Climate change is linked to changes in the distribution and phenology of plankton through several trophic levels, with potential impacts on pelagic food webs. Global warming and other anthropogenic stresses have also been implicated in proposed large-scale increases in jellyfish numbers. We have analyzed a 1992 to 2011 time series on occurrence of common scyphozoan jellyfish at a Skagerrak location for changes in their abundance and phenology, and attempted to relate these observations to environmental and climatic variables including temperature, salinity, chl a concentration, zooplankton prey biomass and the North Atlantic Oscillation index using generalized linear models (GLMs). Cyanea spp. abundances showed no significant trend during the study period. Their fluctuations were positively related to temperature and prey availability, with possible impacts on the polyp generation. The central tendency of annual Cyanea spp. abundance showed a borderline significant shift towards a later date, while zooplankton biomass showed the opposite trend, suggesting potential change in trophic match. Aurelia aurita abundance exhibited a considerable decline and 5-yr cyclicity, but neither pattern was explained by any of the investigated explanatory variables. Unidentified factors causing cyclicity may include predatory interactions or density-dependent processes, possibly involving the benthic stages. Observed changes in abundance may also be related to a possible regime shift in the Norwegian Skagerrak area around the turn of the millennia. Our results highlight the importance of considering changes in jellyfish abundances with regard to locality and species. Improved understanding of polyp ecology seems necessary for understanding fluctuations in jellyfish numbers.