III. BIO-ENVIRONMENTAL ASPECTS

This is a shortened version of the report on the desk study on bio-environmental aspects of aquaculture in rural development, submitted

by

L. Nyman
Director
Freshwater Research Institute
Dnottninglolm, Sweden

1. INTRODUCTION

Tropical lakes in Africa typically have a high biological productivity, even exceeding the productivity of husbandry on fertile farm land (Curry-Lindahl, 1980). However, the capture fisheries are often under pressure both from overexploitation and the degradation of water quality through increased pollution. Priority, therefore, should be given to the restoration of water quality and proper management of fisheries to obtain maximum production of fish protein. Capture fisheries will, however, in the long term not be able to satisfy the increasing demand and need for fish protein. Aquaculture, therefore, seems to be the logical complement to capture fisheries. Both capture fisheries and aquaculture are completely dependent on adequate water resources and good water quality.

This paper deals with the bio-environmental aspects of aquaculture. It is intended to serve as a background for the planning and implementation of activities under the Aquaculture for Local Community Development Programme. It is a shortened version of a report of a desk study carried out for the Programme.

2. FRESHWATER RESOURCES AND WATER QUALITY

The freshwater resources are mainly found as groundwater, and these resources are rapidly being depleted in many arid and semi-arid regions in the tropical and semi-tropical zones. A lowering of the groundwater table, intrusion of salt, and locally diminishing resources of groundwater are frequent causes of human interference at the same time as population increases escalates the demand for high-quality water. This also increases the demand for the utilization of wetlands, rivers and lakes and calls for stringent planning of the utilization of available water resources.

In this context each river basin must be treated as an entity, and every measure taken has implications for the users downstream. The creation of dams for hydroelectrical or irrigation purposes might result in the disappearance of economically valuable fish species, which are not compensated for by other species. After the initial increase during the damming-up phase there is a decrease in fish production. One of the main reasons for this is the fluctuating water levels causing erosion of the productive littoral zone.

Degradation of water quality is dangerous with hazards both to capture fisheries and aquaculture. Industrial wastes and sewage from urban areas, mining operations, etc., add poisonous substances to the water. Equally harmful are effluents from chemical plants and pesticides emanating from agriculture.

Pollution of tropical waters is a general problem. Direct fishkills, reduction of breeding, fry survival and growth are evident results. Harmful are also secondary effects, such as genetic damage to future generations of organisms. A further result is the reduction in species diversity, which seems to favour smaller and short-lived species, since the rapid generation turnover reduces the time each individual is subjected to accumulation of harmful substances (Welcomme, 1979).

Eutrophication is another result of overexploitation of water resources. Even though it is a natural process it is accelerated by nutrient overloading from human activities. Increased primary production and subsequent oxygen demand reduces the number of tolerant fish species and thus the total fish production.

3. ENVIRONMENTAL IMPACT ON/OF AQUACULTURE

Wastes

In cage and pen culture the fish depends on the water flow to supply food and oxygen and also to remove wastes. Enclosures are part of the open water system and as such more interacting with the outside environment than ponds and tanks. In ponds water passes typically only once, and “downstream” environmental impact is experienced where effluents are discharged. Settlement ponds and infiltration techniques may be employed to control the water quality in such cases. For cages and pens, which are an integral part of the ecosystem, there is little scope for the treatment of wastes.

Predation

All enclosures and ponds where high densities of fish are kept are subject to predation. This can be by predatory fish entering unnoticed. Birds, reptiles and mammals may also tap the concentrated resource. In pens and cages the nets may be damaged by predators causing considerable losses of fish.

Wild fish populations

The impact of aquaculture on wild fish can be both direct and indirect. Effluents from the site may enrich downstream waters to a degree that fish may be adversely affected. Feromones emitted by cultured fish may either repel or attract wild fish, hence making them more susceptible to infection or negatively, affecting capture fisheries. The aquaculture system may act as a transmittal of parasites and bacterial and viral diseases. Escaping fish or fish sold and released into natural waters may exterminate local fish species by direct predation or ecological competition. Hybridization between escapes and local fish stocks may disrupt stable genetic systems, i.e., locally adapted gene pools may be lost (Nyman, 1985).

Alien fish species

Introduction of alien fish species has produced well-established effects. A number of species have been introduced by accident. One example is ornamental fish species in Europe and North America. Others, like species of the poeciliid genera Gambusia and Lebistes, have been introduced to control disease by predation on organisms that are intermediate hosts for the disease-producing species. These fish have done well in their new environment and become pests. Still other species have performed well, for instance by controlling snail populations without any known negative effects (Welcomme, 1979).

The most common reason to introduce new species is to fill what is considered a “vacant niche”. This is particularly common in Asia where natural lakes are rare and thus also lacustrine species. The pelagic genera Stolothrissa and Limnothrissa are good examples from Africa, where many introductions have been made in newly created river reservoirs. Many species of tilapiine fish have been spread around the world. They have proven superior to local species under intensive culture conditions. Accidental releases into natural waters have earned them a bad reputation by the very success of their adaptation. They readily replace local fish species and become so prolific and hence stunted, that they are considered as pests. Management schemes for those, often mixed Tilapia stocks, have yet to be formulated and proven.

Parasites and diseases

Fish in high densities, as in culture systems, may have severe impact on the development and spreading of parasites and diseases. Also fish in high densities are more liable to be affected by outbreaks of diseases than are wild fish of the same species. These problems have, however, seldom been reported from the tropics (Beveridge, 1984).

An environmental problem is the control of water-borne diseases like malaria, bilharzia, sleeping sickness and river blindness.

The mosquitos carrying malaria spend their early stages in fresh water. Increasing the area of open waters by the construction of reservoirs or ponds will favour the mosquitos. If, however, water plant development is curbed, fish larvae will readily feed on mosquito larvae. If ponds are developed in formerly swampy areas there is even hope for reduced malaria problems.

The bilharzia produced by parasitic blood flukes rely on aquatic snails as intermediate hosts. These snails are also favoured by dense vegetation. To avoid the problem ponds should have steep sides and be cleared of weeds to disfavour the snails. In heavily infected areas, snail-eating fish-like Tilapia macrochir may be cultivated (Grover et al., 1980).

The tsetse fly which transmits sleeping sickness is also favoured by shaded, moist, areas. Clearance of trees on the pond banks should thus be encouraged.

River blindness is transmitted by black flies (Simulium sp.). Their larvae are, however, favoured by rapidly running water. There is thus little problem of river blindness associated with pond development. Actually, dam constructions in rivers may reduce the incidence of this disease.

Chemicals and drugs are frequently administered to cultured fish to control ectoparasitic and fungal diseases. This is mainly true of intensive units with recycled water. Uncontrollable release of various chemicals in small water bodies, like ponds, may cause health hazards to those eating the fish.

It seems that tropical species, like tilapias, are less prone to develop diseases when cultured than temperate species. This may be due to greater hardiness in these tropical fish species compared to, for example, salmons and trouts, but may also be attributed for by the lower densities of cultured populations employed in most tropical systems (Beveridge, 1984).

4. ENVIRONMENTAL ASPECTS ON SITE SELECTION

The main limiting factor in fish farming is the total amount of water available and its distribution over the year. Proximity to a good water supply thus is a prerequisite. To reduce seepage the ground should hold enough clay. Sandy soils or those rich in gravel or rocks should be avoided.

Water and soil properties are the most crucial, but also climatological and biological factors need to be evaluated before any engineering or economic consideration are made (Klemetsen and Rogers, 1985).

Water resources

Perennial water supplies are rare in tropical regions. Low or seasonal rainfall combined with high evaporation makes all-year round production of fish exceptional. Apart from sites along the banks of rivers with permanent water flow, aquaculture will either depend on water diverted from water storage reservoirs or will have to rely on seasonal production during the months with adequate rainfall. Reservoirs may also be used for fish culture by using net or pen enclosures or onshore ponds.

Water quality, in particular, with regard to pollutants, is important to monitor as is the pH and the alkalinity of the water.

In open water systems (lakes, reservoirs or rivers) where pen and cage culture may be developed, the siting of the units will depend on the direction of water flow or predominant winds, since these factors will determine not only availability of natural feeds and oxygen level, but also the removal of toxic metabolites (Beveridge, 1984; Schmittou, 1970). A wide dispersion of units will on the other hand increase the interference with other users of the water, like capture fisheries.

Climatic conditions

No climatic conditions (except adequate rainfall to maintain water resources) will make aquaculture impossible. Temperature, i.e., the length of the growing period, will on the other hand determine which type of species should be cultured.

Topography and soil conditions

Flat ground or a slight gradient are the best topographical features of a potential site. Not only will flat ground facilitate the excavation of the pond but also decrease the risk of erosion and silting.

Apart from the necessary quality of clay content, pond construction is favoured by poor drainage even though this will occasionally produce swamps which may be flooded during the rainy seasons. This in turn will preclude the draining of ponds and may also increase the risk of fish escaping from the ponds.

Principal environmental factors for site selection

The diagram below illustrates the main environmental factors influencing site selection.

The direction of arrows indicate direction of impact of each component. Those on the left have implications both into and out of the culture system. Diseases and parasites can enter the system by contaminated seed or from the natural water source, but the pond/pen/cage may also act as a potential source of downstream impact on other culture units, natural fish populations and wildlife. The cultured species may have direct or indirect environmental effect on natural populations downstream, by potential release of genetically or ecologically superior species. Pollution of upstream water sources may have severe impact on the culture system, but wastes that are freely dispersed from the culture unit may also cause oxygen depletion of downstream waters and enhance conditions for parasitic development.

Remote sensing techniques

These techniques may prove of value in siting and regional mapping because soil structure and water availability may vary enormously between sites only a few hundred meters apart (Henderson, 1985). Of particular interest is this technique to inland capture fisheries (Howard, 1985). Photogrammetric mapping of sites, identification of turbidity classes, aquatic plant development, phytoplankton production, fish species habitats and environmental change are among the factors which may be monitored. Other areas where remote sensing may be employed include calculation of dimensions of reservoirs and the geomorphology of potential sites as well as sources of point pollution (Welcomme, 1985).

5. SELECTION OF FISH SPECIES

The fish available for rural pond culture fall into three categories -tilapias, carp and trout. For all three production systems, including seed production and feed, have been developed. Also, since these types may be naturalized under most environmental conditions there is little immediate need for research aiming at identifying local species before embarking on aquaculture in new regions. The one exception to this rule, from an environmental point of view, is when neither type of species is naturally present, or where introduction of any of them might cause environmental hazard. Since Tilapia is the most common species cultivated in Africa, its systematics and biology is described below. More details also on other groups are given in Annex 1.

Tilapia systematics and biology

The group of species collectively known as tilapias actually comprise four genera of interrelated cichlid fish, endemic to tropical and subtropical Africa (Trewavas, 1982a, b). The three main genera are separated according to breeding types. Oreochromis species are maternal mouth brooders, and the Tilapia group proper consists of substrate spawners. Sarotherodon species are paternal or biparental mouth brooders.

Tilapias are warm water species generally thriving at temperatures exceeding 22°C and succumbing at temperatures below 10°C. Outside the tropics tilapias are also grown, but the culture facilities must include access to warm water effluents. Certain species can grow and survive in both fresh and salt water which opens possibilities for aquaculture both in inland localities and along sea coasts (Balarin, 1984a). Tilapias are bilaterally compressed, deep-bodied fishes with a more or less distinct spot posterior of the dorsal fin - the “tilapia-mark”. The dressed weight is relatively low, around 50% with a fillet fraction of roughly 40%. The consumer range varies with size of traditional local species and also with economic status of the buyer, hence a range of 25 g to 3 kg is possible (Balarin, 1984a).

Recent studies employing electrophoretic techniques (McAndrew and Majumdar, 1984) seem to refute some of Trewavas's views, but on the other hand it was argued that difficulties in obtaining pure species from the wild are likely to hinder progress in evolutionary studies. Electrophoresis uses the migration of proteins in an electric field to detect differences in molecular size and charge between species, and scanning of numerous polymorphic loci within each species will allow for stock discrimination. Because a large number of proteins (enzymes) may be analysed within a single tissue, e.g., a piece of lateral muscle, biopsy techniques will allow live fish to be identified without losses of potential broodstock (McAndrew, 1981; McAndrew and Majumdar, 1983). Because this method is independent of morphological and meristic variability and also size, age and sex of the individual fish, pure species as well as hybrids and introgressed strains may be detected and classified. In areas where several species of tilapias are known to have been introduced or elsewhere spawning areas of endemic species have been altered, electrophoretic classification must be applied before a brood stock is established. A random sample of 50 fingerlings is required for stock classification.

Tilapia culture

Two problems limit the success of tilapia culture; the prolific breeding habit, which creates stunting, and the ease with which most species interbreed when artificially combined in a new habitat. Genetically pure strains or species and controlled breeding are basic ingredients in any tilapia culture system. One can state with confidence that there is at least one tilapia species for each type of feed that may be locally available, and omnivorous behaviour is displayed by many species. This is also one of the main reasons while tilapia, like carps, can be integrated with almost any type of agricultural production and animal husbandry.

Selection for late maturity in tilapias

Tilapias reach sexual maturity long before they reach marketable size (Kronert, Hörstgen-Schwark and Langholz, 1986). This leads to uncontrolled fry production within the growing stock followed by stunting and negative effects on pond productivity. Numerous methods have been attempted to minimize this problem - manual sexing to obtain pure male or female stocks, hormone treatment to obtain sex reversal and hybridization to obtain sterility. Selection for late maturation revealed that family selection was promising (Kronert, Hörstgen-Schwark and Langholz, 1986) mainly due to the extreme variability of the trait. It also seems possible to select for high body weight without a simultaneous increase in gonadal weight. Kronert, Hörstgen-Schwark and Langholz, provide a three-step selection programme for O. niloticus with the aim of producing a strain which will not mature before reaching a marketable size.

Sex-reversed hybrids

Pure strains of O. mossambicus and O. hornorum will, when hybridized in a certain manner, produce all-male offspring (McAndrew, pers.comm.). If however, gene introgression is widespread in either parental stock there is little point in selection for pure lines. The only reliable tool for establishment of purity of stocks is by electrophoresis of body tissues.

Sex reversal may also be achieved by adding hormones to the feed of young fish (Majumdar and McAndrew, 1986; MacIntosh, 1986). Fry which are treated for 40 days with a male hormone (methyl testosterone) became all male, since the potential female fish are sex-reversed. Elimination of the hormone (99%) was achieved in 100 hours after withdrawal of the hormone from the diet (Johnstone, MacIntosh and Wright, 1983). In all species tested hormone treatment improved growth performance considerably (Majumdar and McAndrew, 1986).

Other advantages of establishing all-male stocks is that they seem to grow faster than normal stocks and besides that unwanted natural production is reduced or even eliminated (MacIntosh, 1986).

6. INTEGRATION WITH AGRICULTURE

Plant wastes from agriculture and manure from animal husbandry can be used as food and fertilizers for fish in ponds. The objective is to increase the total fish production without utilizing expensive artificial feeds, hence an adaptation of the aquaculture system to the local agricultural system. Since most of the fish species which are cultured are either omnivorous or plankton feeders (most tilapias) or benthic feeders (common carp) a wide range of agricultural waste products may be composted or directly used in the ponds. Manure from pigs or ducks act very well as fertilizers, but so do other types of manure as well. When soil conditions and water quality are favourable and the pond is well managed with adequate food and fertilizers, occasional liming is the only addition needed to maintain production at an acceptable level.

Such integration can also be applied for a variety of carp species. Animal manure may thus be used to fertilize ponds where some nutritional pathways (bacteria - microbenthos - meiobenthos/worms) favour benthic feeders like common carp. Others lead to better growth of higher plants, favouring grass carp - phytoplankton - silver carp - and finally zooplankton and insect larvae for big head carp (Pullin and Shehadeh, 1980).

7. GENERAL CONCLUSIONS

Environmental issues

(1) Environmental parameters should be evaluated before undertaking aquaculture development and be monitored during project life.

(2) The carrying capacity of the environment should be assessed when planning and managing renewable natural resources.

(3) In order for adverse environmental impact to be low, small-scale ventures utilizing the generally limited resources of good water quality should be encouraged.

(4) It should be determined what the cultured fish produces and consumes in terms of food and waste products. Environmental reactions to such changes should be evaluated.

Biological issues

(1) The major factors determining the suitability of a species for culture are: (a) species purity, (b) high genetic variability, (c) rapid growth, (d) ease of breeding, (e) low food conversion ratio.

(2) Species should be selected according to (a) which species are indigenous or already established and suitable for pond culture (b) temperature preferences, length of production cycle and type of feed and fertilizer locally available.

(3) Tilapia production should be restricted to areas lower than 1 500 m (for all-year production lower than 1 000 m), carp should be cultured between 1 000 and 2 000 m, and rainbow trout at altitudes exceeding 2 000 m.

(4) Each brood stock should comprise a minimum of 25 pairs of unrelated fish to maintain a large effective population size and minimize inbreeding and loss of genetic variability.

(5) Electrophoretic biopsy analysis should be performed to test potential brood stock for species purity. Fifty randomly chosen fingerlings should be tested of each potential brood stock.

(6) Selection response in local tilapia according to the scheme suggested by Kronert, Hörstgen-Schwark and Langholz, 1986, should be determined.

(7) All-male fry production should be encouraged, either by hybridization or by hormone treatment.

8. FACTORS TO BE ANALYSED DURING PROJECT PREPARATIONS

(1) The water supply: quantity, quality and annual distribution, potential conflicting uses of the water resource.

(2) Physical characteristics: the gradient of potential sites determining what kind of ponds to be built, soil conditions.

(3) Climatic conditions (other than water supply): altitude and/or latitude determining which species will be most successful. Local rainfall patterns determines the extent of forestation which in turn bear on the availability of suitable sites.

(4) Capture fisheries: perennial lakes, rivers and reservoirs support capture fisheries. Where a well managed capture fishery exists there is little scope for aquaculture of any kind.

(5) Animal husbandry and agriculture: cattle, sheep, goats, pigs, ducks and poultry provide manure to be used as feed/fertilizers. Vegetation of most kinds including green grass, leaves and stems of many plants, and corn and rice wastes can be utilized as feeds. The same applies to most household wastes of organic origin.

(6) Fish species to be cultured: a fairly wide range of species can be cultured. Selection of species depends on climatic conditions, availability of seed and feed, and which indigenous species occur. Only as an exception should species alien to the water system be imported, because of the risk of transferring new parasites and diseases, elimination of local species because of superior ecological adaptation by the new species, and lastly, because of the risk that locally favoured fish for consumption may be severely affected.

(7) Waterborne parasites and diseases: even though most African freshwater fish are hardy with regard to surviving parasites and diseases there is a risk of improving the conditions for a number of parasite organisms affecting man when establishing aquaculture. Pond siting, pond structure, clearing of trees and water weeds are part of the system for minimizing these hazards. Introduction of fish species specializing in feeding on mosquito larvae or snails may help break the parasite's host cycle.

(8) Environmental hazards and resource conservation: genetic and ecological interference of cultured fish with local species must be considered, as must effects of waste water from the pond and the potential spread of parasites and diseases. The impact of the culture system on the natural biological resources must be evaluated and minimized.

Each of the topics above require a separate set of factors to be scrutinized. For a general evaluation a checklist is given in Appendix II.

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APPENDIX I

Selected Species of Tropical Origin

Oreochromis mossambicus

This species is the most widespread of the tilapias and although it can attain a large size under culture conditions it is also associated with overcrowding and stunting as a result of early maturation and excessive fecundity. It also hybridizes easily with most other tilapiine fish, which makes it even more difficult to control growth and prolific breeding. Adults are omnivorous, but feed mainly on plankton, benthic algae and higher vegetation. Fry feed predominantly on zooplankton (Jauncey and Ross, 1982).

Oreochromis niloticus

Comparative growth rate studies between different species of tilapias and their hybrids clearly indicated that O. niloticus was the fastest growing species under these given environmental and culture conditions (Majumdar and McAndrew, 1986). It is also noteworthy that neither species hybrid exceeded its parental species in growth rate significantly, and that the best hybrid also involved O. niloticus (and O. spilurus). Others have also noted that O. niloticus shows the best growth (Pullin, 1983). Adults are omnivorous, but feed mainly on phytoplankton (even blue-greens). Fry may utilize a wider range of food items (Jauncey and Ross, 1982).

Oreochromis spilurus

Although only limited information on comparative growth rates is available there are indications that this species even may outperform O. niloticus at raised water salinities (Majumdar and McAndrew, 1986).

Oreochromis aureus

More salinity tolerant and slightly more tolerant to low temperature than O. niloticus, this species does not seem to perform well when pure stocks are used (Majumdar and McAndrew, 1986) but O. aureus have been second only to O. niloticus in many trials (Wohlfarth and Hulata, 1981) and even suggested as the top performer (Coche, 1982) in both extensive and intensive culture systems. The reason for these aberrant results may be that wild and cultured strains have been used, and selection for growth performance over decades may have created a strain with vastly improved growth conditions in culture when compared to uncultured, wild strains of the same species. O. aureus is a microphagous feeder.

Oreochromis andersonii

This species is most resistent to low temperatures of those cultured in ponds, and as such may prove to be the only feasible tilapia candidate at altitudes exceeding 1 000 m. It also appears to react well to semi-intensive culture and integrated with duck and pig raising (Anon., 1985a).

Eels (Anguilla)

In the lower reaches of more permanent rivers at least four species of eels occur - the large mottled eels (A. marmorata and A. nebulosa) and the two smaller plain eels (A. mossambica and A. bicolor). All species may be caught on a larger scale than at present and the market for export is good; however, eel culture even in its most extensive form will not yield enough protein at a low cost for local consumption. If a market for eels may be developed rather intensive eel production in ponds using stunted tilapias as feed may prove to yield a better economic return than any semi-intensive tilapia culture.

Clarias sp.

This genus is characterized by an auxiliary breathing apparatus, which enables them to live on dry land for a long time, and also in water with low oxygen content. All species are predatory, mainly on small fish, while stunted tilapia populations may serve as food in culture conditions. Clarias are highly valued fish in many parts of Africa, the major constraint to its culture being the predatory habit and the difficulty in inducing breeding.

Labeo sp.

This genus is of great economical importance, but the various species are vulnerable because of their spawning migrations, when they are easily caught in dense shoals. Despite the fact that they are not suited in culture conditions, their resemblance to Chinese and common carps make the latter species easily adaptable as food fish on the African market.

Carp hybrids

Two species of Indian carps, Labeo rohita and Cirrhinus mrigala, with chromosome numbers in both species of 50 have been reported to produce triploid, sterile hybrids with the common carp, Cyprinus carpio (Kowtal, 1986). The flesh quality of the hybrids was also reported better than in the parental species and the sterility may be an effective means of curbing the prolific breeding and stunting in the common carp so frequently found in the tropics.

APPENDIX II

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