Malcolm C.M. Beveridge and J.Alan Stewart
Institute of Aquaculture, University of Stirling, Stirling, Scotland FK9 4LA, U.K.
ABSTRACT
Cage aquaculture in lakes and reservoirs continues to exp and and intensify, especially in Asia, despite conflicts in resource use and social inequity, problems arising from waste production and questions regarding sustainability.
Environmental impacts arise from the consumption of resources (environmental goo ds) and the production and release of wastes into the environment, which is relied upon to disperse and assimilate those wastes (environmental services). Cages may have slightly greater demands than ponds in terms of consumption of environmental goods and services per unit fish production. However, environmental impacts are much more strongly related to intensity of production methods and scale of development within a lake or reservoir. In view of the likely scale of operation, cage-based hatcheries and nurseries are unlikely to pose much of an environmental threat. In some circumstances such impacts may be beneficial in terms of enhanced fisheries production.
Problems of resource use conflict may be anticipated by addressing questions of ownership of the lake or reservoir and the use and control of resources within the water body.
1. INTRODUCTION
The focus of the present paper is the use of cages for production of seed for enhancement of inland fisheries production. It begins with a brief review of cage aquaculture in lakes and reservoirs and considers technology, status and trends. Lake-based hatchery/nursery systems are discussed and comparisons made with land-based systems. Environmental goods and services required to support cage-based aquaculture, and hatchery/nursery systems in particular, are highlighted, as is the economic, social and institutional context of cage aquaculture development. Guidelines are proposed.
No mention is made of problems posed by spread of disease or the introduction of e xotic species, as these are covered elsewhere.
2. INLAND WATER CAGE CULTURE: TECHNOLOGY, STATUS AND TRENDS
Cage culture involves the rearing of animals in a structure enclosed on all or all but the top sides by wooden, mesh or net screens, whilst maintaining a free movement of water (Beveridge, 1996). The use of cages for rearing fish and other aquatic animals is thought to have begun in China nearly one thousand years ago (Hu, 1994), and to have become locally widespread in China, Cambodia and Indonesia during the closing years of the last century and early decades of the present century (Lafont and Savoeun, 1951; Hickling, 1962; Ling, 1977; Pantulu, 1979; Reksalegora, 1979). In Europe and North America, cages began to be widely used in the 1960s in the emerging trout and catfish farming industries.
Most cage culture activities in freshwaters are related to the production of food fish. However, cage-based aquaculture has also been used as a cost-effective biomanipulation technique for addressing anthropogenic eutrophication problems (Yang, 1982; Beveridge, 1984a; Graneli, 1987; Milstein et al., 1989; Arcifa et al., 1995; Starling et al., 1997) and as a means of producing seed to enhance fishery production (De Silva, 1988; De Silva et al., 1991; Lu, 1992; Li and Xu, 1995; Lorenzen, 1996).
Aquaculture in lakes and reservoirs is often viewed as desirable as it can generate employment, income and food, support or complement fisheries or other activities, capitalising on previously under-exploited resources such as waste agricultural and industrial materials (Gonzales, 1984; Beveridge and Phillips, 1988; Beveridge, 1996). Foremost among its merits is the possibility of using existing water bodies, subdividing what is usually a common-property resource into parcels over which rights of tenure may be given. In theory this facilitates the involvement in aquaculture of members of the community with scant land resource (Gonzales, 1984; Conway et al., 1989; Beveridge and Muir, 1997; CARE, 1996). Freshwater cages also tend to be inexpensive and easy to construct and manage and, entrepreneurial abilities aside, require modest managerial skills. A further advantage of using cages to produce fingerlings support a culture-based fishery is that fish are readily acclimated to conditions within the lake or reservoir prior to release.
However, cage aquaculture has its disadvantages too. It is not always easy to establish cages in reservoirs, the presence of submerged trees and the practice and extent of drawdown restricting the availability of suitable sites. Drawdown also increases the risks of a breakdown in stratification, with the consequent upwelling of deoxygenated hypolimnetic water that has caused caged fish mortalities in Indonesia and elsewhere (Soemarwoto et al., 1990; Zoran et al., 1994). Cages are vulnerable to adverse weather, vandalism and theft and pollution. There are problems with allocation of rights in what are often publicly owned and used water bodies. All wastes are released untreated into the environment, potentially affecting not only the farmers themselves, but also other resource users. As a result, critics have argued that the development of cage aquaculture in inland waters has brought about problems of social inequity and caused serious environmental impacts and that developments are often unsustainable (Beveridge, 1984a, 1996; Beveridge and Phillips, 1988; Santiago, 1995; CARE, 1996; Beveridge and Muir, 1997; Beveridge et al., 1997).
2.1 Cage technology
Cages for use in freshwaters tend to be smaller than those used in the sea: structures of 10 to 100 m2 surface area and between 2 and 6 m deep are most common. Those used for fry/fingerling stages are usually smaller than those used for production, largely because of the flexibility this gives to operators.
Cages can be broadly categorised into two types: fixed and floating. In the former, the cage bag is attached to posts driven into the lake or reservoir bottom while with floating cages, the bag is secured to a floating collar. Although less costly to construct, fixed cages are intrinsically weaker and are thus limited in size. Their use tends to be confined to sheltered, shallow areas (<10 m depth). However, both types are used for fingerling production.
Few species can be spawned in lake-based hatcheries. A notable exception is the tilapias. Lake-based hatcheries and nurseries for these species were first developed in the Philippines in the late 1970s, and by the mid-1980s there were a number of commercial lake-based fry and fingerling units in southern Luzon (Beveridge, 1984). Most operators used large (5×5×2 m) fine mesh net bags (1–3 mm mesh size) attached to stout posts driven into the lake substrate. They were managed in much the same way as hapa-in-pond systems. Broodstock were introduced to the cage at stocking densities of 2–5 m-2, with ratios of 3–7 females per male. Fry were removed regularly and transferred to fingerling production units (4–8 mm mesh size). Vulnerability to storms and security were identified as key problems by operators, forcing a number to return to land-based systems. In an attempt to reduce typhoon damage top nets were sometimes used so that cages could be sunk below the water surface during adverse weather conditions. A number of modified designs which tried to reduce the incidence of cannibalism among siblings were also developed but were subsequently abandoned as either too expensive or impractical (Beveridge, 1984). Cage-based tilapia hatchery systems are currently being developed in other parts of Asia and Africa.
2.2 Production methods
As in other aquaculture systems cage culture can be by extensive, semi-intensive or intensive means. Extensive cage culture relies solely on the exploitation of natural food and, illuminated cage systems apart, to date has proved economically feasible in only a few, highly productive environments, such as heavily organically loaded canals (Lafont and Savoeun, 1951; Costa-Pierce and Effendi, 1988; Christensen, 1989; Redding and Midlen, 1991) and eutrophic lakes (Pantastico and Baldia, 1981; Aquino and Nielsen, 1983; Li and Mathias, 1994; Santiago et al.., 1991; Santiago, 1995; Beveridge, 1996; Costa-Pierce, 1996). Sustainability in the latter, however, has been a problem (see below). In semi-intensive cage culture, poor quality food materials, usually agricultural by-products or aquatic plants, are given to supplement the natural diet. This method of cage culture prevails throughout Asia (Li and Mathias, 1994; FAO/NACA, 1995; Beveridge et al.., 1997). Intensive cage aquaculture, which involves the provision of all or almost all of the species' nutritional requirements through the use of formulated feeds, was developed in Europe and North America, principally in the rearing of salmonids and catfish. In the tropics and sub-tropics intensive cage culture methods have been implemented where there are large urban populations willing and able to pay the high prices for fish, or where there is a strong export market. In Zimbabwe, for example, a large-scale commercial intensive tilapia cage operation is currently being planned (Berg et al., 1996; Troell and Berg, 1997). In sub-tropical countries intensive methods may be necessary in order to grow a product to market size within the short growing season.
2.3 Cage production and trends
Ninety-seven percent of the current 13 million tonnes of cultured fish production is from inland waters (FAO, 1996) and although there are few reliable data, it is estimated that less than 10% - say, 0.8–1.0 million t - comes from reservoir or lake-based cages (Beveridge, 1996). This statistic obscures the fact that in certain parts of the world and certain sectors of the aquaculture industry cage culture assumes an extremely important role. In the Philippines, for example, more than 35% of tilapia production is from freshwater cages compared to 58% in ponds, the remainder being in pens (Aypa, 1995). In terms of hatchery or nursery production, cages play only a minor role, even in China, where such methods are widely employed.
Several trends are apparent, both in terms of production and in the methods used. Cage aquaculture throughout Asia is increasing. With the expansion in reservoir construction, particularly in the newly-industrialised countries, cage aquaculture is increasingly seen as a means of resettlement of displaced people and of complementing or increasing returns from fishing (Beveridge and Muir, 1987; Costa-Pierce and Soemarwoto, 1987, 1990; Sevilleja et al., 1993; Lorenzen, 1996). For example, in Malaysia, the area of freshwater cages increased from 2.14 ha to 4.87 ha between 1990 and 1993, compared with only a very slight increase in pond area (Ferdouse, 1995). Cage aquaculture in the Philippines, currently amounting to 5000 ha, is also increasing (Aypa, 1995). By contrast, cage aquaculture in inland waters in the northern hemisphere is facing increasingly stringent regulation and in some countries, such as Germany, prohibition, because of environmental concerns (Rosenthal et al., 1993). The growing dependence of inland fisheries on stocking combined with shortages of land is leading to increases in the use of cage-based nursery systems in China and elsewhere (Lu, 1992; Manni, 1992; Li and Xu, 1995; Lorenzen, 1995).
There is some evidence of changes in cage culture practices. In China, cage fish culture has been successfully integrated with waste treatment and other forms of aquatic food production (Li and Mathias, 1994). Taihu lake is very eutrophic as a result of agricultural wastes and domestic sewage. Crops of water chestnut, water spinach and lotus are harvested, excess plant material being fed to caged herbivorous grass carp and Wuchang bream (Chang, 1989). However, the over-riding trend in production methods is towards intensification, especially in Asia, in part facilitated by increasing differentials between market prices and feed costs and in part because of declining aquatic productivity caused by over-exploitation by extensive methods (Aquino and Nielsen, 1983; Beveridge, 1984a, 1996; Mekong Secretariat 1992; FAO/NACA, 1995; Santiago, 1995).
3. AQUACULTURE AND ITS RELATIONSHIP WITH THE ENVIRONMENT
To a greater or lesser extent, all agriculture and managed livestock production, including that of fish, manipulates natural systems in order to increase the output of desired products (Beveridge et al., 1994). Through inputs of energy (or labour or capital), natural resources, or ‘environmental goods’, are transformed into products (fish) valued by society. In so doing, wastes are inevitably produced which the fish farmer requires the environment to dissipate and assimilate. These functions are often termed ‘environmental services’. There is an entire spectrum of dependency upon the environment, at one end of which lies subsistence activities, at the other, industrial-scale production. Industrial food production systems are made possible by the availability of energy subsidies through fossil fuel combustion (Slessor, 1974): through the use of technology and resources imported from other systems these processes have attained a measure of independence from the checks and balances found in natural systems (Giampietro et al., 1992). Such production systems have been described as “flow through”, which, in addition to generating desired products at rates well above the capacity of local resources, also produce wastes which may be beyond the capacity of the local ecosystem to assimilate (Folke and Kautsky, 1989; Giampietro et al., 1992).
The last two decades has seen a rapid growth in the application of industrial production methods to aquaculture, and this has been responsible for a significant proportion of the increase in aquaculture sector output. Such technologies have also been the subject of increasing environmental concerns and associated social and economic impacts beyond the production systems boundaries. Similar concerns have been raised with industrial agriculture, which although substantially increasing food production, has been accompanied by problems of soil erosion, water pollution and biodiversity loss.
The principal environmental goods and services required in aquaculture include:-
The range of environmental goods and services is to some extent system dependent, but is overwhelmingly dependent upon the intensity of production methods employed (see Beveridge et al., 1994, for review).
3.1 Cage aquaculture versus pond aquaculture
Although demand for environmental goods and services is principally driven by production method i.e. whether extensive, semi-intensive, intensive, there are inherent differences between cage and pond systems. Inland water cage fish farms are usually sited in multi-purpose, public water bodies and are rarely integrated with other aspects of the natural, economic and social environments to the same degree as pond aquaculture. The linkages between production system and environment are arguably stronger for cages than for other aquaculture systems and it is the failure to recognise this that has led to problems of over-exploitation (see below).
3.1.1 Environmental goods and services
Some have argued that cage systems are inherently inefficient, making greater demands on environmental goods and services per unit production than other aquaculture systems. In a comparison of semi-intensive culture of tilapias in ponds and intensive culture in cages, Troell and Berg (1997) demonstrated that unlike ponds, cages rely upon construction materials and, especially, feed from outside the immediate ecosystem and that this has significant implications in terms of energy consumption per unit fish production. However, this is not comparing like with like. There have been no ecological or industrial energy consumption-based comparisons of similar intensities of cage and pond aquaculture (e.g. semi-intensively managed cages with semi-intensively managed ponds, etc.). While it is true that cage materials do represent a cost in resource use terms, it is typically small; Stewart (1995) estimates this at 2–5% industrial energy costs per unit output for a range of intensive cage culture systems. Seed requirements are the same as for other systems and, in some cases, can be produced from within the same ecosystem (see above). The argument that caged fish cannot exploit autochthonous plankton and seston production to the same degree as in ponds because of lack of access to benthos and low food densities in the water column is misleading. Column feeders, such as tilapias and plankton-feeding carps, graze on plankton and seston as it passes through the cage and when cages are sited in productive lakes, high yields with no additional food resources can be attained (Table 1).
Table 1. Extensive cage tilapia production, Philippines.
| Lake | Stocking density (m-3) | Size at stocking (g) | Culture period (month) | Size at harvest (g) | Production (kg m-3month-1) |
|---|---|---|---|---|---|
| Bunot | 4 | - | 4 | 250 | 0.24 |
| Laguna | 4 – 8 | 21 | 4 – 5 | 100 | 0.07 – 0.18 |
| Sampaloc | 1.6 – 2.0 | 12.5 – 16.0 | 6 – 9 | 225 – 300 | 0.05 – 0.08 |
| Taal | 50 | - | 4 | 100 | 1.25 |
| Bato | 50 | - | 4 | 70 – 200 | 0.18 – 0.50 |
| Buluan | 10 | 21 | 5 | 200 | 0.40 |
(modified from Beveridge, 1987)
In conclusion, efficiency of resource use is unlikely to be an issue in deciding between cage and pond systems. Of crucial importance, however, is the local context with regard to resource availability (see Section 4 below).
3.2 Extensive cage culture
The problem with such extensively managed cage systems is that they do not appear to be sustainable. Experience from the Philippines, China, Indonesia and elsewhere, shows that the natural food resources within the lake or reservoir quickly become over-exploited, resulting in a total decrease in standing crop or a shift in community dominance towards smaller species which are less susceptible to predation. As the food supply (plankton, seston) is a commonly-owned resource, the tendency is for each owner to maximise his/her share of that resource (c.f. ‘the tragedy of the commons’; Clark, 1981). With time there is an increasing reliance on supplemental feeds and in many instances intensive methods begin to prevail (FAO/NACA, 1995).
3.3 Semi-intensive cage aquaculture
Most aquaculture in the tropics is semi-intensive and operates on the principle that shortfalls in natural food are made up for through use of fertilisers and feeds. Trials have been conducted in Sri Lanka and China using fertilisers in reservoirs, the latter with some success. Qinfang et al. (1992) detail the growth of caged tilapias held in open water and in a fertilised bay within the reservoir. Although caged fish growth was significantly higher in the fertilised bay, fertilisation is usually neither economically feasible nor desirable and in practice, all nutrition over and above that derived from plankton and seston must come from the supply of food. Wastes in semi-intensive pond culture are largely dealt with within the pond ecosystem, stimulating further production of food. Small quantities may be discharged during harvesting (Beveridge et al., 1991, 1997; Beveridge and Phillips, 1993). By contrast, wastes from semi-intensively managed cages are released into the lake or reservoir system at large. Thus, demands for environmental goods and services are likely to be higher for cages than for ponds.
Initially, many semi-intensively managed cage farms are heavily reliant on the supply of natural food. However, experience has shown that unless lakes are subject to a continued high degree of anthropogenic eutrophication (e.g. Lake Pokhara, Nepal; FAO/NACA, 1995), caged fish growth and production fall over time through over-grazing and there is an increasing reliance on supplemental foods and reduced profitability (Beveridge, 1984a, 1996; Sevilleja et al., 1993; Santiago, 1995).
3.4 Intensive cage aquaculture
Like other types of intensive aquaculture, intensive cage aquaculture is more heavily dependent upon environmental goods and services per unit fish production than less intensive methods. With regard to resources, intensive production is heavily reliant on fishmeal-based diets which are largely derived from materials imported from outside the ecosystem boundaries. There is little potential for exploitation of fish resources from within the system to make fishmeal for aquaculture diets as the ecosystem support areas would be very large. Berg et al. (1996), for example, assessed the ‘ecological footprint’ (i.e. the area of resource) of intensive cage tilapia culture in Kariba reservoir, Zimbabwe. Cages were 5 m deep and they estimated that for each 1 m2 of fish cage, 2.1 ha of lake area is required to support the food requirements of the caged fish, a ratio of some 20,000 : 1 between the ecosystem support system and the cage production system. Not only are there impacts associated with the exploitation of marine ecosystems for fishmeal production, but also demands on other resources, including industrial (fossil fuel) energy required to produce and transport the diets. In western Europe, the focus of intensive cage culture in inland waters, around 10,000 tonnes of fish is produced in this way. Some 33,000 t of industrial fish are utilised as feed (see Folke and Kautsky, 1989, for rationale). In the tropics, the amounts used are a fraction of this, although undoubtedly increasing. However, while it can be argued that in total intensive cage culture is responsible for only a very small proportion of the world's fishmeal consumption it is, nonetheless, a very inefficient way to produce food in terms of use of both industrial energy and biological resources (Folke and Kautsky, 1989).
Wastes from cages generally include uneaten food, faecal and urinary wastes and chemicals, although comparatively little use is made of the latter in tropical inland cage aquaculture (Beveridge and Phillips, 1993). It is well-known that uneaten food, faecal and urinary wastes from intensive systems can result in hypernutrification and eutrophication in the water column and an increase in organic matter inputs to sediments (Beveridge 1984a, 1996). Merican and Phillips (1985) typically found that around 30% C, 20% N and 60% P inputs were lost in solid form from trout cages in Scotland. Estimates from an intensive cage tilapia farm in Lake Kariba, Zimbabwe, suggest figures comparable to or even higher than this (Troell and Berg, 1997). Here, approximately 80–90% of P from intensively-managed tilapia cages in Lake Kariba was lost to the environment. By contrast, Costa-Pierce and Roem (1990), who collected sedimenting solid wastes from underneath carp cages in Saguling reservoir, West Java, Indonesia, estimated that only 5% C, 3.5% N, and 0% P (equivalent to 3.7 g C, 0.5 g N and 0 g P m-2 cage area day-1) given to the fish in the form of food were lost in solid form from the cages. The apparent discrepancies are due to differences in species and food composition as well as to feeding rate and proportion of food ingested, digestibilities, stability of food and faecal wastes in water, water temperatures and depth underneath cages.
Wastes from cages are freely released into the environment, potentially interacting with the entire water body. Intensive cage culture, when unregulated, can cause severe environmental problems (Lu, 1992; FAO/NACA, 1995; Santiago, 1995; Beveridge et al., 1991, 1997; Beveridge and Phillips, 1993). Cage farms in lakes and reservoirs are thus vulnerable, both to general pollution and to self-inflicted water quality-related problems. Problems were first observed in Laguna de Bay, Philippines, in the early 1980s (Barica, 1976; Beveridge, 1984a) and have since then been reported in lakes and reservoirs throughout the region (Lu, 1992; Santiago, 1995; Beveridge et al., 1997). With increasing production and intensification of production methods oxygen demand in both the hypolimnion and epilimnion of stratified lakes and reservoirs grows, resulting in fish kills caused by upwelling of anoxic hypolimnetic waters during mixing. Properly managed, taking an integrated systems view, there is potential for beneficial impacts from wastes through enhanced fisheries production, although evidence for this is scant. Furthermore, changes in plankton quality require consideration.
4. ECONOMIC, SOCIAL AND INSTITUTIONAL CONSIDERATIONS
The approach to the identification of potential for cage aquaculture in inland waters, to produce food fish or to provide fingerlings for ongrowing or restocking, is reasonably well developed in terms of technological aspects of such systems. As outlined above, there is a broad understanding of the relationship between systems and their production environment. Thus, it is in theory a relatively simple task to identify potentially suitable technologies, and to design these within specified environmental capacities, for any inland water resource. In practice the development process is much more complex, with examples above of over-development leading to serious environmental problems, which can also lead to economic failure and negative impacts on other resource users, or failure of development efforts despite clear technical potential. What has often been lacking in supporting such developments is an understanding of the linkages between aquaculture activities and the social, economic and institutional environment in which they operate. There is a long history of aquaculture development support being “producer led”, with inadequate consideration given to the markets and marketing. The problem of production focus in aquaculture development activities has also been discussed by Harrison (1994), in the context of production not necessarily equating with the specified economic and welfare objectives.
It is clear that while technical knowledge is vital to assess the range of options which might be feasible, this must also be closely tied in with the perspectives of those who will potentially use the technologies, and those whose lives may be directly or indirectly affected by such change. While this has long been recognised in the development literature (e.g. Chambers, 1983) and has lead to a wide range of participatory approaches to research and development (e.g. Chambers et al., 1989; Scoones and Thompson, 1994) including activities involving fish culture (Brummett and Noble, 1995; CARE, 1996) it is still the case that most rural development support is technology led. Even where participatory activities are involved, the extent to which these new evolving methods are capable of catching the complexity of social and institutional structures and interrelationships can be highly variable. The critical question is who is participating and how, and whether these individuals are representative of the cross section of the community interests and power/regulatory structures.
Such issues are particularly critical when considering aquaculture in inland lakes and reservoirs, which may support a wide range of agricultural, fisheries, industrial and domestic activities. Key questions include issues of ownership, control, users and associated uses and rights of use. These different aspects of resource use and management can involve a wide range of stakeholders for a single water body, ranging from rural landless individuals to government institutions. Furthermore, the interrelationships between these factors and the stakeholders themselves can be both complex and highly variable from place to place. In seeking to assess the potential role for cage culture technologies to enhance the benefits from these water resources understanding these social dimensions is critical.
For example, in Bangladesh and India owners of lakes, reservoirs or ponds can include Government institutions, whole communities, co-operative groups, kin groups, and individuals/ individual households (O'Riorden, 1993; Stewart, 1995). Control of water resources is often use related, and may involve stakeholders other than owners, while general rights of access might involve a wider range of stakeholders again. Thus a particular group may have the right to manage the fish culture/capture, or even a particular capture method (Middendorp et al., 1996), while others will have rights to abstraction for irrigation, and the community as a whole may have rights of use for domestic and livestock needs.
In addition to establishing ownership and rights of use, there are different aspects of resource use conflict which can influence success of cage or other aquaculture development. First, there is the potential for direct and physical resource use conflict, relating to space and access to, or competition for, water or inputs (Beveridge, 1984a). Conflicts may arise due to social or cultural values (e.g. prohibiting use of animal manures in ponds) or perceived conflicts of interest due to misunderstanding of the changes occurring (CARE, 1996), reported pond owners suspecting fish farms taking stock from their ponds. There may also be social conflicts within communities which although not related to the specific activities or resources, may influence the success, resulting in malicious damage, for example. In addition to conflict, simple theft is a major problem for fish culture, and cages, with concentrations of stock, may be particularly vulnerable.
While there is considerable literature associated with social aspects of pond and coastal aquaculture developments, there is limited documentation associated with these aspects in inland water bodies. One case is the development of cage culture technology in baors and ponds in Bangladesh, introduced as an income generating activity for landless women by CARE (CARE, 1996). The results of initial activities suggested technical and economic viability. However, these activities were not sustained due to problems of resource use conflict (or at least perceived conflict of interest), mistrust between stakeholders and security problems. The fish farmers themselves were reportedly still interested, but were unable to continue. It was concluded that there was a need for greater stakeholder involvement in the early planning stages, not only to assess the resource use and control patterns, but also to encourage greater understanding the technology, for example in terms of the potential benefits of cage culture to the fisheries.
In Lake Toba, Indonesia, Pollnac and Sihombing (1996) found minimal evidence of resource use conflict between the newly developed cage culture industry and other stakeholders, including transport and tourism and capture fisheries. There were two cases of malicious damage and stock escapes which farmers blamed on fishermen. The lack of conflict in this case was in part attributed to the large size of the lake and the relatively low population in the area, and planning controls limited the scale of development, unlike other reported cases in Indonesia (Beveridge et al., 1997). However, the authors noted the potential for conflict in the future, and thus the need for appropriate regulation, based on a much better understanding of the technical, environmental and socio-economic conditions.
The key point from this brief examination of these issues is that in seeking to develop cage culture systems as a sustainable activity in inland, multi-use water bodies, the social and economic aspects are likely to be highly critical to the success, and the most difficult to assess. On the one hand, there is a need for planning control to limit the negative consequences of over-development where the technical and economic conditions encourage rapid growth. On the other, there is a need to understand the potential for real and perceived conflict, and through participative development processes, assist in the process of resolution of such conflicts. The role of the specialist here is in both facilitating increased understanding of the implications of technological change, and the development of technological alternatives or modifications to overcome real conflicts. Ultimately, the only way such developments will be sustained is where the technical and social constraints to their success can be resolved among those involved at the local and resource user levels.
5. DISCUSSION AND RECOMMENDATIONS
Cage-based nursery systems are cheap and easy to construct and, from a technical perspective, could be more widely used to produce fingerlings for culture-based fisheries. However, it is unlikely that cage systems can be used as hatcheries for species other than tilapias, given that most tropical species require to be induced to spawn. Cages suffer from disadvantages by comparison with pond-based hatchery systems; they are vulnerable to damage by storms and to vandalism and theft. Costs of guarding cages can be high and was cited as a major reason why some lake-based hatchery owners in the Philippines had reverted to using hapa-in-pond systems (Beveridge, 1984; Smith et al., 1985). Cages established near a family homestead are less likely to suffer losses.
The culture of fish in cages for food and for fingerlings is dependent upon environmental goods and services and, although yet to be subjected to rigorous analysis, environmental demands are unlikely to be much higher for cages than for pond-based production systems. While demands per unit biomass production of cage-based hatchery/nursery operations and on-growing operations are probably similar, total demands exerted by hatcheries within a lake or reservoir are likely to be much smaller, given the biomass being produced. Environmental issues are unlikely to arise, provided that cage culture operations are restricted to the provision of fingerlings for the same system. Environmental capacity models, although never applied to tropical systems, could be used to assess whether proposed cage nursery/hatchery systems are likely to have serious impacts or not (Beveridge, 1984a, 1996). Models could also be developed to assess the potential for beneficial increases in fisheries production through enhanced productivity.
While cage culture technology is undoubtedly capable of producing fingerlings to support fisheries, the key questions are what are the objectives of the activity, is it the best means to achieve these objectives, what are the costs/benefits/risks and uncertainties associated with this approach, i.e. will it work? In considering the adoption of cage-based nurseries or hatcheries, an integrated resource management or watershed perspective is essential. Only in this way can all the ramifications - environmental, social and economic - be considered. Among other things, adequate skills, good markets and clear legal tenure of sites are prerequisites and this implies the full participation of resource users.
ACKNOWLEDGEMENTS
We thank Dr David Little, AIT, for information used in the compilation of this paper.
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