Fundamentals of Aquatic Veterinary Medicine. Группа авторов

      Monitoring of the system may be fully automated or may be performed by individuals. The benefits of automated monitoring equipment include the ability to set alarms for power outages, changes in dissolved oxygen and large changes in ammonia. Some systems are able to calculate flow rates and water usage, which is beneficial for production calculations. Monitoring should include ammonia, nitrite, nitrate, dissolved oxygen, temperature, total alkalinity and pH to assure proper water quality parameters.

      1.2.1 Extensive Culture System

      The extensive culture system is characterized by low to no inputs (food, fertilizer, etc.) and low stocking densities. Extensive aquaculture is practiced in lakes, reservoirs, lagoons, ponds and tanks.

      1.2.1.1 Ponds

      The most common production system in use is the earthen pond. Earthen ponds are extremely popular among fish growers due in part to ease of construction, low maintenance, relatively small area requirements and ability to grow a wide variety of species. Additionally, because earthen ponds mimic nature, they may produce fish of an overall healthier appearance than other techniques. A natural supply of food is often available in earthen ponds, which may lead to better fish health. Pond culture can vary, from all life stages naturally occurring in a single pond to elaborate systems with discrete ponds for holding broodstock, spawning, rearing, growing and catch‐out or harvest.

      1.2.1.2 Tanks

      Tanks essentially act as ponds but are generally constructed of concrete or fiberglass. Wood can also be used but must first be treated to prevent rotting. Concrete tanks have the advantage of being less expensive, easily constructed and formed into various shapes.

       Plastic, fiberglass or glass tanks are ordinary tanks which are either designed and constructed for rearing fish or used for storing water. They can be moved from one place to another.

       Wooden troughs are constructed with planks. They vary in size and depth. After construction, the tank is lined with nylon to prevent leakage. It can be moved from one place to other.

      1.2.2 Semi‐Intensive Culture System

      The semi‐intensive culture system is distinguished by increased stocking rates and the requirement for some level of input, such as food, fertilizer, chemicals, etc.

       Cage culture uses existing water resources (i.e. lakes or ponds) but encloses the fish in a cage or basket, which allows water to pass freely between the fish and the pond or lake. One of the main advantages is the ease of harvesting.

       Fish pens are enclosures installed in open waters that are not too deep for raising fish. The fish have access to the water bottom, unlike in cages.

       Rafts/trays or long lines are mostly used for oyster culture.

       Some improvised containers such as bathroom tubs, big plastic containers, abandoned plank canoe, earthen pots are used to culture fish where the farmer cannot afford the conventional rearing facilities. Some fish farmers in Lagos use these containers for breeding. Apart from breeding purposes, this type of rearing facility is not suitable for commercial fish farming.

      1.2.3 Intensive Culture System

      Intensive culture system is where animals are maintained in systems such as ponds, tanks and raceways, where the support parameters are carefully controlled and dependence on the natural environment is minimal. Such systems require a high degree of management and usually involve substantial investment and operating costs, resulting in high yields per unit area. They typically feature recirculating systems, which filter and reuse all or a portion of their water. Currently, this technique is in limited use due to its high start‐up and maintenance costs. It has several advantages, however, in that this system is highly desirable in areas where a constant water supply may be questionable or a discharge is not appropriate. Other advantages of this type of system include the ability to incorporate the growth of a second product through hydroponics and the use of settled material as fertilizer. As the technology evolves and water withdrawal and discharge requirements become more stringent, this technique is bound to see an increase in use. Ponds are also very popular for housing ornamental fish, such as koi or goldfish. Typically lined with EPDM rubber, these can be fairly simple in having only a recirculating system and waterfall or extremely complex, with sophisticated filtration and water conditioning systems.

1.3 Physical Features of Life‐Support Systems

      Multiple designs from simple box filters to multistage proprietary units can be found in use for home aquaria. For freshwater systems, these typically include a mechanical filter to remove particulate and a biofilter placed afterwards in the return water flow. Often an ultraviolet (UV) light unit is installed to control planktonic algae. In saltwater systems, a protein skimmer is also used to remove dissolved organic matter. Adequate gas exchange is generally assured by the simple water turnover provided by the filter system, although additional aeration is often provided with the use of air pumps and air stones. Ponds will usually have the same components as freshwater aquaria, scaled up to meet the needs of larger fish. The most modern designs will typically feature a prefilter, mechanical filter, biofilter and UV light. Water is returned to the pond via a spillway or waterfall, which assures adequate gas exchange. Designs for these components vary, with some rudimentary systems having only a biofilter and waterfall. As this system relies on in‐pond sedimentation, regular depopulation and cleanout is needed. Public aquaria differ only by size and scale from home aquarium systems. However, some large‐scale water purification equipment is described below. Some examples of aquaculture open and closed husbandry systems (raceways, recirculating, ponds, net pens) are given in Chapter 3. Different systems for different life stages are described in Chapter 10.

      1.3.1 Mechanical, Chemical and Other Types of Filtration

Recirculating systems can vary in complexity and design. Depending on the water source, varying types of water purification may be required or recommended to prevent pathogens at the point of entry as well as within the system itself. Mechanical filters are often used to improve water quality and reduce pathogen loads by removing particulate matter in the system. Recirculating systems that are considered self‐cleaning remove waste solids and provide some water purification prior to system reentry. Mechanical filter design based on water flow direction can be described as “downflow”, where the water to be processed passes by gravity or under pressure down through the media or “upflow”, where it is forced under pressure up through the media. The advantage of this upflow design is to allow sediment to fall away from the media, making it somewhat self‐cleaning (Tepper, 2000). Media used may be sand, polypropylene or fiberglass mesh or polypropylene brushes in downflow systems and polypropylene beads in upflow systems.

      Prefilters can be used to separate settleable solids by rotating the water in an upflow direction. This strips off the heavier particulate, enabling more efficient water processing before entering the mechanical filter.

      Industrial aquaculture facilities may use self‐cleaning tanks in recirculating systems to remove waste solids and provide some water purification using large rotating drum filters. Also, water sources with high total solids prior to system entry may also require large screen filtration to remove solids. Other large facilities may employ pressurized canisters of upflow design for this purpose (floating bead filters). These canisters are also often seen in home pond filtration, although downflow box filters with filter mesh for particulate removal are also used, where they are the most common type of filter for small aquasystems like freshwater and marine home aquaria. Sand canister filters are sometimes used due to their high efficiency at removing solids and low cost. They have the disadvantage of being hard to maintain over time due to “caking” of the sand media.