Selection of sites and farming practices
Although there are several reasons for the opposition to the steady increase in aquaculture, there is also recognition that the capture fisheries in many parts of the world have not been managed to ensure sustainability and that aquaculture is the only growth sector in fisheries. Therefore to maintain aquatic production it is necessary to expand aquaculture. This situation has occurred at a time when social and environmental problems have become dominant issues in development. Sustainability has achieved worldwide recognition as a policy to be followed.Aquaculture, like any other human activity, has been looking at ways in which it can be promoted as a sustainable activity. In this search for the solution to the environmental and economic problems encountered, it has become quite clear that most of the problems are related to the sites where individual farms are situated, and therefore site selection has become an important part of aquaculture. Sites have to be selected to ensure that the activities in the farm do not exceed the carrying capacity of the environment. The precautionary approach that UNCED (1987) recommended recognized that many development projects may have uncertain and potentially damaging implications for the environment that are not readily observable and should therefore be carefully and rigorously evaluated for sustainability, particularly those projects utilizing natural resources. Aquaculture and aquaculture-based fisheries fall into this category of natural resource-based development. In the past, research and experimentation have been guided by the objectives of obtaining increased yields by intensifying aquacultural practices. Lack of tested sustainable practices was viewed as another impediment to the emerging infant industry, without a clear idea of the dimensions of sustainability. Aquaculture had been based on the principle of short-term economic viability. When this was affected by disease outbreaks as a result of self-pollution or external waste discharges, there was general recognition that environmental sustainability is a valid idea to be considered.
From the definitions quoted earlier, it is clear that sustainability can be interpreted and understood differently according to interest in the various aspects involved. The practical meaning of sustainable development will rarely be agreed in relation to practical development decisions. This results in specific discussions of the trade-offs between different development and conservation objectives and their associated activities (GESAMP, 2001).
The estimation of environmental capacity is basic to the selection of zones for aquaculture sites and is relevant to the allocation of appropriate areas for the promotion of aquaculture by the state. Assessment may not be as detailed as in the case of environmental perturbation. With increasing efforts to eradicate poverty, succeeding generations may not be poor, and their needs may change in line with future economic development.
In aquaculture the main forms of wastes that are of importance in environmental management are suspended solids and dissolved nutrients, especially sources of nitrogen and phosphorus. The major sources of these wastes are accumulations of uneaten or spilled feeds and faecal matter. For example, shrimp farm waste is mainly composed of uneaten feed and faecal matter which account for 15–20 per cent and 20–25 per cent of feed given respectively (Primavera, 1994).
In tidal ponds, the inflows may contain appreciable quantities of organic matter. This, along with the nutrients and unutilized primary production resulting from fertilization, may give rise to algal blooms. The need to avoid over-fertilizing of farms through excessive application of organic or chemical fertilizers is widely recognized but over-feeding is not so apparent, especially when automatic feeding techniques are used. It has been estimated that feed losses in processed feeds may vary from 5 to 20 per cent, and over-feeding can reduce feed digestibility and increase faecal production significantly. The use of computerized feeding systems, based on automatic monitoring of the environment and food conversion ratios, are effective in minimizing feed losses.
Ackeforce and Ennel (1994) consider that the discharge of nutrients and organic material to surrounding waters is inevitable in the open cage system used in Nordic countries. When assessing the environmental impact of aquaculture the feed coefficient and the content of phosphorus in the feed are two important factors to be considered. Mass balance calculations are used to assess the discharge of polluting substances. The feed coefficient in many north European aquaculture units has been reduced from 2.3 to less than 1.3 as a result of experience in the formulation of improved feeds. The nitrogen content in the commercial feeds has been decreased from 7.8 per cent and the phosphorus content from 1.7 to <1 per cent. As a result, for every ton of fish produced, discharges of phosphorus now are <10kg and nitrogen <53kg (Ackeforce and Ennel, 1994). Though imposing restrictions in feed-making at entry point through policy and regulation for reducing pollution is thus possible, regulating effluent quality (exit point) is preferred since this would give more avenues for diversifying feeds according to the availability, quality and costs of ingredients and ingenuity of the farmer (Tacon and Forster, 2003).
The processing method adopted in commercial feeds is of importance in reducing the pollutive effects of feed-derived wastes. Extruded pellets have a slow sinking rate and higher water stability and availability. The ingredients that compose the feeds are also important from the point of view of waste production. Commercial salmon feed now has the composition of 30 per cent fat, 40 per cent protein and 13 per cent carbohydrate, with an energy content of 19.2mJ/kg (Wilson, 1994). The nitrogen content is now about 7 per cent and the fish utilizes fat instead of protein for energy, with lesser volumes of nitrogenous compounds such as ammonia being excreted. There is less excretion of phosphorus, since its content has been reduced to about 1 per cent in the diet.
Feed management includes the regulation of the size of feed according to the size of biomass and age composition and intervals of feeding according to environmental conditions. To avoid wastes and feed spillage many advanced farms, whether land-based or off-shore, hatcheries or rearing facilities, use computer programs to regulate feeding according to daily variations in the weather conditions. By the use of such adjusted feeding procedures, feed conversion efficiencies have been increased and quality of effluent discharged into waterways enhanced.
Commercial fish feeds generally use fish meal as a major component even in improved formulations (Ennel, 1995) to reduce waste discharge. In the light of the controversial prediction that there may be a shortage of fish meal (Wickjstrom and New, 1989) and alleged overformulation (De Silva, 1999), the search for suitable substitutes has to be continued. Besides reducing fish meal as a source of proteins, manufacturers use meat meal, bone meal, blood meal, poultry meal and dried brewer’s yeast to reduce fish meal in aquafeeds. Kaushik et al. (1995) reported the effects on growth,protein utilization, and potential estrogenic or antigenic effects of its partial or total replacement by soybean meal.
Environmental impacts of aquaculture are very much associated with the type of farming adopted and the species under culture. The sites where farms are located have a considerable role in determining the environmental impacts of culture operations, so it is important to bring to bear the impact assessment data that are significant to the selection of sites for farm development. GESAMP (1996a; 1996b) recommended estimating the amount of effluent from the farms discharged into neighbouring waterways and the ability of these water bodies to disperse/assimilate the wastes. The quantity of wastes from aquatic farms will varywith the intensity of farming operations, but the assimilative capacity of waste discharges will depend very much on the flushing rate of the receiving water, or regular removal of farm sediment. Since many farms are provided with water inlets and outlets, it is considered beneficial to have sedimentation tanks associated with inlets or outlets of farms. Where regulations have been practised, one important condition to be satisfied in the design of the farm may be to reserve space for settling tanks to the extent of at least 10 per cent of the farm area.
Negroni (2000) considers constructed wet-lands an attractive option for the disposal of fish farm effluents. Macrophytes can clean waste water containing potential pollutants by direct assimilation. The major removal mechanisms for nitrogen are nitrification and denitrification, mediated mainly through bacteria. Phosphorus removal occurs as a result of adsorption. Pathogens are removed during passage of waste water through sedimentation and filtration. Some use probiotics to displace pathogens responsible for the occurrence of shrimp diseases, but Sonnenholzner and Boyd (2000) found this ineffective with commercially available probiotics.
It has been pointed out that site selection in aquatic farming has a significant role in social impacts. If not properly located aquaculture farms can affect the present livelihood of neighbouring villages. Very often large coastal aquaculture farms prevent easy access to the beaches where small-scale fishermen beach their boats and dry their nets. Farm operations may obstruct fishermen from carrying out their fishing activities. Complaints about attempts to privatise common property resources have to be avoided to prevent adverse social impacts.
If proper care is not taken in farm siting as well as in the design and construction of adequately wide buffer zones and embankments, it is likely that neighbouring agriculture fields may be affected by the salinization of soils. Drinking water sources may also be affected by salinization. Where ground water has to be pumped for the reduction of salinity of farm ponds, there is a risk of land subsidence. In coastal sites the construction of farms may give rise to soil erosion and the destruction of mangroves.