Aquaculture – environment
Housing requirements for farmed fish.
Tanks, cages or ponds must be of adequate size to house the number of animals and must take into consideration their species, size and expected growth. Water must be of an appropriate temperature, pH, salinity, oxygen level and be sufficiently covered, filtered and cleaned.
- Fish must be stocked at a density in accordance with their housing conditions, Cage; 20 kg/m³, Tank; 10 kg/m³ and Pond 20 t/ha
- Tanks must have appropriate covers
- Adequate water quality must be monitored and maintained, including suitable pH, dissolved oxygen level, temperature, ammonia and salinity and must be sufficiently filtered and cleaned. Water quality data must be recorded
- A high level of hygiene and cleanliness must be kept at all times and sufficient water exchange must be maintained
- Natural environment should be replicated in the tank where possible
- Normal diurnal pattern of lighting must be provided with 12 hours of dark and 12 hours of light each day
- Air surrounding tanks and ponds must be of acceptable quality with respect to dust, chemicals and smells with special care taken when spraying insecticides.
All facilities used to house fish must be operated in a manner that optimizes conditions for the particular fish species. Suitable facilities for holding fish include ponds, raceways, tanks, cages and aquaria.
All facilities should be aerated. Tanks and aquaria should be aerated continuously with diffused air and ponds with mechanical aerators such as paddlewheels for around 8 hours/day.
In circular, self-cleaning tanks, a constant flow of water must be used to facilitate the removal of solids and dissolved wastes, e.g. ammonia to supplement aeration. If tanks need to be static, e.g. during chemical treatment, fish should not be fed and water (10–30%) should be exchanged daily.
Space
The stocking density for fish is dependent on the prevailing water quality, the species and size of the fish, the temperature of the water and the oxygen supply. The table below lists the optimum stocking density for each of the housing types.
| Housing type | Optimum stocking density |
|---|---|
| Tank | 10 kg/m³ |
| Cage | 20 kg/m³ |
| Pond | 20 t/ha |
Covers or shelter for tank
Tanks should be placed undercover or in a building out of direct sunlight to provide an environment with relatively low light intensity.
During winter months the need to heat individual tanks can be avoided by keeping tanks in a closed environment where the air temperature of the room can be maintained. Heating should be used before the water temperature drops. This reduces the amount of heating required and saves power and money.
Water quality
Maintenance of good water quality is essential. Fish cannot be easily and effectively monitored on a regular basis but monitoring of the environment can be done daily and provides the best method of monitoring fish health.
Maintenance of good water quality requires the regular monitoring of temperature, dissolved oxygen, pH and ammonia, and for marine and brackish water species, salinity.
Schools should purchase appropriate equipment that allows accurate regular monitoring of the water quality and the data must be recorded.
Temperature
Fish are ectotherms because heat is obtained from outside the animal unlike endotherms (e.g. mammals) that generate their own body heat. Usually, the body temperature of ectotherms is close to that of their surroundings; they are often described as poikilothermic (having variable temperature).
Temperature affects all chemical and biological processes. The metabolic rate of fish doubles for every rise of 10°C. Therefore, temperature has a direct effect on important factors such as growth, oxygen demand, food requirements and food conversion efficiency. The higher the temperature, the greater the requirement for oxygen and food and the faster the growth rate.
Temperature partly determines the concentration of oxygen in water. The solubility of oxygen decreases with increasing temperature, and so concentrations are usually lower in summer.
Silver perch have a temperature tolerance range of 2 to 38°C with optimum growth occurring between 23 and 28°C. During winter when water temperatures are lower, silver perch will require less food and have a slower growth rate. At temperatures below 10°C the fish may enter a state of torpor, with greatly reduced appetite and activity. As the water temperature increases in spring and summer, the fish will require a larger quantity of food due to the increase in their metabolic rate. If the temperature is to exceed the critical level for a particular species, fish may become stressed, more vulnerable to disease, stop growing and can die.
Dissolved oxygen
Dissolved oxygen is the most critical and limiting variable in fish husbandry and culture. Like all animals, fish cannot live without oxygen. Although fish can survive at levels of 4 mg/L, they may suffer stress, reduced growth and increased susceptibility to disease.
Oxygen enters water through diffusion at the air-water interface and as a result of photosynthesis when there are plants in the water. For aquaria, tanks and raceways, dissolved oxygen is usually supplied through low pressure compressors or blowers (through diffusers like air stones). In ponds, paddlewheel aerators are among the most efficient methods of transferring oxygen from the air to the water. This also helps with mixing water throughout the pond.
Salinity
Salinity refers to the total concentrate of all dissolved ions. Many Australian native fish tolerate a wide range in salinity, with freshwater species coping with up to 5 g/L and many estuarine species coping with salinity as low as 10 g/L. Fish need to be given time to adjust to changing salinity.
pH
The desirable range for fish is around 6–9, depending on the species. A pH of 4 is lethal for most species while prolonged exposure to pH levels of above 10 is also lethal. Other variables that influence the water quality include alkalinity, hardness, turbidity and ammonium, nitrite, hydrogen sulphide and carbon dioxide levels.
Water exchange
Poor water quality can result from inadequate water exchange. Water exchange can be achieved through:
- Partial draining of the pond or tank and then replacing the lost water.
- Flow-through systems with the pond, tank or raceway remaining full through water entering and leaving the system at the same time from different locations.
- Recirculating systems.
Filtration
The maintenance of water quality in tanks and aquariums can be assisted through a filtration system. The different types of filtration include:
- Mechanical
- Chemical
- Biological.
Cleaning
Tanks should be cleaned regularly, by siphon or vacuum pump, to reduce problems with the accumulation of organic matter (uneaten food, faeces) and fouling organisms, bacteria and algae. Filters need to be backwashed regularly to prevent build up and decomposition of accumulating waste material. Floors and drains associated with tank rooms should be cleaned and sterilized on a regular basis. Dilute pool chlorine or sodium hypochlorite (NaOCl 20 ppm) or caustic soda (NaOH 1%) are suitable cleaning agents for this purpose.