Fish selection
Several fish species have recorded excellent growth rates in aquaponic units. Fish species suitable for aquaponic farming include: tilapia, common carp, silver carp, grass carp, barramundi, jade perch, catfish, trout, salmon, Murray cod, and largemouth bass. Some of these species, which are available worldwide, grow particularly well in aquaponic units and are discussed in more detail in the following sections. In planning an aquaponic facility it is critical to appreciate the importance of the availability of healthy fish from reputable local suppliers.
Some cultured fish have been introduced to areas outside of their natural habitat, such as tilapia and a number of carp and catfish species. Many of these introductions have been through aquaculture. It is also important to be aware of local regulations governing the importation of any new species. Exotic (i.e. non-native) species should never be released into local bodies of water. Local extension agents should be contacted for more information regarding invasive species and native species suitable for farming.
Tilapia
Main commercial types:
Blue tilapia (Oreochromis aureus)
Nile tilapia (Oreochromis niloticus)
Mozambique tilapia (Oreochromis mossambicus)
Various hybrids combining these three species.
Description
Native to East Africa, tilapias are one of the most popular freshwater species to grow in aquaculture systems worldwide (Figure 7.6). They are resistant to many pathogens and parasites and handling stress. They can tolerate a wide range of water quality conditions and do best in warm temperatures. Although tilapias briefly tolerate water temperatures extremes of 14 and 36 °C, they do not feed or grow below 17 °C, and they die below 12 °C. The ideal range is 27-30 °C, which ensures good growth rates. Therefore, in temperate climates, tilapias may not be appropriate for winter seasons unless the water is heated. An alternate method for cool climates is to grow multiple species throughout the year, rearing tilapias during the warmest seasons and switching to carp or trout during the winter. In ideal conditions, tilapias can grow from fingerling size (50 g) to maturity (500 g) in about 6 months.
Tilapias are omnivores, meaning they eat both plant- and animal-based feed. Tilapias are candidates for many alternative feeds, discussed in Section 9.1.2. Tilapias have been fed duckweed, Azolla spp*.*, Moringa olifera and other high-protein plants, but care must be used to ensure a whole feed (i.e. nutritionally complete). Tilapias eat other fish, especially their own young; when breeding, the tilapia should be separated by size. Tilapias less than 15 cm eat smaller fish, though when larger than 15 cm they are generally too slow and cease to be a problem.
Tilapias are easy to breed in small-scale and medium-scale aquaponic systems. More information is available in the section on Further Reading, but a brief discussion is outlined below. One method is to use a large aquaponic system for the grow-out stage. Two smaller separate fish tanks can then be used to house the broodstock and juveniles. Small separate aquaponic systems can be used to manage the water quality in these two tanks, but may not be necessary with a low stocking density. Broodstock fish are hand-selected adults that are not harvested, and they are chosen as healthy specimens for breeding. Tilapias breed readily, especially where the water is warm, oxygenated, algae-filled and shaded, and in a calm and quiet environment. Rocky substrate on the bottom encourages nest building. The optimal ratio of males to females also encourages breeding; often, 2 males are paired with 6-10 females to initiate spawning. Tilapia eggs and fry are seen either in the mouths of the females or swimming on the surface. These fry can be transferred into juvenile rearing tanks, ensuring that no larger fingerlings are present that will eat them, and grown until they are large enough to enter the main culture tanks.
Tilapias can be aggressive, especially in low densities, because males are territorial. Therefore, the fish should be kept at high densities in the grow-out tanks. Some farms only use male fish in the grow-out tanks; all male cultures of the same age grow larger and faster, because males do not divert energy in developing ovaries and do not stop feeding when spawning eggs as females do. Moreover, the growth rate in all-male tanks is not reduced by competition for food from fry and fingerlings, which are continuously produced if sexually mature males and females are left growing together. Monosex male tilapia can be obtained through hormone treatment or hand sexing of fingerlings. In the first case, fry are fed a testosterone-enriched feed during their first three weeks of life. High levels of the hormone in the blood cause a sex reversal in female fry. This technique, widely used in Asia and America but not in Europe (owing to different regulations), allows farmers to stock same-size male tilapia in ponds in order to avoid any problems of spawning and growth depression by feed competition from newer juveniles.
Hand sexing simply consists of separating males from females by looking at their genital papilla when fish are about 40 g or larger. The process of identification is quite straightforward. In the vent region the males have only a single opening whereas females have two slits. The vent of the female is more “C” shaped, while in males the papilla is more triangular. As the fish grow larger, secondary characteristics can help identify males from females. Male fish have larger heads with a more pronounced forehead region, a humped back and more squared-off features. Females are sleeker and have smaller heads. Moreover, the fish’s behaviour can indicate the sex because males chase other males away and then court the females. Hand-sexing can be performed with small numbers of fish, as it does not take much time. However, this technique may not be practical in large-scale systems owing to the large numbers of fish being cultured. Nevertheless, mixed-sex tilapia can be reared in tanks until fish reach sexual maturity at the age of five months. Although females are relatively underperforming, they still do not cause problems with spawning and can be harvested at an earlier stage (200 g or more), leaving the males to grow further.
Carp
Main commercial types:
Common carp (Cyprinus carpio)
Silver carp (Hypophthalmichthys molitrix)
Grass carp (Ctenopharyngodon idella)
Description
Native to eastern Europe and Asia, carps are currently the most cultured fish species globally (Figure 7.7). Carp, like tilapia, are tolerant to relatively low DO levels and poor water quality, but they have a much larger tolerance range for water temperature. Carp can survive at temperatures as low as 4 °C and as high as 34 °C making them an ideal selection for aquaponics in both temperate and tropical regions. Best growth rates are obtained when temperatures are between 25 °C and 30 °C. In these conditions, they can grow from fingerling to harvest size (500-600 g) in less than a year (10 months). Growth rates dramatically decrease with temperatures below 12 °C. Male carp are smaller than females, yet can still grow up to 40 kg and 1-1.2 m in length in the wild.
In the wild, carps are bottom-feeding omnivores that eat a large range of foods. They have a preference for feeding on invertebrates such as water insects, insect larvae, worms, molluscs and zooplankton. Some herbivorous carp species also eat the stalks, leaves and seeds of aquatic and terrestrial plants, as well as decaying vegetation. Cultured carp can be easily trained to eat floating pellet feed.
Carp fingerlings are best obtained from hatcheries and dedicated breeding facilities. The procedure to obtain juveniles is more complicated than tilapia because spawning in female carps is induced by hormone injection, a technique requiring additional knowledge of fish physiology and experience.
Carps can easily be polycultured and this has been done for centuries. It mainly consists in culturing herbivorous fish (grass carp), planktivorous fish (silver carp) and omnivorous/detritivorous fish (common carp) together in order to cover all the food niches. In aquaponics, the combination of these three species, or at least grass carp with common carp, would result in a better use of food, as the former would feed on both pellet and crop residues while the latter would also seek for wastes accumulating at the bottom of the tank. The supply of roots, among other crop residues, would be also extremely beneficial to the nutrient pool in the aquaponic system, because their digestion by the fish and the successive waste mineralization would return most of the micronutrients back to the plants.
Other carp species (ornamental fish)
Gold or Koi carps are mainly produced for the ornamental fish industry rather than food fish (Figure 7.8). These fish also have a high tolerance to a variety of water conditions and therefore are good candidates for an aquaponic system. They can be sold to individuals and aquarium stores for considerably more money than fish sold as food. Koi carps and other ornamental fish are a popular choice for vegetarian aquaponic growers.
Beyond the climatic characteristics and fish management issues, the choice of a carp species to be cultured in aquaponics should follow a cost-benefit analysis that takes into account the convenience in culturing a fish that is bonier and generally fetches lower market prices than other species.
Catfish
Main commercial types:
Channel catfish (Ictalurus punctatus)
African catfish (Clarias gariepinus)
Description
Catfish are an extremely hardy group of fish tolerating wide swings in DO, temperature and pH (Figure 7.9). They are also resistant to many diseases and parasites, making them ideal for aquaculture. Catfish can be easily stocked at very high densities, up to 150 kg/m3. These stocking densities require comprehensive mechanical filtration and solids removal beyond that discussed in this publication. The African catfish is one of many species in the Clariidae family. These species are air breathers, making them ideal for aquaculture and aquaponics as a sudden and dramatic drop in DO would not result in any fish mortalities. Catfish are the easiest species for beginners or for aquaponists who want to grow fish in areas where the supply of electricity is not reliable. Given the high tolerance to low DO levels and high ammonia levels, catfish can be stocked at higher densities, provided there is adequate mechanical filtration. Regarding waste management, it is worth noting that suspended solid waste produced by catfish is less voluminous and more dissolved than that of tilapia, a factor that facilitates greater mineralization. Like tilapia, catfish grow best in warm water and prefer a temperature of 26 °C; but in the case of African catfish growth stops below 20-22 °C. The physiology of catfish is different from other fish, as they can tolerate high levels of ammonia, but, according to recent literature, nitrate concentrations of more than 100 mg/litre may reduce their appetite due to an internal regulatory control trigged by high levels of nitrate in their blood.
Catfish are benthic fish, meaning they occupy only the bottom portion of the tank. This can cause difficulties in raising them at high densities because they do not spread out through the water column. In overcrowded tanks, catfish can hurt each other with their spines. When raising catfish, one option is to use a tank with greater horizontal space than vertical space, thereby allowing the fish to spread out along the bottom. Alternatively, many farmers raise catfish with another species of fish that utilize the upper portion of the tank, commonly bluegill sunfish, perch or tilapia. Catfish can be trained to eat floating pellets.
Trout
Main commercial type:
Rainbow trout (Oncorhynchus mykiss)
Description
Trout are carnivorous cold-water fish that belong to the salmon family (Figure 7.10). All trout require colder water than the other species previously mentioned, preferring 10-18 °C with an optimum temperature of 15 °C. Trout are ideal for aquaponics in Nordic or temperate climate regions, especially in winter. Growth rates significantly decrease as temperatures increase above 21 °C; above this temperature trout may not be able to properly utilize DO even if available. Trout require a high protein diet compared with carp and tilapia meaning greater amounts of nitrogen in the overall nutrient pool per unit of fish feed added. This occurrence allows for more cultivable areas of leafy vegetables while maintaining a balanced aquaponic unit. Trout have a very high tolerance to salinity, and many varieties can survive in freshwater, brackish water and marine environments. Overall, trout require better water quality than tilapia or carp, particularly with regard to DO and ammonia. Successful aquaculture of trout also requires frequent water quality monitoring as well as backup systems for air and water pumps.
Rainbow trout is the most common trout species grown in aquaculture systems in the United States of America and Canada and in sea cages or flow-through tanks and ponds in central or northern Europe (Norway, Scotland [the United Kingdom]), in parts of South America (Chile, Peru), in many upland areas in tropical and subtropical Africa and Asia (Islamic Republic of Iran, Nepal, Japan) and Australia. Rainbow trout are long, thin and scale-less fish, usually blue-green and spotted on top with a red stripe on the sides. Trout are also cultured and released into streams and lakes to supplement sport fishing.
Trout require a high-protein diet with substantial amount of fats. Trout are considered an “oily fish”, a nutritional description indicating a high amount of vitamin A, vitamin D and omega-3 fatty acid, making them an excellent choice to grow for domestic consumption. Trout command higher prices in some markets for the same reason, but they require diets comparatively rich in fish oil.
Largemouth bass
Main commercial type:
Largemouth bass (Micropterus salmoides)
Description
Largemouth bass are native to North America but are widely spread throughout the world, occurring in many water bodies and ponds (Figure 7.11). They belong to the order Perciformes (perch-like fish) which also includes striped bass, Australian bass, the black sea bass, the European sea bass and many others.
Largemouth bass tolerates a wide temperature range as growth will only cease at less than 10 °C or more than 36 °C; they will stop feeding at temperatures less than 10 °C. The optimal growth temperatures are in the range of 24-30 °C for all fish stages. They tolerate low DO and pH, although for a good FCR the optimal DO is above 4 mg/litre.
Largemouth bass prefer clean water with a concentration of suspended solids less than 25 mg/litre, yet growth has been observed in ponds with turbidity as high as 100 mg/ litre. As with trout, largemouth bass are carnivorous fish, demanding high protein diets; thus size cohorts should be separated to prevent the consumption of fry and very small juveniles by larger fish. Growth rates are highly dependent on temperature and quality of feed; in temperate climates most of the growth is obtained during the warmer seasons (late spring, summer and early fall). Given their high tolerance to DO as well as good resistance to high nitrite levels, largemouth bass are an excellent choice for aquaponic farmers, particularly for those who cannot change species between cold and warm seasons. Attempts have been carried out to culture this species in polyculture with tilapia. Nutritionally speaking, largemouth bass contain relatively high levels of omega-3 fatty acids compared with other freshwater fish.
Prawns
Main commercial type:
Giant river prawn (Macrobrachium rosenbergii)
Description
The term prawn refers to a very diverse group of stalk-eyed freshwater decapod crustaceans with long narrow muscular abdomens, long antenna and slender legs (Figure 7.12). They can be found feeding on the bottom of most coastlines and estuaries, as well as in freshwater systems. They usually live from one to seven years, and most species are omnivores. Shrimp and prawns, respectively, commonly refer to saltwater and freshwater species, although these names are often confused, especially in the culinary sense.
Prawns can be a great addition to an aquaponic system. They consume uneaten fish food, fish waste and whatever organic material they find in the water or on the bottom. As such, they help to clean and support system health, and accelerate organic material decomposition. It is better to grow prawns and mid-water fish simultaneously in an aquaponic system, as prawns cannot be grown in high enough densities to produce adequate wastes for the plants. Prawns are very territorial, so they need a substantial allocation of lateral space; the horizontal surface area determines the number of individuals that can be raised, although stacked layers of netting can increase surface area and increase quantity. Some polyculture systems with tilapia have been tested with various degree of success, although the number of individual that can be stocked is low. Most prawns have similar needs, which include hard water, warm temperatures (24-31 C°) and good water quality, but the conditions should be adjusted for the particular species grown.
In ideal conditions, prawns have a four-month growing cycle, meaning it is theoretically possible to grow three crops annually. Prawn post-larvae need to be purchased from a hatchery. The larval cycle of prawns is fairly complex, requiring carefully monitored water quality and special feed. Although possible on a small-scale, breeding prawns is only recommended for experts. Because they can eat the roots of the plants, prawns should be grown in the fish tanks only.
Source: Food and Agriculture Organization of the United Nations, 2014, Christopher Somerville, Moti Cohen, Edoardo Pantanella, Austin Stankus and Alessandro Lovatelli, Small-scale aquaponic food production, http://www.fao.org/3/a-i4021e.pdf. Reproduced with permission.