Background
The authors write that there is "concern about the deterioration of the world's oceans," and that one line of evidence for this concern is "an increasing incidence of jellyfish blooms." However, they say that this "perception," as they describe it, is "largely based on reports of increases in a few disparate regions (Condon et al., 2012)," as well as on "an analysis of media reports and perceptions of scientific experts and fishers (Brotz et al., 2012)."

What was done
In revisiting this important subject, Condon et al. (2013) say they analyzed "all available long-term datasets on changes in jellyfish abundance across multiple coastal stations, using linear and logistic mixed models and effect-size analysis," in order "to test the null hypothesis that jellyfish population sizes and the occurrence of blooms have not significantly increased in the world's oceans."

What was learned
The ultimate result of the several analyses of the 22 researchers - who hail from Argentina, Australia, Canada, Japan, Norway, Peru, Slovenia, Spain, the United Kingdom and the United States - was that "there is no robust evidence for a global increase in jellyfish." Although they acknowledge "there has been a small linear increase in jellyfish since the 1970s," they say that "this trend was unsubstantiated by effect-size analysis that showed no difference in the proportion of increasing vs. decreasing jellyfish populations over all time periods examined." Instead, they report that "the strongest non-random trend indicated jellyfish populations undergo larger, worldwide oscillations with an approximate 20-year periodicity, including a rising phase during the 1990s that contributed to the perception of a global increase in jellyfish abundance."

What it means
The results of Condon et al.'s study do not support the view that the global abundance of jellyfish is increasing as a result of "the deterioration of the world's oceans." What they do imply, as the international research team concludes, is the continuance of normal "recurrent phases of rise and fall in jellyfish populations that society should be prepared to face."

References
Botz, L., Cheung, W.W.L., Kleisner, K., Pakhomov, E. and Pauly, D. 2012. Increasing jellyfish populations: Trends in large marine ecosystems. Hydrobiologia 690: 3-20.

Condon, R.H., Graham, W.M., Duarte, C.M., Pitt, K.A., Lucas, C.H., Haddock, S.H.D., Sutherland, K.R., Robinson, K.L., Dawson, M.N., Decker, M.B., Mills, C.E., Purcell, J.E. Malej, A., Mianzan, H., Uye, S.-I., Gelcich, S. and Madin, L.P. 2012. Questioning the rise of gelatinous zooplankton in the world's oceans. BioScience 62: 160-169.