Nile tilapia

Subspecies

Although FishBase considers the species as monotypic,^[2] several distinctive populations often are recognized as valid subspecies:^[1][8][10]

• O. n. niloticus (Linnaeus, 1758) – most of species' range
• O. n. baringoensis Trewavas, 1983 – Lake Baringo in Kenya
• O. n. cancellatus (Nichols, 1923) – Awash basin in Ethiopia
• O. n. eduardianus (Boulenger, 1912) – Albertine Rift Valley lakes
• O. n. filoa Trewavas, 1983 – hot springs in Awash basin in Ethiopia
• O. n. sugutae Trewavas, 1983 – Karpeddo soda springs at Suguta River in Kenya
• O. n. tana Seyoum & Kornfield, 1992 – Lake Tana in Ethiopia
• O. n. vulcani Trewavas, 1933 – Lake Turkana in Ethiopia and Kenya

While the species is overall very widespread and common, the IUCN considers O. n. baringoensis as endangered, O. n. sugutae as vulnerable, and O. n. filoa as data deficient.^[1]

A population found in Lake Bogoria appears to be an undescribed subspecies.^[8]

The forms referred to as Oreochromis (or Tilapia) nyabikere and kabagole seem to belong to this species, too. An undescribed population found at, for example, Wami River, Lake Manyara, and Tingaylanda seems to be a close relative.^[11]

Feeding

The Nile tilapia is mostly a herbivore, but with omnivorous tendencies, especially when young.^[2] They mostly feed on phytoplankton and algae, and in some populations other macrophytes also are important.^[1] Other recorded food items are detritus and aquatic insect larvae,^[2] including those of mosquitoes, making it a possible tool in the fight against malaria in Africa.^[12] However, when introduced outside its native range, it often becomes invasive, threatening more localized species.^[2]

The Nile tilapia typically feeds during daytime, which suggests that, similar to trout and salmon, it exhibits a behavioral response to light as a main factor contributing to feeding activity. Due to its 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 daylight. A recent study found evidence that, contrary to popular belief, size dimorphism between the sexes results from differential food conversion efficiency rather than different 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 body weight.^[13]

Social organization

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.^[14] Like other fish, Nile tilapia travel 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.^{[citation needed]}

Like other tilapias, such as Mozambique tilapia, dominance between the males is established first through noncontact displays such as lateral display and tail beats. Unsuccessful attempts to reconcile the hierarchy results in contact fighting to inflict injuries. Nile tilapia have 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.^[15] 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.^[13]

Reproduction

Typical of most fish, Nile tilapia reproduce 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 in 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.^[16] 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.^[17]

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.^[18] For males, reproductive advantage goes to the more dominant males. 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.^[19] Furthermore, visual communication between Nile tilapia mates both stimulates and modulates reproductive behavior between partners such as courtship, spawning frequency, and nest building.^[14]

Parental care

Species belonging to the genus Oreochromis typically care for their young through mouthbrooding, oral incubation of the eggs and larvae. Similar to other tilapia, Nile tilapia are maternal mouthbrooders and extensive care is, therefore, provided almost exclusively by the female. After spawning in a nest made by a male, the young fry or eggs are carried in the mouth of the mother for a period of 12 days. Sometimes, the mother pushes the young back into her mouth if she believes they are not ready for the outside. Nile tilapias also demonstrate parental care in times of danger. When approached by a danger, the young often swim back into the protection of their mother's mouth.^[20] However, mouthbrooding leads to significant metabolic modifications for the parents, usually the mother, as reflected by fluctuations in body weight and low fitness. Thus, parental-offspring conflict can be observed through the costs and benefits of mouthbrooding. Protection of the young ensures passage of an individual's genes into the future generations, but caring for the young also reduces an individual's own reproductive fitness.^[17]

Since female Nile tilapia exhibiting parental care show extended interspawning periods, one of the benefits is slowing down vitellogenesis (yolk deposition) to increase the survival rate of one's own young. The size of spawned eggs correlates directly with advantages concerning hatching time, growth, survival, and onset of feeding, since increased egg size means increased nutrients for the developing young. Thus, one of the reasons behind a delayed interspawning period by female Nile tilapia may be for the benefit of offspring survival.^[18][21]