Only a tiny percentage of fish reach adulthood; most either starve or become food themselves.Those that survive this long do so by a combination of good luck and good genes. Having reached sexual maturity, they now have the chance to pass on those successful genes to their own offspring.
When does a juvenile fish become an adult?
The transition from juvenile to adult comes when the fish reaches sexual maturity. Its appearance changes; males and females become easier to tell apart and often advertise their maturity through color patterns. Breeding adults are often far more colorful than juveniles, and adult males are very often more colorful than adult females. But as adulthood can change the way that fishes look, it also changes the way they behave. As juveniles, fishes' main concerns were to find food and avoid predators; once they reach adulthood, reproduction goes straight to the top of their list. As a result, fish may often become more aggressive as they seek to stake their claim, especially with members of their own gender who are now competitors for breeding opportunities. They may also start to defend territories and attempt to court potential mates.
What changes when a fish becomes an adult?
As juveniles, fish use most of their energy to grow. Once they reach adulthood this energy is switched to producing eggs or sperm and growth slows. Female livebearers, such as swordtails (Xiphophorus helleri), can reach considerable sizes if they remain unmated. If they start to reproduce as soon as they become mature, they may only reach half the size. Larger females produce more offspring, so it can be beneficial for an individual to delay breeding. Alternatively, if there are many predators in the environment, prey fish sometimes respond by breeding at younger ages.This phenomenon is seen in guppy populations: those that live alongside pike cichlids and blue acaras reach maturity considerably earlier than those that live in predator-free populations.
Do fish ever stop growing?
Fish continue to grow throughout their lives, although their rate of growth slows with every passing year. It is for this reason that biologists are able to determine the age of fish by counting rings on the scales or, for greater accuracy, doing the same using the fish's earbone, the otolith. A fish's age can also be estimated from its appearance: as fish get older, their muscles tighten and this can be seen in the way that superannuated fish often have characteristic curved spines.
What is old for a fish?
Fish in the wild seldom die of old age.This may be because they have been caught by predators -- an ever-present risk -- or parasites or simply that the wear and tear of everyday life -- raising young and fighting rivals -- has weakened them, perhaps fatally. For example, an extensive study on cichlids in Lake Tanganyika showed their average age to be three years and the oldest to be 10. However, there are records of these same species reaching over 20 years of age in the aquarium. The age at which fish can be considered old varies from species to species. Fish such as guppies and African killifish would be lucky to attain 12 months, whereas some of the larger catfish and cypnnids can continue for two or three decades.