Fish were reluctant to accept water hyacinth leaf meal in pelleted diets (Hasan et al., 2009a). A major problem with the use of water hyacinth meal in fish diets is its relatively high fibre content, as fish do not appear to produce cellulase directly and, therefore, their ability to maintain a symbiotic gut flora capable of hydrolyzing cellulose is limited (Buddington, 1980). A number of growth studies have been conducted using dried water hyacinth in pelleted fish feeds, partially or completely replacing fishmeal or other conventional protein sources. Whole water hyacinth or its leaf meal was evaluated as a major ingredient in pelleted diets for Nile tilapia, Java tilapia (Oreochromis mossambicus), grass carp, common carp, rohu (Labeo rohita), stinging catfish (Heteropneustes fossilis), Java barb, sepat rawa (Trichogaster sp.), matrincha (Brycon sp.) and African catfish (Clarias gariepinus). The inclusion rate of water hyacinth meal used varied widely, ranging from as low as 2.5% to as high as 100%. Growth responses have been highly variable. Significant reductions in growth responses have been reported in rohu fry and fingerlings when 27-30% leaf meal was included to replace the fishmeal protein of the control diet (Hasan et al., 1990; Hasan et al., 1994). There was a similar response in Nile tilapia when fed a test diet containing 40% water hyacinth meal (Klinavee et al., 1990). Better or similar growth responses than those from the control diet have been observed in grass carp and common carp fed 50% water hyacinth (Murthy et al., 1990), Java tilapia fed only water hyacinth (Dey et al., 1982), and in matrincha (Brycon sp.) with an 18.5% inclusion rate (Saint-Paul et al., 1981). However, in some of these studies, the control diet consisted only of a rice bran-oil cake mixture, which itself may not have generated good growth. The specific growth rate was only slightly reduced (10-15%) in tilapia fed a diet replaced at 75 or 100% by water hyacinth meal, but the fish may also have obtained extra nutrients from plankton (Edwards et al., 1985), an indirect benefit that may also have been present in other studies (Dey et al., 1982; Murthy et al., 1990).
Composting, fermentation and other processes
Composting is one of the most widely used processing techniques to prepare water hyacinth for use as a fertilizer or fish feed. A large quantity of inorganic nitrogen and phosphorus accumulates in the roots, which makes it suitable as a compost or inorganic fertilizer. Composting increases the nutritive value and acceptability of water hyacinth: composted water hyacinth was found to have a protein content similar to that of the corresponding meal, half the crude fibre and fat content and twice the amount of ash (Edwards et al., 1985). However, in China, when fresh whole water hyacinth was applied to ponds as feed and fertilizer, the fish were reluctant to accept it and it took a long time to decompose, eventually resulting in inefficient utilization (Hasan et al., 2009a).
Other processes have been reported. In China, fresh water hyacinth is mashed into a liquid form with a high-speed beater and added to ponds containing carp fingerlings. The paste can also be mixed with rice bran and fermented before adding to the pond. Leaf extracts have also been tried. In one experiment, leaves were crushed with water and the resulting solution was sieved to remove the fibrous material. Rohu spawn that received the extract (100 ml/d for 120 spawn in 40 L tanks for 30 days) had a growth rate of 8.59 vs. 9.04 g/d for the control diet (phytoplankton) (Kumar et al., 1991). A dried protein concentrate made from pressed leaves of water hyacinth increased growth and reduced mortality in channel catfish fingerlings (Ictalurus punctatus) when included at up to 10% of the diet. The water hyacinth meal could be used as a replacement for alfalfa meal (Liang et al., 1971). In a series of experiments in Thailand, water hyacinths were added in three forms to a series of earthen ponds stocked with Nile tilapia: fresh whole plants that decomposed beneath the water in situ; freshly chopped water hyacinth spread on the surface; and composted water hyacinth. Extrapolated yields of 5 to 6 t/ha/year of fish were obtained with all three treatments at the same dry matter loading rate of 200 kg/ha/day (Edwards, 1987).