Environmental impact assessment
In order to regulate unsustainable aquaculture enterprises many governments have instituted the system of licensing as mentioned earlier. Some national farmers’ groups/ alliances have also formulated codes of management practices for aqua farmers, to ensure sustainability of aquaculture systems. Application of licences will require appropriate investigations of the sites for environmental impacts and the socio-economic implications of proposed aquaculture practices. In selecting culture systems aquaculturists seldom give adequate weighting to waste production and their disposal, compared to economic yields and steady returns on investments. Waste reduction is an integral part of good husbandry and disregard of this can be counter-productive as, in the long run, the sustainability of the operations will be affected by the degradation of the environment (Pillay, 1992). It is therefore important to estimate the environmental capacity of the proposed farm to handle the consequences of discharges from a farm in receiving water.
Environmental capacity measures the resilience of the natural environment to the aquaculture activities. It is essential to estimate(1) the rate at which nutrients can be added without triggering eutrophication, (2) the rate of organic flux to the benthos without major disruption to natural benthic processes, and (3) the rate of dissolved oxygen depletion that can be accommodated without causing mortality of the biota (GASAMP, 2001). The main forms of pollutants to be considered are suspended solids and dissolved nutrients, especially nitrogen and phosphorus.The major sources of these pollutants are uneaten or spilled feeds and faecal matter. Inflows of water may contain varying quantities of organic matter. This, as well as unutilised primary production resulting from intentional fertilization or the degradation of organic matter, may add to the loads of solid and dissolved wastes in effluents originating from the farms.Algal blooms may be caused by over-feeding and over-fertilization. It has been estimated that feed losses may amount to 5–20 per cent. Excess feeding could reduce assimilation and significantly increase faecal production. Automatic feeding devices and computerized feeding systems have proved effective in improving feed conversion ratios and minimizing feed losses. Dry processed feed pellets are much superior to moist feeds of high water-stability, and would improve consumption and reduce losses.
The soluble contents of effluents depend on the feed ingredients. As mentioned above, nitrogen and phosphorus are of greater environmental concern and as such their content should be kept as low as possible. De Silva (1999) contends that many of the feeds presently in use are over-formulated. An easily digestible feed, with the proper protein and correct amino acid balance to energy ratio, would reduce the quantity of nitrogen discharged. Present-day Atlantic salmon feeds contain upto 35 per cent lipids with a concurrent reduction in protein in the diet and reduction in effluent nitrogen load by 58 per cent and phosphorus by about 85 per cent (Enell, 1995). Reduction in nitrogen content by 10 per cent and phosphorus by 40 per cent in salmon feed reduced FCR from 2 to 1.4 per cent, which in turn reduced environmental pollution (Makinen, 1991). Extruded pellets are less pollutive and preferred in aquaculture, though they are more expensive. The use of poor-quality carbohydrates in feed manufacture can result in increased solids and BOD in farm waters. It is clear that the feed quality and conversion ratio of feeds to be used in a farm is, along with species of fish or shellfish to be reared, of significant importance in estimating the nutrient loading of effluents to be treated.
There are a number of ways of treating effluents, but the most cost-effective means is simple sedimentation in pond farms. In commercial pond culture large settling or sedimentation tanks or basins have to be used and 10 per cent of the farm area may be required, which in areas where the land is costly or rare is not easy to find. A length-width ratio higher than 4 and the installation of baffles, which increase the mean residence time of suspended solids, is of importance in the design of sedimentation tanks. The importance of the sedimentation pond and biofiltration units have also been stressed (Avnilamech, 1998). In Thailand some shrimp farms have adopted recirculation systems incorporating an intakereservoir and biofiltration in pond units, where compatible finfish such as mullet and milkfish and seaweeds (Gracilaria) are grown.
The quantity of water consumed in aquaculture depends on the culture system selected and the stock density maintained in the farm. Tanks and raceway systems normally require large quantities of water, but a good percentage of this is discharged back into the ground or surface waters. In pond farms under stagnant and semistagnant conditions loss of water due to seepage and evaporation can be high depending on the climatic conditions and soil properties. Boyd (1981) reported an average loss of 1.3–21.5 cm/day in experimental ponds in Auburn (USA). Direct rainfall and in some cases groundwater inflows compensate for the loss and therefore rainfall in the area has to be taken into account.
Selection of farming system is governed by a number of considerations. From the point of view of environmental impact, semi-intensive systems have many favourable features as, when properly designed and maintained, pond farms merge well with the landscape and in many cases enhance the scenic beauty of coun-tryside. Polyculture as in conventional systems, when it is feasible, is intended to make full use of farm resources, including wastes. This serves to reduce the load of faecal matter and thereby of soluble nutrients, in order to improve the environmental profile.