Changes in water temperature may affect the aggressive behavior of aquatic organisms, such as fish, either by changing some physiological mechanisms or by increasing the probability of encounters between individuals as a result of variation in their swimming activity. In our study, we evaluated the influence of increasing and decreasing temperature on the aggressive behavior of the Neotropical cichlid fish Cichlasoma paranaense. Firstly, we tested the critical thermal maximum (CTMax) tolerated by this species. Then, we tested the effect of decreasing or increasing the water temperature in 6o C (starting at 27° C) on the aggressive interactions of fish under isolation or housed in groups. We found a CTMax value of 39° C for C. paranaense. We also observe that a 6° C decrease in water temperature lowers swimming activity and aggressive interactions in both isolated and group-housed fish, as expected. On the other hand, the increase in temperature had no effect on the fish’s aggressive behavior, neither for isolated nor for grouped fish. We concluded that C. paranaense shows high tolerance to elevated temperatures and, in turn, it does not affect aggressive behavior. Nevertheless, we cannot dismiss possible effects of elevated temperatures on aggressive interactions over longer periods.

Fish social behavior can be affected by artificial environments, particularly by factors that act upon species that show aggressive behavior to set social rank hierarchy. Although aggressive interactions are part of the natural behavior in fish, if constant and intense, such interactions can cause severe body injuries, increase energy expenditure, and lead the animals to suffer from social stress. The immediate consequence of these factors is a reduced welfare in social fish species. In this paper, we consider the factors that impact on the social behavior and welfare of Nile tilapia, an African cichlid fish widely used both in fish farms and in research; this species is frequently used as a model for physiology and behavior research. This is a polygynous species whose males interact aggressively, establishing a territorial based hierarchy, where a dominant male and several subordinate males arise. When social stability is shrunk, the negative effects of prolonged fighting emerge. In this paper, we summarized how some of the common practices in aquaculture, such as classifying individuals by matching their sizes, water renewal, stock density, and environment lighting affect Nile tilapia social aggressive interactions and, in turn, impact on its welfare. We also discuss some ways to decrease the effects of aggressive interactions in Nile tilapia, such as environment color and body tactile stimulation.

In variable environments, animals can learn to alter their behavior to adjust to changes. Sometimes, however, this learning ability can be impaired. For example, challenges in the social or physical environment can trigger physiological responses that compromise an individual's capacity to learn and these can prevent the animal from modifying its behavior to cope with the altered environment. One such disruption that might affect learning ability is the isolation of an individual that would normally live in a social group. To investigate the effects of isolation on cognition, we tested whether social isolation impairs learning skills in a cichlid fish, Cichlasoma paranaense. Two treatments were compared, one with isolated individuals and another with fish housed as part of a social group. Cognition was assessed in terms of the fish learning to associate a visual landmark with an accessible food reward. Fish searched for a food reward in a T-maze: a green or a yellow visual landmark signaled which chamber contained accessible food. Learning was assumed when a fish was able to find the food in nine out of ten trials. All fish, regardless of treatment, found it challenging to reach the learning criterion, but fewer isolated fish (3 out of 15 fish) were likely to learn the task compared to fish housed socially (7 out of 14 fish) (Bayesian logistic regression with 95% of chance that the two treatments differ). The results therefore suggest that social isolation for a normally social species of fish can impair learning. Furthermore, given that fish sometimes need to be isolated to treat them in a specific way, or to monitor an individual's response during a test, our results suggest that there may be some welfare implications for the way we treat and use social species of fish in empirical studies.

Inhibitory control is a way to infer cognitive flexibility in animals by inhibiting a behavioral propensity to obtain a reward. Here we tested whether there are differences in inhibitory control between females and males of the fish Nile tilapia owing to their distinct reproductive roles. Individuals were tested under a detour-reaching paradigm, consisting of training fish to feed behind an opaque barrier and, thereafter, testing them with a transparent one. Fish is expected to avoid trying to cross through the transparent barrier to achieve food (reward), thus showing inhibitory control by recovering the learned detour with the opaque apparatus. Both males and females learned to detour the transparent barrier with similar scores of correct responses, whereas females reached the food faster. This result is probably associated to their different sex roles in reproduction: females care for the eggs and fry inside their mouth (thus requiring a high inhibitory control not to swallow them), whereas males have to stay inside the territory defending it against intruder males, which also demands some inhibitory ability not to leave the spawning site and take the risk of losing it. Furthermore, this evidence of cognitive flexibility can enable social fish to deal with unpredictable interactions.

Nests are the key elements of the reproductive repertoire of several fish species. Understanding the physiological aspects behind their nesting behaviour is imperative to improve our knowledge about behavioural mechanisms focusing on species conservation. Nevertheless, current knowledge on fish nesting is still underexplored. Here, we show a synthesis based on an integrative approach for understanding the diversity of nesting behaviour in fish. Because a nest sometimes does not involve new buildings (as in birds), we suggest a more comprehensive definition based on the behaviour instead of the structure per se. Forty fish families were recorded as they made nests, which were sorted into six main types: bowls, burrows, foam nests, nests made of plant matter and animal parts, besides nests made on clean rock surfaces. Besides spawning and parental care, these nests also serve as a refuge against predators and male competitors, as targets for sexual selection, including bowers with no parental care function, and badges of social status. We showed that, although it is advantageous to what the Darwinian fitness of the species is concerned, nesting also requires time and energy. We propose an evolution of nest types and functions, from the ancestral bowl used as refuge to derived foam nests with sexual selection functions. Physiologically, nesting is controlled by the HPG axis and particular brain circuits, this probably being the less explored subject. Finally, we highlighted that, in the changing world of the Anthropocene, nesting behaviour emerges as a framework to indicate the quality of the aquatic environments.