Methods of aggression in fish vary widely by species, but some common examples are chasing, charging, biting, fin display, color changes, and flared gills.
[1] Aggression is an important evolutionary pressure that increases an individual's access to resources while reducing overall conflict within the social group.
[2] Fish use aggressive behaviors to defend a territory, establish dominance, appeal to potential mates, and protect their young.
[1] Attacking is how two competitors can directly compare their strength by biting or head-bumping, but it comes with significant drawbacks: it's energetically costly, time consuming, and risking bodily injury.
A threat display, on the other hand, allows competitors to assess the other's strength indirectly, making it a safer way to settle conspecific conflict.
[3] During a confrontation, threat displays communicate the factors that would tip a physical fight, such as body-size, dominance status, motivation, and territoriality.
So, characteristics like a long 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.
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 eggs in large territories.
At lower light levels, the risk of losing resources like food and mates gives rise to aggressive behavior among the fish belonging to a territory.
Additionally, for further understanding on how the rate of aggression and distance among neighboring fish varies with nighttime light intensities in the same area, Sveinn Valdimarsson and Neil Metcalfe conducted an experiment with the juveniles of Atlantic salmon (Salmo salar).
This is because when the level of light was intense, due to darkness of night the territorial fish failed to detect their food or other members in the same area.
S. Josefin Dahlbom and colleagues experimented on zebra fish (Danio rerio) to study the difference in aggression level between males and females if they are put together under similar environment.
To see gender-related changes in aggression, they tested if intersexual agonistic events take place between isolated males and females, who were not previously paired to each other as breeding partners.
In short, various forces affecting each sex can result in different aggressive behaviors among male and female fish.
To examine the relationship between aggression and genetic makeup, a team from CNRS/Laboratoire Neurobiologie et Développement conducted an experiment on zebra fish.
In another experiment Katrina Tiira performed a different test on land-locked Atlantic salmon (Salmo salar) to see if juveniles with low estimated genetic diversity showed less aggression.
In conclusion, it can be explained that genetic variation in salmon can manipulate the agnostic syndrome if the individuals in a group are closely related.
In summary, growth hormone plays a vital role in controlling aggression in rainbow trout and other fish.