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.
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