Do Differences in Conspecific Body Size Induce Social Stress in Domestic Rainbow Trout?

Little is known about the cognitive and subjective experiences of fish that are confined with conspecifics of varying body sizes. Plasma cortisol and several behavioural variables were recorded when a ȁ8mediumȁ9 sized fish had its familiar social group (comprised of medium, like-sized individuals) replaced with a group of fish that were either medium sized, smaller or larger than itself. Interestingly, the medium sized test fish showed very few behavioural responses indicative of stress when exposed to a new social cohort. Fish did not use a particular part of their tank, or water column, nor did they show any significant change in locomotory behaviour. There was no difference in aggressive ȁ8chasingȁ9 behaviour in any of the treatments, however, medium sized fish were frequently chased by their tank-mates when exposed to the ȁ8largeȁ9 fish treatment, and were almost never chased by fish smaller than themselves. Similarly, plasma cortisol concentrations did not differ between fish that were exposed to different size treatment groups, although there was a high increase above baseline levels; this suggests that encounters with unfamiliar fish were stressful, regardless of size.