Fish aggression
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Aggression refers to agonistic behaviors characterized by threats and physical force. 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.
Aggressive behavior comes in two forms: threat displays and attacks.[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. While displays are less risky than attacks, they still come with a cost. Many fish will make their heads look larger by pushing out their opercula, but this also makes it difficult to breathe. Aggressive behaviors might invite the attention of predators,[4] and time spent challenging a competitor means time lost foraging, courting, or caring for young. Therefore, some fish will choose to end an aggressive encounter immediately by signaling submission.[3]
Fish territories are defended areas 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. For example, the three-spine stickleback (Gasterosteus aculeatus) is polygynous, with males preferring two or more female mates in their 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, 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.
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 is attracted to another male, she can dump the previous partner without any hesitation. In this situation, the ditched male fish becomes 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 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.
Territorial aggression can take place not only due to the pressure of mating, producing offspring, or intruders, but also from light intensity. The term intensity is used to describe the rate at which light spreads over a surface of a given area some distance from a source. 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). At the beginning, S. salar were exposed to four different nighttime light intensities (0.00, 0.01, 0.50 and 1.00 lx) for 24 hours. From the result, it was concluded that fish showed less aggression when the intensity of the light was lower. 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. So they decreased their territory size and remained closer to each other rather than attacking. This was a good example of evolution of cooperation among the fish. Additionally, as the light intensities of the light increased, those fish could see each other to defend their space. The territory size increases during bright lights. In short, the aggressive behavior of the salmon toward their rivals is highly manipulated by light intensities. Thus, the size of the space that the fish is defending increases or decreases between day and night.[5]