7.4.1 Tilapia

Main commercial types:
Blue tilapia (Oreochromis aureus)
Nile tilapia (Oreochromis niloticus)
Mozambique tilapia (Oreochromis mossambicus)
Various hybrids combining these three species.

 

FIGURE 7.6
Line drawing and photograph of a Nile tilapia (Oreochromis niloticus)

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, Az olla 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.