ABSTRACT Aggression, especially territorial is a serious problem in farmed and ornamental fish. Especially the tropical species like Siamese fighting fish Betta splendens or marine species like blu head Thalassoma bifasciatum. Also farmed species of Salmoniformes (Coho salmon Oncorhynchus kisutch, rainbow trout Oncorhynchus mykiss) exhibit territorial aggression which is correlated with growth rate of the fish. In these species native territorial aggression occurs, and in ethology it is called stereotypical agonistic behaviour or conspecific aggression. In this type modulation of aggression serotonin (5-HT) plays a main role. A decrease of 5-HT in brain intensifies these type of aggression and in opposite an increase of 5-HT reduces it. The purpose of this study was to examine the effects of different doses of fluoxetine on male aggressive behaviors of Betta splendens fish. It was concluded that fluoxetine added to aquarium water in the doses of 4, 40 and/or 100 μg·ga–1 BW during 14 - 28 days increased synaptic levels of 5-HT what in turn resulted in the reduction of the specific aggressive behaviors. Fluoxetine caused periodic, and sometimes even total weakening of male-male type fight, which is a standard trial applied in ethological research on Siamese fighting fish. In current study, the most effective was the dose of 40 μg·g–1 BW. The mechanism of this antiaggressivity depended on an increase of serotonergic system activity in animal brain (especially, in raphe nuclei), however synaptic levels of 5-HT in brain were not measured in this experiment. Thus, the hypothesis that exposure from day 1 - 21 would reduce aggression was rejected; as was the hypothesis that exposure on fluoxetine from day 10 (16) - 14 - 28 would rapidly reduce aggression in teleost fishes. The results obtained suggests that a complex role of serotonin in the expression of aggression in teleost fish because acute treatment with 5-HT1A receptor agonist WAY-100635 did not increase aggression in fish . We emphasize the physiological concepts that can be addressed with this experiment, including the role of the serotonergic system in regulation of aggression, and the interplay of environmental contaminants and physiology in regulating the expression of behavior of fishes.
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