[Smart Farmers] Do fish have tempers? A brief analysis of fish aggression

What is fish aggression ?

Aggressive behavior refers to the adaptive behavior exhibited by individuals of the same species in competition for resources such as space or food. Like other animals, aggressive behavior is an innate and significant characteristic of fish.

The aggressive behavior of fish is usually divided into two stages: the first stage is mainly mutual evaluation, in which fish determine the opponent's attack potential through approach, display, circling and other behaviors, until the first demonstration or avoidance behavior occurs, marking the end of the first stage; in the second stage, all aggressive behaviors (such as demonstration, pecking and chasing) are initiated by the "winner", while the "loser" only shows submission and avoidance behaviors.

Figure 1. Betta fish attack behavior (Photo from the WeChat account of the Aquaculture and Equipment Engineering Research Center)

What are the factors that influence fish aggressive behavior?

1. Ecological factors

Environmental enrichment: Environmental enrichment can provide a visual barrier for fish, making it difficult for attacking individuals to quickly find their targets, thus improving the fish's defense capabilities. At the same time, it can provide cover for the attacked objects, limit the range of activities of attacking individuals, and reduce the fish's basic stress level.

Competition for food: Food is an important resource for fish. Uneven food distribution can lead to increased aggressive behavior in fish and even cause physical damage and increased mortality. Even when food supply is sufficient, competition for food will occur, exacerbating the occurrence of aggressive behavior.

Breeding process: Fish will attack to increase their chances of getting a mate and improve their mating success rate. After courtship fails, fish will attack their mate. For example, during the breeding process of the redtail crown fish (Aequidens rivulatus), strong aggressive behavior indicates a failed mate selection (Figure 2).

Figure 2. Aggression behavior of redtail crownfish (Image from Li et al. 2022. [1])

2. Individual Differences

Species/sex: While aggression is common, there is great variation in the “fighting power” between species, with pelagic fish being less aggressive, for example, while aggression is more common and intense in spatially confined coral reef fish, such as Amphiprion ocellaris (Figure 3). Within a given species, the intensity of aggression also varies between the sexes, with males generally being more aggressive than females.

Figure 3. The attacking behavior of the eye-spotted double-sawfish (picture from the Internet)

Personality: Even within individuals of the same species, stock, sex, age group, and size, some fish are more prone to aggressive behavior than others, a reflection of the different personalities of fish. Aggression in fish is often closely associated with personality traits such as boldness and exploratory nature, and together with these personality traits forms a behavioral syndrome.

3. Physiological and chemical factors

The physiological factors that regulate fish aggressive behavior can be roughly divided into two categories, namely monoamine neurotransmitters and hormones, including dopamine (DA), 5-hydroxytryptamine (5-HT), histamine (HA), growth hormone (GH), HPI axis-mediated corticosteroids, and HPG axis-mediated sex hormones.

5-HT is one of the most important neurotransmitters regulating aggressive behavior. The precursor of 5-HT is tryptophan (Trp), which is catalyzed by tryptophan hydroxylase (TPH) to form 5-HT, which is then oxidized and deaminated by monoamine oxidase (MAO) to form 5-hydroxyindole acetic acid (5-HIAA), which is finally excreted from the body in the urine.

6- The 5-HT energy utilization rate (5-HIAA/5-HT) in fish is negatively correlated with aggressiveness. For example, the brain 5-HIAA/5-HT of highly aggressive individuals of Sebastes schlegelii juveniles is significantly lower than that of low-aggressive individuals.

Figure 4. 5-HT synthesis pathway (picture from the Internet)

4. Genetic factors

From a molecular genetics perspective, fish aggressive behavior is a quantitative trait that results from the combined action of multiple separate genes that are able to respond rapidly to the environment.

Some phenotypic differentiation of fish aggressive behavior may be caused by genetic mutations. The genes related to the various neural pathways that affect fish aggressive behavior have also become the focus of research. For example, fibroblast growth factor receptor (FGFR1A), dopamine transporter gene (SLC6A3), tyrosine hydroxylase gene (TH), histamine production enzyme gene (HNT), histamine receptor gene (HRH2 and HRH3), 5-HT receptor gene (HTR1A, HTR1B and HTR2A), arginine oxytocin receptor gene (V1A1 and V1A2), corticotropin releasing hormone (CRH), glucocorticoid receptor (NR3C1) and neuropeptide gene (NPY).

Figure 5. Diagram of the neural pathways that regulate aggressive behavior in fish (Image from Filby et al. 2010. [2])

References:

[1] Li Haixia, Wang Jie, Zhang Xu, Hu Yu, Cai Qinglin, Liu Ying, Ma Zhen. Sex Differences in Mate Choice Preference Characteristics of Aequidens rivulatus[J]. Animals, 2022, 12(9).

[2] Filby Amy ,Paull Gregory ,Hickmore Tamsin ,Tyler Charles. Unravelling the neurophysiological basis of aggression in a fish model[J]. BMC Genomics, 2010,11(1).

Author: Ge Xiaoyu (graduate student at the Aquaculture and Equipment Engineering Research Center of Dalian Ocean University)

Scientific review: Liu Ying (doctoral supervisor at the School of Biosystems Engineering and Food Science, Zhejiang University), Ma Zhen (associate professor and graduate supervisor at the Aquaculture and Equipment Engineering Research Center, Dalian Ocean University)

Planning: Liu Yadan, Liu Ying (doctoral supervisors at the College of Biosystems Engineering and Food Science, Zhejiang University)

Editor: Wu Yuetong and Xie Yun