Reports on jellyfish blooms and their negative impacts, at local and global scale, have increased recently (Condon et al., 2012). Although Aurelia aurita, or moon jellyfish, is abundant in the North Sea coastal waters (Hay et al., 1990), detailed information on its abundance in the Southern North Sea is scarce. Qualitative information is available mainly for the Netherlands and German waters (Barz and Hirch, 2007) and limited for Belgian waters. The impact of water temperature on the abundance of A. aurita through asexual reproduction, was shown from the laboratory experiments of Liu (2009), Han and Uye (2010), Holst (2012) and Purcell et al. (2012). In Belgian waters, asexual reproduction of A. aurita happens in winter (De Blauwe, 2003), so winter sea surface temperature (SST) can play a crucial role on its abundance. Three hypothesises related to A. aurita population in Belgian waters are tested in the present study:(H1) A. aurita medusae occur seasonally in Zeebrugge harbour, from April to August; (H2) A decrease of the minimum winter SST increases the medusa population of A. aurita; and (H3) A. aurita medusae on Belgian beaches originate from the population in Zeebrugge harbour. Data of A. aurita medusa abundances, collected in Zeebrugge harbour (1998-2013) and along the Belgian beaches (2007-2013) by members of the StrandwerkgroepBelgïeand made available via the waarnemingen web site, are used to study the seasonal pattern and the year-to-year variability. In addition, a protocol of A. aurita observation in Zeebrugge harbour (Spring-Summer 2013) was set up with the aim to check the three hypotheses. Data analysis shows that A. aurita medusae in Zeebrugge harbour (Belgium) occur seasonally from April to August, with a peak of abundance in May. It also suggests an inverse tendency between winter SST and medusa abundance in Zeebrugge harbour. No clear correlation is found between winter SST and medusa abundance on the Belgian beaches because the stranding of moon jellyfish on the beaches requires specific conditions (e.g. onshore winds and currents). The cold winter SST in 2013 could partly explain the massive stranding in Oostende of A. aurita medusae on the 25th of May 2013. The OSERIT model interface, developed to forecast the impact of oil pollution, has been used in backtracking mode to assess this stranding. According to the results, the medusae could come from the east coast of United Kingdom (simulation including the wind effect) or from France (simulation without wind effect). An intermediate situation such that the medusae stranding could come from the southern-east coast of United Kingdom is probably more realistic than two presented situations. Due to weaknesses of both datasets, no meaningful statistical analysis was applied in this study. To get a better insight into A. aurita dynamics and its link with winter SST, further researches should be conducted. Suggestions for future observation protocol are given. In addition to in situ observations, the OSERIT model interface, providing adaptations, would be a useful tool to assess the dispersal of medusa population in the Southern North Sea.