Presented by
Dr. C. Agius
Director, National
Aquaculture Centre, Malta
1. BACKGROUND
Aquaculture can be defined as the rearing of aquatic organisms under controlled conditions.
Aquaculture has a long history in certain parts of the world but whilst in the past it has often been conducted in an extensive manner, modern methods seek to intensify production due to increasing demand for fishery products. This is particularly relevant to the Mediterranean Sea where fisheries are fast declining due to inherent poverty of stocks and over-exploitation. In any event aquaculture should be viewed a supplement for rather than a replacement of fisheries at least for the foreseeable future. It is unlikely that aquaculture will ever replace fisheries totally, considering that the sea covers 80% of the earth's surface and that fish meal is still the major component of fish feeds.
In developing countries aquaculture is often pursued as a subsistence-level activity whereas in industrialised countries it tends to be conducted for financial profits. Biologically it is advantageous to culture aquatic organisms because their buoyancy and poïkilothermy render them more efficient users for food than terrestrial, warm-blooded animals.
Shellfish show further savings of energy since they are sessile and filter feeders.
Production figures show a steady rise in aquaculture production worldwide. In particular there has been a steep rise in production of Mediterranean species, notably sea bass and sea bream.
2. SPECIES CULTURED
The main aquatic species cultured are as follows:
Finfish such as sea bass, sea bream, eels and tilapia.
Shellfish such as mussels, clams and oysters.
Crustaceans such a shrimps and lobsters.
Algae.
3. TYPES OF AQUACULTURE SYSTEMS
The production cycle is usually split up into the hatchery and the fattening stages.
In the hatchery, broodstock produce eggs which hatch into larvae which are then grown into juveniles or fingerlings, as they are better known. Fingerlings are then grown to market size in fattening units. Hatchery operations are usually conducted indoors as they require controlled conditions.
Finfish fattening is usually carried out either in ponds, raceways tanks or cages. The materials from which these are constructed vary enormously depending on many factors such a species being cultured, material availability, etc. Ponds and raceways are usually dug into the earth (they may also be plastic lined) or built of concrete. Tanks are usually constructed of concrete or fibreglass.
Shellfish fattening is usually carried out in bags or other containers placed on the sea bed or in off-bottom culture.
Crustacean culture is usually earned out in tanks, trays or earthen or concrete ponds.
Algae are usually cultured in tanks or in ponds.
4. WATER QUALITY AND QUANTITY
4.1 Quality Parameters
Main water quality parameters relevant for fish fanning operations include salinity temperature, oxygen, carbon dioxide, ammonia, turbidity, levels of organic matter, productivity, and pH.
4.1.1 Salinity
Water for aquaculture may be fresh, brackish or salty (seawater). Sometimes a species is restricted to a particular salinity, whereas others are able to adapt to different conditions. For example:
Salmon spawn in freshwater, migrate extensively in the open sea and return to spawn in freshwater.
Eels spawn in the open sea, migrate up rivers and return to the open sea for spawning.
Tilapia show varying tolerances to a wide range of salinities depending on the species.
4.1.2 Temperature
Water temperature is very important as it:
dictates whether farming is possible,
dictates growth; and
dictates oxygen levels.
It is important to note the inverse relationship that exists between temperature and oxygen levels.
4.1.3 Temperature Fluctuations
Water temperature fluctuations are critical since fish are cold-blooded and their physiological rate is directly temperature dependent. General points related to water temperature are as follows:
Ambient, open water usually fluctuates with season.
Ambient, enclosed or semi-enclosed water such as in a lagoon will generally go to extremes being colder in winter and warmer in summer. This is a very relevant consideration in many North African coastal settings.
Borehole water usually has a constant temperature year-round.
4.1.4 Oxygen
Oxygen is essential as it enables food utilisation.
Different species tolerate different levels of oxygen, eg. Tilapia is more tolerant than sea-bass to low oxygen levels-
Borehole water is usually low in oxygen content.
Enclosed water bodies could experience algal blooms which can be lethal to fish.
4.2 Quantity Parameters
4.2.1 Water exchange
Water exchange determines three important processes, viz:
Provision of food;
Provision of oxygen; and
Removal of wastes.
Water quantity may dictate whether to select an open flow system as opposed to a recycling system. Open flow systems involve simple flow-through, therefore requiring large quantities of water. Recycling systems reuse water and require a minimum volume replenishment of 5–10% per day. Though they are very economical on water quantities, recycling systems are less widely used as much stricter control of water quality and more sophisticated waste removal technology are necessary.
4.2.2 Recycling system
Basically two types of wastes have to be removed — viz., solid and soluble wastes.
Solid wastes arc usually removed mechanically or by sedimentation under gravity.
Soluble wastes are usually removed through the biological activity of bacterial filters (or biofilters) wherein bacteria convert ammonia into harmless nitrogenous substances.
The basic set-up of a recycling system is as follows:
| Header tank | → | Fish tanks |
| ↑ | ↓ | |
| Pump | Sedimentation chamber | |
| ↑ | ↓ | |
| Collecting tank | ← | Biological filter |
Possible additions to this basic design would be an aeration system, temperature control, and ultra-violet treatment.
4.2.3 Calculating water flows
Water flow requirements depend on many factors but are primarily dictated by oxygen demand which will in turn dictate the need for waste removal.
The more sensitive the species under culture the higher the required flow.
The younger the fish the higher the demand for oxygen.
The higher the temperature the lower is the level of dissolved oxygen per unit volume for water and therefore the higher the flows required.
Oxygen demand increases with increasing metabolic activity, therefore higher flows are required after feeding.
The higher the stocking densities of fish stocks the higher the demand for oxygen and therefore the higher the flows required.
5. MAIN CRITERIA FOR SITE ASSESSMENT
5.1 Land-Based Farms
For land-based farms the following factors are important:
source of water, head of pumping.
effluent discharge.
area available.
topography.
soil/rock type.
accessibility.
services (power, tap-water, telephone, etc.).
5.2 Sea-Based Farms
For sea-based farms important factors to take into account include:
depth.
exposure.
prevailing winds.
operational base.
nearest safe port.
bottom type for anchors/moorings
5.2.1 Cage culture
Cages can be of the inshore or the offshore type. Formerly most cage culture was conducted along inshore protected coastal zones due to the reduced risk of storm damage. In recent years however there is the realisation that environmental damage and disease incidence are exacerbated in sheltered waters there has been increased interest in offshore cages.
Cages can be of many different types of material including wood, plastic, metal and rubber (used lorry tires even!). They can be laid down singly or in groups.
The siting of the cages is crucial as topographical and other features dictate water exchange which is vital for maintaining good water quality in order to efficiently remove wastes and replenish oxygen levels.
Nowadays various offshore-type cages are available which have proven to function reliably for a number of years in very exposed sites. A vital requirement of such systems is a good diving team, since regular checking of moorings and nets is essential. Usually nets have a shorter life span than the cage collars. An important consideration is that breams have been observed to chew nets and escape through the holes; this is not a problem with bass.
6. POSSIBLE EFFECTS ON THE ENVIRONMENT
6.1 Negative Effects
It is important to attempt to minimise the negative environmental impact in designing a fish farm. Possible negative impacts include:
Competing uses such as tourism and pleasure diving.
Aesthetic appearance, eg. farm land base in a touristic area.
The impact of solid wastes.
in land-based farms it should be possible to remove most solid wastes whereas in cage farms they should be dissipated by currents.
The impact of soluble wastes.
Soluble wastes should be removed by adequate water flows in land-based farms; they are likely to be diffused quickly in the open sea.
6.2 Positive Effects
Possible positive effects include:
Increase of fish in the sea which decreases pressure on wild stocks.
Increase in food production for human consumption.
7. MANAGEMENT
7.1 Reacting to Emergency Situations
Simple but useful tips to bear in mind under emergency situations are the following:
Think hard before taking any drastic decisions.
Suspension of feeding is usually safe and at times recommended.
Avoid unnecessary panic.
It may be too late to apply antibiotics.
7.2 Stocking and Transfers
Before commencing any fish stocking operation it is essential to be fully prepared to accept the fish being transferred in.
It should be determined whether quarantine is required in order to ensure the introduction of a disease-free stock. A health certificate should always be requested wherever relevant.
All details of fish movements including stocking densities to be employed, etc., should be worked out well in advance of accepting any shipment of new stock.
Fish should be starved at least 24 hours prior to transfer to reduce metabolic stress. Large fish can even be starved for 48 hours.
Appropriate containers should be used for fish transfers including an adequate aeration system wherever applicable.
It is important to remain alert for temperature changes during transport, noting temperature difference between point of origin and final destination.
7.3 Grading
Regular fish grading or selection is necessary to ensure more uniform growth and to avoid cannibalism and repression of growth by dominant individuals.
Grading is usually also accompanied by thinning.
Before grading, fish should be starved to minimise stress.
The frequency for grading depends on many factors such as species, size or age, season and farm type.
7.4 Feeding: Methods, Frequency and Quantity
The main factors of relevance in terms of feeds and feeding are the following:
Feed type.
Pellet size.
Quantity of feed that should be given.
Frequency for feeding throughout the day.
Careful note of any reduction in feeding rate.
Possibility of using automatic feeders.
7.5 Diseases
Diseases can be caused by:
Parasites.
Bacteria.
Virus.
Fungi.
Dietary deficiencies.
Avoidance of stressful factors/situations minimises the risk of disease outbreaks.
7.6 Prophylaxis
Prophylaxis may be achieved by:
Cleanliness.
Proper disposal of carcasses.
Regular disinfection.
Taking special care about new introductions.
7.7 Treatment
Treatment should only be resorted to if all else fails. It is much more advisable to prevent than to cure. Treatment inevitably stresses fish and has a detrimental effect on the environment, particularly microbial flora.
Treatment can be carried out by the bath, oral or injection methods. It is logistically impossible to treat under some particular conditions such as offshore cages.
8. FISH HARVESTING AND PACKAGING
Panic selling and premature harvesting of fish should always be avoided as this is normally uneconomical. Proper management and planning is therefore required.
Be fully prepared with all the requirements such as boxes, ice, etc., before commencing to cull fish.
Starve fish for at least 24 hours prior to harvesting.
Take special care in summer when temperatures are high; maintainance of top flesh quality is ensured by keeping the fish body temperature low at all times from the moment of capture onwards.
Part harvesting can be especially stressful.
Seabass are particularly sensitive and extra care is needed with this species.
