ABSTRACT. – The population density and individual size of the common jellyfish Aurelia aurita were studied during 2014 and 2015 in two Danish fjord systems, Kertinge Nor and Mariager Fjord in order to obtain a better understanding of the driving forces for somatic degrowth (shrinkage) of medusa during late summer and autumn. In both fjord systems the numerous medusae were characterized by their small body size and by a distinct phase of degrowth. The population predation impact of A. aurita, with estimated zooplankton half-lives of only about 1 to 3 d, indicated that shortage of prey controls the maximum umbrella size of only 60 (Kertinge Nor) to 100 mm (Mariager Fjord) and the subsequent degrowth. When jellyfish were brought into the laboratory in early spring and the late degrowth period and continuously fed with zooplankton (Artemia salina), equivalent to 5.8 μg C l–1, this resulted in initial growth rates of 11.3 and 24.4 % d–1 in two series of experiments with 56.9 and 5.5 mm umbrella diameter, respectively, and considerably longer survival than of jellyfish in their natural environment. The degrowth rates in Kertinge Nor (–1.2 % d–1) and Mariager Fjord (−1.5 and −0.7 % d−1 in 2014 and 2015, respectively) were slightly lower than observed in laboratory starvation experiments (−3.2 and −4.2 % d−1), indicating that the starvation of jellyfish in nature was less pronounced, i.e. some prey organisms may have been available although the short estimated half-lives of zooplankton suggested a pronounced predation impact exerted by the jellyfish.