Stress, cortisol, and feed intake in rainbow trout (Oncorhynchus mykiss) - Can you stomach the stress?

Fish in aquaculture is known to ingest less feed after exposure to stressors such as handling, crowding, vaccination and transportation. In scientific studies, this phenomenon has been ascribed to an appetite-reducing effect of stress, namely the stress hormone cortisol, or other aspects of the physiological stress response. However, preliminary findings have suggested that during recovery from stress fish has not necessarily lost the will to feed, but rather the ability to feed. To test the hypothesis that decreased feed intake in salmonids subjected to stressful situations is at least in part caused by other factors than a reduction in appetite, feed intake and attempts at feed intake in isolated rainbow trout was closely monitored after transfer to a novel environment. In a follow-up study known appetite markers were measured in the hypothalamus and plasma in rainbow trout receiving exogenous cortisol through feed. Furthermore, I also investigated alteration of the gastrointestinal (GI) tract in rainbow trout, by measuring stomach volume after exogenous cortisol exposure. From these studies I found that the anorectic stage following subjection to stress is, at least partly, mediated through some physical obstruction in the upper GI tract. I saw that nearly all fish went through three distinct phases of feeding behavior subsequent to stress exposure: Passive/anorectic, active but unable, and active and able. The second stage is an intermediate phase between being completely passive and successfully ingesting feed. The intermediate phase of feeding behavior has not been reported earlier. Here, fish are seen actively trying to ingest feed, but appearing unable to swallow, and thus repeatedly spitting grabbed pellets back out. I also noted that reduced feed intake in response to cortisol treatment was not caused by the alteration of neither corticotropin-releasing factor, cocaine- and amphetamine-regulated transcript, neuropeptide Y, leptin or arginine vasotocin, all which are known appetite-regulating factors. When examining stomach volume of rainbow trout intraperitoneally injected with cortisol, I found that stomach volume was significantly reduced compared to control. In conclusion, the sum of these findings indicates that the observed reduction of feed intake in stressed salmonids is not caused by the lack of appetite per se, but rather a physical obstruction somewhere along the upper GI tract, rendering the fish willing but unable to ingest feed. The mechanisms contributing to this behavior are still to be elucidated.