Fish welfare issues are predicated on understanding whether fish are sentient beings. Therefore, we analyzed the logic of the methodologies used for studying this attribute. We conclude that empirical science is unable to prove or to disprove that fish are sentient beings. Thus, we propose a combined ethical-scientific approach for considering fish as sentient beings. The most difficult ongoing question is to determine which conditions fish prefer. Approaches to assess fish preferences should be rigorously and cautiously employed. In light of these considerations, attempts to establish physiological standards for fish welfare are discouraged, and a preference-based definition of fish welfare is proposed.

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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.

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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.

In this review, we provide a brief retrospective history of the science of animal welfare and recognize the sentience of non-human animals; however, we emphasize that crucial problems remain regarding how to define and measure animal welfare. In general, the use of physiological measures to assess welfare is discouraged. Furthermore, there is a theoretical background for measures of stress, but not for welfare states because life may not be at risk. Instead, a preference or choice-based approach, which is based on the animal decision, is recommended. To this end, welfare is discussed and then contrasted with disease, health, stress and distress. In addition, the importance of prospective capacities for the welfare of human and non-human animals is discussed.

Four substrata were offered to groups of adult Nile tilapia Oreochromis niloticus (one male and two females) simultaneously: pure sand, a mixture of sand and shells, stones and no substratum. The results showed that males chose to dig nests in a lighter and more homogeneous substratum.