written by Professor Hans Ackefors and Mr. Patrick White  (Part 2)

This paper was published earlier in World Aquaculture - June 2002, Volume 33, No2 and has been provided with permission from the authors

Environmental issues are currently much in focus in aquaculture, and therefore the following sections discuss only preparation of a CBP with respect to the environment and the production process.
Aquaculture technology is very diverse as indicated in the following table.

Table 3.  The Diversity of Aquaculture's Technological Systems and Practices

It entails many different systems and practices, depending on the fundamental resources of water, nutrition, and the species or species group.  It is easy to forget that the husbandry process, especially for aquatic organisms, is extraordinarily complex.  Therefore producing a CBP is not a simple task, and may have to be constructed from a series of operational elements.  The scientific and technical skills which may be involved can be visualized from an overview of the elements of the rearing process (see Table 4).

Table 4.  The Diversity of Aquaculture as Indicated by Production Elements

The Aquatic Ecosystem

Almost all aquaculture functions within an aquatic ecosystem, therefore the long-term commitment for aquaculture to be a legitimate sharer and user of the natural resources of water and land is an important issue for governments.  In countries where water-use and land-use policies are weak or unclear, aquaculture development has been severely constrained and subject to problems since infancy.
The environment is the starting- point for all aquaculture, and good site selection is the pre-requisite for a successful farming enterprise.  Factors for consideration include the quantitative and qualitative parameters of the original site before location of the enterprise, the parameters of the internal operations of the proposed enterprise, and the parameters when the enterprise is functioning.

Water Management

A reliable, high quality water source, and subsequently its careful management, is fundamental to successful aquatic farming.  Production technology must therefore be adapted to the available water source.  In some cases this may start as pre-treatment, particularly when the water is shared with other users up-stream, such as light industry, forestry, agriculture, and even other types of aquaculture.
In many areas the water is too acidic and outside optimal levels for survival and growth of certain organisms, in which case liming might be necessary.  In other places the water has too much iron or manganese, or other heavy metals, in which case simple aeration or sand filters are needed.  Finally, the water may be supersaturated with gases, like carbon dioxide, and degassing is required.
The supply of water might also be polluted, in which case pre-treatment is more difficult.  Here, the potential farmer has to identify the pollutant and its concentration before proposing a solution.

Physical and Chemical Environment

Site selection is the most critical factor bearing on the success of any farming operation, not only because of water quality but also security. 
Firstly, both water temperature and salinity regimes of any site have an important bearing on the choice of species.  The temperature ranges and optimal temperatures for growth differ from species to species, and even between strains, and it is also important to know lethal limits.  Similar, some species have wider tolerances of salinity, such as salmonids smolts and Mediterranean sea bass, whereas others are more restricted, such as gilthead seabream.  Then, together, both temperature and salinity may negatively impact a possible site; for example, there are coastal sites with a 20 - 30 ppt salinity range but which may be super-cooled in winter, making them unsuitable for rearing salmon and rainbow trout.
Secondly, water depth, tidal currents, and wind velocity are all factors which help maintain good water quality conditions at any site, but together they are the physical forces which are risks both to farm security and production.  Critical production factors, such as growth rate, food conversion efficiency, and resistance to disease, can all be impaired by the mechanical stress on and around a site.  A strong moorage system is vital for security and to give stability to the net-pens.  Therefore, every coastal complex must be designed to withstand exposure to above-average waves and winds, and perhaps even ice-loading in winter.
An option to help increased fish production is to use deeper or sunken cages.  These offer better protection from the physical forces near the surface, and mitigate the influence of high and low temperature regimes found in extreme summer and winter conditions.  But, this is not the case for shellfish production, such as mussel and oysters suspended beneath rafts.  Light penetration is necessary to generate phytoplankton production, and water turbidity stirs up the sedimentary organic material, both of which are necessary for these filter-feeders.

Biotic Factors

Most bacterial flora are useful and harmless food organisms for most aquatic species, but some are pathogens and cause diseases and impair growth.  A few are vectors and may be harmful to human health if the carriers are consumed.  Shellfish are particularly notorious for concentrating marine bacteria and viruses to levels which are a risk to human health.  Consequently there are stringent water quality standards imposed for the harvesting of shellfish for human consumption.  Most countries base their bio-quality criteria on the number of fecal coliform bacteria per 100 ml water.
There are a number of common microalgae which, when concentrated in shellfish, can be toxic to humans.  For example, dinoflagellates cause diarrhetic shellfish poisoning (DSP), and other planktonic algae cause paralytic shellfish poisoning (PSP) and amnesic shellfish poison (ASP).  The only way to avoid these risks is by regularly sampling and analyzing the shellfish quantitatively for the presence of these toxins.
Marine phytoplankton may at times be directly hazardous to fish.  Fish farmers in many countries have experienced heavy mortality when weather conditions favor massive phytoplankton production.  These concentrations, or blooms, are known to kill fish by removing oxygen from the water, overly concentrating oxygen, mechanically impairing the gill tissue, or directly producing a toxin.
Another critical biotic factor for any site is the presence of larger predators, such as marine mammals and birds, which can be a significant problem to fish and shellfish farmers.  Marine mammals, such as seals and sea-lions, are opportunistic predators around fish farms, and otters are predators of both fish and shellfish.  All can do significant physical damage to nets and other structures.  Almost all marine mammals are now protected by international convention and national jurisdictions, although in some countries dangerous or nuisance individuals can be shot under license.  All avian predators, such as ospreys, herons, cormorants, and ducks, are also protected by law, although recently there was a move to license the killing of cormorants in inland waters as they were outside their natural range.  Nonetheless, there is considerable opposition to the authorities by fishermen and farmers over the protection of certain predators as their population numbers have passed historic levels.
On the substrate, starfish and some other invertebrates are common predators of shellfish and present a difficult problem for cultivators.  Off-bottom culture is not always a better option, as starfish are known to climb onto structures, and some authorities ban the use of structures in inter-tidal areas.