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The ideal way to utilize aquatic weeds is to employ animals to harvest the plants by eating them and thus convert them into useful products, the only expense involved then being the periodic harvesting and processing of the animals. Herbivorous fish are obviously candidates for this task, as well as aquatic and herbivorous mammals and birds. Because of the attractiveness of this concept there is a considerable body of literature on the subject, especially on the white amur, also known as the Chinese grass carp, Ctenopharyngodon idella.

In the selected papers on grass carp summarized in this chapter, and in the review by Opuszynski (1972), are references to the remarkable capacity of this fish to eat aquatic weeds, its rapid growth, and the palatable and nutritious flesh it produces. Because it is herbivorous when mature, and eats a wide range of aquatic plants, it is easy to assume that the grass carp is potentially ideal as a method of both controlling and utilizing aquatic plants. Moreover, the difficulty of breeding the fish appears to reduce, but not eliminate, the possible hazards of introducing it into new areas. In spite of the advantages, great caution is being shown in its release into new habitats because there is still considerable doubt as to whether the carp might not profoundly disturb the aquatic environment, and especially other valuable species such as trout and other game fish, which may be adversely affected. An early but still useful review of observations on feeding habits of grass carp is given by Singh et al. (1967), and a bibliography on the grass carp published by FAO gives 243 references (Nair, 1968).

A more recent bibliography prepared by the Agricultural Research Council, Weed Research Organization, U.K. (1976) entitled “Selected references to herbivorous fish”, contains 162 annotations.

The grass carp is a good example of the difficulties inherent in the use of aquatic animals: how to control what they eat and predict the effect they may have on the other organisms in the water. With birds (like ducks and geese) it is possible to control their activities to some degree, as can be done with domestic livestock. But careful introduction and restrictive measures are necessary in order to impose some degree of control on aquatic animals. This is the main disadvantage of biological control of plants by animals as distinct from mechanical harvesting or destruction by chemicals. Moreover, the use of animals may be unreliable because disease, climate, migration or other influences may reduce their activity when it is most needed.

The large aquatic mammal, the manatee, was suggested (Allsopp, 1960, 1961) as a potential method of aquatic weed control following its successful use in Guyana. The idea aroused world-wide interest, but this flagged when the scarcity of the manatee became apparent, and the difficulties involved in its management were described (Bertram and Bertram, 1962).

Shell (1962) describes trials in ponds in the U.S.A. with a strain of the common carp (Cyprinus carpio) and three species of tilapia, T. nilotica, T. mossambica and T. melanopleura. The author suggests that control of aquatic weeds by the carp may be as much due to the fish dislodging the plants while looking for food, and covering algae with mud, as actually eating them. Tilapia in the U.S.A. are useful for eating weeds but are vulnerable to cold and predators. T. melanopleura was probably the most successful. The general conclusion from the trials was that while the fish tested may be of some value in controlling aquatic weeds they were not the total answer to the problem.

Bennett (1974) has given a brief review on the use of fish, including grass carp, tilapia and two fish both known as silver dollar, Metynnis roosevelti and Mylossoma argenteum. He comments that the dual role of fish as weed control agents and as a source of protein for human or animal food gives added initiative for investigating the more promising species. However, there is considerable concern over the possible detrimental effects on the various components of the ecosystem that may follow the introduction of exotic fish. The author also reviews briefly the limitations of the manatee and the coypu.

No comprehensive review of the literature on the use of fish and aquatic animals appears to have yet been published.

However, in the U.S. National Academy of Sciences publication (1976) on “Making aquatic weeds useful: some perspectives for developing countries”, Part I is devoted to the subject. The chapter headings are: grass carp (14 references); other herbivorous fish (12 references); manatees (four references); ducks, geese, and swans (two references); other herbivorous animals, with sections on water buffalo (one reference) and the rodents Capybara (one reference) and Nutria (four references); crayfish (11 references). The crayfish is an unusual animal as a potentially lucrative exploiter of aquatic vegetation. It is pointed out that “crayfish should be thought of more in terms of an available crop associated with aquatic weeds, not as a weed-control agent.”

The red crayfish, Procambrus clarkii, which is widely farmed in Louisiana, U.S.A., is described as an omnivorous type which does not prefer plants “but will consume all vegetation available - including water hyacinth - if there is nothing better to eat (a situation that commonly occurs when their populations are very dense)”. There are many hazards associated with crayfish and the potential impact of any crayfish which it is intended to introduce must be thoroughly studied under strict security before they are released into an aquatic environment. A world-wide search for edible size herbivorous crayfish is needed. Among the species which exist some are likely to prove good food and valuable aquatic weed control agents. The food habits of crayfish, techniques for rearing and harvesting, and the effectiveness of crayfish in removing vegetation under different aquatic situations, such as rice and taro paddies, waterways and reservoirs, all need thorough review and analysis. Only a part of the crayfish is eaten, most of the body being discarded. The carcase is high in calcium and nitrogen so research is needed to determine the usefulness of the carcase for fertilizer and animal feed.

The information given by the National Academy of Sciences is succinct, authoritative and well illustrated. The book is recent and readily available.

Achmad, S., 1971 Problems and control of aquatic weeds in Indonesian open waters. In Tropical weeds; some problems, biology and control. Proceedings of the 1st Indonesian weed Science Conference, Bogor. Biotrop.Bull., (2):107–13

The author points out that the presence of a certain amount of aquatic plants can stabilize fish production. A balanced population of predators and forage fish cannot be maintained if there are no hiding places for the fingerlings. A list of fishes, in addition to grass carp, which feed on aquatic plants in Indonesia is given as follows (local name in brackets):

Leptoharbus hoeveni (djelawat)
Puntius javanicus (tawes)
Puntius bramoides (lelawak)
Ost eochilus melanopleura (ketabau)
Cyclocheilichthyes apogon (seren)
Tilapia mossambica (mudjair)

Plants preferred by such fish include Ipomoea aquatica, Azolla spp., Salvinia spp., Hydrilla spp. and Pistia stratiotes.

Adams, A.E. and V. Tifolko, 1970 A progress report on the introduction of grass carp (Ctenopharyngodon idella) in Fiji. Fiji Agric.J., 32(2):43–6

Hydrilla verticillata is a serious weed in rivers in Fiji. Grass carp were imported from Malaysia as fingerlings and fed on the weed in a pond, and released into a netted portion of theRewa river. Weight increased from 9.75 g in December 1968 to 3 069 g (length 56 cm) in April 1970. This weight gain was noted as being much higher than those recorded in Malaysia, China, Russia and various European countries. It was concluded that grass carp are able to control H. vertioillata but that large-scale introduction of the fish to do this was dependent on the successful development in Fiji of induced breeding.

Ahling, B. and A. Jernelöv, 1971 Weed control with grass carp in Lake Osbysjön. Rep.Swed. Water Air pollut.Res.Lab., (B94a):24 p. Issued also in Vatten, 27(2):253–64

The authors describe an experiment to control Myriophyllum verticillatum in a lake in Sweden with grass carp. The area of the lake was 4.6 ha and contained about 16 tonnes (dry weight) of the weed. In May, 250 grass carp, average weight 380 g, were introduced. After 100 days they had increased in weight to about 1 kg average, and had consumed about half of the weed. At the same time some Potamogeton natans was eliminated, while Nymphaea alba, Typha angustifolia and phragmites sp. were largely unaffected. A curious feature was that the Myriophyllum reduction was uneven, some areas being cleared and others apparently unchanged. The clearance was more pronounced in deeper water. There was a slight increase in plankton but no effect on turbidity.

Blackburn, R.D. and D.L. Suton, 1971 Growth of white amur (Ctenopharyngodon idella) on selected species of aquatic plants. Proc.Eur.Weed Res.Counc.Int.Symp.Aquat. Weeds, 3:87–93

Tests with grass carp in tanks showed that the fish gained more weight on a diet of fresh Najas guadalupensis and Hydrilla verticillata than on a dried fish food diet. Contrary to the observations of other authors cited in this chapter grass carp were observed to eat water hyacinth. The small fish ate only the roots and did not grow so fast as the large fish which ate both leaves and roots. (From Weed Abstracts)

Chapman, V.J. and B.J. Coffey, 1971 Experiments with grass carp in controlling exotic Macrophytes in New Zealand. Hidrobiologia, Bucharest, 12:313–23

The authors review the introduction and spread in New Zealand of the submerged aquatic weeds Lagarosiphon major, Ceratophyllum demersum, Egeria densa and Elodea canadensis. Methods of control are discussed including possible utilization by harvesting for stock food. Though harvesting was considered practicable the use of the plants as fodder was believed to be unsuitable because of the content of arsenic accumulated by the plants from the thermal waters that enter the lakes. Arsenic in amounts of 35–75 ppm dry weight are common, and extreme values up to 2 000 ppm have been recorded. The alternative possibility of the use of grass carp was investigated as considered more promising, so a few fish were imported from Malaysia.

Trials showed that carp would eat the problem weeds, though not uniformly. The order of preference was Egeria, Lagarosiphon, Ceratophyllum. Further trials were carried out which included, in addition, 18 other aquatic plants. The order of preference of these plants is listed. It was noted that plants of the same species from different sources varied apparently in their palatability. Lagarosiphon, especially the young tips, was preferred to Egeria, which was superior to older Lagarosiphon, the tough stams of which were rejected. Other plants rejected were Typha angustifolia, Myriophyllum brasiliense and the rootstock of Vallisneria gigantea, and, surprisingly, pasture grasses. The authors observe that grass carp have the capacity to survive periods of starvation by losing weight. Therefore in the field it may be better to overstock mildly rather than understock. Arsenic accumulation in carp after feeding on Lagarosiphon from arsenic–rich water was recorded.

Denike, T.J. and R.W. Geiger,1974. The utilization of Chara in water management. Hyacinth Control J., 12:18–20

Chara spp. are considered as useful aquatic plants because they provide excellent habitats for fish; their fragile stems do not impede boats, and the plants maintain environmental balance. But the Chara requires control and can, in shallow water, become undesirable. With the use of suitable herbicides unwanted weeds such as Hydrilla, Najas and Utricularia can be suppressed and thus encourage the growth of Chara. Some species of Chara present in Florida, U.S.A., are: C.vulgaris, C. schweinitzii, C. lornemanni, C. foliosa and C.fragilis.

Hill, H.R. and R.E. Rintz, 1972 Observations of declining water lettuce populations in Lake Izabal, Guatemala. Proc.Annu.Meet.South.Weed Sci.Soc., 25:374–80

Turtles are reported to feed onPistia stratiotes in Lake Izabal. The authors cite reports of the herbivorous turtles, Dermatemys and Pseudemys, being caught by local inhabitants for food. It is supposed that turtle feeding could account for observed root damage to Pistia plants.

*Hora, S.L., 1951 The water hyacinth problem and pig farming. Sci.Cult., 17(6):231–2

The author describes the water hyacinth–pig–fish–human cycle adopted as a farming system in ponds in Malaya.

“The fish caught from the pond were Trichogaster pectoralis (corresponding to Kholisha of Bengal) and Clarias (Magur of Bengal). We were informed that other varieties of Jiol Machh (Live Fish, species of Ophicephalus) were also present. The pond was of a small size, probably less than 2 000 sq. feet [1802] in area, but it was thickly covered with a luxuriant growth of water hyacinth. The water was dirty and thick and full of sludge matter. There was a small clearance in the middle where the net for catching fish was used. On one side of the pond, there was a piggery, the washings from which were being drained into the pond. Beside there were overhanging latrines over the pond. The primary object of heavily manuring the pond was to keep going the luxuriant growth of water hyacinth which was being harvested every day for use as pig fodder. We saw it being cooked with oil cake and other ingredients of the usual Chinese pig–feed. The pigs are kept in enclosures and do not roam about and feed on dirty matter as is seen in certain Indian villages.

“Fish taken out from such heavily polluted ponds are kept in tubs or small masonry pits for a few days and fed on wholesome food. The fishes thus get cleaned of the sludge matter and gain excellent flavour and taste.”

The paper urges the introduction of similar methods into Bengal, especially in sewage-irrigated areas near Calcutta. The author states that the use of water hyacinth for pig fodder and the production of live fish, so highly valued in Bengal for their nutritional qualities, would change the weed from a pest to a blessing.

Legner, E.F., 1975 et al., Biological aquatic weed control by fish in the lower Sonoran Desert of California. Calif.Agric., 29(11):8–10

Irrigation channels heavily infested with Potamogeton pectinatus, Myriophyllum spicatum, Najas guadalupensis and Chara sp. were attacked by the introduction of the fish Tilapia hornorum, T. mossambica and T. zillii. In one trial the T. zillii were introduced at the rate of 4 700/ha. Within four months almost all of the weeds had been disposed of. The fish numbers during this time had increased to 11 000/ha. An important observation in this and other trials was that the large rise in tilapia population also increased the game fish numbers. Tilapia play an important role in the game fish food chain. Overwintering survival of T. mossambica and T. zillii is poor because they are then lethargic and are taken by predators, so that continuous annual stocking is essential for maximum biological control of the aquatic plants. The tilapia breed and grow fast if the water temperature is around 21°C. The lower critical temperature is about 10°C. Tilapia have the additional advantage of reducing mosquito breeding habitats. Because of the successful trials it is expected that the use of tilapia will be extended over most of the region of Southern California.

*Le Mare, D.W., 1952 Pig rearing, fish farming and vegetable growing. Malay.Agric.J., 35(3): 156–66

The paper describes a fish farm in Malaysia about 1 ha in area, consisting of eight ponds about 1 m in depth and supplied with water from a stream which was diverted to flow through piggeries. Six ponds were stocked in December 1949 with 16 500 Tilapia mossambica, and four species of carp (Ctenopharyngodon idella, Aristichthys nobilis, Hypophthalmichthys molixtrix and Cyprinus carpio) were placed in the other two ponds in February 1950.

“The fish were, at first, irregularly fed with rice bran. In April 1950, pig manure became available from the pig-fattening section and this rapidly caused a luxuriant growth of algae, diatoms and crustacea (all microscopic life) on which the fish fed, and feeding of the fish with bran was stopped. At the same time, kangkong (Ipomoea reptans) was introduced into the ponds. This plant grew well and within six weeks of planting it was possible to harvest the vegetable daily as pig fodder. It is important to note that the Health Department have kept a careful watch on the ponds and, because the young of the fish Tilapia are effective larval feeders, there has been no undue breeding of mosquitoes in the rots of the kangkong.”

Liang, J.K. and R.T. Lovell, 1971 Nutritional value of water hyacinth in channel catfish feeds. Hyacinth Control J., 9(1):40–4

A series of feeding trials were carried out to find the usefulness of Eichhornia crassipes (water hyacinth), Justicia americana, and Alternanthera philoxeroides as food for fingerling channel catfish (Ictalurus punctatus). The plant material was pressed to extract protein in the express liquid which was then separated to prepare a dry concentrate. This was fed in increasing amounts until it became 40% of the fish diet. J. americana proved the most palatable, followed by water hyacinth, then A. philoxeroides. It was found that the most favourable use of water hyacinth was as a supplement to vitamin-deficient diets when, if added at the rate of 5–10%, it increased growth and reduced mortality of the fingerlings. The water hyacinth meal in these respects was better than commercial alfalfa meal, and thus it could be used as a replacement for alfalfa when it is known that alfalfa meal has a value in these feeds.

*Ling, S.W., 1960 Control of aquatic vegetation. In Lectures presented at the Third International Inland Fisheries Training Centre, Bogor, Indonesia, 31 October-10 December 1955, conducted by the Government of Indonesia and FAO with the cooperation of IPFC. Vol.1. Rome, FAO (3.23):12 p.

“Aquatic plants are so important as fish food that, in China and other countries where herbivorous fish are stocked and cultured in combination with other non–predatory species, fish farmers make special efforts to promote the growth of suitable kinds. It is therefore to be stressed that measures for the control of aquatic plants are warranted only when the growth of such plants has become so excessive as to interfere with the desired fisheries production and management.”

Small and tender plants such as Lemna and Wolffia are excellent feed for certain kinds of fish. Salvinia may be collected in the autumn for grass carp. In Thailand grass carp will eat young shoots of Typha, Cyperus and Nymphaea and roots and young leaves of Pistia and Eichhornia. In Indonesia lakes densely covered with Hydrilla, Ceratophyllum, Ipomoea, Potamogeton, etc. have been converted to highly productive waters by the plant-eating fish Puntius javanicus and Trichogaster pectoralis.

McVea, C. and C.E. Boyd, 1975 Effects of waterhyacinth cover on water chemistry, phytoplankton and fish in ponds. J.Environ.Qual., 4(3):375–8

Water hyacinth growing on ponds stocked with the fish Tilapia aurea decreased phytoplankton production by shading and by removal of phosphorus from the water. When hyacinth had 25% cover of the water the dissolved oxygen was less than when the cover was smaller but there was still enough oxygen for survival and growth of fish. Reduction of phytoplankton progressively lowered the numbers of fish down to 5% water hyacinth cover, after which the effect on the fish could not be measured.

Mehta, I. and R.K. Sharma, 1972, Control of aquatic weeds by the white amur in Rajasthan, India. Hyacinth Control J., 10:16–9

Grass carp is reported as a voracious feeder of Najas foveolata, Chara sp., Vallisneria spiralis, Potamogeton nodosus, P. pectinatus, P. perfoliatus and Hydrilla verticillata. Fish weighing about 65 g preferred Chara sp., N. foveolata and V. spiralis. Larger fish (200 g) preferred P. pectinatus, P. nodosus, V. spiralis and N. foveolata, and only ate small amounts of Hydrilla sp., P. perfoliatus and Typha sp. However, large fish may flourish on these weeds and may be able to chew leaves of tough plants and fibrous grasses. The observations on grass carp to date are most favourable and show how useful they can be in place of herbicides. More information is needed on best stocking rates. (From author's summary)

Mitchell, C., 1977 The use of carp for submerged weed control. Proc.N.Z.Weed Pest Control Conf., 30:145–8

A lake in the North Island of New Zealand, heavily infested with Egeria densa, was divided by a net and grass carp were introduced at the rate of 22/ha to one half of the lake. A year later it was found that the carp had severely grazed a band of rushes (Eleocharis sphacelata) along the margin. The Egeria had been largely ignored. It was concluded that the carp stocking rate was too low, so much higher rates are to be tried in order to force the carp to eat the Egeria.

Opoku, A.K. and P.C. Pierce, 1971 Summary of results for a fishing strip clearing programme conducted at Volta Lake during 1970-71. In Report of the 4th Annual Meeting of the National Weed Committee for Ghana. Akosombo, Ghana, Volta River Authority, pp. 21–8

The authors discuss the problems related to fishing as a result of the growth of the emergent plants Vossia cuspidata, Polygonum senegalense and Alternanthera sessilis along the margin of Lake Volta. They point out that the aquatic plant population is beneficial as it provides food and shelter for fish, but the weeds also interfere with fishing and harbour the snail vectors for the disease bilharzia. They have considered the introduction of grass carp but regard it as unlikely to be useful against coarse emergent plants such as Vossia. Control by herbicides was not practicable, so clearance for fishing is being done by cutting and burning.

Opuszynski, K., The use of phytophagous fish to control aquatic plants. Aquaculture, 1(1):61–74

The paper gives a brief reference to silver carp but is mainly concerned with grass carp in poland.

Silver carp (Hypophthalmichthys molitrix) grows up to about 1 m in length and a weight of 20 kg. It has dense gill rakers which enable it to filter plankton of more than 8 μm in diameter. The fish feed mainly on phytoplankton but can also take zooplankton. In pond trials near Warsaw it was found that when silver carp were present algal numbers increased. The reason for this was not understood. Possibilities were that numbers of young algae were encouraged to form by the carp eating the larger ones, or by reducing those zooplankton which also eat algae.

The biology of the grass carp and a wide range of observation and opinions on its diet are reviewed. They are warm–water fish, the upper lethal temperature for fry being 38–39°C, but they can winter at 1–2°C. They have low oxygen requirements, the lethal level for fry being 0.32–0.60 mg/litre. Sexual maturity depends on latitude. In China it is at 4–5 years, in central Europe 6–7 years, and in Moscow, U.S.S.R., 10 years. The eggs are semipelagic and develop while carried along with the river current. Grass carp are suitable for sport fishing and may be caught with plant, animal or spinning bait. The flesh is palatable.

When the fry are 30–40 mm in length they feed on zooplankton and some Chironomid larvae, but as they grow bigger they feed increasingly on macroflora. In fish over 16 g animal food represented less than 1% of all nutrients. The author cites Chinese reports that pond–cultured grass carp are omnivorous including in their diet small fish and insects. As the fish grow they eat different plants. Initially these are filamentous algae, e.g. Fontinalis sp., Chara and small species of flower–bearing plants such as Lemna sp., Potamogeton pectinatus and Elodea canadensis. Fish weighing more than 250 g also eat emergent plants such as Schoenoplectus lacustris, Sagittaria sagittifolia, leaves and shoots of Phragmites communis and Glyceria aquatica. Young fish require more food per unit of body weight increase (food coefficients 92–78) compared with older ones (36–35). Animal protein is apparently essential in the diet but the quantity needed seems to decrease with growth.

In general the food preferred is soft submerged plants. In some instances the emergent Typha has been taken by breaking the base of the shoot and then eating all of it. One report states that Typha and Phragmites over 1 m tall are not eaten. Variation in consumption of filamentous algae has also been observed. In one case, where car were exposed to a massive bloom, only 9% was found to be in their stomachs compared with 72% of macroflora, indicating a clear preference for the larger plants. When fish fodder is added to ponds the carp will take some but most of their food continues to be plants.

High temperatures increase feeding rates, rising to a maximum at 33°c, which is close to the lethal temperature. Daily food consumption is about 50% of body weight at 20°c rising to 100–120% at 22–23°c. Temperature variations are believed to affect food selection; at the higher temperatures the carp will eat harder plants like Typha and Phragmites. It is this characteristic that makes grass carp suitable for stocking in ponds heated by the cooling water from power stations, where the high temperature also encourages excessive growth of aquatic plants. Two species of plants observed to be reluctantly eaten by grass carp, even in heated waters, are Nuphar luteum and Najas sp.

In the moderate temperatures of north–east and middle Europe it seems unlikely that grass carp can be relied on for efficient weed control unless large numbers of big fish were used. But this is not considered economically justified. However grass carp might be useful adjuncts to regular mechanical or chemical methods of control.

The disturbing consequences on the conditions in a water body due to the introduction of grass carp are discussed. Many fish lay their eggs on aquatic plants so that the wholesale destruction of these plants by carp would alter the species composition of fish. The carp have strong masticating teeth so that their food becomes highly disintegrated, also the assimilation of food in the carp is extremely low (only 20%) This can result in large quantities of highly disintegrated and only partly digested plant mass. Also, carp excrement is easily decomposed by micro–organisms. All this may induce an intensive fertilization of the water resulting in a change from pre dominantly large aquatic plants to an undesirable situation such as an algal bloom.

Another feature of grass carp feeding is the turbidity of the water they cause by damage to plant roots. Many more plants are destroyed by grass carp than are eaten because of their habit of tearing out plant segments from the bottom and eating only a portion at the base. This has been observed with Glyceria aquatica which is particularly vulnerable because of a weak rooting system.

Piña R., 1974 Las carpas contra el lirio. (Carp against water hyacinth). Tec. Pesq., 7(80): 12–3

In an attempt to control water hyacinth in the lake formed by the Endho dam in Mexico, one million grass carp fingerlings were released. The problems of maintaining an optimum carp population and the need for supplementary mechanical control methods are discussed. (From Weed Abstracts)

*Singh, S.B., 1963 Preliminary experiments on the relative manurial values of some aquatic weeds as composts. Proc. IPFC, 10(2): 141–5

The author describes the composting of various aquatic weeds in India (see Chapter IX). He mentions the judicious use of these composts for the manuring of fish ponds.

Singh, S.B. et al., 1967 Efficiency of grass carp in controlling and utilizing aquatic weeds in India. Proc.IPFC, 12(2): 220–35(1966)

The feeding habits and preferences of grass carp introduced into ponds are recorded, and also the amount of plants eaten and the weight gain of the fish. Tabulated information is given on the stocking rate and size of fish and their feeding rate.

The authors list 17 plants the carp readily controlled, and five which were less readily taken. The fish did not feed actively on Eichhornia crassipes (water hyacinth) Pistia, Colocasia antiquorum, Nymphoides or Nymphaea. When only water hyacinth, Pistia and Colocasia were available the fish lost condition.

Sirenko, L.A., 1969 (Algal blooms of water). Kiev, Naukova Dumka, 268 p. (in Russian)

This book on blue-green algae contains a section on their effect on fish. (from Weed Abstracts)

Stott, B. and L.D. Orr, 1970 Estimating the amount of aquatic weed consumed by grass carp. Prog.Fish-Cult., 32(1):51–4

In feeding experiments with grass carp in the United Kingdom the authors determined that if the fish had a diet of about 66% Elodea canadensis and 33% Myriophyllum sp., about 10 tonnes of fresh weed would produce about 100 kg of fish. The Kcal content/g was 0.345 for Elodea and 0.552 for Myriophyllum. They cite typical productivity of submerged macrophytes in temperate zones as 2–7 tonnes (dry weight)/ha/annum. This corresponds to the amount measured of Elodea and Myriophyllum at 17–59 tonnes fresh weight/ha/annum.

*Villadolid, D.V. and D.M. Bunag, 1953 New uses for water hyacinths. Philipp.Fish.Yearb., 1953:80–1, 241–2

The authors describe a variety of uses for water hyacinth in the Phillippines, including details of its use for fish and pig feed.

“Some of our own fishpond operators, especially in Bulacan and Pampanga, have started to use the water hyacinth as supplementary food for bangos [milkfish, Chanos chanos]. This water plant is carried down the rivers of the swamplands in great quantities especially during the rainy season. It is stopped by bamboo floating barriers in front of the ponds and thrown high on the dikes every week. Then it is either stored for future use or piled immediately inside the ponds. In two or three days the piles of dry water hyacinths ferment and the bangos start to feed on them greedily.”

“In the Minalin and Guagua area in Pampanga, pond operators have shortened the conversion process. They pile the fresh water hyacinths in the pondand cover them with a layer of mud. In a few days the pile rots and the fish begin to feedon the decaying weed.”

Drying water hyacinth in the sun greatly increases the feed value per unit weight or volume of the plant. It is in this hay-like form that the hyacinth is recommended for use as a feed base, or a feed supplement, for fish.

For details of a feed mixture using water hyacinth given to pigs and fish by Malay farmers see Chapter VII.

*Zobairi, A.R.K., 1959 Manuring of fish ponds. Agric.Pak., 10(1):125–38

The author describes an unusual way to utilize water hyacinth for fish culture. After making the hyacinth into compost it is scattered into fish ponds as fertilizer. Alternatively, the hyacinth is burnt to ash. The ash is then mixed with superphosphate at a ratio of 5:1 and then scattered over the surface of the pond at a rate of 920 kg/ha. The paper recommends weekly treatment until a dense bloom of zooplankton is formed. A further variation, recommended as the best, is to applythe fertilizer to the pond bed when it is dry in heaps or rows at the rate of 1–1.5 tonnes/ha and let the pond fill during the monsoon.

Zolotova, Z.K., 1970 Biological weed control in irrigation channels with the aid of grass carp. Sb.Nauchno-Issled.Rabot Vses.Nauchno-Issled-Inst.Prudov.Rybn.Khoz., 1970(4):112–26 (in Russian)

A canal with 46% infestation of emergent weeds was stocked with yearling grass carp at the rateof 5 400 fish/ha (250 kg/ha). Within two months the weeds had been reduced by 66%, mainly at the expense of Glyceria maxima, Leersia oryzoides, Beckmannia eruciformis and Carex riparia. Soft vegetation including Lemna minor and Salvinia natans was almost completely consumed. Typha latifolia (weighing 5.5 kg/m2) was eliminated. Established reeds and Typha angustifolia were not taken by the young fish but they did eat new shoots as they emerged and thus arrested the spread of the plants. When the preferred plants were all consumed the carp fed on the decomposing stalks of T. latifolia, rhizomes of Phragmites communis, seeds of Typha spp. and roots of bank grasses. It was concluded that the grass carp,at 254 kg/ha, had consumed 15 700 kg of plant material per hectare within two months. (From Weed Abstracts)

Zolotova, Z.K. and L.V. Khromov, 1970 The weeding role of grass carp. Rybovod.Rybolov., 1970(4):8 (in Russian)

A heavily weeded pond was stocked with yearling carp at the rate of 1 250 fish/ha (188 kg/ha). Within 70 days the fish had cleared the pond of weeds totalling 16.2 t/ha. First attacked was the soft vegetation and a thick carpet of Carex riparia on the pond bottom, most of which was removed within a month. Next eaten was Sparganium spp. leaving fibrous vegetation dominated by Scirpus tabernaemontani, Typha latifolia, T. angustifolia and Phragmites communis. The fish then attacked these plants and broke down Scirpus spp 3 m high and chewed off P. communis at the base, consuming also the rhizomes. (From Weed Abstracts)

Anon., 1971 Use grass carp for weed control. Indian Farming, 21(5):45–7

The paper reports the introduction in 1959 of grass carp and its successful induced breeding and distribution in India. “The fish from 25-mm size starts feeding on minute floating plants such as Wolffia and as it grows it feeds on Lemna, Spirodela and Azolla,and cut bits of submerged weeds such as Hydrilla, Najas, etc., as the size of its mouth increases. Fish of about 100 g size can be used to control most of the common water weeds in small farm ponds, if free from predators. Bigger fish of 0.5–1 kg can be employed to control weeds in larger waters.

The submerged weeds preferred by grass carp are Hydrilla, Najas, Ceratophyllum, Potamogeton, Utricularia and Myriophyllum. The fish will also control Ottelia, Nechamandra, Vallisneria, Trapa, Limnophila andSalvinia (to some extent). However, water hyacinthand Pistia are not completely consumed.

For control of floating weeds grass carp of about 10 cm length (about 15 g) are stocked at 1 000-2 000/ha according to weed density. For other weeds use fish of about 20–30 cm(100–250 g) and stocked at 200–1 000/ha. Regular inspections are needed to determine whether control is proceeding satisfactorily. If not more fish can be added. After the weed has been cleared the fish may be carefully netted out of the water and transferred for use elsewhere. As grass carp is good to eat, and easily caught by angling, precautions against poaching are necessary. If predatory fishes are present then the grass carp should be at least 1 kg in weight before being introduced.”

The paper goes on to describe methods of breeding and rearing the carp.

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