8. MEETING REPORTS

REPORT OF MEETING I
Progress of Fish Culture Development

Details of technical problems were assigned to subsequent meetings. The participants at the first Meeting confined their discussions to an evaluation of the present status of fish culture and certain general aspects of fish culture development.

In a review of documents I/RR-1 to 6 and I/E-1 to 5, the Discussion Leader commented on their scope and from the point of view of the progress of fish culture, classified the countries into two groups:

1. Asia and the Far East, Europe and North America characterized by fairly well established piscicultural practices with a long history and tradition, particularly in the Far East, and

2. Africa, the Near East and Latin America where fish culture is, generally speaking, at its initial phase, except in a small number of countries.

In highlighting the phases of progress in these two groups of countries, it was observed that socio-economic and technical limitations had hampered fish culture, especially in the second group of countries, while considerable advance had been achieved mostly in the former group where traditional methods had been systematized and scientific techniques developed, establishing modern fish culture for food, profit and pleasure.

Over-selling and ill-conceived publicity in many areas have led to over-ambitious and ill-balanced fish culture programs, resulting in disillusionment, adversely affecting future development efforts. Appropriate pilot studies, effective extension services and training of personnel are essential pre-requisites to the establishment of a successful fish culture program.

Extension of pond farming has been witnessed in Bulgaria, Czechoslovakia, Hungary, Israel, Poland, Romania, the U.S.S.R. and Yugoslavia. Besides the common carp, the tench (Tinca tinca), the pike (Esox lucius), the pike-perch (Stizostedion lucioperca), tilapia and grey mullets (Mugil spp.) are cultivated. The Chinese carps have been introduced and experimented upon in Hungary, Israel, Poland, the U.S.S.R. and Yugoslavia. Consumer preference of fresh-water species seems to delimit further expansion in some areas. There is a need for discovering new species with additional qualities of acceptability.

In the Far East, even with its age-old traditions in fish culture in fresh as well as brackish waters, the progress has failed to keep pace with the exploding population increase. Remarkable results have, however, been achieved in fish rearing in small holdings, hybridization of tilapia and induced breeding of Indian major carps (Catla catla, Labeo rohita and Cirrhina mrigala) and Chinese carps (Ctenopharyngodon idella, Hypophthalmichthys molitrix and Aristichthys nobilis).

With all the diversity of conditions existing in Africa, the Near East and Latin America, the fundamental problems confronting fish culture in these regions have much in common. Even in the face of protein deficiency, traditional food habits and ethnic and socio-economic patterns have often hampered the development of pisciculture. Populations which could hardly pay for the cost of fish transport to distant consuming centres could ill afford to pay for fish feed and other items of fish farm expenditure. Yet subsistence fish culture, which has indeed served its limited purpose, remains still to be expanded into commercial fish farming, which should not fail to form an integrated part of the overall economy.

In a statement by the World Health Organization (WHO) observer, the danger incident in some areas, of neglected and ill-managed fish ponds harbouring mosquitoes, snails and other hosts of some human parasites was pointed out. In answering this, most fish culturists explained that properly managed fish ponds with weed-free surface and clean margins far from promoting larval incidence, could actually contribute to the control of hosts, especially when suitable larvivorous and molluscivorous fish are added to the stock.

REPORT OF MEETING II
Fertilization and Role of Soil in Fish Pond Productivity

The majority of contributions to this Meeting related to the fertilization of ponds; the scarcity of papers on pond soils reflected the poor status of knowledge on the subject. Present concepts on the role of soil in fish ponds are largely based on knowledge of lake deposits and bottom muds. Although the results of studies on lake muds and, particularly, the extensive studies carried out on rice field soils can be invaluable in understanding the processes of exchange of nutrients between soil and water, it was realized that conditions in fish ponds are far more complex because of the diversity of cultural practices, high densities of fish stocks maintained, differing feeding patterns, and the significant raising of trophic levels through the intensive use of feeds and fertilizers.

Even though it is often believed that fish ponds constructed on fertile soils give higher production, there was no general agreement among workers that this is true. There is not sufficient experimental evidence to prove this. In any case, in many countries of the world a major objective of fish culture development is the utilization of areas unsuitable for agriculture and it is essential to develop appropriate techniques for treatment of such soils to obtain as high a production as possible.

Peaty soils, which often have to be used for fish ponds, are generally very acid, due both to humic acids and free sulphuric acids, with little mineralization of organic matter. Peats are deficient in calcium, magnesium and potassium. Application of lime and magnesium will be necessary to combat acidity, but there is then the danger of mineralization proceeding too rapidly and leading to oxygen deficiency. Relatively little is known about the soil-water interchange of ions in these pond soils and in view of the importance of reclaiming swampy areas for fish culture, intensive studies of such soils are urgently needed. Cat clays with a high content of aluminium sulphides are unproductive and even toxic for agricultural crops. Because of high aluminium and iron content, flocs of hydroxides form on liming. In stagnant ponds these settle quickly and a layer of deposits form on the bottom, isolating the underlying soil from the overlying water. Such soils have proved quite productive when fertilized with phosphates. Alkaline marl soils tend to be less fertile due to precipitation of iron or calcium phosphate. Application of organic manures may be necessary to make up for the low organic contents of such soils. Chelation of the iron with chelating agents has also been recommended.

Phosphorus is generally accepted to be very important in the productivity of ponds and an appreciable amount of work has been done on the phosphorus cycle. There are, however, differences in views on aspects such as the ferrous-ferric redox system. Some workers believe that fixation as calcium phosphate is more important, whereas others conclude that interchange of phosphates take place between the aluminium, iron and organic fractions, but not with the calcium fraction. Similar differences of opinion exist on the fate of nitrogen in ponds. It has been observed by certain workers that nitrate in muds is reduced to ammonia and that ammonia production is harmful to fish. On the contrary, others have found that the use of liquid ammonia as a fertilizer results in increased production of fish. These differences in observations and views appear to be largely due to differences in soil and water conditions as well as other ecological factors that have affected experimental results.

The key note of the discussions on fertilizations was that there is no universal fertilizer. Fertilization must be related to local conditions, and chemical biological and economic conditions should be the determining factors in the selection of fertilizers. Organic as well as inorganic fertilizers have their use in fish culture even though in many cases the latter may be more efficient and advantageous to use. In ponds where benthos feeding fish are cultured and in ponds with marl soils organic fertilizers are useful and necessary.

The Meeting considered the role of calcium in the carbon dioxide-carbonate-bicarbonate buffer system. Experience gained in certain areas shows that it is not advisable to accept rigidly the levels of alkalinity usually considered as optimum. Though in most ponds liming is usually necessary, over-liming may result in precipitation of iron or phosphate and consequent reduction in productivity. Very little is known about the role of magnesium and potassium in productivity.

There was general agreement that phosphate is a very efficient fertilizer in many circumstances, particularly under tropical conditions. It was, however, emphasized that phosphates in water are affected by a number of factors which differ widely in different areas and investigations using radio-active isotopes will be required to study them properly. In the application of phosphate fertilizers, there was agreement that frequent small doses are preferable to one large dose. Excessive applications may lower primary production by autoshading and result in unfavourable algal blooms and excessive growth of submerged macrophytes.

Differences of opinion were expressed about the use of nitrogenous fertilizers. Experience in sub-tropical and temperate climates indicates the need for such fertilizers in ponds. This could be due to the relatively low level of nitrogen fixation in colder climates owing to lower temperatures and lesser light intensities. The lack of adequate information on nitrogen requirements and the nitrogen cycle in ponds was recognized and the need for further studies using radio-active tracers emphasized. As regards the nitrate fertilizers used in fish ponds, it was pointed out that ammonium sulphate might make the water more acid and potassium nitrate more alkaline. Calcium nitrate has the advantage that the calcium is taken in the buffer system. Aqueous ammonia has been used with success in Israel and Poland.

Following the practice in agriculture, N:P:K mixtures are generally used for fertilizing fish ponds without determining the nutrient requirements of soil or water. This was considered wasteful as in most cases, if a mixture is required, it may be only N:P.

The discussions at the Meeting clearly showed the need for more intensive studies on soil and water chemistry of fish ponds, as well as the processes of nutrient exchange. Fertilizer practices have to be formulated on the basis of the ecological and economic conditions prevailing in the area. Adequate soil and water analysis facilities are required in fish culture establishments for the adoption of efficient and economic fertilizer practices.

REPORT OF MEETING III
Feeds and Feeding

The discussion on feeds and feeding followed two general lines: (i) a consideration of the economic justification of feeding warm-water fish in ponds in different areas of the world, and (ii) a consideration of efforts being made in a number of countries to develop cheap, readily available and efficient diets. In addition to these, the meeting also discussed briefly the physiology of digestion in cultivated fishes.

There was general acceptance in principle of the advantages of supplemental feeding of warm-water fish, but there were serious doubts about the feasibility of artificial feeding in some of the developing countries, especially in Africa, at the present time. Absence of a tradition of animal husbandry and the economic condition of the farmers were advanced as the major reasons for supplemental feeding being not adopted as a general practice. The existing high cost of feeds renders feeding uneconomical in many countries.

It was suggested that in discussing the economics of feeding, distinction should be made between subsistence level fish culture and commercial fish farming. In the case of the former economics are of less significance. In commercial farming economic principles can be applied to determine whether feeding is justified. However, it has to be remembered that it is not necessary to depend on artificial feeding to raise fish and in many cases feeding has to be done only to supplement the natural foods available in the ponds.

Development of adequate feeds is of importance in ensuring their effective use and for this purpose much clearer information than now available has to be gathered on the nutritional requirements of cultivated fishes. Research efforts should be intensified on digestion, absorption and metabolism of feed ingredients. With such information, it should be possible to produce cheap feeds composed of locally available ingredients.

A major problem in the development of fish feeds has been the need to include a relatively large amount of animal protein. It appears possible to circumvent this need for animal protein by using a combination of vegetable products which will provide fish with the necessary amino acids in the required quantity for optimum growth.

Research is needed on methods of improving the nutritional quality of waste material or readily available natural products, especially in developing countries. The use of microbial digestion and biosynthesis should, if possible, be tried as techniques for this purpose.

The necessity for standardizing feeding trials to obtain reliable data was also emphasized at the Meeting.

REPORT OF MEETING IV
Breeding and Selection

Breeding and selection are of fundamental importance in fish culture and the eight review papers (documents IV/R-1 to 5; I/RR-1; VIII-IV/R-1; VIII-IV/R-3) and 15 experience papers (documents IV/E-1 to 14 and VIII-IV/E-8) presented at the Symposium gave a fair picture of the present status of our knowledge on these subjects.

Breeding Techniques

Pond breeding techniques have probably made the minimum advance in the case of fishes that spawn only in salt water, such as the milkfish (Chanos chanos) and most grey mullets (Mugil spp.) and the industry still depends on natural resources for stocking material. Partial success achieved recently in the breeding of Mugil cephalus by hypophysation is encouraging and with the efforts that are now being made in a number of countries, it can be justifiably expected that successful procedures will soon be evolved to breed salt-water fishes in ponds.

Fishes like tilapias, pearl spot (Etroplus suratensis) and certain species of grey mullets that spawn both in fresh as well as salt water can easily be bred in ponds, without the use of elaborate procedures. In the tilapias prolific breeding has necessitated controls and sex direction and sterility have been achieved through hybridization.

Major developments in techniques have occurred in the pond breeding of fresh-water fishes, especially the common carp, the Chinese carps, the Indian major carps and certain catfishes. Hypophysation is being increasingly practised to induce early or late spawning as desired and to induce spawning in confined waters in the case of species that breed only under riverine conditions.

Common carp and the Chinese carps attain sexual maturity in the first year of life in tropical waters, as against the three to four years they take in temperate waters. This, coupled with the development of methods for their controlled breeding in confined waters, has made rapid selective breeding of these species possible in the tropics.

Certain secretions of fish are reported to have inhibitory influence on reproduction and under conditions of heavy stocking these secretions might repress reproduction. Avoidance of overcrowding in brood ponds and provision of facilities for complete change of water in breeding ponds are recommended under such situations.

Selection

Extensive work on the selection of common carp has been done in Europe and Asia to develop strains suited to local requirements. The Meeting noted with interest work now in progress in this field in Israel, Poland and the U.S.S.R. The need for similar intensive studies in other parts of the world was recognized. Hypophysation provides an efficient tool in such work and in warm waters selection can be achieved in a much shorter time. It will be very advantageous if the selective breeding of fishes is done in the tropics and the progeny testing undertaken in tropical as well as temperate areas. Work carried out in Israel has indicated the need for frequent exchange of brood stock between farms as continued inbreeding rapidly decreases growth capacity.

Hybridization

Uncontrolled spawning of pond breeding fishes (e.g. species of Tilapia, common carp, etc.) often upset population structure and adversely affect production. A considerable amount of work has been done to combat this, in the case of tilapias by inducing sterility or by mono-sex production. Hybridization between (i) female Tilapia mossambica and male T. hornorum, (ii) male T. hornorum and female T. nilotica, and (iii) female T. nilotica and male T. hornorum have resulted in the production of 100 percent male progeny. These hybrid males show better growth rates. Other hybrids produced in Africa, Israel and U.S.A. are also reported to grow faster than the parent species. As these hybrids are fertile, it is necessary in mono-sex culture to remove the parent stock from the spawning ponds to prevent the mature hybrids mating with them.

Successful spawning of fish by hypophysation has made hybridization easier. As indiscriminate hybridization is generally undesirable and may lead to dangerous consequences, it was generally felt that such work should be purposeful. It should be dictated by recognized local needs and any hybrid produced should be fully tested for its value. If the hybrid does not come up to expectations, it should be destroyed, leaving no possibility of its escape into a natural waterway.

The Meeting noted that the existing difficulties in determining the identity of strains and species introduced into countries are bound to increase with the increasing use of hybrids in fish culture and therefore considered it advisable to establish an international registry of fish introductions and their consequences. In view of nomenclatural difficulties, a system of designating strains and stocks by code numbers was considered advantageous.

REPORT OF MEETING V
Biological and Other Methods of Increasing Production

The use of quick-growing species for culture and the acceleration of growth rates by artificial feeding are generally accepted as means of increasing production from ponds. There is no reason to believe that quick-growing species are poor converters of food, as often suspected by fish culturists.

In discussing mixed culture of species as a means of increasing production, it was pointed out that in Israel synergistic action may develop among fishes in properly treated ponds when the density of fish population is adjusted to the carrying capacity. The selection of compatible species has to be based on an evaluation of their efficiency in combined culture, and it is necessary to formulate standard and comparable methods for this purpose. The importance of biological waste disposal in growth promotion was also emphasized by some of the speakers.

Supplemental feeding increases production but over-feeding initiates unfavourable biological processes which result in the decline of production.

Even though there are chemical ways to “clean” the aquatic environment, biological methods are more suitable and less expensive. In mixed culture, plankton feeders will achieve this at least partially. Various known techniques of increasing natural food production, including the provision of sub-strata for the development of bottom fauna were discussed. The full utilization of natural food produced in ponds must be made possible by adequate stocking of fish of different sizes/ages, or of different feeding habits, to build up an efficient food chain.

The advantages of species combinations in fish ponds have been well demonstrated in traditional fish culture as well as in modern fish culture in many parts of the world. The need for development of suitable species combinations for tilapia in Africa was emphasized as in many areas only 20–30 percent of the production is of commercial value at present.

Manipulation of stock densities and increasing the number of crops per year are very efficient means of increasing production. Alternating fish crop with agricultural crops can be beneficial under certain conditions. The practice of raising ducks in combination with fish has yielded good results in parts of Eastern Europe.

The influence of environmental factors, particularly temperature on production, was emphasized. Some cold-water species like trouts adapt themselves well to higher temperatures and can be raised in warm water with advantage. In temperate zones particular attention is required to prolong the growing season. Improvements in over-wintering techniques may be of help in this direction.

REPORT OF MEETING VI
Standardization of Research Techniques

Chemical Methods

The methods adopted for the determination of dissolved oxygen, free carbon dioxide, total alkalinity, bicarbonate alkalinity, total hardness, total calcium and pH were discussed and the methods found most suitable in fish culture research pointed out.

Although the sulphamic acid and azide modifications of the Winkler method are suitable for determination of dissolved oxygen for most pond waters, Ohle's iodine difference method would be necessary for waters where ferrous or ferric iron, nitrate, sulphate and/or organic matter render the modifications ineffective.

For the determination of free carbon dioxide, titration may be done with standard Na₂Co₃ or NaOH to the phenolphthalein end point. Titration before and after boiling will differentiate CO₂ from strong acids.

Total alkalinity (total amount of bound CO₂) is generally determined by colorimetry. The sample is first titrated with standard H Cl to the phenolphthalein end point (pH 8.3) to determine carbonate and hydroxide alkalinities. Then, the titration is continued to the appropriate methyl orange end point (pH 4.5–5.1 according to the total alkalinity value), which gives a measure of the total alkalinity. The bicarbonate alkalinity, which is more important because of its relationship to photosynthesis, is the difference between the total alkalinity and the phenolphalein alkalinity. The results will be more accurate if the titrations are made to electrometrically determined end points.

Total hardness should be determined by EDTA titration using chrome black T or calmagite as indicator, and expressed as ppm CaCO₃ equivalent. Since calcium is important in the aquatic environments, it is often desirable to determine this cation separately from total hardness. This can be done by EDTA titration with ammonium purpurate (murexide) indicator.

Hydrogen ion concentration should be determined only electrometrically by using glass electrodes. Colorimetric methods are too inaccurate for use in fish culture work.

For the analysis of bottom muds from ponds, composited and dried samples should be used. These are mixed with water at a standard proportion and the pH determined with glass electrode. To determine the lime requirement of pond soil, the sample is mixed with a standard buffer solution (pH 7.0) and the decrease in pH indicates the amount of lime required.

Biological Methods

The diurnal movements, vertical distribution patterns, and rapid fluctuations in abundance of plankters as well as wind action make determination of average plankton abundance difficult, if not impossible, because of the number of samples and the time required for analysis. The method of rapid estimation of seston (plankton plus detritus) by nephlometric analysis, which requires only one to two minutes per sample, appears to be well suited for use in fish ponds.

In population analysis and in estimation of fish foods available to individuals of various sizes, length-weight, length-maximum body depth, length-maximum body width, and length-mouth width equations and tables are very useful. Except in special cases, fish length should be expressed as total length and if and when standard length is used, a conversion factor should be worked out and included in published reports.

In describing the condition of fish, the relative condition (when W = average weight of fish of length L₁ and Ŵ = standard average weight computed for fish of length L₁) should preferably be used.

Standardization of methods for evaluating the yield and efficiency in food conversion of various cultivated and cultivable species was suggested. Tests have to be made at three levels of stocking in unfertilized ponds, fertilized ponds and in unfertilized ponds with artificial feeding.

Rapid transportation and changes in scientific names of tilapias and other fishes have led to the suggestion that a numbering system be established to follow the current scientific name, coded to indicate the continent, country, river watershed and year when the fish was first obtained from a habitat. When a scientific name was changed, this should allow literature published in prior years to be credited to the scientific “name of the moment”.

The use of standard forms to record routine research records may facilitate collection and recording of data.

Statistical Methods

The usefulness of statistical methods in planning fish culture research and analyzing experimental data cannot be over-emphasized. Unfortunately, adequate use of these methods has not been made by many fish culture scientists.

A single test is often indicative and useful, but replications are essential to determine the significance of the results. For the variations to be expected in most fishery problems, a minimum of three replications are necessary and four to five or more are desirable. Where insufficient ponds are available for adequate replication at one time, it is usually necessary to repeat the experiment in succeeding months or years to ensure results of sufficient reliability. Even where sufficient ponds are available for adequate replication at one time, repetition of an experiment over a several year period is often necessary to take into account the effects of climatic or other variations on the results.

The use of randomized blocks, latin squares and other methods of replication of experiments can be most advantageous in experimental studies. Increased efficiency in certain comparisons can be obtained by the use of sectioned areas of the same pond for experiments. The ponds are divided by nets or screens into two or more equal sections. This procedure reduces effects of variations between ponds, but such a procedure introduces the problem of interaction between the groups within the sectioned pond. Since fish wastes and the oxygen demand of the pond biota are very important factors limiting fish production, this procedure must be used with caution.

For most experiments it is necessary to use in all ponds fish of the same size, preferably of the same age and from the same parents when possible. The experiments should be started in such a sequence so that each pond can be drained after the same number of days where possible.

The results of a short-term experiment cannot be extrapolated to determine what the production might have been had the experiment been continued. Production values obtained by short-term experiments, expressed as production per ha per year are valid only if harvestable-sized fish were grown in the short-term experiment. The extrapolation of a calculated curve beyond the limits of observed points is not permissible and may lead to completely false interpretations.

In general, the Meeting felt that more than the standardization of techniques, the immediate need is to ensure that there is much more uniformity in reporting the results of studies, so that they will be comparable and of benefit to workers in all parts of the world. In research reports, the methods used should be adequately described. The Meeting was of the opinion that a “Manual of Fish Culture Research Methods” for the use of fish culture scientists and a “Manual of Methods of Chemical Analyses” for the use of laboratory and field workers would be extremely valuable.

REPORT OF MEETING VII
Weed Control

The discussions at the Meeting covered mechanical, biological and chemical methods of aquatic weed control in fish ponds.

Mechanical Control

Manual clearance is the oldest and probably the most frequently used method of weed control. This method is rapidly being replaced in many areas of the world by chemical and biological methods because of the high cost of labour. Motor-powered weed cutters have been used in many countries to control aquatic vegetation. However, these cutters have proved to be very expensive and beyond the means of the average fish farmer in a number of countries. Control is usually temporary and must be repeated several times during the growing season. The problems of transporting mechanical cutters and diggers from one area to another make their use unsatisfactory for small ponds.

Biological Control

Biological agents for the control of aquatic weeds are employed in many regions. The herbivorous fish, Tilapia, is widely used in Africa and other countries. Several kinds of carps are used in Israel, India, U.S.A. and elsewhere. The rodent, nutria (Myocastor), and geese, were reported to be effective in some countries. In the United States a large freshwater snail (Marisa) and the manatee (Trichechus) are being investigated for control of aquatic weeds. Also, in the U.S.A., insects are being evaluated for biological control of many aquatic weeds.

It was suggested that the ideal herbivorous fish, to be used for weed control, should meet the following requirements: (i) control a wide variety of weeds; (ii) not interfere with other fish species; (iii) be hardy and easy to handle; (iv) be economical to use, and (v) add to the fishery. The attainment of a biological equilibrium, in which the energy trapped by vegetation is returned to the biological cycle in the water by a relatively rapid process, has great importance in the maintenance of fertility in fish culture and in the continuing high yields of fish harvest.

It was generally agreed that biological methods of weed control will play an important role in the future, especially in fish culture. The encouraging results obtained by the use of herbivorous fishes such as Tilapia melanopleura in Africa, Ctenopharyngodon idella in Asia, Cyprinus carpio in Europe and Asia, Tilapia mossambica and Puntius gonionotus in Indonesia were pointed out. There is, however, an urgent need to intensify studies on the subject to enable further extension of the methods successfully under different ecological conditions.

Chemical Control

Herbicides can effectively control many species of aquatic weeds in ponds and along shorelines. In the use of this method, it is important that the fish culturist knows the weed species, the appropriate herbicide, the recommended dosage, the time for treatment, and the water volume or area to be treated. Once the control program is initiated, it is equally important that it is continued. The high cost of herbicides is a limiting factor in many parts of the world, but it was reported that they have proven economical for use in fish ponds in the Near East, Europe, and the United States.

The Meeting expressed the need for further research on the persistence of herbicides in water, soil and fish flesh, and for more studies on the movement of herbicides through the fish food chain. The field of chemical weed control has made tremendous progress in the last five years and the number of new herbicides available for control of aquatic weeds has increased many fold. The increased volume of research in this area promises to solve many of the problems in aquatic weed control for fish ponds. In this connection, it was pointed out that in papers published on this subject there is a lack of consistency in the chemical nomenclature of herbicides and a suggestion was made that the full chemical name of the herbicide used should be given the first time it is referred to in a report and all treatment rates be based on percentage composition of the active ingredient. Fish toxicity data on herbicides should include formulation, water temperature, chemical and physical characteristics of the water and the location of the laboratory or pond where the study was made.

It may be dangerous to recommend use of herbicides to fish culturists until they are trained to understand the chemicals and the proper methods of application. In all areas of the world there is public opposition to the addition of chemicals to water resources. The confidence of the public can only be gained through wise and safe use of these methods. An important part of chemical weed control is the education of the public with regard to the safety of herbicides for fish, birds, mammals and human beings when properly used.

The fish culturist often combines the three control methods discussed above. This is essential in many cases, as different types of weeds may occur in the pond and they will require different control methods. It must be remembered, however, that the best combination of methods may not bring about the eradication of a weed. Man has seldom brought about the total eradication of a plant species. The best results will probably be obtained by taking advantage of the natural environment through plant competition and biotic balance. This should be supplemented by a diligent herbicidal and management program. It is recognized that proper construction and good management of ponds are often the best methods of weed control.

The Meeting took note that aquatic weeds found in fish ponds are widely disseminated, and that regulatory measures should be taken wherever possible to prevent the introduction of harmful species. Shipments of fish between countries may provide a means of spreading seeds or bits of plant material. A suggestion was made that upon receipt of a fish shipment, a period of feeding might follow so that seeds carried in the gut may be collected and destroyed.

REPORT OF MEETING VIII
New Systems and New Fishes for Culture

New Systems of Fish Culture

For purposes of discussion, a ‘system of fish culture’ was defined as the utilization of a fish cultural technique under particular ecological and socio-economic conditions.

The two entirely new systems of culture discussed at the Meeting were: (i) culture of common carp and other fishes in floating pens made of plastic netting, and (ii) the culture of carp in concrete tanks with a closed water circulating system.

The use of floating pens has made it possible to utilize the water cooling reservoirs of power stations in the U.S.S.R. for fish production. These new systems open up very great possibilities for intensive fish husbandry, if they prove economically sound. The culture of common carp and the catfish in cages or large baskets in running water, though an old system in some countries of the East, is spreading to other areas and is being improved. The culture of channel catfish (Ictalurus punctatus) in rice fields and the raising of the crayfish Procambarus clarkii in combination with rice or in embanked swamp-land are new systems that have developed in the U.S.A.

Though fish culture in brackish waters is an age-old industry in certain countries of Asia and the Far East, it is a new system in most other areas. The successful culture of grey mullets (Mugil spp.), shrimps (Penaeus spp.) and the pompano (Trachinotus carolinus) in brackish-water ponds is of special interest and the Meeting considered that the vast possibilities of the system of brackish-water fish culture should receive much more attention than heretofore. This system enables the utilization of extensive areas of mangrove swamps and tidal lands that are unsuitable for agricultural or other productive purposes. The results of transplantation of the sole, Solea vulgaris into lagoons in the U.A.R. led to a brief consideration of the potentialities of the culture of euryhaline fishes in what may be termed “marine” environments or brackish waters. It was recognized that coastal aquiculture may in the future play a very important role in the maintenance or enhancement of fish production throughout the world.

New Fishes for Culture

In discussing new fishes for culture, the Meeting took into account species that are entirely new in fish culture and fishes that are new to certain areas. In the former group comes the various hybrids of tilapia, that have been developed to enable mono-sex culture and increased yields from ponds. The culture of channel catfish in the U.S.A. has already been mentioned. The African fishes Heterotis niloticus, Hemichromis fasciatus and Lates niloticus have begun to be utilized in pond culture, some of them being used as predators for controlling populations of tilapia or carp. There is wide-spread interest in the culture of fresh- and salt-water shrimps and prawns. In view of the prevailing high demand for shrimps and prawns in the world market, there appears to be very great possibilities for establishment of their culture on commercial lines. Intensification of research and experiments under different ecological conditions are urgently required to evolve suitable techniques for this purpose.

In recent years there has been very wide-spread international exchange of species for culture. The Chinese carps, especially the grass carp, Ctenopharyngodon idella and the silver carp, Hypophthalmichthys molitrix have been introduced into a number of Eastern European, Asian and Central American countries. The culture of common carp is now being tried in many countries in Africa, where their introduction and culture were subjects of considerable controversy. The culture of grey mullets, in brackish or fresh waters is new in Israel, the U.A.R. and the U.S.A.

During discussions it was emphasized by many speakers that greater attention should be devoted to select and evaluate new species for culture as for lack of suitable local species, fish cultural programs in many countries have been greatly handicapped. The need is particularly great in Latin America and Africa.

REPORT OF MEETING IX
Diseases and Parasites

The review and experience papers presented at the Meeting summarized the present knowledge on diseases and parasites of warm-water pond fishes. It was clear from these that there has been marked-increase in parasitism and mortality in cultivated fishes with the intensification of fish cultural operations. This presents a problem of serious magnitude in all countries of the world where fish culture has made appreciable progress.

The Meeting devoted some attention to the pond or water conditions that are conducive to a high incidence of parasitism and fish disease. It was suggested that in many areas there is a correlation between pollution of the water and appearance of fish diseases. Heavily fertilized and densely stocked ponds in Malaysia have shown a relatively high degree of infection of Lernaea, Argulus and Dactylogyrus. In India, it has been observed that infections with Lernaea and Argulus are greater during summer when the water levels are low and organic materials concentrated. On the contrary, it was reported that in Israel Lernaea are more abundant in unfertilized and clean water.

Among the most wide-spread parasitic diseases are those caused by protozoan infection, especially infection by Trichodina, Chilodonella, Costia and Ichthyophthirius. Under tropical and sub-tropical conditions these diseases assume more dangerous proportions. Ichthyophthiriasis is one of the most wide-spread diseases, but fortunately the control measures are fairly well known now and only proper application of these measures at the right time have to be ensured. Quarantine measures have been introduced to prevent the spread of this disease in a number of European countries. Though pond fishes are infected by a variety of trematodes, only a few of them cause serious injury or mortality. Dactylogyrus and Gyrodactylus are of importance but prophylactic as well as curative measures to combat their infection have been developed. Copepod parasites, especially Lernaea and Argulus, are very common in warm-water ponds and high densities of fish stocks promote higher rates of infection. Some progress has been achieved in the study of these parasites, but further work is necessary to evolve suitable prophylactic and curative measures.

There is considerable difference of opinion among workers on the nature and methods of control of diseases of viral or bacterial origin. It is interesting to note that there are no records of any diseases of viral or bacterial origin in Africa, Israel and other Middle East countries. Infectious dropsy though a very common disease of the common carp in other parts of the world, is conspicuous by its absence in these areas. It is not clear whether this is related to the climatic conditions or the techniques of culture. Whether dropsy is caused by a bacteria or a virus is still a matter of controversy, but the majority view appeared to be in favour of considering a virus as the causal agent. The use of only healthy breeders, drying of ponds in winter, prevention of skin injuries, treatment with antibiotics in suspected cases (intraperitoneal injection or baths in antibiotic solution in spring or adding antibiotics in feeds in summer) are measures successfully adopted in Yugoslavia.

There is inadequate information available on bacterial diseases of warm-water fishes and even in cases that have been investigated, as for example those described from Europe, the relationship with the infection of dropsy has not been taken into account. The causal agent of dropsy in carp ponds in India has been identified as Aeremonas, but the symptoms of the disease are very similar to those of dropsy as observed in Europe.

A relatively new disease of carp reported at the Meeting was the air bladder disease, which is a major cause of high mortality in ponds in Hungary, Poland and the U.S.S.R. It is characterized by inflammation of brown-coloured concretions in the middle layer of the walls. In its chronic state a large air cyst is formed around the bladder and this upsets the centre of gravity of the fish and it rests belly upwards. The etiology of this disease is not properly known and in view of the serious nature of this disease and the losses sustained by farmers on account of this, the Meeting felt that study of this should receive high priority. Another new disease of carps, the etiology of which is not properly known, is infectious necrosis of the gills, affecting all age groups and often resulting in largescale mortality.

In conclusion the Meeting considered the dangers of spreading fish diseases and introducing new parasites into countries as a result of the large-scale international exchange of live fish and fish eggs. It was noted that only a few countries in the world have effective regulations for inspection and control of fish introductions or quarantine arrangements. This is a field in which there is need for international cooperation. The Meeting took note of the recommendation of the European Inland Fisheries Advisory Commission at its Fourth Session, Belgrade (1966), that a survey of the laws and regulations governing import and export of live fish and fish eggs in different countries of Europe should be carried out by FAO and in supporting this recommendation suggested that this survey be made world-wide. The need for a general intensification of research on diseases and parasites was repeatedly emphasized at the Meeting. Close cooperation between research institutions and, if possible, coordination of programs was generally urged; this need was particularly stressed in the study and control of infectious dropsy.

The Meeting also felt that better means of dissemination of information on the diagnosis of diseases and both prophylactic as well as curative treatments, should be devised as a large majority of fish culturists are less proficient in this aspect of fish culture than others.