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Introduction

1.1 Aquaculture engineering

During the past few years there has been considerable growth in the global aquaculture industry. Many factors have made this growth possible. One is developments within the field of aquaculture engineering, for example improvements in technology that allow reduced consumption of fresh water and development of re-use systems. Another is the development of offshore cages: sites that until a few years ago not were viable for aquaculture purposes can be used today with good results. The focus on economic efficiency and the fact that salaries are increasing have also resulted in the increased use of technology to reduce staff numbers.

The development of new aquaculture species would not have been possible without the contribution of the fisheries technologist. Even if some techniques can be transferred for the farming of new species, there will always be a need for technology to be developed and optimized for each species. An example of this is the development of production tanks for flatfish with a larger bottom surface area than those used for pelagic fish.

Aquaculture engineering covers a very large area of knowledge and involves many general engineering specialisms, such as mechanical engineering, environmental engineering, materials technology, instrumentation and monitoring, and building design and construction. The primary aim of aquaculture engineering is to utilize technical engineering knowledge and principles in aquaculture and biological production systems. The production of fish has little in common with the production of nails, but the same technology can be used in both production systems. It is therefore a challenge to bring together both technological and biological knowledge within the aquaculture field.

1.2 Classification of aquaculture

There are a number of ways to classify aquaculture facilities and production systems, based on the technology or the production system used.

'Extensive', 'intensive' and 'semi-intensive' are common ways to classify aquaculture based on production per unit volume (m3) or unit area (m2) farmed. Extensive aquaculture involves production systems with low production per unit volume. The species being farmed are kept at a low density and there is minimal input of artificial substances and human intervention. A low level of technology and very low investment per unit volume farmed characterize this method. Pond farming without additional feeding, like some carp farming, is a typical example. Sea ranching and restocking of natural lakes may also be included in this type of farming.

In intensive farming, production per unit volume is much higher and more technology and artificial inputs must be used to achieve this. The investment costs per unit volume farmed will of course also be much higher. The maintenance of optimal growth conditions is necessary to achieve the growth potential of the species being farmed. Additional feeding, disease control methods and effective breeding systems also characterize this type of farming. The risk of disease outbreaks is higher than in extensive farming because the organism is continuously stressed for maximal performance. Salmon farming is a typical example of intensive aquaculture.

It is also possible to combine the above production systems and this is called semi-intensive aquaculture. An example is intensive fry production combined with extensive on-growing. Aquacultural systems can also be classified according to the life stage of the species produced on the farm, for instance eggs, fry, juvenile or on-growing. Farms may also cover the complete production process, and this is called full production.

Depemding on the type of farming technology used, there are also a number of classifications based on the design and function of the production unit. This will of course be species and life-stage dependent. For fish the following classifications may be used: (1) closed production units, where the fish are kept in an enclosed production unit separated from the outside environment; (2) open production units, where the unit has permeable walls (e.g. nets) and so the fish are partly affected by the surrounding environment. It is also possible to classify the farm based on where it is located: within the sea, in a tidal zone or on land.

Land-based farms may be classified by the type of water supply for the farm: water may be gravity-fed or pumped. In gravity-fed systems the water source is at a higher altitude than the farm and the water flows by gravity from the source to the farm. In pumped systems, the source can be at an equal or lower altitude compared with the farm. For tidal through-flow farms, water supply and exchange are achieved using the tide.

Farms can also be classified by how the water supplied to a farm is used. If the water is used once, flowing directly through, it is named a flow-through system. If the water is used several times, with the outlet water being recycled, it is a water re-use or recirculating aquaculture system (RAS). It is also possible to separate production systems as monoculture or polyculture: monoculture involves the production of only one species (e.g. fish), whereas polyculture involves the production of two or more (e.g. fish and rice). This is also named 'integrated aquaculture'.

1.3 The farm: technical components in a system

In a farm the various technical components included in a system can be roughly separated as follows:

• Production units
• Water transfer and treatment
• Additional equipment (feeding, handling and monitoring equipment).

To illustrate this, two examples are given: a land-based hatchery and juvenile farm, and an on-growing sea cage farm.

1.3.1 Land-based hatchery and juvenile production farm

Land-based farms normally utilize much more technical equipment than sea cage farms, especially intensive production farms with a number of tanks. The major components are as follows (Fig. 1.1):

• Water inlet and transfer
• Water treatment facilities
• Production units
• Feeding equipment
• Equipment for internal fish transport and size grading
• Equipment for transport of fish from the farm
• Equipment for waste and wastewater treatment
• Instrumentation and monitoring systems.

Figure 1.1 Example of major components in a land-based hatchery and juvenile production plant.

Water inlet and transfer

The design of the inlet depends on the water source: sea water or fresh water (lakes, rivers), or surface water or groundwater. It is also quite common to have several water sources in use on the same farm. Further, it depends whether the water is fed by gravity or whether it has to be pumped, in which case a pumping station is required. Water is normally transferred in pipes, but open channels may also be used.

Water treatment facilities

Water is usually treated before it is delivered to the fish. Equipment for removal of particles prevents excessively high concentrations reaching the fish; additionally, large microorganisms may be removed by the filter. Water may also be disinfected to reduce the burden of microorganisms, especially that used on eggs and small fry. Aeration may be necessary to increase the concentration of oxygen and to remove possible supersaturation of nitrogen and carbon dioxide. If there is lack of water or the pumping height is large, pure oxygen gas may be added to the water. Another possibility if the water supply is limited is to re-use the water, although this will involve considerable water treatment. For optimal development and growth of the fish, heating or cooling of the water may be necessary; in most cases this will involve a heat pump or a cold-storage plant. If the pH in a freshwater source is too low, pH adjustment may be part of the water treatment.

Production units

The production units necessary and their size and design will depend on the species being grown. In the hatchery there will either be tanks with upwelling water (fluidized eggs) or units where the eggs lie on the bottom or on a substrate. After hatching the fish are moved to some type of production tank. Usually there are smaller tanks for weaning and larger tanks for further on-growing until sale. Start-feeding tanks for weaning are normally under a roof, while on-growing tanks can also be outside.

Feeding equipment

Some type of feeding equipment is commonly used, especially for dry feed. Use of automatic feeders will reduce manual work on the farm. Feeding at intervals throughout the day and night may also be possible; the fish will then always have access to food, which is important at the fry and juvenile stages.

Internal transport and size grading

Because of fish growth it is necessary to divide the group to avoid fish densities becoming too high. It is also common to size grade to avoid large size variations in one production unit; for some species this will also reduce the possibilities for cannibalism.

Transport of fish

When juvenile fish are to be transferred to an on-growing farm, there is a need for transport. Either a truck with water tanks or a boat with a well is normally used. The systems for loading may be an integral...