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In this study, we described immediate/acute reactions to stress-related chemical cues (SC – Stress Cue) in fish. Specifically, we evaluated the effects of SC on ventilation rate (VR) and feeding behavior in Nile tilapia (Oreochromis niloticus), a highly relevant species for world aquaculture, therefore, to understand the diversity of stressful contexts and stress responses in this species have important practical applications (stress reduction). Stress cue was obtained from conspecifics exposed to a handling stressor. Stress was confirmed by measuring plasma cortisol levels. The responses to SC were contrasted to chemical control cues: a cue originated from non-stressed conspecifics and pure water (vehicle control). We observed that Nile tilapia exposed to SC had an increase in VR, but without effects on feeding behavior (feeding latency and ingestion). Thus, the SC is a stressor that induces readily stress response (VR increase), suggesting sympathetic activation, but did not change feeding performance. In practical terms, it is positive because although social propagation of stress via SC elicits a stress response, it did not harm appetite. [ABSTRACT FROM AUTHOR]

The selective serotonin reuptake inhibitor fluoxetine is one of the most commonly administered psychotropic medications; however, it has been recognized as toxic to aquatic life. In this study, we showed that stress responses and feeding motivation in Nile tilapia were affected by acute exposure to fluoxetine. To reach that conclusion, we exposed Nile tilapia to 0, 1 or 10 µg/L (environmentally/biologically relevant doses) of fluoxetine over a 24 h period and then exposed them to a handling stressor. We found that the 10 µg/L dose enhanced cortisol response to stress but caused an earlier decrease in the ventilation boost induced by that stressor. An immediate ventilation boost after stressful stimuli indicates sympathetic activation. Thus, this suggests that fluoxetine decreased sympathetic nervous system activity but augmented hypothalamus–pituitary–interrenal axis activity in the fish. Both feeding latency and ingestion were similar among the tested conditions; however, a multiple logistic regression model revealed that in the presence of a stressor or fluoxetine, the Nile tilapia tended to ingest less food but there was a higher probability of this decrease to be associated with fluoxetine. We concluded that acute exposure to environmentally/biologically relevant fluoxetine concentrations over 24 h acted as a modifying factor for Nile tilapia stress physiology and tended to interfere with feeding motivation. An acute stress response is an emergency reaction that contributes to the recovery of homeostasis. In the presence of fluoxetine, modifications of acute stress responses and the tendency to reduce food intake, which restricts the ability to replace the energy spent on stress responses, could compromise the resumption of homeostasis and an animal's adjustment to different environmental contexts, such as those associated with aquaculture, in which anthropogenic stressors inevitably occur. [ABSTRACT FROM AUTHOR]