Food competition and its relation to aquaculture in Perca fluviatilis

In this thesis, explanations for, and predictors of, the large growth heterogeneity, in juvenile perch in aquaculture and natural environments, were studied. A large variation between individuals in competitive ability, estimated as feeding success or growth, was found. Relative feeding success (in aquaria) and growth rates (in tanks) of individuals were consistent over time within environments but not positively correlated between them. It was indicated that individual feeding success was negatively correlated between environments. Individuals may be differently prone to be successful in specific environments due to variations in innate characteristics, but this has not been tested. Predation risk decreased boldness (risk proneness) but did not affect food intake at the individual level. Better competitors were also bolder. Feeding success was positively correlated to growth rate within aquaria, but not between environments (aquaria vs. tanks). Group composition affected individual boldness but not food intake. Strong social hierarchies were not found, but aggressive behaviour was observed in one study. Thus, a large variation in boldness and competitive ability was found, and this variation may be of importance for the large variation in growth. There was a seasonal variation in growth. This knowledge is important for optimising feed management, thereby maximising growth and economics and minimising environmental impact of future perch farms. The consistency in the differences in individual growth rate indicates that perch have a large potential for successful breeding programs. Boldness and competitive success in aquaria experiments can not be used as indicators of individual growth potential. High light intensities were found to stress the perch, indicated by an increased swimming activity. Tank wall colour did not seem to affect behaviour or growth, and no colour was selected over others.