Nile tilapia

The Nile tilapia (Oreochromis niloticus) is a species of tilapia, a cichlid fish native to Africa from Egypt south to east and central Africa, and as far west as Gambia. It is also native to Israel, and numerous introduced populations exist outside its natural range (e.g., Brazil). It is also commercially known as mango fish, nilotica, or boulti. The first name leads to easy confusion with another tilapia traded commercially, the mango tilapia (Sarotherodon galilaeus). The Nile tilapia has distinctive, regular, vertical stripes extending as far down the body as the bottom edge of the caudal fin, with variable coloration. Adults reach up to 60 cm (24 in) in length and up to 4.3 kg (9.5 lb). It lives for up to nine years. It tolerates brackish water and survives temperatures between 8 and 42 °C (46 and 108 °F). It is an omnivore, feeding on plankton as well as on higher plants. Introduced tilapia can easily become an invasive species (see Tilapia as exotic species). It is a species of high economic value and is widely introduced outside its natural range; probably next to the Mozambique tilapia (O. mossambicus), it is the most commonly cultured cichlid. In recent research done in Kenya, this fish has been shown to feed on mosquito larvae, making it a possible tool in the fight against malaria in Africa. The Nile tilapia is an omnivore that feeds on both plankton and aquatic plants. It generally feeds in shallow waters, as harmful gases (such as carbon dioxide, hydrogen sulfide, and ammonia) and temperature fluctuations found in deep waters create problems for the physiology of the fish. The Nile tilapia thrives on the warmer temperatures commonly found in shallow waters compared to the colder environment of the deep lake. In general, tilapias are macrophyte-feeders, feeding on a diverse range of filamentous algae and plankton. The Nile tilapia typically feeds during daytime hours. This suggests that, similar to trout and salmon, it exhibits a behavioral response to light as a main factor contributing to feeding activity. Due to their fast reproductive rate, however, overpopulation often results within groups of Nile tilapia. To obtain the necessary nutrients, night feeding may also occur due to competition for food during the daylight hours. A recent study found evidence that, contrary to popular belief, size dimorphism between the sexes results from differential food conversion efficiency rather than differential amounts of food consumed. Hence, although males and females eat equal amounts of food, males tend to grow larger due to a higher efficiency of converting food to energy. Groups of Nile tilapia establish social hierarchies in which the dominant males have priority for both food and mating. Circular nests are built predominantly by males through mouth digging to become future spawning sites. These nests often become sites of intense courtship rituals and parental care. Like other fish, the Nile tilapia travels almost exclusively in schools. Although males settle down in their crafted nesting zones, females travel between zones to find mates, resulting in competition between the males for females. Like other tilapias, such as Mozambique tilapia, dominance between the males is established first through non-contact displays such as lateral display and tail beats. Unsuccessful attempts to reconcile the hierarchy results in contact fighting to inflict injuries. Nile tilapia has been observed to modify their fighting behavior based upon experiences during development. Thus, experience in a certain form of agonistic behavior results in differential aggressiveness among individuals. Once the social hierarchy is established within a group, the dominant males enjoy the benefits of both increased access to food and an increased number of mates. However, social interactions between males in the presence of females results in higher energy expenditures as a consequence of courtship displays and sexual competition. Typical of most fish, the Nile tilapia reproduces through mass spawning of a brood within a nest made by the male. In such an arrangement, territoriality and sexual competition amongst the males lead to large variations in reproductive success for individuals among a group. The genetic consequence of such behavior is reduced genetic variability in the long run, as inbreeding is likely to occur among different generations due to differential male reproductive success. Perhaps driven by reproductive competition, tilapias reproduce within a few months after birth. The relatively young age of sexual maturation within Nile tilapia leads to high birth and turnover rates. Consequently, the rapid reproductive rate of individuals can actually have a negative impact on growth rate, leading to the appearance of stunted tilapia as a result of a reduction in somatic growth in favor of sexual maturation. Female Nile tilapia, in the presence of other females either visually or chemically, exhibit shortened interspawning intervals. Although parental investment by a female extends the interspawning period, female tilapia that abandon their young to the care of a male gain this advantage of increased interspawning periods. One of the possible purposes behind this mechanism is to increase the reproductive advantage of females that do not have to care for young, allowing them more opportunities to spawn. For males, reproductive advantage goes to the more dominant males. Studies have found that males have differential levels of gonadotropic hormones responsible for spermatogenesis, with dominant males having higher levels of the hormone. Thus, selection has favored larger sperm production with more successful males. Similarly, dominant males have both the best territory in terms of resources and the greatest access to mates. Furthermore, visual communication between Nile tilapia mates both stimulates and modulates reproductive behavior between partners such as courtship, spawning frequency, and nest building.