Fish aggression

Aggression refers to negative behavior or attitudes toward another, mainly by applying physical force. Evolution on the other hand, is any process of formation or development of something like habit, trait or character in a population from generation to generation. Evolution can explain why fish exhibit aggression because it is a simple emotion. This emotion increases an individual’s survival or reproduction. Aggressive behavior can derive in fish species due to territory, sex specific selection and genetic variation. There is no specific fish species who display invasive behavior. Almost all the fish are aggressive sometimes in their lives depending on their surroundings.

Fish territory, or in another word, defended areas are generally ruled by a single individual or by breeding pairs. The guarded resource may include food, shelter, sexual partner or offspring. While protecting their regions, fish often display aggressive behavior against their intruders. The territory owner strikes at competing fish directly ending in a bite, or a bump. Such aggressive behavior is seen in large juveniles, females and other fish of the same kind from the same area. To better understand this topic, consider the male three-spine stickleback, Gasterosteus aculeatus, as an example. Theo Bakker's paper claims the three-spine stickleback male fish is polygynous—meaning they prefer two or more female mates in a territory. Thus, males are highly aggressive so they have access to females in a particular territory, which also leads to intrasexual selection among males. Intrasexual selection is selection within the same sex. For instance, some male animals compete against one another, physically, for access to females for their kind. So, characters like big tail, sharp teeth or similar weaponry that can be used against other males of the same species as means of mating with females is a selective advantage.

There is a deep relationship between the aggression in fish and the size of the regions owned by them. In a smaller territory, female fish often disappear before mating. Female fish are not bound to mate with a particular fish. If by chance a female get attracted to another male, she can dump the previous partner without any hesitation. In this situation, the ditched male fish become more aggressive to find mates in order to reproduce. Their levels of aggression increase more when the rates of sneaking by rival males go up too. The sneaking males enter the nest and release their own sperm over the eggs of the breeding fish. The rival fish here are using alternative reproduction methods like parental, sneaker or satellite to avoid being hurt by breeding males. On the other hand, the breeding males have higher mating success and endure less looting of the eggs in large territories. Once the eggs have been gathered, the breeding males decrease their territories to protect their offspring from the predators during their parental phase. After the eggs have hatched, the males’ continue to show similar or more invasive behavior due to increased reproductive value of offspring and the awareness of newly hatched young fish against enemies.

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