AQUATIC ECOSYSTEMS AND AQUACULTURE PRODUCTION

Mr. J.F. MUIR

INTRODUCTION

Aquatic ecosystems provide the basis for aquatic production, eg in traditional fisheries, where the stocks we capture represent the upper levels of the aquatic food chain, in fresh waters ranging from herbivorous species such as silver carp, through to omnivores such as blackhead carp, catfish, to carnivores such as the salmonids, pike and pike -perch. Most fisheries supply however, originates in coastal and marine waters. Most prognoses suggest that after significant growth in the last two decades, capture fisheries are unlikely to grow by more than 1 or 2% annually, and will be increasingly limited by physical and biological capacity, by deteriorating environments, and by resources and energy costs of exploitation. In fresh water, fisheries are less important, but the same trends can be observed.

AQUACULTURE PRODUCTION

Managed ecosystems form the basis for aquaculture production, which has been widely offered as a means to compensate and ultimately supplement traditional fisheries (see eg Pillay, 1990). Here, unlike fisheries, inputs, production processes and quality of output can be at least partially controlled, and ownership, care and environmental responsibility might be might be more easily established. By removing natural constraints to survival and productivity, and by husbandry and management, production need be limited only by availability of simple inputs such as land, water, seed, fertiliser and feeds. According to FAO statistic, aquaculture has grown steadily in recent decades (Table 1). On present trends by the year 2000, farmed aquatic production might account for some 20 million tonnes per annum, some 16–18% by weight, perhaps as much as 45% by value. In terms of quantity, this might at least compensate for some deficits in demand (New, 1991)

Table 1 Fishery sector production 1989, by environment/habitat

Sources FAO Fisheries Statistics, 1991, modified from New, 1991.

There is sizeable variation in different species group : as indicated in Figure 1, which shows the increasing role of fish and to smaller extent, crustacea, while seaweed production remains relatively static. Though the carps dominate in production terms, species as tilapia and salmonids have greatest production growth rates. Production of species such as channel catfish and milkfish has grown only modestly. Species such as Clarias catfish, though not yet produced in sufficient quantities to record prominently, have shown considerable increases.

For crustaceans, production in freshwater has been relatively static. In wide development terms, the potential of aquaculture deserves closer examination. More than 80% of production comes from Asia; Latin America contributes less than 5%, while Africa - where aquaculture might make its greatest social impact - produced only 60,000 tonnes, a more 0.2% of global production, a decline over the past three decades;

Figure 1 Aquaculture production by major species category, 1984, 1990

Source: FAO, 1992

The areas most likely to benefit from aquaculture have been less successful to date in supporting it, though many of the physical resources, including good levels of photosynthetic energy, may be equally available. One of the key issues under current examination is the role of social and institutional factors in adopting aquaculture, and the specific features of technology which may affect this. Although as Table 2 indicates, the rate of growth in aquaculture production has been higher in the developing countries, much of the newer investment in aquaculture production involves higher value species - as it is often only these that can support the input costs. It can thus be difficult for aquaculture to offer low enough costs for many societies, and indeed those traditionally dependent on capture fisheries base their consumption on significantly lower prices than are common for many forms of aquaculture.

Table 2: Aquaculture Production by country group ('000 tonnes)

Source: FAO 1992; note: figures include seaweeds, exclude mammals.

¹ according to standard UN categories of GDP

In the many areas where inland aquaculture has traditionally offered lower-cost supplies - eg carp culture of China, SE Asia and India, products are either increasing in price beyond the everyday means of the poorer rural communities, or are being supplanted by other higher-value cash crop products. Within a given range of aquatic resource potential, lower value products are unlikely to develop at the same rate as crops for export or the more prosperous sectors of the society. While lower value production from simple traditional methods continues to be significant in volume terms, its importance may become eclipsed.

SYSTEM DESCRIPTIONS

Two fundamental types of system can be used for aquaculture production:

-  Land-based, in which production units are installed in fixed positions on land, with water supplies provided to them, and;

-  Water-based, in which structures are set up within defined bodies of water.

Each type of system involves different considerations for siting, construction, operation and management. Although the range of systems, species and techniques used is very large, for any given set of circumstances the options will quickly be narrowed by a small number of major constraints. Thus the use of a particular species may define the types of systems suitable (eg because of environmental or feeding requirements, or market value), which will in turn suggest particular species. A particular system will support certain species and require particular sites.

Although it may seem that there could be infinite combinations of these main elements, certain quite clear relationships exist between them which serve to narrow down the possibilities, and once understood, allow the designer or developer to concentrate on ensuring that species, system and site match satisfactorily. A successful combination of these basic elements will do much to ensure the economic viability of the aquaculture project. Systems can be defined according to two main criteria;

-  the type of system; ie ponds, lagoons, tanks raceways, cages, enclosures,

-  the intensity of production; typically described as extensive, semi-intensive, etc.

Typical characteristics are given in Table 3, as an example of how systems can be classified by their main features. In selecting an appropriate system for a particular set of circumstances, the developer will need to assess the importance of each factor and put a weighting on it.

Table 3 : Aquaculture systems; outline design characteristics

Thus if water area is in short supply, a system requiring less water area is a major consideration. If there is a shortage of skilled labour, a less complex design may be important. Table 4 summarises some of the other design consideration for coastal aquaculture systems. Once the major features of the system have been selected, the scale of production and the intensity of operation must be determined. The system employed will clearly have a significant bearing on the overall capital and operating requirements, and hence the costs, of a project. Thus for the same quantity of product using different intensity of production, the capital and operating inputs vary substantially. It is this variation in characteristics which makes it so important to select systems appropriate to the sites and resources available.

Table 4 Aquaculture systems; outline design constraints

Note : the fewer * the better;
- means negligible constraint

WATER RESOURCES AND ECOSYSTEMS

Surface freshwaters including streams, rivers and temporary bodies such as floodplains; can be particularly vulnerable to human influence, including physical obstruction, impoundment, containment, pollution; support wide range of species including anadromous/ catadromous migratory fish; often subject to complex seasonal/water level interactions, with dramatic short-term changes in productivity, as related to water quality, flow regimes, diversity of flowing water habitat. Conventional aquaculture potential primarily relates to pond, tank and raceway culture; also cages and enclosures in river, and hatchery/fingerling production for stocking temporary water bodies. Water quality is determined largely by topography and solubility of catchment, climate and the nature and extent of any pollution sources. Pollutant and natural solute concentrations tend to increase from source. Pollution of remote catchments by acid rain and radiation products has shown man's influence may occur considerable distance away from source. Water flow also tends to increase and the amplitudes of short and long-term flow important in suitability for aquaculture; eg development may be constrained by minimum flows, rather than mean flows.

Static freshwaters such as ponds, lakes, reservoirs normally have more stable physical and chemical characteristics. Related to flushing time, controlled fertilisations allows highly fertile pond ecosystems, successfully used in many tropical and subtropical regions. These systems are also of considerable interest for cage culture, aquaculture-based fisheries, and water supply or storage. These can however he problems for waste accumulation from intensive cage aquaculture. Water bodies can be characterised by factors including overall shape, average and distributed depth, permanence of volume, their nutrient characteristics and trophic states, incident solar and wind energy, the degree and effects of mixing and turnover of water, seasonal effects, as well as their importance for other or economic objectives. Overall, they are complex systems, very heavily biologically mediated in their potential. For fisheries-management based aquaculture, the priorities are to identify the biological capacity and its constraints and to identify the forms of aquaculture most suitable to assit in effective management and development. For more intensive forms of aquaculture, the main priorities are to ensure adequate exchange of water and sufficient ‘processing capacity’ to handle the additional nutrient loads generated, within specified criteria for lake condition.
Additionally there should be sufficient space for physical and service facilities involved, and there should be sufficient ‘activity capacity’ to accept the needs of other users;

Groundwater supplies (artesian or other wells, springs and groundwater fed ponds) are of interest for aquaculture, particularly for hatcheries, and usually have more stable chemical characteristics than surface waters, although they may contain undesirable levels of dissolved salts and metal ions, low levels of dissolved oxygen and high levels of toxic gases (carbon dioxide, hydrogen sulphide). There is increasing concern about contamination of groundwater supplies by persistent pollutants which may also prove detrimental to aquaculture (there is also concern about aquaculture contributing to this pollution; e.g. nitrogen from operations in Bavaria).

Industrial sources; aquaculture has been successful in using water e.g. warm cooling water from nuclear power stations and municipal tap water supplies. Heated effluents are frequently supersaturated with nitrogen and other gases and other industrial and municipal waters may be contaminated with various inorganic and organic compounds which may require pre-treatment or culture of appropriately tolerant organisms.

Wastewaters : hold considerable potential in the longer term, and approaches for wastewater use are being developed widely throughout the world, particularly in association with increased integration of municipal-level waste-water treatment and disposal. Possibilities may include; direct use of fish culture in association with waste-water treatment, eg as a component in secondary or tertiary lagooning systems; need to limit, eg with specific processing or to use as feed or fishmeal resource, or to use eg for broodstock, where pathogen transfer succeeding generations could be avoided; use of wastewaters in irrigation schemes, with integration of fish culture in supply channels and/or intermediate storage ponds; similar constraints as above; use of wastewaters to reservoirs, eg for irrigation purposes; use of reservoirs for cage culture; stock could be transferred to ‘cleaning reservoirs’ prior to harvest. Although technical means are available for carrying out these approaches, there remain problems concerning public perception and acceptability, even if wastewater is treated, let alone using it for direct production of food. However if might be noted that fertilisation and in some cases enrichment already contributes to fish production in many water bodies.

ECOSYSTEM MANAGEMENT CONSIDERATIONS

The extensive to intensive continuum; the discontinuities; the role of the aquatic ecosystem in maintaining environmetal conditions - primarily in an aerobic regime, and in supporting natural feed supply. The approaches to operation moving from:

-  untreated; no feed, fertiliser, relying on natural ecosystem productivity; these are essentially 'natural habitats, with very low yields, but with very little ecosystem-based risk.

-  partially fertilised systems, simplest managed systems, often described as ‘low-input’ systems, receiving occasional inputs, or low level of additional fertility; these are slightly modified habitats, have moderate yields, and usually require very little management or monitoring

-  fertilised systems, in which specific management intervention is involved in stimulating natural productivity, manipulating habitats to optimise yields, but generally still dependent on conventional ecosystem processes to maintain environmental conditions.

-  partially fed, sometimes in additional to fertilisation; more management intervention, and gradual removal of the food supply role, increasing depending on external input; water quality/ecosystem quality becomes more critical, and higher loading may make the system more unstable, requiring increasing levels of water management - water exchange, aeration, etc.

-  completely fed systems; assumed no role for ecosystem in food supply; stock and waste loading increasingly overload normal aerobic processing capacity, make more explicit water management increasingly necessary, to the point where ecosystem processes play a negligible role.

Extensive, low input systems are generally relatively stable with respect to aquaculture environment; intensive, highly managed systems, though highly unstable in ecological terms can also be operated in a stable manner due to the management input.

However there are many dangers of misplacing the approach, and operating outside the constraints of the intensity level.

The most common danger is to operate too intensively, with declining growth rates, risk of disease, mortalities due to poor environmental quality.

Another danger however, is to operate under the capacity of the system - ie inefficiently, wasting water and nutrient resources, eg through understoking, poor water management, nutrient imbalances with respect to optimum natural feeding organisms.

The importance of understanding the ecosystem as a whole is quite apparent; the extent to which it can be understood in real terms may however be limited, ie the cost - time, equipment, consumables, of acquiring information, as the gains to be achieved.

AQUACULTURE SYSTEMS AND RESOURCE USE

Managed systems for aquaculture have developed significantly over recent years, moving from traditional semi-natural methods towards modern intensive tank and cage-based techniques. The pace of technical change has accelerated, with increasing potential for environmental consequence. Traditional systems are also evolving, in response to changes in resource availability, economic demand, and technology. A substantial research and development activity has arisen, in the public and the private sector; while some of this has been effective, there are many areas where this support may have shown little result, and where concerns for application of resources might suggest better focusing. The implications of longer-term development are worth considering further. Table 3 estimates the additional water and land resources needed to support the extra production require to maintain average per capital supply at 1989 level to the year 2000. These are based on estimates of the type/intensity of system used, and typical water land usage (see eg Muir, 1992).

Table 3 : Projected fish water aquaculture resources use by major species/production system

Source/notes: table based on projections by New (1991), assuming basis of 1989 per capita supply, with stable capture fishery production at 100 million tonnes. Net increase in production excludes molluscs and aquatic plants, and is based on species distribution in 1989, with author's assumptions for percentages of catch species group grown in particular environments or systems. I includes common, grass, silver and bighead carp 2 Tilapia also grown in coastal/marine areas but assumed negligible here 3 includes Macrobrachium and red swamp crawfishs 4 Onshore (ie coastal) production of trout and salmon occurs, but assumed negligible 5 Salmon also grown in freshwater, but insignificant